Thinking in Java, 2nd edition, Revision 9

©2000 by Bruce Eckel

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Preface

I suggested to my brother Todd, who is making the leap from hardware into programming, that the next big revolution will be in genetic engineering.

We’ll have microbes designed to make food, fuel, and plastic; they’ll clean up pollution and in general allow us to master the manipulation of the physical world for a fraction of what it costs now. I claimed that it would make the computer revolution look small in comparison.

Then I realized I was making a mistake common to science fiction writers: getting lost in the technology (which is of course easy to do in science fiction). An experienced writer knows that the story is never about the things; it’s about the people. Genetics will have a very large impact on our lives, but I’m not so sure it will dwarf the computer revolution (which enables the genetic revolution)—or at least the information revolution. Information is about talking to each other: yes, cars and shoes and especially genetic cures are important, but in the end those are just trappings. What truly matters is how we relate to the world. And so much of that is about communication.

This book is a case in point. A majority of folks thought I was very bold or a little crazy to put the entire thing up on the Web. “Why would anyone buy it?” they asked. If I had been of a more conservative nature I wouldn’t have done it, but I really didn’t want to write another computer book in the same old way. I didn’t know what would happen but it turned out to be the smartest thing I’ve ever done with a book.

For one thing, people started sending in corrections. This has been an amazing process, because folks have looked into every nook and cranny and caught both technical and grammatical errors, and I’ve been able to eliminate bugs of all sorts that I know would have otherwise slipped through. People have been simply terrific about this, very often saying “Now, I don’t mean this in a critical way...” and then giving me a collection of errors I’m sure I never would have found. I feel like this has been a kind of group process and it has really made the book into something special.

But then I started hearing “OK, fine, it’s nice you’ve put up an electronic version, but I want a printed and bound copy from a real publisher.” I tried very hard to make it easy for everyone to print it out in a nice looking format but that didn’t stem the demand for the published book. Most people don’t want to read the entire book on screen, and hauling around a sheaf of papers, no matter how nicely printed, didn’t appeal to them either. (Plus, I think it’s not so cheap in terms of laser printer toner.) It seems that the computer revolution won’t put publishers out of business, after all. However, one student suggested this may become a model for future publishing: books will be published on the Web first, and only if sufficient interest warrants it will the book be put on paper. Currently, the great majority of all books are financial failures, and perhaps this new approach could make the publishing industry more profitable.

This book became an enlightening experience for me in another way. I originally approached Java as “just another programming language,” which in many senses it is. But as time passed and I studied it more deeply, I began to see that the fundamental intention of this language is different from all the other languages I have seen.

Programming is about managing complexity: the complexity of the problem you want to solve, laid upon the complexity of the machine in which it is solved. Because of this complexity, most of our programming projects fail. And yet, of all the programming languages of which I am aware, none of them have gone all-out and decided that their main design goal would be to conquer the complexity of developing and maintaining programs [1] . Of course, many language design decisions were made with complexity in mind, but at some point there were always some other issues that were considered essential to be added into the mix. Inevitably, those other issues are what cause programmers to eventually “hit the wall” with that language. For example, C++ had to be backwards-compatible with C (to allow easy migration for C programmers), as well as efficient. Those are both very useful goals and account for much of the success of C++, but they also expose extra complexity that prevents some projects from being finished (certainly, you can blame programmers and management, but if a language can help by catching your mistakes, why shouldn’t it?). As another example, Visual Basic (VB) was tied to BASIC, which wasn’t really designed to be an extensible language, so all the extensions piled upon VB have produced some truly horrible and unmaintainable syntax. Perl is backwards-compatible with Awk, Sed, Grep, and other Unix tools it was meant to replace, and as a result is often accused of producing “write-only code” (that is, after a few months you can’t read it). On the other hand, C++, VB, Perl, and other languages like Smalltalk had some of their design efforts focused on the issue of complexity and as a result are remarkably successful in solving certain types of problems.

What has impressed me most as I have come to understand Java is what seems like an unflinching goal of reducing complexity for the programmer. As if to say “we don’t care about anything except reducing the time and difficulty of producing robust code.” In the early days, this goal has resulted in code that doesn’t run very fast (although there have been many promises made about how quickly Java will someday run) but it has indeed produced amazing reductions in development time; half or less of the time that it takes to create an equivalent C++ program. This result alone can save incredible amounts of time and money, but Java doesn’t stop there. It goes on to wrap all the complex tasks that have become important, such as multithreading and network programming, in language features or libraries that can at times make those tasks trivial. And finally, it tackles some really big complexity problems: cross-platform programs, dynamic code changes, and even security, each of which can fit on your complexity spectrum anywhere from “impediment” to “show-stopper.” So despite the performance problems we’ve seen, the promise of Java is tremendous: it can make us significantly more productive programmers.

One of the places I see the greatest impact for this is on the Web. Network programming has always been hard, and Java makes it easy (and they’re working on making it easier all the time). Network programming is how we talk to each other more effectively and cheaper than we ever have with telephones (email alone has revolutionized many businesses). As we talk to each other more, amazing things begin to happen, possibly more amazing even than the promise of genetic engineering.

In all ways—creating the programs, working in teams to create the programs, building user interfaces so the programs can communicate with the user, running the programs on different types of machines, and easily writing programs that communicate across the Internet—Java increases the communication bandwidth between people. I think that perhaps the results of the communication revolution will not be seen from the effects of moving large quantities of bits around; we shall see the true revolution because we will all be able to talk to each other more easily: one-on-one, but also in groups and, as a planet. I've heard it suggested that the next revolution is the formation of a kind of global mind that results from enough people and enough interconnectedness. Java may or may not be the tool that foments that revolution, but at least the possibility has made me feel like I'm doing something meaningful by attempting to teach the language.

Preface to the 2nd edition

People have made many, many wonderful comments about the first edition of this book, which has naturally been very pleasant for me. However, every now and then someone will have complaints, and for some reason one complaint that comes up periodically is “the book is too big.” In my mind it is faint damnation indeed if “too many pages” is your only complaint. (One is reminded of the Emperor of Austria’s complaint about Mozart’s work: “Too many notes!” Not that I am in any way trying to compare myself to Mozart.) In addition, I can only assume that such a complaint comes from someone who is yet to be acquainted with the vastness of the Java language itself, and has not seen the rest of the books on the subject—for example, my favorite reference is Cay Horstmann & Gary Cornell’s Core Java (Prentice-Hall), which grew so big it had to be broken into two volumes. Despite this, one of the things I have attempted to do in this edition is trim out the portions that have become obsolete, or at least nonessential. I feel comfortable doing this because the original material remains on the Web site and the CD ROM that accompanies this book, in the form of the freely-downloadable first edition of the book (at www.BruceEckel.com). If you want the old stuff, it’s still there, and this is a wonderful relief for an author. For example, you may notice that the original last chapter, “Projects,” is no longer here; two of the projects have been integrated into other chapters, and the rest were no longer appropriate. So, by all rights the book should be thinner.

But alas, it is not to be.

The biggest issue is the continuing development of the Java language itself, and in particular the expanding APIs that promise to provide standard interfaces for just about everything you’d like to do (and I won’t be surprised to see the “JToaster” API eventually appear). Covering all these APIs is obviously beyond the scope of this book and is a task relegated to other authors, but some issues cannot be ignored. The biggest of these include server-side Java (primarily Servlets & Java Server pages, or JSPs), which is truly an excellent solution to the World Wide Web problem, wherein we’ve discovered that the various Web browser platforms are just not consistent enough to support client-side programming. In addition, there is the whole problem of easily creating applications to interact with databases, transactions, security, and the like, which is involved with Enterprise Java Beans (EJBs). These topics are wrapped into the chapter formerly called “Network Programming” and now called “Distributed Computing,” a subject that is becoming essential to everyone. You’ll also find this chapter has been expanded to include Jini (pronounced “genie,” and it isn’t an acronym, just a name) and JavaSpaces, two cutting-edge technologies that allow us to change the way we think about interconnected applications. And of course the book has been changed to use the Swing GUI library throughout. Again, if you want the old Java 1.0/1.1 stuff you can get it from the freely-downloadable book at www.BruceEckel.com (it is also included on this edition’s new CD ROM, bound into the book; more on that a little later).

Aside from additional small language features added in Java 2 and corrections made throughout the book, the other major change is in the collections chapter (9), which now focuses on the Java 2 collections used throughout the book. I’ve also improved that chapter to more deeply go into some of the important issues of collections, in particular how a hash function works (so that you can know how to properly create one). There have been other movements and changes, including a rewrite of Chapter 1, and removal of some appendices and other material that I consider no longer necessary for the printed book, but those are the bulk of them. In general, I’ve tried to go over everything, remove from the 2nd edition what is no longer necessary (but which still exists in the electronic first edition), include changes, and improve everything I could. As the language continues to change—albeit not quite at the same breakneck pace as before—there will no doubt be further editions of this book.

For those of you who still can’t stand the size of the book, I do apologize. Believe it or not, I have worked hard to keep it small. Despite the bulk, I feel like there may be enough alternatives to satisfy you. For one thing, the book is available electronically (from the Web site, and also on the CD ROM that accompanies this book), so if you carry your laptop you can carry the book on that with no extra weight. If you’re really into slimming down, there are actually Palm Pilot versions of the book floating around. (One person told me he would read the book in bed on his Palm with the backlighting on to keep from annoying his wife. I can only hope that it helps send him to slumberland.) If you need it on paper, I know of people who print a chapter at a time and carry it in their briefcase to read on the train.

Java 2

At this writing, the release of Sun’s Java Development Kit (JDK) 1.3 is imminent, and the proposed changes for JDK 1.4 have been publicized. Although these version numbers are still in the “ones,” the standard way to refer to any version of the language that is JDK 1.2 or greater is to call it “Java 2.” This indicates the very significant changes between “old Java”—which had many warts that I complained about in the first edition of this book—and this more modern and improved version of the language, which has far fewer warts and many additions and nice designs.

This book is written for Java 2. I have the great luxury of getting rid of all the old stuff and writing to only the new, improved language because the old information still exists in the electronic 1st edition on the Web and on the CD ROM (which is where you can go if you’re stuck using a pre-Java-2 version of the language). Also, because anyone can freely download the JDK from java.sun.com, it means that by writing to Java 2 I’m not imposing a financial hardship on someone by forcing them to upgrade.

There is a bit of a catch, however. JDK 1.3 has some improvements that I’d really like to use, but the version of Java that is currently being released for Linux is JDK 1.2.2. Linux (see www.Linux.org) is a very important development in conjunction with Java, because it is fast becoming the most important server platform out there—fast, reliable, robust, secure, well-maintained, and free, a true revolution in the history of computing (I don’t think we’ve ever seen all of those features in any tool before). And Java has found a very important niche in server-side programming in the form of Servlets, a technology that is a huge improvement over the traditional CGI programming (this is covered in the “Distributed Programming” chapter).

So although I would like to only use the very newest features, it’s critical that everything compiles under Linux, and so when you unpack the source code and compile it under that OS (with the latest JDK) you’ll discover that everything will compile. However, you will find that I’ve put notes about features in JDK 1.3 here and there.

The CD ROM

Another bonus with this edition is the CD ROM that is packaged in the back of the book. I’ve resisted putting CD ROMs in the back of my books in the past because I felt the extra charge for a few Kbytes of source code on this enormous CD was not justified, preferring instead to allow people to download such things from my Web site. However, you’ll soon see that this CD ROM is different.

The CD does contain the source code from the book, but it also contains the book in its entirety, in several electronic formats. My favorite of these is the HTML format, because it is fast and fully indexed—you just click on an entry in the index or table of contents and you’re immediately at that portion of the book.

The bulk of the 300+ Megabytes of the CD, however, is a full multimedia course called Thinking in C: Foundations for C++ & Java. I originally commissioned Chuck Allison to create this seminar-on-CD-ROM as a stand-alone product, but decided to include it with the second editions of both Thinking in C++ and Thinking in Java because of the consistent experience of having people come to seminars without an adequate background in C. The thinking apparently goes “I’m a smart programmer and I don’t want to learn C, but rather C++ or Java, so I’ll just skip C and go directly to C++/Java.” After arriving at the seminar, it slowly dawns on folks that the prerequisite of understanding C syntax is there for a very good reason. By including the CD ROM with the book, we can ensure that everyone attends a seminar with adequate preparation.

The CD also allows the book to appeal to a wider audience. Even though Chapter 3 (Controlling program flow) does cover the fundamentals of the parts of Java that come from C, the CD is a gentler introduction, and assumes even less about the student’s programming background than does the book. It is my hope that by including the CD more people will be able to be brought into the fold of Java programming.

[1] I take this back on the 2nd edition: I believe that the Python language comes closest to doing exactly that. See www.Python.org.

Last Update:03/13/2000


Introduction

Like any human language, Java provides a way to express concepts. If successful, this medium of expression will be significantly easier and more flexible than the alternatives as problems grow larger and more complex.

You can’t look at Java as just a collection of features—some of the features make no sense in isolation. You can use the sum of the parts only if you are thinking about design, not simply coding. And to understand Java in this way, you must understand the problems with it and with programming in general. This book discusses programming problems, why they are problems, and the approach Java has taken to solve them. Thus, the set of features I explain in each chapter are based on the way I see a particular type of problem being solved with the language. In this way I hope to move you, a little at a time, to the point where the Java mindset becomes your native tongue.

Throughout, I’ll be taking the attitude that you want to build a model in your head that allows you to develop a deep understanding of the language; if you encounter a puzzle you’ll be able to feed it to your model and deduce the answer.

Prerequisites

This book assumes that you have some programming familiarity: you understand that a program is a collection of statements, the idea of a subroutine/function/macro, control statements such as “if” and looping constructs such as “while,” etc. However, you might have learned this in many places, such as programming with a macro language or working with a tool like Perl. As long as you’ve programmed to the point where you feel comfortable with the basic ideas of programming, you’ll be able to work through this book. Of course, the book will be easier for the C programmers and more so for the C++ programmers, but don’t count yourself out if you’re not experienced with those languages (but come willing to work hard; also, the multimedia CD that accompanies this book will bring you up to speed on the basic C syntax necessary to learn Java). I’ll be introducing the concepts of object-oriented programming (OOP) and Java’s basic control mechanisms, so you’ll be exposed to those, and the first exercises will involve the basic control-flow statements.

Although references will often be made to C and C++ language features, these are not intended to be insider comments, but instead to help all programmers put Java in perspective with those languages, from which, after all, Java is descended. I will attempt to make these references simple and to explain anything that I think a non- C/C++ programmer would not be familiar with.

Learning Java

At about the same time that my first book Using C++ (Osborne/McGraw-Hill, 1989) came out, I began teaching that language. Teaching programming languages has become my profession; I’ve seen nodding heads, blank faces, and puzzled expressions in audiences all over the world since 1989. As I began giving in-house training with smaller groups of people, I discovered something during the exercises. Even those people who were smiling and nodding were confused about many issues. I found out, by chairing the C++ track at the Software Development Conference for a number of years (and later the Java track), that I and other speakers tended to give the typical audience too many topics too fast. So eventually, through both variety in the audience level and the way that I presented the material, I would end up losing some portion of the audience. Maybe it’s asking too much, but because I am one of those people resistant to traditional lecturing (and for most people, I believe, such resistance results from boredom), I wanted to try to keep everyone up to speed.

For a time, I was creating a number of different presentations in fairly short order. Thus, I ended up learning by experiment and iteration (a technique that also works well in Java program design). Eventually I developed a course using everything I had learned from my teaching experience—one that I would be happy giving for a long time. It tackles the learning problem in discrete, easy-to-digest steps, and in a hands-on seminar (the ideal learning situation) there are exercises following each of the short lessons. I now give this course in public Java seminars, which you can find out about at www.BruceEckel.com. (The introductory seminar is also available as a CD ROM. Information is available at the same Web site.)

The feedback that I get from each seminar helps me change and refocus the material until I think it works well as a teaching medium. But this book isn’t just seminar notes—I tried to pack as much information as I could within these pages, and structured it to draw you through onto the next subject. More than anything, the book is designed to serve the solitary reader who is struggling with a new programming language.

Goals

Like my previous book Thinking in C++, this book has come to be structured around the process of teaching the language. In particular, my motivation is to create something that provides me with a way to teach the language in my own seminars. When I think of a chapter in the book, I think in terms of what makes a good lesson during a seminar. My goal is to get bite-sized pieces that can be taught in a reasonable amount of time, followed by exercises that are feasible to accomplish in a classroom situation.

My goals in this book are to:

  1. Present the material one simple step at a time so that you can easily digest each concept before moving on.
  2. Use examples that are as simple and short as possible. This sometimes prevents me from tackling “real world” problems, but I’ve found that beginners are usually happier when they can understand every detail of an example rather than being impressed by the scope of the problem it solves. Also, there’s a severe limit to the amount of code that can be absorbed in a classroom situation. For this I will no doubt receive criticism for using “toy examples,” but I’m willing to accept that in favor of producing something pedagogically useful.
  3. Carefully sequence the presentation of features so that you aren’t seeing something that you haven’t been exposed to. Of course, this isn’t always possible; in those situations, a brief introductory description is given.
  4. Give you what I think is important for you to understand about the language, rather than everything I know. I believe there is an information importance hierarchy, and that there are some facts that 95 percent of programmers will never need to know and that just confuse people and adds to their perception of the complexity of the language. To take an example from C, if you memorize the operator precedence table (I never did), you can write clever code. But if you need to think about it, it will also confuse the reader/maintainer of that code. So forget about precedence, and use parentheses when things aren’t clear.
  5. Keep each section focused enough so that the lecture time—and the time between exercise periods—is small. Not only does this keep the audience’s minds more active and involved during a hands-on seminar, but it gives the reader a greater sense of accomplishment.
  6. Provide you with a solid foundation so that you can understand the issues well enough to move on to more difficult coursework and books.

Online documentation

The Java language and libraries from Sun Microsystems (a free download) come with documentation in electronic form, readable using a Web browser, and virtually every third party implementation of Java has this or an equivalent documentation system. Almost all the books published on Java have duplicated this documentation. So you either already have it or you can download it, and unless necessary, this book will not repeat that documentation because it’s usually much faster if you find the class descriptions with your Web browser than if you look them up in a book. (Plus it will be up-to-date.) This book will provide extra descriptions of the classes only when it’s necessary to supplement the documentation so you can understand a particular example.

Chapters

This book was designed with one thing in mind: the way people learn the Java language. Seminar audience feedback helped me understand the difficult parts that needed illumination. In the areas where I got ambitious and included too many features all at once, I came to know—through the process of presenting the material—that if you include a lot of new features, you need to explain them all, and this easily compounds the student’s confusion. As a result, I’ve taken a great deal of trouble to introduce the features as few at a time as possible.

The goal, then, is for each chapter to teach a single feature, or a small group of associated features, in such a way that no additional features are relied upon. That way you can digest each piece in the context of your current knowledge before moving on.

Here is a brief description of the chapters contained in the book, which correspond to lectures and exercise periods in my hands-on seminars.

Chapter 1: Introduction to Objects

This chapter is an overview of what object-oriented programming is all about, including the answer to the basic question “What’s an object?”, interface vs. implementation, abstraction and encapsulation, messages and functions, inheritance and composition, and the all-important polymorphism. You’ll also get an overview of issues of object creation such as constructors, where the objects live, where to put them once they’re created, and the magical garbage collector that cleans up the objects that are no longer needed. Other issues will be introduced, including error handling with exceptions, multithreading for responsive user interfaces, and networking and the Internet. You’ll learn what makes Java special, why it’s been so successful, and about object-oriented analysis and design.

Chapter 2: Everything is an Object

This chapter moves you to the point where you can write your first Java program, so it must give an overview of the essentials, including the concept of a reference to an object; how to create an object; an introduction to primitive types and arrays; scoping and the way objects are destroyed by the garbage collector; how everything in Java is a new data type (class) and how to create your own classes; functions, arguments, and return values; name visibility and using components from other libraries; the static keyword; and comments and embedded documentation.

Chapter 3: Controlling Program Flow

This chapter begins with all of the operators that come to Java from C and C++. In addition, you’ll discover common operator pitfalls, casting, promotion, and precedence. This is followed by the basic control-flow and selection operations that you get with virtually any programming language: choice with if-else; looping with for and while; quitting a loop with break and continue as well as Java’s labeled break and labeled continue (which account for the “missing goto” in Java); and selection using switch. Although much of this material has common threads with C and C++ code, there are some differences. In addition, all the examples will be full Java examples so you’ll get more comfortable with what Java looks like.

Chapter 4: Initialization & Cleanup

This chapter begins by introducing the constructor, which guarantees proper initialization. The definition of the constructor leads into the concept of function overloading (since you might want several constructors). This is followed by a discussion of the process of cleanup, which is not always as simple as it seems. Normally, you just drop an object when you’re done with it and the garbage collector eventually comes along and releases the memory. This portion explores the garbage collector and some of its idiosyncrasies. The chapter concludes with a closer look at how things are initialized: automatic member initialization, specifying member initialization, the order of initialization, static initialization and array initialization.

Chapter 5: Hiding the Implementation

This chapter covers the way that code is packaged together, and why some parts of a library are exposed while other parts are hidden. It begins by looking at the package and import keywords, which perform file-level packaging and allow you to build libraries of classes. The subject of directory paths and file names is also examined. The remainder of the chapter looks at the public, private, and protected keywords, the concept of “friendly” access, and what the different levels of access control mean when used in various contexts.

Chapter 6: Reusing Classes

The concept of inheritance is standard in virtually all OOP languages. It’s a way to take an existing class and add to its functionality (as well as change it, the subject of Chapter 7). Inheritance is often a way to reuse code by leaving the “base class” the same, and just patching things here and there to produce what you want. However, inheritance isn’t the only way to make new classes from existing ones. You can also embed an object inside your new class with composition. In this chapter you’ll learn about these two ways to reuse code in Java, and how to apply them.

Chapter 7: Polymorphism

On your own, you might take nine months to discover and understand polymorphism, a cornerstone of OOP. Through small, simple examples you’ll see how to create a family of types with inheritance and manipulate objects in that family through their common base class. Java’s polymorphism allows you to treat all objects in this family generically, which means the bulk of your code doesn’t rely on specific type information. This makes your programs extensible, so building programs and code maintenance is easier and cheaper.

Chapter 8: Interfaces & Inner Classes

Java provides a third way to set up a reuse relationship, through the interface, which is a pure abstraction of the interface of an object. The interface is more than just an abstract class taken to the extreme, since it allows you to perform a variation on C++’s “multiple inheritance,” by creating a class that can be upcast to more than one base type.

At first, inner classes look like a simple code hiding mechanism: you place classes inside other classes. You’ll learn, however, that the inner class does more than that—it knows about and can communicate with the surrounding class—and that the kind of code you can write with inner classes is more elegant and clear, although it is a new concept to most and takes some time to become comfortable with design using inner classes.

Chapter 9: Holding your Objects

It’s a fairly simple program that has only a fixed quantity of objects with known lifetimes. In general, your programs will always be creating new objects at a variety of times that will be known only while the program is running. In addition, you won’t know until run-time the quantity or even the exact type of the objects you need. To solve the general programming problem, you need to create any number of objects, anytime, anywhere. This chapter explores in depth the container library that Java 2 supplies to hold objects while you’re working with them: the simple arrays and more sophisticated containers (data structures) such as ArrayList and HashMap.

Chapter 10: Error Handling with Exceptions

The basic philosophy of Java is that badly-formed code will not be run. As much as possible, the compiler catches problems, but sometimes the problems—either programmer error or a natural error condition that occurs as part of the normal execution of the program—can be detected and dealt with only at run-time. Java has exception handling to deal with any problems that arise while the program is running. This chapter examines how the keywords try, catch, throw, throws, and finally work in Java; when you should throw exceptions and what to do when you catch them. In addition, you’ll see Java’s standard exceptions, how to create your own, what happens with exceptions in constructors, and how exception handlers are located.

Chapter 11: The Java IO System

Theoretically, you can divide any program into three parts: input, process, and output. This implies that IO (input/output) is an important part of the equation. In this chapter you’ll learn about the different classes that Java provides for reading and writing files, blocks of memory, and the console. The distinction between “old” IO and “new” Java IO will be shown. In addition, this section examines the process of taking an object, “streaming” it (so that it can be placed on disk or sent across a network) and reconstructing it, which is handled for you with Java’s object serialization. Also, Java’s compression libraries, which are used in the Java ARchive file format (JAR), are examined.

Chapter 12: Run-Time Type Identification

Java run-time type identification (RTTI) lets you find the exact type of an object when you have a reference to only the base type. Normally, you’ll want to intentionally ignore the exact type of an object and let Java’s dynamic binding mechanism (polymorphism) implement the correct behavior for that type. But occasionally it is very helpful to know the exact type of an object for which you have only a base reference. Often this information allows you to perform a special-case operation more efficiently. This chapter explains what RTTI is for, how to use it and how to get rid of it when it doesn’t belong there. In addition, the Java reflection mechanism is introduced.

Chapter 13: Creating Windows and Applets

Java comes with the “Swing” GUI library, which is a set of classes that handle windowing in a portable fashion. These windowed programs can either be applets or stand-alone applications. This chapter is an introduction to Swing and the creation of World Wide Web applets. The important “Java Beans” technology is introduced. This is fundamental for the creation of Rapid-Application Development (RAD) program-building tools.

Chapter 14: Multiple Threads

Java provides a built-in facility to support multiple concurrent subtasks, called threads, running within a single program. (Unless you have multiple processors on your machine, this is only the appearance of multiple subtasks.) Although these can be used anywhere, threads are most apparent when trying to create a responsive user interface so, for example, a user isn’t prevented from pressing a button or entering data while some processing is going on. This chapter looks at the syntax and semantics of multithreading in Java.

Chapter 15: Distributed Computing

All the Java features and libraries seem to really come together when you start writing programs to work across networks. This chapter explores communication across networks and the Internet, and the classes that Java provides to make this easier. It introduces the very important concepts of Servlets and JSPs (for server-side programming), along with Java DataBase Connectivity (JDBC), and Remote Method Invocation (RMI). Finally, there’s an introduction to the new technologies of JINI, JavaSpaces, and Enterprise JavaBeans (EJBs).

Appendix A: Passing & Returning Objects

Since the only way you talk to objects in Java is through references, the concepts of passing an object into a function and returning an object from a function have some interesting consequences. This appendix explains what you need to know to manage objects when you’re moving in and out of functions, and also shows the String class, which uses a different approach to the problem.

Appendix B: The Java Native Interface (JNI)

A totally portable Java program has serious drawbacks: speed and the inability to access platform-specific services. When you know the platform that you’re running on, it’s possible to dramatically speed up certain operations by making them native methods, which are functions that are written in another programming language (currently, only C/C++ is supported). This appendix gives you enough of an introduction to this feature that you should be able to create simple examples that interface with non-Java code.

Appendix C: Java Programming Guidelines

This appendix contains suggestions to help guide you while performing low-level program design and writing code.

Appendix D: Recommended Reading

A list of some of the Java books I’ve found particularly useful.

Exercises

I’ve discovered that simple exercises are exceptionally useful to complete a student’s understanding during a seminar, so you’ll find a set at the end of each chapter.

Most exercises are designed to be easy enough that they can be finished in a reasonable amount of time in a classroom situation while the instructor observes, making sure that all the students are absorbing the material. Some exercises are more advanced to prevent boredom for experienced students. The majority are designed to be solved in a short time and test and polish your knowledge. Some are more challenging, but none present major challenges. (Presumably, you’ll find those on your own—or more likely they’ll find you).

Multimedia CD ROM

There are two multimedia CDs associated with this book. The first is bound into the book itself: Thinking in C, described at the end of the preface, which prepares you for the book by bringing you up to speed on the necessary C syntax you need to be able to understand Java.

A second Multimedia CD ROM is available, which is based on the contents of the book. This CD ROM is a separate product and contains the entire contents of the week-long “Hands-On Java” training seminar. This is more than 15 hours of lectures that I have recorded, synchronized with hundreds of slides of information. Because the seminar is based on this book, it is an ideal accompaniment.

The CD ROM contains all the lectures (with the important exception of personalized attention!) from the five-day full-immersion training seminars. We believe that it sets a new standard for quality.

The Hands-On Java CD ROM is available only by ordering directly from the Web site www.BruceEckel.com.

Source code

All the source code for this book is available as copyrighted freeware, distributed as a single package, by visiting the Web site www.BruceEckel.com. To make sure that you get the most current version, this is the official site for distribution of the code and the electronic version of the book. You can find mirrored versions of the electronic book and the code on other sites (some of these sites are found at www.BruceEckel.com), but you should check the official site to ensure that the mirrored version is actually the most recent edition. You may distribute the code in classroom and other educational situations.

The primary goal of the copyright is to ensure that the source of the code is properly cited, and to prevent you from republishing the code in print media without permission. (As long as the source is cited, using examples from the book in most media is generally not a problem.)

In each source code file you will find a reference to the following copyright notice:

//:! :CopyRight.txt
Copyright ©2000 Bruce Eckel
Source code file from the 2nd edition of the book
"Thinking in Java." All rights reserved EXCEPT as
allowed by the following statements:
You can freely use this file
for your own work (personal or commercial),
including modifications and distribution in
executable form only. Permission is granted to use
this file in classroom situations, including its
use in presentation materials, as long as the book
"Thinking in Java" is cited as the source.
Except in classroom situations, you cannot copy
and distribute this code; instead, the sole
distribution point is http://www.BruceEckel.com
(and official mirror sites) where it is
freely available. You cannot remove this
copyright and notice. You cannot distribute
modified versions of the source code in this
package. You cannot use this file in printed
media without the express permission of the
author. Bruce Eckel makes no representation about
the suitability of this software for any purpose.
It is provided "as is" without express or implied
warranty of any kind, including any implied
warranty of merchantability, fitness for a
particular purpose or non-infringement. The entire
risk as to the quality and performance of the
software is with you. Bruce Eckel and the
publisher shall not be liable for any damages
suffered by you or any third party as a result of
using or distributing software. In no event will
Bruce Eckel or the publisher be liable for any
lost revenue, profit, or data, or for direct,
indirect, special, consequential, incidental, or
punitive damages, however caused and regardless of
the theory of liability, arising out of the use of
or inability to use software, even if Bruce Eckel
and the publisher have been advised of the
possibility of such damages. Should the software
prove defective, you assume the cost of all
necessary servicing, repair, or correction. If you
think you've found an error, please submit the
correction using the form you will find at
www.BruceEckel.com. (Please use the same
form for non-code errors found in the book.)
///:~

You may use the code in your projects and in the classroom (including your presentation materials) as long as the copyright notice that appears in each source file is retained.

Coding standards

In the text of this book, identifiers (function, variable, and class names) will be set in bold. Most keywords will also be set in bold, except for those keywords that are used so much that the bolding can become tedious, such as “class.”

I use a particular coding style for the examples in this book. This style follows the style that Sun itself uses in virtually all of the code you will find at its site (see http://java.sun.com/docs/codeconv/index.html), and seems to be supported by most Java development environments. If you’ve read my other works, you’ll also notice that Sun’s coding style coincides with mine—this pleases me, although I had nothing to do with it. The subject of formatting style is good for hours of hot debate, so I’ll just say I’m not trying to dictate correct style via my examples; I have my own motivation for using the style that I do. Because Java is a free-form programming language, you can continue to use whatever style you’re comfortable with.

The programs in this book are files that are included by the word processor in the text, directly from compiled files. Thus, the code files printed in the book should all work without compiler errors. The errors that should cause compile-time error messages are commented out with the comment //! so they can be easily discovered and tested using automatic means. Errors discovered and reported to the author will appear first in the distributed source code and later in updates of the book (which will also appear on the Web site www.BruceEckel.com).

Java versions

I generally rely on the Sun implementation of Java as a reference when determining whether behavior is correct.

Over time, Sun has released three major versions of Java: 1.0, 1.1 and 2 (which is called version 2 even though the releases of the JDK from Sun continue to use the numbering scheme of 1.2, 1.3, 1.4, etc.). Version 2 seems to finally bring Java into the prime time, in particular where user interface tools are concerned. This book focuses on and is tested with Java 2, although I do sometimes make concessions to earlier features of Java 2 so that the code will compile under Linux (via the Linux JDK that was available at this writing).

If you need to learn about earlier releases of the language that are not covered in this edition, the first edition of the book is freely downloadable at www.BruceEckel.com and is also contained on the CD that is bound in with this book.

One thing you’ll notice is that, when I do need to mention earlier versions of the language, I don’t use the sub-revision numbers. In this book I will refer to Java 1.0, Java 1.1, and Java 2 only, to guard against typographical errors produced by further sub-revisioning of these products.

Seminars and mentoring

My company provides five-day, hands-on, public and in-house training seminars based on the material in this book. Selected material from each chapter represents a lesson, which is followed by a monitored exercise period so each student receives personal attention. The audio lectures and slides for the introductory seminar are also captured on CD ROM to provide at least some of the experience of the seminar without the travel and expense. For more information, go to www.BruceEckel.com.

My company also provides consulting, mentoring and walkthrough services to help guide your project through its development cycle—especially your company’s first Java project.

Errors

No matter how many tricks a writer uses to detect errors, some always creep in and these often leap off the page for a fresh reader.

There is an error submission form linked from the beginning of each chapter in the HTML version of this book (and on the CD ROM bound into the back of this book, and downloadable from www.BruceEckel.com) and also on the Web site itself, on the page for this book. If you discover anything you believe to be an error, please use this form to submit the error along with your suggested correction. If necessary, include the original source file and note any suggested modifications. Your help is appreciated.

Note on the cover design

The cover of Thinking in Java is inspired by the American Arts & Crafts Movement, which began near the turn of the century and reached its zenith between 1900 and 1920. It began in England as a reaction to both the machine production of the Industrial Revolution and the highly ornamental style of the Victorian era. Arts & Crafts emphasized spare design, the forms of nature as seen in the art nouveau movement, hand-crafting, and the importance of the individual craftsperson, and yet it did not eschew the use of modern tools. There are many echoes with the situation we have today: the turn of the century, the evolution from the raw beginnings of the computer revolution to something more refined and meaningful to individual persons, and the emphasis on software craftsmanship rather than just manufacturing code.

I see Java in this same way: as an attempt to elevate the programmer away from an operating-system mechanic and toward being a “software craftsman.”

Both the author and the book/cover designer (who have been friends since childhood) find inspiration in this movement, and both own furniture, lamps, and other pieces that are either original or inspired by this period.

The other theme in this cover suggests a collection box that a naturalist might use to display the insect specimens that he or she has preserved. These insects are objects, which are placed within the box objects. The box objects are themselves placed within the “cover object,” which illustrates the fundamental concept of aggregation in object-oriented programming. Of course, a programmer cannot help but make the association with “bugs,” and here the bugs have been captured and presumably killed in a specimen jar, and finally confined within a small display box, as if to imply Java’s ability to find, display, and subdue bugs (which is truly one of its most powerful attributes).

Acknowledgements

Thanks to the Doyle Street Cohousing Community for putting up with me for the two years that it took me to write the first edition of this book (and for putting up with me at all). Thanks very much to Kevin and Sonda Donovan for subletting their great place in gorgeous Crested Butte, Colorado for the summer while I worked on the book. Also thanks to the friendly residents of Crested Butte and the Rocky Mountain Biological Laboratory who made me feel so welcome.

Thanks to Claudette Moore at Moore Literary Agency for her tremendous patience and perseverance in getting me exactly what I wanted.

My first two books were published with Jeff Pepper as editor at Osborne/McGraw-Hill. Jeff appeared at the right place and the right time at Prentice-Hall and has cleared the path and made all the right things happen to make this the most pleasant publishing experience I’ve ever had. Thanks, Jeff—it means a lot to me.

I’m especially indebted to Gen Kiyooka and his company Digigami, who graciously provided my Web server for the first several years of my presence on the Web. This was an invaluable learning aid.

Thanks to Cay Horstmann (co-author of Core Java, Prentice Hall, 2000), D’Arcy Smith (Symantec), and Paul Tyma (co-author of Java Primer Plus, The Waite Group, 1996), for helping me clarify concepts in the language.

Thanks to people who have spoken in my Java track at the Software Development Conference, and students in my seminars, who ask the questions I need to hear in order to make the material more clear.

Special thanks to Larry and Tina O’Brien, who helped turn my seminar into the original Hands-On Java CD ROM. (You can find out more at www.BruceEckel.com.)

Lots of people sent in corrections and I am indebted to them all, but particular thanks go to (for the first edition): Kevin Raulerson (found tons of great bugs), Bob Resendes (simply incredible), John Pinto, Joe Dante, Joe Sharp (all three were fabulous), David Combs (many grammar and clarification corrections), Dr. Robert Stephenson, Franklin Chen, Zev Griner, David Karr, Leander A. Stroschein, Steve Clark, Charles A. Lee, Austin Maher, Dennis P. Roth, Roque Oliveira, Douglas Dunn, Dejan Ristic, Neil Galarneau, David B. Malkovsky, Steve Wilkinson, and a host of others.

Prof. Ir. Marc Meurrens put in a great deal of effort to publicize and make the electronic version of the first edition of the book available in Europe.

There have been a spate of smart technical people in my life who have become friends and have also been both influential and unusual in that they do yoga and practice other forms of spiritual enhancement, which I find quite inspirational and instructional. They are Kraig Brockschmidt, Gen Kiyooka, and Andrea Provaglio, (who helps in the understanding of Java and programming in general in Italy, and now in the United States as an associate of the MindView team).

It’s not that much of a surprise to me that understanding Delphi helped me understand Java, since there are many concepts and language design decisions in common. My Delphi friends provided assistance by helping me gain insight into that marvelous programming environment. They are Marco Cantu (another Italian—perhaps being steeped in Latin gives one aptitude for programming languages?), Neil Rubenking (who used to do the yoga/vegetarian/Zen thing until he discovered computers), and of course Zack Urlocker, a long-time pal whom I’ve traveled the world with.

My friend Richard Hale Shaw’s insights and support have been very helpful (and Kim’s, too). Richard and I spent many months giving seminars together and trying to work out the perfect learning experience for the attendees. Thanks also to KoAnn Vikoren, Eric Faurot, Marco Pardi, and the rest of the cast and crew at MFI. Thanks especially to Tara Arrowood, who re-inspired me about the possibilities of conferences.

The book design, cover design, and cover photo were created by my friend Daniel Will-Harris, noted author and designer (www.Will-Harris.com), who used to play with rub-on letters in junior high school while he awaited the invention of computers and desktop publishing, and complained of me mumbling over my algebra problems. However, I produced the camera-ready pages myself, so the typesetting errors are mine. Microsoft® Word 97 for Windows was used to write the book and to create camera-ready pages. The body typeface is Georgia and the headlines are in Verdana. The cover typeface is ITC Rennie Mackintosh.

Thanks to the vendors who created the compilers: Borland, the Blackdown group (for Linux), and of course, Sun.

A special thanks to all my teachers and all my students (who are my teachers as well). The most fun writing teacher was Gabrielle Rico (author of Writing the Natural Way, Putnam, 1983). I’ll always treasure the terrific week at Esalen.

The supporting cast of friends includes, but is not limited to: Andrew Binstock, Steve Sinofsky, JD Hildebrandt, Tom Keffer, Brian McElhinney, Brinkley Barr, Bill Gates at Midnight Engineering Magazine, Larry Constantine and Lucy Lockwood, Greg Perry, Dan Putterman, Christi Westphal, Gene Wang, Dave Mayer, David Intersimone, Andrea Rosenfield, Claire Sawyers, more Italians (Laura Fallai, Corrado, Ilsa, and Cristina Giustozzi), Chris and Laura Strand, the Almquists, Brad Jerbic, Marilyn Cvitanic, the Mabrys, the Haflingers, the Pollocks, Peter Vinci, the Robbins Families, the Moelter Families (and the McMillans), Michael Wilk, Dave Stoner, Laurie Adams, the Cranstons, Larry Fogg, Mike and Karen Sequeira, Gary Entsminger and Allison Brody, Kevin Donovan and Sonda Eastlack, Chester and Shannon Andersen, Joe Lordi, Dave and Brenda Bartlett, David Lee, the Rentschlers, the Sudeks, Dick, Patty, and Lee Eckel, Lynn and Todd, and their families. And of course, Mom and Dad.

Internet contributors

Thanks to those who helped me rewrite the examples to use the Swing library: Jon Shvarts, Thomas Kirsch, Rahim Adatia, Rajesh Jain, Ravi Manthena, Banu Rajamani, Jens Brandt, Nitin Shivaram, and everyone who expressed support. This really helped me jump-start the project.


1: Introduction to Objects

1: Introduction
to Objects

The genesis of the computer revolution was in a machine. The genesis of our programming languages thus tends to look like that machine.

But computers are not so much machines as they are mind amplification tools (“bicycles for the mind,” as Steve Jobs is fond of saying) and a different kind of expressive medium. As a result, the tools are beginning to look less like machines and more like parts of our minds, and also like other expressive mediums such as writing, painting, sculpture, animation, and filmmaking. Object-oriented programming (OOP) is part of this movement toward using the computer as an expressive medium.

This chapter will introduce you to the basic concepts of OOP, including an overview of development methods. This chapter, and this book, assume that you have had experience in a procedural programming language, although not necessarily C. If you think you need more preparation in programming and the syntax of C before tackling this book, you should work through the Thinking in C: Foundations for C++ and Java training CD ROM, bound in with this book and also available at www.BruceEckel.com.

This chapter is background and supplementary material. Many people do not feel comfortable wading into object-oriented programming without understanding the big picture first. Thus, there are many concepts that are introduced here to give you a solid overview of OOP. However, many other people don’t get the big picture concepts until they’ve seen some of the mechanics first; these people may become bogged down and lost without some code to get their hands on. If you’re part of this latter group and are eager to get to the specifics of the language, feel free to jump past this chapter—skipping it at this point will not prevent you from writing programs or learning the language. However, you will want to come back here eventually to fill in your knowledge so you can understand why objects are important and how to design with them.

The progress of abstraction

All programming languages provide abstractions. It can be argued that the complexity of the problems you’re able to solve is directly related to the kind and quality of abstraction. By “kind” I mean, “What is it that you are abstracting?” Assembly language is a small abstraction of the underlying machine. Many so-called “imperative” languages that followed (such as Fortran, BASIC, and C) were abstractions of assembly language. These languages are big improvements over assembly language, but their primary abstraction still requires you to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer must establish the association between the machine model (in the “solution space,” which is the place where you’re modeling that problem, such as a computer) and the model of the problem that is actually being solved (in the “problem space,” which is the place where the problem exists). The effort required to perform this mapping, and the fact that it is extrinsic to the programming language, produces programs that are difficult to write and expensive to maintain, and as a side effect created the entire “programming methods” industry.

The alternative to modeling the machine is to model the problem you’re trying to solve. Early languages such as LISP and APL chose particular views of the world (“All problems are ultimately lists” or “All problems are algorithmic”). PROLOG casts all problems into chains of decisions. Languages have been created for constraint-based programming and for programming exclusively by manipulating graphical symbols. (The latter proved to be too restrictive.) Each of these approaches is a good solution to the particular class of problem they’re designed to solve, but when you step outside of that domain they become awkward.

The object-oriented approach goes a step further by providing tools for the programmer to represent elements in the problem space. This representation is general enough that the programmer is not constrained to any particular type of problem. We refer to the elements in the problem space and their representations in the solution space as “objects.” (Of course, you will also need other objects that don’t have problem-space analogs.) The idea is that the program is allowed to adapt itself to the lingo of the problem by adding new types of objects, so when you read the code describing the solution, you’re reading words that also express the problem. This is a more flexible and powerful language abstraction than what we’ve had before. Thus, OOP allows you to describe the problem in terms of the problem, rather than in terms of the computer where the solution will run. There’s still a connection back to the computer, though. Each object looks quite a bit like a little computer; it has a state, and it has operations that you can ask it to perform. However, this doesn’t seem like such a bad analogy to objects in the real world—they all have characteristics and behaviors.

Some language designers have decided that object-oriented programming by itself is not adequate to easily solve all programming problems, and advocate the combination of various approaches into multiparadigm programming languages. [2]

Alan Kay summarized five basic characteristics of Smalltalk, the first successful object-oriented language and one of the languages upon which Java is based. These characteristics represent a pure approach to object-oriented programming:

  1. Everything is an object. Think of an object as a fancy variable; it stores data, but you can “make requests” to that object, asking it to perform operations on itself. In theory, you can take any conceptual component in the problem you’re trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program.
  2. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you “send a message” to that object. More concretely, you can think of a message as a request to call a function that belongs to a particular object.
  3. Each object has its own memory made up of other objects. Put another way, you create a new kind of object by making a package containing existing objects. Thus, you can build complexity in a program while hiding it behind the simplicity of objects.
  4. Every object has a type. Using the parlance, each object is an instance of a class, in which “class” is synonymous with “type.” The most important distinguishing characteristic of a class is “What messages can you send to it?”
  5. All objects of a particular type can receive the same messages. This is actually a loaded statement, as you will see later. Because an object of type “circle” is also an object of type “shape,” a circle is guaranteed to accept shape messages. This means you can write code that talks to shapes and automatically handle anything that fits the description of a shape. This substitutability is one of the most powerful concepts in OOP.

An object has an interface

Aristotle was probably the first to begin a careful study of the concept of type; he spoke of “the class of fishes and the class of birds.” The idea that all objects, while being unique, are also part of a class of objects that have characteristics and behaviors in common was used directly in the first object-oriented language, Simula-67, with its fundamental keyword class that introduces a new type into a program.

Simula, as its name implies, was created for developing simulations such as the classic “bank teller problem.” In this, you have a bunch of tellers, customers, accounts, transactions, and units of money—a lot of “objects.” Objects that are identical except for their state during a program’s execution are grouped together into “classes of objects” and that’s where the keyword class came from. Creating abstract data types (classes) is a fundamental concept in object-oriented programming. Abstract data types work almost exactly like built-in types: You can create variables of a type (called objects or instances in object-oriented parlance) and manipulate those variables (called sending messages or requests; you send a message and the object figures out what to do with it). The members (elements) of each class share some commonality: every account has a balance, every teller can accept a deposit, etc. At the same time, each member has its own state, each account has a different balance, each teller has a name. Thus, the tellers, customers, accounts, transactions, etc., can each be represented with a unique entity in the computer program. This entity is the object, and each object belongs to a particular class that defines its characteristics and behaviors.

So, although what we really do in object-oriented programming is create new data types, virtually all object-oriented programming languages use the “class” keyword. When you see the word “type” think “class” and vice versa [3] .

Since a class describes a set of objects that have identical characteristics (data elements) and behaviors (functionality), a class is really a data type because a floating point number, for example, also has a set of characteristics and behaviors. The difference is that a programmer defines a class to fit a problem rather than being forced to use an existing data type that was designed to represent a unit of storage in a machine. You extend the programming language by adding new data types specific to your needs. The programming system welcomes the new classes and gives them all the care and type-checking that it gives to built-in types.

The object-oriented approach is not limited to building simulations. Whether or not you agree that any program is a simulation of the system you’re designing, the use of OOP techniques can easily reduce a large set of problems to a simple solution.

Once a class is established, you can make as many objects of that class as you like, and then manipulate those objects as if they are the elements that exist in the problem you are trying to solve. Indeed, one of the challenges of object-oriented programming is to create a one-to-one mapping between the elements in the problem space and objects in the solution space.

But how do you get an object to do useful work for you? There must be a way to make a request of the object so that it will do something, such as complete a transaction, draw something on the screen, or turn on a switch. And each object can satisfy only certain requests. The requests you can make of an object are defined by its interface, and the type is what determines the interface. A simple example might be a representation of a light bulb:

Light lt = new Light();
lt.on();

The interface establishes what requests you can make for a particular object. However, there must be code somewhere to satisfy that request. This, along with the hidden data, comprises the implementation. From a procedural programming standpoint, it’s not that complicated. A type has a function associated with each possible request, and when you make a particular request to an object, that function is called. This process is usually summarized by saying that you “send a message” (make a request) to an object, and the object figures out what to do with that message (it executes code).

Here, the name of the type/class is Light, the name of this particular Light object is lt, and the requests that you can make of a Light object are to turn it on, turn it off, make it brighter, or make it dimmer. You create a Light object by defining a “reference” (lt) for that object and calling new to request a new object of that type. To send a message to the object, you state the name of the object and connect it to the message request with a period (dot). From the standpoint of the user of a predefined class, that’s pretty much all there is to programming with objects.

The diagram shown above follows the format of the Unified Modeling Language (UML). Each class is represented by a box, with the type name in the top portion of the box, any data members that you care to describe in the middle portion of the box, and the member functions (the functions that belong to this object, which receive any messages you send to that object) in the bottom portion of the box. Often, only the name of the class and the public member functions are shown in UML design diagrams, and so the middle portion is not shown. If you’re interested only in the class name, then the bottom portion doesn’t need to be shown, either.

The hidden implementation

It is helpful to break up the playing field into class creators (those who create new data types) and client programmers [4] (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only what’s necessary to the client programmer and keeps everything else hidden. Why? Because if it’s hidden, the client programmer can’t use it, which means that the class creator can change the hidden portion at will without worrying about the impact to anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs. The concept of implementation hiding cannot be overemphasized.

In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger library.

If all the members of a class are available to everyone, then the client programmer can do anything with that class and there’s no way to enforce rules. Even though you might really prefer that the client programmer not directly manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the world.

So the first reason for access control is to keep client programmers’ hands off portions they shouldn’t touch—parts that are necessary for the internal machinations of the data type but not part of the interface that users need in order to solve their particular problems. This is actually a service to users because they can easily see what’s important to them and what they can ignore.

The second reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, and then later discover that you need to rewrite it in order to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this easily and require only a relink by the user.

Java uses three explicit keywords to set the boundaries in a class: public, private, and protected. Their use and meaning are quite straightforward. These access specifiers determine who can use the definitions that follow. public means the following definitions are available to everyone. The private keyword, on the other hand, means that no one can access those definitions except you, the creator of the type, inside member functions of that type. private is a brick wall between you and the client programmer. If someone tries to access a private member, they’ll get a compile-time error. protected acts like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be introduced shortly.

Java also has a “default” access, which comes into play if you don’t use one of the aforementioned specifiers. This is sometimes called “friendly” access because classes can access the friendly members of other classes in the same package, but outside of the package those same friendly members appear to be private.

Reusing the implementation

Once a class has been created and tested, it should (ideally) represent a useful unit of code. It turns out that this reusability is not nearly so easy to achieve as many would hope; it takes experience and insight to produce a good design. But once you have such a design, it begs to be reused. Code reuse is one of the greatest advantages that object-oriented programming languages provide.

The simplest way to reuse a class is to just use an object of that class directly, but you can also place an object of that class inside a new class. We call this “creating a member object.” Your new class can be made up of any number and type of other objects, in any combination that you need to achieve the functionality desired in your new class. Because you are composing a new class from existing classes, this concept is called composition (or more generally, aggregation). Composition is often referred to as a “has-a” relationship, as in “a car has an engine.”

(The above UML diagram indicates composition with the filled diamond, which states there is one car. I will typically use a simpler form: just a line, without the diamond, to indicate an association. [5] )

Composition comes with a great deal of flexibility. The member objects of your new class are usually private, making them inaccessible to the client programmers who are using the class. This allows you to change those members without disturbing existing client code. You can also change the member objects at run-time, to dynamically change the behavior of your program. Inheritance, which is described next, does not have this flexibility since the compiler must place compile-time restrictions on classes created with inheritance.

Because inheritance is so important in object-oriented programming it is often highly emphasized, and the new programmer can get the idea that inheritance should be used everywhere. This can result in awkward and overly complicated designs. Instead, you should first look to composition when creating new classes, since it is simpler and more flexible. If you take this approach, your designs will be cleaner. Once you’ve had some experience, it will be reasonably obvious when you need inheritance.

Inheritance:
reusing the interface

By itself, the idea of an object is a convenient tool. It allows you to package data and functionality together by concept, so you can represent an appropriate problem-space idea rather than being forced to use the idioms of the underlying machine. These concepts are expressed as fundamental units in the programming language by using the class keyword.

It seems a pity, however, to go to all the trouble to create a class and then be forced to create a brand new one that might have similar functionality. It’s nicer if we can take the existing class, clone it, and then make additions and modifications to the clone. This is effectively what you get with inheritance, with the exception that if the original class (called the base or super or parent class) is changed, the modified “clone” (called the derived or inherited or sub or child class) also reflects those changes.

(The arrow in the above UML diagram points from the derived class to the base class. As you will see, there can be more than one derived class.)

A type does more than describe the constraints on a set of objects; it also has a relationship with other types. Two types can have characteristics and behaviors in common, but one type may contain more characteristics than another and may also handle more messages (or handle them differently). Inheritance expresses this similarity between types using the concept of base types and derived types. A base type contains all of the characteristics and behaviors that are shared among the types derived from it. You create a base type to represent the core of your ideas about some objects in your system. From the base type, you derive other types to express the different ways that this core can be realized.

For example, a trash-recycling machine sorts pieces of trash. The base type is “trash,” and each piece of trash has a weight, a value, and so on, and can be shredded, melted, or decomposed. From this, more specific types of trash are derived that may have additional characteristics (a bottle has a color) or behaviors (an aluminum can may be crushed, a steel can is magnetic). In addition, some behaviors may be different (the value of paper depends on its type and condition). Using inheritance, you can build a type hierarchy that expresses the problem you’re trying to solve in terms of its types.

A second example is the classic “shape” example, perhaps used in a computer-aided design system or game simulation. The base type is “shape,” and each shape has a size, a color, a position, and so on. Each shape can be drawn, erased, moved, colored, etc. From this, specific types of shapes are derived (inherited): circle, square, triangle, and so on, each of which may have additional characteristics and behaviors. Certain shapes can be flipped, for example. Some behaviors may be different, such as when you want to calculate the area of a shape. The type hierarchy embodies both the similarities and differences between the shapes.

Casting the solution in the same terms as the problem is tremendously beneficial because you don’t need a lot of intermediate models to get from a description of the problem to a description of the solution. With objects, the type hierarchy is the primary model, so you go directly from the description of the system in the real world to the description of the system in code. Indeed, one of the difficulties people have with object-oriented design is that it’s too simple to get from the beginning to the end. A mind trained to look for complex solutions is often stumped by this simplicity at first.

When you inherit from an existing type, you create a new type. This new type contains not only all the members of the existing type (although the private ones are hidden away and inaccessible), but more important, it duplicates the interface of the base class. That is, all the messages you can send to objects of the base class you can also send to objects of the derived class. Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class. In the previous example, “a circle is a shape.” This type equivalence via inheritance is one of the fundamental gateways in understanding the meaning of object-oriented programming.

Since both the base class and derived class have the same interface, there must be some implementation to go along with that interface. That is, there must be some code to execute when an object receives a particular message. If you simply inherit a class and don’t do anything else, the methods from the base-class interface come right along into the derived class. That means objects of the derived class have not only the same type, they also have the same behavior, which isn’t particularly interesting.

You have two ways to differentiate your new derived class from the original base class. The first is quite straightforward: You simply add brand new functions to the derived class. These new functions are not part of the base class interface. This means that the base class simply didn’t do as much as you wanted it to, so you added more functions. This simple and primitive use for inheritance is, at times, the perfect solution to your problem. However, you should look closely for the possibility that your base class might also need these additional functions. This process of discovery and iteration of your design happens regularly in object-oriented programming.

Although inheritance may sometimes imply (especially in Java, where the keyword that indicates inheritance is extends) that you are going to add new functions to the interface, that’s not necessarily true. The second and more important way to differentiate your new class is to change the behavior of an existing base-class function. This is referred to as overriding that function.

To override a function, you simply create a new definition for the function in the derived class. You’re saying, “I’m using the same interface function here, but I want it to do something different for my new type.”

Is-a vs. is-like-a relationships

There’s a certain debate that can occur about inheritance: Should inheritance override only base-class functions (and not add new member functions that aren’t in the base class)? This would mean that the derived type is exactly the same type as the base class since it has exactly the same interface. As a result, you can exactly substitute an object of the derived class for an object of the base class. This can be thought of as pure substitution, and it’s often referred to as the substitution principle. In a sense, this is the ideal way to treat inheritance. We often refer to the relationship between the base class and derived classes in this case as an is-a relationship, because you can say “a circle is a shape.” A test for inheritance is to determine whether you can state the is-a relationship about the classes and have it make sense.

There are times when you must add new interface elements to a derived type, thus extending the interface and creating a new type. The new type can still be substituted for the base type, but the substitution isn’t perfect because your new functions are not accessible from the base type. This can be described as an is-like-a [6] relationship; the new type has the interface of the old type but it also contains other functions, so you can’t really say it’s exactly the same. For example, consider an air conditioner. Suppose your house is wired with all the controls for cooling; that is, it has an interface that allows you to control cooling. Imagine that the air conditioner breaks down and you replace it with a heat pump, which can both heat and cool. The heat pump is-like-an air conditioner, but it can do more. Because the control system of your house is designed only to control cooling, it is restricted to communication with the cooling part of the new object. The interface of the new object has been extended, and the existing system doesn’t know about anything except the original interface.

Of course, once you see this design it becomes clear that the base class “cooling system” is not general enough, and should be renamed to “temperature control system” so that it can also include heating—at which point the substitution principle will work. However, the diagram above is an example of what can happen in design and in the real world.

When you see the substitution principle it’s easy to feel like this approach (pure substitution) is the only way to do things, and in fact it is nice if your design works out that way. But you’ll find that there are times when it’s equally clear that you must add new functions to the interface of a derived class. With inspection both cases should be reasonably obvious.

Interchangeable objects
with polymorphism

When dealing with type hierarchies, you often want to treat an object not as the specific type that it is, but instead as its base type. This allows you to write code that doesn’t depend on specific types. In the shape example, functions manipulate generic shapes without respect to whether they’re circles, squares, triangles, or some shape that hasn’t even been defined yet. All shapes can be drawn, erased, and moved, so these functions simply send a message to a shape object; they don’t worry about how the object copes with the message.

Such code is unaffected by the addition of new types, and adding new types is the most common way to extend an object-oriented program to handle new situations. For example, you can derive a new subtype of shape called pentagon without modifying the functions that deal only with generic shapes. This ability to extend a program easily by deriving new subtypes is important because it greatly improves designs while reducing the cost of software maintenance.

There’s a problem, however, with attempting to treat derived-type objects as their generic base types (circles as shapes, bicycles as vehicles, cormorants as birds, etc.). If a function is going to tell a generic shape to draw itself, or a generic vehicle to steer, or a generic bird to move, the compiler cannot know at compile-time precisely what piece of code will be executed. That’s the whole point—when the message is sent, the programmer doesn’t want to know what piece of code will be executed; the draw function can be applied equally to a circle, a square, or a triangle, and the object will execute the proper code depending on its specific type. If you don’t have to know what piece of code will be executed, then when you add a new subtype, the code it executes can be different without requiring changes to the function call. Therefore, the compiler cannot know precisely what piece of code is executed, so what does it do? For example, in the following diagram the BirdController object just works with generic Bird objects, and does not know what exact type they are. This is convenient from BirdController’s perspective because it doesn’t have to write special code to determine the exact type of Bird it’s working with, or that Bird’s behavior. So how does it happen that, when move( ) is called while ignoring the specific type of Bird, the right behavior will occur (a Goose runs, flies, or swims, and a Penguin runs or swims)?

The answer is the primary twist in object-oriented programming: the compiler cannot make a function call in the traditional sense. The function call generated by a non-OOP compiler causes what is called early binding, a term you may not have heard before because you’ve never thought about it any other way. It means the compiler generates a call to a specific function name, and the linker resolves this call to the absolute address of the code to be executed. In OOP, the program cannot determine the address of the code until run-time, so some other scheme is necessary when a message is sent to a generic object.

To solve the problem, object-oriented languages use the concept of late binding. When you send a message to an object, the code being called isn’t determined until run-time. The compiler does ensure that the function exists and performs type checking on the arguments and return value (a language in which this isn’t true is called weakly typed), but it doesn’t know the exact code to execute.

To perform late binding, Java uses a special bit of code in lieu of the absolute call. This code calculates the address of the function body, using information stored in the object (this process is covered in great detail in Chapter 7). Thus, each object can behave differently according to the contents of that special bit of code. When you send a message to an object, the object actually does figure out what to do with that message.

In some languages (C++, in particular) you must explicitly state that you want a function to have the flexibility of late-binding properties. In these languages, by default, member functions are not dynamically bound. This caused problems, so in Java dynamic binding is the default and you don’t need to remember to add any extra keywords in order to get polymorphism.

Consider the shape example. The family of classes (all based on the same uniform interface) was diagrammed earlier in this chapter. To demonstrate polymorphism, we want to write a single piece of code that ignores the specific details of type and talks only to the base class. That code is decoupled from type-specific information, and thus is simpler to write and easier to understand. And, if a new type—a Hexagon, for example—is added through inheritance, the code you write will work just as well for the new type of Shape as it did on the existing types. Thus, the program is extensible.

If you write a method in Java (as you will soon learn how to do):

void doStuff(Shape s) {
  s.erase();
  // ...
  s.draw();
}

This function speaks to any Shape, so it is independent of the specific type of object that it’s drawing and erasing. If in some other part of the program we use the doStuff( ) function:

Circle c = new Circle();
Triangle t = new Triangle();
Line l = new Line();
doStuff(c);
doStuff(t);
doStuff(l);

The calls to doStuff( ) automatically work correctly, regardless of the exact type of the object.

This is actually a pretty amazing trick. Consider the line:

doStuff(c);

What’s happening here is that a Circle is being passed into a function that’s expecting a Shape. Since a Circle is a Shape it can be treated as one by doStuff( ). That is, any message that doStuff( ) can send to a Shape, a Circle can accept. So it is a completely safe and logical thing to do.

We call this process of treating a derived type as though it were its base type upcasting. The name cast is used in the sense of casting into a mold and the up comes from the way the inheritance diagram is typically arranged, with the base type at the top and the derived classes fanning out downward. Thus, casting to a base type is moving up the inheritance diagram: “upcasting.”

An object-oriented program contains some upcasting somewhere, because that’s how you decouple yourself from knowing about the exact type you’re working with. Look at the code in doStuff( ):

  s.erase();
  // ...
  s.draw();

Notice that it doesn’t say “If you’re a Circle, do this, if you’re a Square, do that, etc.” If you write that kind of code, which checks for all the possible types that a Shape can actually be, it’s messy and you need to change it every time you add a new kind of Shape. Here, you just say “You’re a shape, I know you can erase( ) and draw( ) yourself, do it, and take care of the details correctly.”

What’s impressive about the code in doStuff( ) is that, somehow, the right thing happens. Calling draw( ) for Circle causes different code to be executed than when calling draw( ) for a Square or a Line, but when the draw( ) message is sent to an anonymous Shape, the correct behavior occurs based on the actual type of the Shape. This is amazing because, as mentioned earlier, when the Java compiler is compiling the code for doStuff( ), it cannot know exactly what types it is dealing with. So ordinarily, you’d expect it to end up calling the version of erase( ) and draw( ) for the base class Shape, and not for the specific Circle, Square, or Line. And yet the right thing happens because of polymorphism. The compiler and run-time system handle the details; all you need to know is that it happens, and more important how to design with it. When you send a message to an object, the object will do the right thing, even when upcasting is involved.

Abstract base classes and interfaces

Often in a design, you want the base class to present only an interface for its derived classes. That is, you don’t want anyone to actually create an object of the base class, only to upcast to it so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents them. This is a tool to enforce a particular design.

You can also use the abstract keyword to describe a method that hasn’t been implemented yet—as a stub indicating “here is an interface function for all types inherited from this class, but at this point I don’t have any implementation for it.” An abstract method may be created only inside an abstract class. When the class is inherited, that method must be implemented, or the inheriting class becomes abstract as well. Creating an abstract method allows you to put a method in an interface without being forced to provide a possibly meaningless body of code for that method.

The interface keyword takes the concept of an abstract class one step further by preventing any function definitions at all. The interface is a very handy and commonly used tool, as it provides the perfect separation of interface and implementation. In addition, you can combine many interfaces together, if you wish. (You cannot inherit from more than one regular class or abstract class.)

Object landscapes and lifetimes

Technically, OOP is just about abstract data typing, inheritance, and polymorphism, but other issues can be at least as important. The remainder of this section will cover these issues.

One of the most important factors is the way objects are created and destroyed. Where is the data for an object and how is the lifetime of the object controlled? There are different philosophies at work here. C++ takes the approach that control of efficiency is the most important issue, so it gives the programmer a choice. For maximum run-time speed, the storage and lifetime can be determined while the program is being written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, and control of these can be very valuable in some situations. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while you're writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictive.

The second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you don't know until run-time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run-time, the amount of time required to allocate storage on the heap is significantly longer than the time to create storage on the stack. (Creating storage on the stack is often a single assembly instruction to move the stack pointer down, and another to move it back up.) The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact on the creation of an object. In addition, the greater flexibility is essential to solve the general programming problem.

Java uses the second approach, exclusively [7] . Every time you want to create an object, you use the new keyword to build a dynamic instance of that object.

There's another issue, however, and that's the lifetime of an object. With languages that allow objects to be created on the stack, the compiler determines how long the object lasts and can automatically destroy it. However, if you create it on the heap the compiler has no knowledge of its lifetime. In a language like C++, you must determine programmatically when to destroy the object, which can lead to memory leaks if you don’t do it correctly (and this is a common problem in C++ programs). Java provides a feature called a garbage collector that automatically discovers when an object is no longer in use and destroys it. A garbage collector is much more convenient because it reduces the number of issues that you must track and the code you must write. More important, the garbage collector provides a much higher level of insurance against the insidious problem of memory leaks (which has brought many a C++ project to its knees).

The rest of this section looks at additional factors concerning object lifetimes and landscapes.

Collections and iterators

If you don’t know how many objects you’re going to need to solve a particular problem, or how long they will last, you also don’t know how to store those objects. How can you know how much space to create for those objects? You can’t, since that information isn’t known until run-time.

The solution to most problems in object-oriented design seems flippant: you create another type of object. The new type of object that solves this particular problem holds references to other objects. Of course, you can do the same thing with an array, which is available in most languages. But there’s more. This new object, generally called a container (also called a collection, but the Java library uses that term in a different sense so this book will use “container”), will expand itself whenever necessary to accommodate everything you place inside it. So you don’t need to know how many objects you’re going to hold in a container. Just create a container object and let it take care of the details.

Fortunately, a good OOP language comes with a set of containers as part of the package. In C++, it’s part of the Standard C++ Library and is sometimes called the Standard Template Library (STL). Object Pascal has containers in its Visual Component Library (VCL). Smalltalk has a very complete set of containers. Java also has containers in its standard library. In some libraries, a generic container is considered good enough for all needs, and in others (Java, for example) the library has different types of containers for different needs: a vector (called an ArrayList in Java) for consistent access to all elements, and a linked list for consistent insertion at all elements, for example, so you can choose the particular type that fits your needs. Container libraries may also include sets, queues, hash tables, trees, stacks, etc.

All containers have some way to put things in and get things out; there are usually functions to add elements to a container, and others to fetch those elements back out. But fetching elements can be more problematic, because a single-selection function is restrictive. What if you want to manipulate or compare a set of elements in the container instead of just one?

The solution is an iterator, which is an object whose job is to select the elements within a container and present them to the user of the iterator. As a class, it also provides a level of abstraction. This abstraction can be used to separate the details of the container from the code that’s accessing that container. The container, via the iterator, is abstracted to be simply a sequence. The iterator allows you to traverse that sequence without worrying about the underlying structure—that is, whether it’s an ArrayList, a LinkedList, a Stack, or something else. This gives you the flexibility to easily change the underlying data structure without disturbing the code in your program. Java began (in version 1.0 and 1.1) with a standard iterator, called Enumeration, for all of its container classes. Java 2 has added a much more complete container library that contains an iterator called Iterator that does more than the older Enumeration.

From a design standpoint, all you really want is a sequence that can be manipulated to solve your problem. If a single type of sequence satisfied all of your needs, there’d be no reason to have different kinds. There are two reasons that you need a choice of containers. First, containers provide different types of interfaces and external behavior. A stack has a different interface and behavior than that of a queue, which is different from that of a set or a list. One of these might provide a more flexible solution to your problem than the other. Second, different containers have different efficiencies for certain operations. The best example is an ArrayList and a LinkedList. Both are simple sequences that can have identical interfaces and external behaviors. But certain operations can have radically different costs. Randomly accessing elements in an ArrayList is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a LinkedList it is expensive to move through the list to randomly select an element, and it takes longer to find an element that is further down the list. On the other hand, if you want to insert an element in the middle of a sequence, it’s much cheaper in a LinkedList than in an ArrayList. These and other operations have different efficiencies depending on the underlying structure of the sequence. In the design phase, you might start with a LinkedList and, when tuning for performance, change to an ArrayList. Because of the abstraction via iterators, you can change from one to the other with minimal impact on your code.

In the end, remember that a container is only a storage cabinet to put objects in. If that cabinet solves all of your needs, it doesn’t really matter how it is implemented (a basic concept with most types of objects). If you’re working in a programming environment that has built-in overhead due to other factors, then the cost difference between an ArrayList and a LinkedList might not matter. You might need only one type of sequence. You can even imagine the “perfect” container abstraction, which can automatically change its underlying implementation according to the way it is used.

The singly rooted hierarchy

One of the issues in OOP that has become especially prominent since the introduction of C++ is whether all classes should ultimately be inherited from a single base class. In Java (as with virtually all other OOP languages) the answer is “yes” and the name of this ultimate base class is simply Object. It turns out that the benefits of the singly rooted hierarchy are many.

All objects in a singly rooted hierarchy have an interface in common, so they are all ultimately the same type. The alternative (provided by C++) is that you don’t know that everything is the same fundamental type. From a backward-compatibility standpoint this fits the model of C better and can be thought of as less restrictive, but when you want to do full-on object-oriented programming you must then build your own hierarchy to provide the same convenience that’s built into other OOP languages. And in any new class library you acquire, some other incompatible interface will be used. It requires effort (and possibly multiple inheritance) to work the new interface into your design. Is the extra “flexibility” of C++ worth it? If you need it—if you have a large investment in C—it’s quite valuable. If you’re starting from scratch, other alternatives such as Java can often be more productive.

All objects in a singly rooted hierarchy (such as Java provides) can be guaranteed to have certain functionality. You know you can perform certain basic operations on every object in your system. A singly rooted hierarchy, along with creating all objects on the heap, greatly simplifies argument passing (one of the more complex topics in C++).

A singly rooted hierarchy makes it much easier to implement a garbage collector (which is conveniently built into Java). The necessary support can be installed in the base class, and the garbage collector can thus send the appropriate messages to every object in the system. Without a singly rooted hierarchy and a system to manipulate an object via a reference, it is difficult to implement a garbage collector.

Since run-time type information is guaranteed to be in all objects, you’ll never end up with an object whose type you cannot determine. This is especially important with system level operations, such as exception handling, and to allow greater flexibility in programming.

Collection libraries and support for easy collection use

Because a container is a tool that you’ll use frequently, it makes sense to have a library of containers that are built in a reusable fashion, so you can take one off the shelf and plug it into your program. Java provides such a library, which should satisfy most needs.

Downcasting vs. templates/generics

To make these containers reusable, they hold the one universal type in Java that was previously mentioned: Object. The singly rooted hierarchy means that everything is an Object, so a container that holds Objects can hold anything. This makes containers easy to reuse.

To use such a container, you simply add object references to it, and later ask for them back. But, since the container holds only Objects, when you add your object reference into the container it is upcast to Object, thus losing its identity. When you fetch it back, you get an Object reference, and not a reference to the type that you put in. So how do you turn it back into something that has the useful interface of the object that you put into the container?

Here, the cast is used again, but this time you’re not casting up the inheritance hierarchy to a more general type, you cast down the hierarchy to a more specific type. This manner of casting is called downcasting. With upcasting, you know, for example, that a Circle is a type of Shape so it’s safe to upcast, but you don’t know that an Object is necessarily a Circle or a Shape so it’s hardly safe to downcast unless you know that’s what you’re dealing with.

It’s not completely dangerous, however, because if you downcast to the wrong thing you’ll get a run-time error called an exception, which will be described shortly. When you fetch object references from a container, though, you must have some way to remember exactly what they are so you can perform a proper downcast.

Downcasting and the run-time checks require extra time for the running program, and extra effort from the programmer. Wouldn’t it make sense to somehow create the container so that it knows the types that it holds, eliminating the need for the downcast and a possible mistake? The solution is parameterized types, which are classes that the compiler can automatically customize to work with particular types. For example, with a parameterized container, the compiler could customize that container so that it would accept only Shapes and fetch only Shapes.

Parameterized types are an important part of C++, partly because C++ has no singly rooted hierarchy. In C++, the keyword that implements parameterized types is “template.” Java currently has no parameterized types since it is possible for it to get by—however awkwardly—using the singly rooted hierarchy. However, a current proposal for parameterized types uses a syntax that is strikingly similar to C++ templates.

The housekeeping dilemma: who should clean up?

Each object requires resources in order to exist, most notably memory. When an object is no longer needed it must be cleaned up so that these resources are released for reuse. In simple programming situations the question of how an object is cleaned up doesn’t seem too challenging: you create the object, use it for as long as it’s needed, and then it should be destroyed. It’s not hard, however, to encounter situations in which the situation is more complex.

Suppose, for example, you are designing a system to manage air traffic for an airport. (The same model might also work for managing crates in a warehouse, or a video rental system, or a kennel for boarding pets.) At first it seems simple: make a container to hold airplanes, then create a new airplane and place it in the container for each airplane that enters the air-traffic-control zone. For cleanup, simply delete the appropriate airplane object when a plane leaves the zone.

But perhaps you have some other system to record data about the planes; perhaps data that doesn’t require such immediate attention as the main controller function. Maybe it’s a record of the flight plans of all the small planes that leave the airport. So you have a second container of small planes, and whenever you create a plane object you also put it in this second container if it’s a small plane. Then some background process performs operations on the objects in this container during idle moments.

Now the problem is more difficult: how can you possibly know when to destroy the objects? When you’re done with the object, some other part of the system might not be. This same problem can arise in a number of other situations, and in programming systems (such as C++) in which you must explicitly delete an object when you’re done with it this can become quite complex.

With Java, the garbage collector is designed to take care of the problem of releasing the memory (although this doesn’t include other aspects of cleaning up an object). The garbage collector “knows” when an object is no longer in use, and it then automatically releases the memory for that object. This (combined with the fact that all objects are inherited from the single root class Object and that you can create objects only one way, on the heap) makes the process of programming in Java much simpler than programming in C++. You have far fewer decisions to make and hurdles to overcome.

Garbage collectors vs. efficiency and flexibility

If all this is such a good idea, why didn’t they do the same thing in C++? Well of course there’s a price you pay for all this programming convenience, and that price is run-time overhead. As mentioned before, in C++ you can create objects on the stack, and in this case they’re automatically cleaned up (but you don’t have the flexibility of creating as many as you want at run-time). Creating objects on the stack is the most efficient way to allocate storage for objects and to free that storage. Creating objects on the heap can be much more expensive. Always inheriting from a base class and making all function calls polymorphic also exacts a small toll. But the garbage collector is a particular problem because you never quite know when it’s going to start up or how long it will take. This means that there’s an inconsistency in the rate of execution of a Java program, so you can’t use it in certain situations, such as when the rate of execution of a program is uniformly critical. (These are generally called real time programs, although not all real time programming problems are this stringent.)

The designers of the C++ language, trying to woo C programmers (and most successfully, at that), did not want to add any features to the language that would impact the speed or the use of C++ in any situation where programmers might otherwise choose C. This goal was realized, but at the price of greater complexity when programming in C++. Java is simpler than C++, but the tradeoff is in efficiency and sometimes applicability. For a significant portion of programming problems, however, Java is the superior choice.

Exception handling: dealing with errors

Ever since the beginning of programming languages, error handling has been one of the most difficult issues. Because it’s so hard to design a good error handling scheme, many languages simply ignore the issue, passing the problem on to library designers who come up with halfway measures that can work in many situations but can easily be circumvented, generally by just ignoring them. A major problem with most error handling schemes is that they rely on programmer vigilance in following an agreed-upon convention that is not enforced by the language. If the programmer is not vigilant—often the case if they are in a hurry—these schemes can easily be forgotten.

Exception handling wires error handling directly into the programming language and sometimes even the operating system. An exception is an object that is “thrown” from the site of the error and can be “caught” by an appropriate exception handler designed to handle that particular type of error. It’s as if exception handling is a different, parallel path of execution that can be taken when things go wrong. And because it uses a separate execution path, it doesn’t need to interfere with your normally executing code. This makes that code simpler to write since you aren’t constantly forced to check for errors. In addition, a thrown exception is unlike an error value that’s returned from a function or a flag that’s set by a function in order to indicate an error condition—these can be ignored. An exception cannot be ignored, so it’s guaranteed to be dealt with at some point. Finally, exceptions provide a way to reliably recover from a bad situation. Instead of just exiting you are often able to set things right and restore the execution of a program, which produces much more robust programs.

Java’s exception handling stands out among programming languages, because in Java, exception handling was wired in from the beginning and you’re forced to use it. If you don’t write your code to properly handle exceptions, you’ll get a compile-time error message. This guaranteed consistency makes error handling much easier.

It’s worth noting that exception handling isn’t an object-oriented feature, although in object-oriented languages the exception is normally represented with an object. Exception handling existed before object-oriented languages.

Multithreading

A fundamental concept in computer programming is the idea of handling more than one task at a time. Many programming problems require that the program be able to stop what it’s doing, deal with some other problem, and then return to the main process. The solution has been approached in many ways. Initially, programmers with low-level knowledge of the machine wrote interrupt service routines and the suspension of the main process was initiated through a hardware interrupt. Although this worked well, it was difficult and nonportable, so it made moving a program to a new type of machine slow and expensive.

Sometimes interrupts are necessary for handling time-critical tasks, but there’s a large class of problems in which you’re simply trying to partition the problem into separately running pieces so that the whole program can be more responsive. Within a program, these separately running pieces are called threads, and the general concept is called multithreading. A common example of multithreading is the user interface. By using threads, a user can press a button and get a quick response rather than being forced to wait until the program finishes its current task.

Ordinarily, threads are just a way to allocate the time of a single processor. But if the operating system supports multiple processors, each thread can be assigned to a different processor and they can truly run in parallel. One of the convenient features of multithreading at the language level is that the programmer doesn’t need to worry about whether there are many processors or just one. The program is logically divided into threads and if the machine has more than one processor then the program runs faster, without any special adjustments.

All this makes threading sound pretty simple. There is a catch: shared resources. If you have more than one thread running that’s expecting to access the same resource you have a problem. For example, two processes can’t simultaneously send information to a printer. To solve the problem, resources that can be shared, such as the printer, must be locked while they are being used. So a thread locks a resource, completes its task, and then releases the lock so that someone else can use the resource.

Java’s threading is built into the language, which makes a complicated subject much simpler. The threading is supported on an object level, so one thread of execution is represented by one object. Java also provides limited resource locking. It can lock the memory of any object (which is, after all, one kind of shared resource) so that only one thread can use it at a time. This is accomplished with the synchronized keyword. Other types of resources must be locked explicitly by the programmer, typically by creating an object to represent the lock that all threads must check before accessing that resource.

Persistence

When you create an object, it exists for as long as you need it, but under no circumstances does it exist when the program terminates. While this makes sense at first, there are situations in which it would be incredibly useful if an object could exist and hold its information even while the program wasn’t running. Then the next time you started the program, the object would be there and it would have the same information it had the previous time the program was running. Of course, you can get a similar effect by writing the information to a file or to a database, but in the spirit of making everything an object it would be quite convenient to be able to declare an object persistent and have all the details taken care of for you.

Java provides support for “lightweight persistence,” which means that you can easily store objects on disk and later retrieve them. The reason it’s “lightweight” is that you’re still forced to make explicit calls to do the storage and retrieval. In addition, JavaSpaces (described in Chapter 15) provide for a kind of persistent storage of objects. In some future release more complete support for persistence might appear.

Java and the Internet

If Java is, in fact, yet another computer programming language, you may question why it is so important and why it is being promoted as a revolutionary step in computer programming. The answer isn’t immediately obvious if you’re coming from a traditional programming perspective. Although Java is very useful for solving traditional stand-alone programming problems, it is also important because it will solve programming problems on the World Wide Web.

What is the Web?

The Web can seem a bit of a mystery at first, with all this talk of “surfing,” “presence,” and “home pages.” There has even been a growing reaction against “Internet-mania,” questioning the economic value and outcome of such a sweeping movement. It’s helpful to step back and see what it really is, but to do this you must understand client/server systems, another aspect of computing that’s full of confusing issues.

Client/Server computing

The primary idea of a client/server system is that you have a central repository of information—some kind of data, often in a database—that you want to distribute on demand to some set of people or machines. A key to the client/server concept is that the repository of information is centrally located so that it can be changed and so that those changes will propagate out to the information consumers. Taken together, the information repository, the software that distributes the information, and the machine(s) where the information and software reside is called the server. The software that resides on the remote machine, communicates with the server, fetches the information, processes it, and then displays it on the remote machine is called the client.

The basic concept of client/server computing, then, is not so complicated. The problems arise because you have a single server trying to serve many clients at once. Generally, a database management system is involved so the designer “balances” the layout of data into tables for optimal use. In addition, systems often allow a client to insert new information into a server. This means you must ensure that one client’s new data doesn’t walk over another client’s new data, or that data isn’t lost in the process of adding it to the database. (This is called transaction processing.) As client software changes, it must be built, debugged, and installed on the client machines, which turns out to be more complicated and expensive than you might think. It’s especially problematic to support multiple types of computers and operating systems. Finally, there’s the all-important performance issue: you might have hundreds of clients making requests of your server at any one time, and so any small delay is crucial. To minimize latency, programmers work hard to offload processing tasks, often to the client machine, but sometimes to other machines at the server site, using so-called middleware. (Middleware is also used to improve maintainability.)

The simple idea of distributing information to people has so many layers of complexity in implementing it that the whole problem can seem hopelessly enigmatic. And yet it’s crucial: client/server computing accounts for roughly half of all programming activities. It’s responsible for everything from taking orders and credit-card transactions to the distribution of any kind of data—stock market, scientific, government, you name it. What we’ve come up with in the past is individual solutions to individual problems, inventing a new solution each time. These were hard to create and hard to use, and the user had to learn a new interface for each one. The entire client/server problem needs to be solved in a big way.

The Web as a giant server

The Web is actually one giant client/server system. It’s a bit worse than that, since you have all the servers and clients coexisting on a single network at once. You don’t need to know that, since all you care about is connecting to and interacting with one server at a time (even though you might be hopping around the world in your search for the correct server).

Initially it was a simple one-way process. You made a request of a server and it handed you a file, which your machine’s browser software (i.e., the client) would interpret by formatting onto your local machine. But in short order people began wanting to do more than just deliver pages from a server. They wanted full client/server capability so that the client could feed information back to the server, for example, to do database lookups on the server, to add new information to the server, or to place an order (which required more security than the original systems offered). These are the changes we’ve been seeing in the development of the Web.

The Web browser was a big step forward: the concept that one piece of information could be displayed on any type of computer without change. However, browsers were still rather primitive and rapidly bogged down by the demands placed on them. They weren’t particularly interactive, and tended to clog up both the server and the Internet because any time you needed to do something that required programming you had to send information back to the server to be processed. It could take many seconds or minutes to find out you had misspelled something in your request. Since the browser was just a viewer it couldn’t perform even the simplest computing tasks. (On the other hand, it was safe, since it couldn’t execute any programs on your local machine that might contain bugs or viruses.)

To solve this problem, different approaches have been taken. To begin with, graphics standards have been enhanced to allow better animation and video within browsers. The remainder of the problem can be solved only by incorporating the ability to run programs on the client end, under the browser. This is called client-side programming.

Client-side programming

The Web’s initial server-browser design provided for interactive content, but the interactivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HTML contains simple mechanisms for data gathering: text-entry boxes, check boxes, radio boxes, lists and drop-down lists, as well as a button that can only be programmed to reset the data on the form or “submit” the data on the form back to the server. This submission passes through the Common Gateway Interface (CGI) provided on all Web servers. The text within the submission tells CGI what to do with it. The most common action is to run a program located on the server in a directory that’s typically called “cgi-bin.” (If you watch the address window at the top of your browser when you push a button on a Web page, you can sometimes see “cgi-bin” within all the gobbledygook there.) These programs can be written in most languages. Perl is a common choice because it is designed for text manipulation and is interpreted, so it can be installed on any server regardless of processor or operating system.

Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with it. However, Web sites built on CGI programs can rapidly become overly complicated to maintain, and there is also the problem of response time. The response of a CGI program depends on how much data must be sent, as well as the load on both the server and the Internet. (On top of this, starting a CGI program tends to be slow.) The initial designers of the Web did not foresee how rapidly this bandwidth would be exhausted for the kinds of applications people developed. For example, any sort of dynamic graphing is nearly impossible to perform with consistency because a GIF file must be created and moved from the server to the client for each version of the graph. And you’ve no doubt had direct experience with something as simple as validating the data on an input form. You press the submit button on a page; the data is shipped back to the server; the server starts a CGI program that discovers an error, formats an HTML page informing you of the error, and then sends the page back to you; you must then back up a page and try again. Not only is this slow, it’s inelegant.

The solution is client-side programming. Most machines that run Web browsers are powerful engines capable of doing vast work, and with the original static HTML approach they are sitting there, just idly waiting for the server to dish up the next page. Client-side programming means that the Web browser is harnessed to do whatever work it can, and the result for the user is a much speedier and more interactive experience at your Web site.

The problem with discussions of client-side programming is that they aren’t very different from discussions of programming in general. The parameters are almost the same, but the platform is different: a Web browser is like a limited operating system. In the end, you must still program, and this accounts for the dizzying array of problems and solutions produced by client-side programming. The rest of this section provides an overview of the issues and approaches in client-side programming.

Plug-ins

One of the most significant steps forward in client-side programming is the development of the plug-in. This is a way for a programmer to add new functionality to the browser by downloading a piece of code that plugs itself into the appropriate spot in the browser. It tells the browser “from now on you can perform this new activity.” (You need to download the plug-in only once.) Some fast and powerful behavior is added to browsers via plug-ins, but writing a plug-in is not a trivial task, and isn’t something you’d want to do as part of the process of building a particular site. The value of the plug-in for client-side programming is that it allows an expert programmer to develop a new language and add that language to a browser without the permission of the browser manufacturer. Thus, plug-ins provide a “back door” that allows the creation of new client-side programming languages (although not all languages are implemented as plug-ins).

Scripting languages

Plug-ins resulted in an explosion of scripting languages. With a scripting language you embed the source code for your client-side program directly into the HTML page, and the plug-in that interprets that language is automatically activated while the HTML page is being displayed. Scripting languages tend to be reasonably easy to understand and, because they are simply text that is part of an HTML page, they load very quickly as part of the single server hit required to procure that page. The trade-off is that your code is exposed for everyone to see (and steal). Generally, however, you aren’t doing amazingly sophisticated things with scripting languages so this is not too much of a hardship.

This points out that the scripting languages used inside Web browsers are really intended to solve specific types of problems, primarily the creation of richer and more interactive graphical user interfaces (GUIs). However, a scripting language might solve 80 percent of the problems encountered in client-side programming. Your problems might very well fit completely within that 80 percent, and since scripting languages can allow easier and faster development, you should probably consider a scripting language before looking at a more involved solution such as Java or ActiveX programming.

The most commonly discussed browser scripting languages are JavaScript (which has nothing to do with Java; it’s named that way just to grab some of Java’s marketing momentum), VBScript (which looks like Visual Basic), and Tcl/Tk, which comes from the popular cross-platform GUI-building language. There are others out there, and no doubt more in development.

JavaScript is probably the most commonly supported. It comes built into both Netscape Navigator and the Microsoft Internet Explorer (IE). In addition, there are probably more JavaScript books available than there are for the other browser languages, and some tools automatically create pages using JavaScript. However, if you’re already fluent in Visual Basic or Tcl/Tk, you’ll be more productive using those scripting languages rather than learning a new one. (You’ll have your hands full dealing with the Web issues already.)

Java

If a scripting language can solve 80 percent of the client-side programming problems, what about the other 20 percent—the “really hard stuff?” The most popular solution today is Java. Not only is it a powerful programming language built to be secure, cross-platform, and international, but Java is being continually extended to provide language features and libraries that elegantly handle problems that are difficult in traditional programming languages, such as multithreading, database access, network programming, and distributed computing. Java allows client-side programming via the applet.

An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page (just as, for example, a graphic is automatically downloaded). When the applet is activated it executes a program. This is part of its beauty—it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. The user gets the latest version of the client software without fail and without difficult reinstallation. Because of the way Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters. (This safely includes the vast majority of machines.) Since Java is a full-fledged programming language, you can do as much work as possible on the client before and after making requests of the server. For example, you won’t need to send a request form across the Internet to discover that you’ve gotten a date or some other parameter wrong, and your client computer can quickly do the work of plotting data instead of waiting for the server to make a plot and ship a graphic image back to you. Not only do you get the immediate win of speed and responsiveness, but the general network traffic and load on servers can be reduced, preventing the entire Internet from slowing down.

One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client. On the other hand, a Java applet can be decompiled without too much trouble, but hiding your code is often not an important issue. Two other factors can be important. As you will see later in this book, a compiled Java applet can comprise many modules and take multiple server “hits” (accesses) to download. (In Java 1.1 and higher this is minimized by Java archives, called JAR files, that allow all the required modules to be packaged together and compressed for a single download.) A scripted program will just be integrated into the Web page as part of its text (and will generally be smaller and reduce server hits). This could be important to the responsiveness of your Web site. Another factor is the all-important learning curve. Regardless of what you’ve heard, Java is not a trivial language to learn. If you’re a Visual Basic programmer, moving to VBScript will be your fastest solution, and since it will probably solve most typical client/server problems you might be hard pressed to justify learning Java. If you’re experienced with a scripting language you will certainly benefit from looking at JavaScript or VBScript before committing to Java, since they might fit your needs handily and you’ll be more productive sooner.

ActiveX

To some degree, the competitor to Java is Microsoft’s ActiveX, although it takes a completely different approach. ActiveX was originally a Windows-only solution, although it is now being developed via an independent consortium to become cross-platform. Effectively, ActiveX says “if your program connects to its environment just so, it can be dropped into a Web page and run under a browser that supports ActiveX.” (IE directly supports ActiveX and Netscape does so using a plug-in.) Thus, ActiveX does not constrain you to a particular language. If, for example, you’re already an experienced Windows programmer using a language such as C++, Visual Basic, or Borland’s Delphi, you can create ActiveX components with almost no changes to your programming knowledge. ActiveX also provides a path for the use of legacy code in your Web pages.

Security

Automatically downloading and running programs across the Internet can sound like a virus-builder’s dream. ActiveX especially brings up the thorny issue of security in client-side programming. If you click on a Web site, you might automatically download any number of things along with the HTML page: GIF files, script code, compiled Java code, and ActiveX components. Some of these are benign; GIF files can’t do any harm, and scripting languages are generally limited in what they can do. Java was also designed to run its applets within a “sandbox” of safety, which prevents it from writing to disk or accessing memory outside the sandbox.

ActiveX is at the opposite end of the spectrum. Programming with ActiveX is like programming Windows—you can do anything you want. So if you click on a page that downloads an ActiveX component, that component might cause damage to the files on your disk. Of course, programs that you load onto your computer that are not restricted to running inside a Web browser can do the same thing. Viruses downloaded from Bulletin-Board Systems (BBSs) have long been a problem, but the speed of the Internet amplifies the difficulty.

The solution seems to be “digital signatures,” whereby code is verified to show who the author is. This is based on the idea that a virus works because its creator can be anonymous, so if you remove the anonymity individuals will be forced to be responsible for their actions. This seems like a good plan because it allows programs to be much more functional, and I suspect it will eliminate malicious mischief. If, however, a program has an unintentional destructive bug it will still cause problems.

The Java approach is to prevent these problems from occurring, via the sandbox. The Java interpreter that lives on your local Web browser examines the applet for any untoward instructions as the applet is being loaded. In particular, the applet cannot write files to disk or erase files (one of the mainstays of viruses). Applets are generally considered to be safe, and since this is essential for reliable client/server systems, any bugs in the Java language that allow viruses are rapidly repaired. (It’s worth noting that the browser software actually enforces these security restrictions, and some browsers allow you to select different security levels to provide varying degrees of access to your system.)

You might be skeptical of this rather draconian restriction against writing files to your local disk. For example, you may want to build a local database or save data for later use offline. The initial vision seemed to be that eventually everyone would get online to do anything important, but that was soon seen to be impractical (although low-cost “Internet appliances” might someday satisfy the needs of a significant segment of users). The solution is the “signed applet” that uses public-key encryption to verify that an applet does indeed come from where it claims it does. A signed applet can still trash your disk, but the theory is that since you can now hold the applet creator accountable they won’t do vicious things. Java provides a framework for digital signatures so that you will eventually be able to allow an applet to step outside the sandbox if necessary.

Digital signatures have missed an important issue, which is the speed that people move around on the Internet. If you download a buggy program and it does something untoward, how long will it be before you discover the damage? It could be days or even weeks. By then, how will you track down the program that’s done it? And what good will it do you at that point?

Internet vs. intranet

The Web is the most general solution to the client/server problem, so it makes sense that you can use the same technology to solve a subset of the problem, in particular the classic client/server problem within a company. With traditional client/server approaches you have the problem of multiple types of client computers, as well as the difficulty of installing new client software, both of which are handily solved with Web browsers and client-side programming. When Web technology is used for an information network that is restricted to a particular company, it is referred to as an intranet. Intranets provide much greater security than the Internet, since you can physically control access to the servers within your company. In terms of training, it seems that once people understand the general concept of a browser it’s much easier for them to deal with differences in the way pages and applets look, so the learning curve for new kinds of systems seems to be reduced.

The security problem brings us to one of the divisions that seems to be automatically forming in the world of client-side programming. If your program is running on the Internet, you don’t know what platform it will be working under, and you want to be extra careful that you don’t disseminate buggy code. You need something cross-platform and secure, like a scripting language or Java.

If you’re running on an intranet, you might have a different set of constraints. It’s not uncommon that your machines could all be Intel/Windows platforms. On an intranet, you’re responsible for the quality of your own code and can repair bugs when they’re discovered. In addition, you might already have a body of legacy code that you’ve been using in a more traditional client/server approach, whereby you must physically install client programs every time you do an upgrade. The time wasted in installing upgrades is the most compelling reason to move to browsers, because upgrades are invisible and automatic. If you are involved in such an intranet, the most sensible approach to take is the shortest path that allows you to use your existing code base, rather than trying to recode your programs in a new language.

When faced with this bewildering array of solutions to the client-side programming problem, the best plan of attack is a cost-benefit analysis. Consider the constraints of your problem and what would be the shortest path to your solution. Since client-side programming is still programming, it’s always a good idea to take the fastest development approach for your particular situation. This is an aggressive stance to prepare for inevitable encounters with the problems of program development.

Server-side programming

This whole discussion has ignored the issue of server-side programming. What happens when you make a request of a server? Most of the time the request is simply “send me this file.” Your browser then interprets the file in some appropriate fashion: as an HTML page, a graphic image, a Java applet, a script program, etc. A more complicated request to a server generally involves a database transaction. A common scenario involves a request for a complex database search, which the server then formats into an HTML page and sends to you as the result. (Of course, if the client has more intelligence via Java or a scripting language, the raw data can be sent and formatted at the client end, which will be faster and less load on the server.) Or you might want to register your name in a database when you join a group or place an order, which will involve changes to that database. These database requests must be processed via some code on the server side, which is generally referred to as server-side programming. Traditionally, server-side programming has been performed using Perl and CGI scripts, but more sophisticated systems have been appearing. These include Java-based Web servers that allow you to perform all your server-side programming in Java by writing what are called servlets. Servlets and their offspring, JSPs, are two of the most compelling reasons that companies who develop Web sites are moving to Java, especially because they eliminate the problems of dealing with differently abled browsers.

A separate arena: applications

Much of the brouhaha over Java has been over applets. Java is actually a general-purpose programming language that can solve any type of problem—at least in theory. And as pointed out previously, there might be more effective ways to solve most client/server problems. When you move out of the applet arena (and simultaneously release the restrictions, such as the one against writing to disk) you enter the world of general-purpose applications that run standalone, without a Web browser, just like any ordinary program does. Here, Java’s strength is not only in its portability, but also its programmability. As you’ll see throughout this book, Java has many features that allow you to create robust programs in a shorter period than with previous programming languages.

Be aware that this is a mixed blessing. You pay for the improvements through slower execution speed (although there is significant work going on in this area—JDK 1.3, in particular, introduces the so-called “hotspot” performance improvements). Like any language, Java has built-in limitations that might make it inappropriate to solve certain types of programming problems. Java is a rapidly evolving language, however, and as each new release comes out it becomes more and more attractive for solving larger sets of problems.

Analysis and design

The object-oriented paradigm is a new and different way of thinking about programming. Many folks have trouble at first knowing how to approach an OOP project. Once you know that everything is supposed to be an object, and as you learn to think more in an object-oriented style, you can begin to create “good” designs that take advantage of all the benefits that OOP has to offer.

A method (often called a methodology) is a set of processes and heuristics used to break down the complexity of a programming problem. Many OOP methods have been formulated since the dawn of object-oriented programming. This section will give you a feel for what you’re trying to accomplish when using a method.

Especially in OOP, methodology is a field of many experiments, so it is important to understand what problem the method is trying to solve before you consider adopting one. This is particularly true with Java, in which the programming language is intended to reduce the complexity (compared to C) involved in expressing a program. This may in fact alleviate the need for ever-more-complex methodologies. Instead, simple methodologies may suffice in Java for a much larger class of problems than you could handle using simple methodologies with procedural languages.

It’s also important to realize that the term “methodology” is often too grand and promises too much. Whatever you do now when you design and write a program is a method. It may be your own method, and you may not be conscious of doing it, but it is a process you go through as you create. If it is an effective process, it may need only a small tune-up to work with Java. If you are not satisfied with your productivity and the way your programs turn out, you may want to consider adopting a formal method, or choosing pieces from among the many formal methods.

While you’re going through the development process, the most important issue is this: Don’t get lost. It’s easy to do. Most of the analysis and design methods are intended to solve the largest of problems. Remember that most projects don’t fit into that category, so you can usually have successful analysis and design with a relatively small subset of what a method recommends [8] . But some sort of process, no matter how limited, will generally get you on your way in a much better fashion than simply beginning to code.

It’s also easy to get stuck, to fall into “analysis paralysis,” where you feel like you can’t move forward because you haven’t nailed down every little detail at the current stage. Remember, no matter how much analysis you do, there are some things about a system that won’t reveal themselves until design time, and more things that won’t reveal themselves until you’re coding, or not even until a program is up and running. Because of this, it’s crucial to move fairly quickly through analysis and design, and to implement a test of the proposed system.

This point is worth emphasizing. Because of the history we’ve had with procedural languages, it is commendable that a team will want to proceed carefully and understand every minute detail before moving to design and implementation. Certainly, when creating a DBMS, it pays to understand a customer’s needs thoroughly. But a DBMS is in a class of problems that is very well-posed and well-understood; in many such programs, the database structure is the problem to be tackled. The class of programming problem discussed in this chapter is of the “wild-card” (my term) variety, in which the solution isn’t simply re-forming a well-known solution, but instead involves one or more “wild-card factors”—elements for which there is no well-understood previous solution, and for which research is necessary [9] . Attempting to thoroughly analyze a wild-card problem before moving into design and implementation results in analysis paralysis because you don’t have enough information to solve this kind of problem during the analysis phase. Solving such a problem requires iteration through the whole cycle, and that requires risk-taking behavior (which makes sense, because you’re trying to do something new and the potential rewards are higher). It may seem like the risk is compounded by “rushing” into a preliminary implementation, but it can instead reduce the risk in a wild-card project because you’re finding out early whether a particular approach to the problem is viable. Product development is risk management.

It’s often proposed that you “build one to throw away.” With OOP, you may still throw part of it away, but because code is encapsulated into classes, during the first pass you will inevitably produce some useful class designs and develop some worthwhile ideas about the system design that do not need to be thrown away. Thus, the first rapid pass at a problem not only produces critical information for the next analysis, design, and implementation pass, it also creates a code foundation.

That said, if you’re looking at a methodology that contains tremendous detail and suggests many steps and documents, it’s still difficult to know when to stop. Keep in mind what you’re trying to discover:

  1. What are the objects? (How do you partition your project into its component parts?)
  2. What are their interfaces? (What messages do you need to send to each object?)

If you come up with nothing more than the objects and their interfaces, then you can write a program. For various reasons you might need more descriptions and documents than this, but you can’t get away with any less.

The process can be undertaken in five phases, and a Phase 0 that is just the initial commitment to using some kind of structure.

Phase 0: Make a plan

You must first decide what steps you’re going to have in your process. It sounds simple (in fact, all of this sounds simple), and yet people often don’t make this decision before they start coding. If your plan is “let’s jump in and start coding,” fine. (Sometimes that’s appropriate when you have a well-understood problem.) At least agree that this is the plan.

You might also decide at this phase that some additional process structure is necessary, but not the whole nine yards. Understandably, some programmers like to work in “vacation mode,” in which no structure is imposed on the process of developing their work; “It will be done when it’s done.” This can be appealing for a while, but I’ve found that having a few milestones along the way helps to focus and galvanize your efforts around those milestones instead of being stuck with the single goal of “finish the project.” In addition, it divides the project into more bite-sized pieces and makes it seem less threatening (plus the milestones offer more opportunities for celebration).

When I began to study story structure (so that I will someday write a novel) I was initially resistant to the idea of structure, feeling that I wrote best when I simply let it flow onto the page. But I later realized that when I write about computers the structure is clear enough to me that I don’t have to think about it very much. But I still structure my work, albeit only semi-consciously in my head. Even if you think that your plan is to just start coding, you still somehow go through the subsequent phases while asking and answering certain questions.

The mission statement

Any system you build, no matter how complicated, has a fundamental purpose; the business that it’s in, the basic need that it satisfies. If you can look past the user interface, the hardware- or system-specific details, the coding algorithms and the efficiency problems, you will eventually find the core of its being—simple and straightforward. Like the so-called high concept from a Hollywood movie, you can describe it in one or two sentences. This pure description is the starting point.

The high concept is quite important because it sets the tone for your project; it’s a mission statement. You won’t necessarily get it right the first time (you may be in a later phase of the project before it becomes completely clear), but keep trying until it feels right. For example, in an air-traffic control system you may start out with a high concept focused on the system that you’re building: “The tower program keeps track of the aircraft.” But consider what happens when you shrink the system to a very small airfield; perhaps there’s only a human controller, or none at all. A more useful model won’t concern the solution you’re creating as much as it describes the problem: “Aircraft arrive, unload, service and reload, then depart.”

Phase 1: What are we making?

In the previous generation of program design (called procedural design), this is called “creating the requirements analysis and system specification.” These, of course, were places to get lost; intimidatingly named documents that could become big projects in their own right. Their intention was good, however. The requirements analysis says “Make a list of the guidelines we will use to know when the job is done and the customer is satisfied.” The system specification says “Here’s a description of what the program will do (not how) to satisfy the requirements.” The requirements analysis is really a contract between you and the customer (even if the customer works within your company, or is some other object or system). The system specification is a top-level exploration into the problem and in some sense a discovery of whether it can be done and how long it will take. Since both of these will require consensus among people (and because they will usually change over time), I think it’s best to keep them as bare as possible—ideally, to lists and basic diagrams—to save time. You might have other constraints that require you to expand them into bigger documents, but by keeping the initial document small and concise, it can be created in a few sessions of group brainstorming with a leader who dynamically creates the description. This not only solicits input from everyone, it also fosters initial buy-in and agreement by everyone on the team. Perhaps most importantly, it can kick off a project with a lot of enthusiasm.

It’s necessary to stay focused on the heart of what you’re trying to accomplish in this phase: determine what the system is supposed to do. The most valuable tool for this is a collection of what are called “use cases.” Use cases identify key features in the system that will reveal some of the fundamental classes you’ll be using. These are essentially descriptive answers to questions like [10] :

  • “Who will use this system?”
  • “What can those actors do with the system?”
  • “How does this actor do that with this system?”
  • “How else might this work if someone else were doing this, or if the same actor had a different objective?” (to reveal variations)
  • “What problems might happen while doing this with the system?” (to reveal exceptions)

If you are designing an auto-teller, for example, the use case for a particular aspect of the functionality of the system is able to describe what the auto-teller does in every possible situation. Each of these “situations” is referred to as a scenario, and a use case can be considered a collection of scenarios. You can think of a scenario as a question that starts with: “What does the system do if...?” For example, “What does the auto-teller do if a customer has just deposited a check within the last 24 hours, and there’s not enough in the account without the check having cleared to provide a desired withdrawal?”

Use case diagrams are intentionally simple to prevent you from getting bogged down in system implementation details prematurely:

Each stick person represents an “actor,” which is typically a human or some other kind of free agent. (These can even be other computer systems, as is the case with “ATM.”) The box represents the boundary of your system. The ellipses represent the use cases, which are descriptions of valuable work that can be performed with the system. The lines between the actors and the use cases represent the interactions.

It doesn’t matter how the system is actually implemented, as long as it looks like this to the user.

A use case does not need to be terribly complex, even if the underlying system is complex. It is only intended to show the system as it appears to the user. For example:

The use cases produce the requirements specifications by determining all the interactions that the user may have with the system. You try to discover a full set of use cases for your system, and once you’ve done that you have the core of what the system is supposed to do. The nice thing about focusing on use cases is that they always bring you back to the essentials and keep you from drifting off into issues that aren’t critical for getting the job done. That is, if you have a full set of use cases, you can describe your system and move onto the next phase. You probably won’t get it all figured out perfectly on the first try, but that’s OK. Everything will reveal itself in time, and if you demand a perfect system specification at this point you’ll get stuck.

If you do get stuck, you can kick-start this phase by using a rough approximation tool: describe the system in a few paragraphs and then look for nouns and verbs. The nouns can suggest actors, context of the use case (e.g., “lobby”), or artifacts manipulated in the use case. Verbs can suggest interactions between actors and use cases, and specify steps within the use case. You’ll also discover that nouns and verbs produce objects and messages during the design phase (and note that use cases describe interactions between subsystems, so the “noun and verb” technique can be used only as a brainstorming tool as it does not generate use cases) [11] .

The boundary between a use case and an actor can point out the existence of a user interface, but it does not define such a user interface. For a process of defining and creating user interfaces, see Software for Use by Larry Constantine and Lucy Lockwood, (Addison-Wesley Longman, 1999) or go to www.ForUse.com.

Although it’s a black art, at this point some kind of basic scheduling is important. You now have an overview of what you’re building, so you’ll probably be able to get some idea of how long it will take. A lot of factors come into play here. If you estimate a long schedule then the company might decide not to build it (and thus use their resources on something more reasonable—that’s a good thing). Or a manager might have already decided how long the project should take and will try to influence your estimate. But it’s best to have an honest schedule from the beginning and deal with the tough decisions early. There have been a lot of attempts to come up with accurate scheduling techniques (much like techniques to predict the stock market), but probably the best approach is to rely on your experience and intuition. Get a gut feeling for how long it will really take, then double that and add 10 percent. Your gut feeling is probably correct; you can get something working in that time. The “doubling” will turn that into something decent, and the 10 percent will deal with the final polishing and details [12] . However you want to explain it, and regardless of the moans and manipulations that happen when you reveal such a schedule, it just seems to work out that way.

Phase 2: How will we build it?

In this phase you must come up with a design that describes what the classes look like and how they will interact. An excellent technique in determining classes and interactions is the Class-Responsibility-Collaboration (CRC) card. Part of the value of this tool is that it’s so low-tech: you start out with a set of blank 3 x 5 cards, and you write on them. Each card represents a single class, and on the card you write:

  1. The name of the class. It’s important that this name capture the essence of what the class does, so that it makes sense at a glance.
  2. The “responsibilities” of the class: what it should do. This can typically be summarized by just stating the names of the member functions (since those names should be descriptive in a good design), but it does not preclude other notes. If you need to seed the process, look at the problem from a lazy programmer’s standpoint: What objects would you like to magically appear to solve your problem?
  3. The “collaborations” of the class: what other classes does it interact with? “Interact” is an intentionally broad term; it could mean aggregation or simply that some other object exists that will perform services for an object of the class. Collaborations should also consider the audience for this class. For example, if you create a class Firecracker, who is going to observe it, a Chemist or a Spectator? The former will want to know what chemicals go into the construction, and the latter will respond to the colors and shapes released when it explodes.

You may feel like the cards should be bigger because of all the information you’d like to get on them, but they are intentionally small, not only to keep your classes small but also to keep you from getting into too much detail too early. If you can’t fit all you need to know about a class on a small card, the class is too complex (either you’re getting too detailed, or you should create more than one class). The ideal class should be understood at a glance. The idea of CRC cards is to assist you in coming up with a first cut of the design so that you can get the big picture and then refine your design.

One of the great benefits of CRC cards is in communication. It’s best done real time, in a group, without computers. Each person takes responsibility for several classes (which at first have no names or other information). You run a live simulation by solving one scenario at a time, deciding which messages are sent to the various objects to satisfy each scenario. As you go through this process, you discover the classes that you need along with their responsibilities and collaborations, and you fill out the cards as you do this. When you’ve moved through all the use cases, you should have a fairly complete first cut of your design.

Before I began using CRC cards, the most successful consulting experiences I had when coming up with an initial design involved standing in front of a team—who hadn’t built an OOP project before—and drawing objects on a whiteboard. We talked about how the objects should communicate with each other, and erased some of them and replaced them with other objects. Effectively, I was managing all the “CRC cards” on the whiteboard. The team (who knew what the project was supposed to do) actually created the design; they “owned” the design rather than having it given to them. All I was doing was guiding the process by asking the right questions, trying out the assumptions, and taking the feedback from the team to modify those assumptions. The true beauty of the process was that the team learned how to do object-oriented design not by reviewing abstract examples, but by working on the one design that was most interesting to them at that moment: theirs.

Once you’ve come up with a set of CRC cards, you may want to create a more formal description of your design using UML [13] . You don’t need to use UML, but it can be helpful, especially if you want to put up a diagram on the wall for everyone to ponder, which is a good idea. An alternative to UML is a textual description of the objects and their interfaces, or, depending on your programming language, the code itself [14] .

UML also provides an additional diagramming notation for describing the dynamic model of your system. This is helpful in situations in which the state transitions of a system or subsystem are dominant enough that they need their own diagrams (such as in a control system). You may also need to describe the data structures, for systems or subsystems in which data is a dominant factor (such as a database).

You’ll know you’re done with Phase 2 when you have described the objects and their interfaces. Well, most of them—there are usually a few that slip through the cracks and don’t make themselves known until Phase 3. But that’s OK. All you are concerned with is that you eventually discover all of your objects. It’s nice to discover them early in the process, but OOP provides enough structure so that it’s not so bad if you discover them later. In fact, the design of an object tends to happen in five stages, throughout the process of program development.

Five stages of object design

The design life of an object is not limited to the time when you’re writing the program. Instead, the design of an object appears over a sequence of stages. It’s helpful to have this perspective because you stop expecting perfection right away; instead, you realize that the understanding of what an object does and what it should look like happens over time. This view also applies to the design of various types of programs; the pattern for a particular type of program emerges through struggling again and again with that problem (This is chronicled in the book Thinking in Patterns with Java, downloadable at www.BruceEckel.com). Objects, too, have their patterns that emerge through understanding, use, and reuse.

1. Object discovery. This stage occurs during the initial analysis of a program. Objects may be discovered by looking for external factors and boundaries, duplication of elements in the system, and the smallest conceptual units. Some objects are obvious if you already have a set of class libraries. Commonality between classes suggesting base classes and inheritance may appear right away, or later in the design process.

2. Object assembly. As you’re building an object you’ll discover the need for new members that didn’t appear during discovery. The internal needs of the object may require other classes to support it.

3. System construction. Once again, more requirements for an object may appear at this later stage. As you learn, you evolve your objects. The need for communication and interconnection with other objects in the system may change the needs of your classes or require new classes. For example, you may discover the need for facilitator or helper classes, such as a linked list, that contain little or no state information and simply help other classes function.

4. System extension. As you add new features to a system you may discover that your previous design doesn’t support easy system extension. With this new information, you can restructure parts of the system, possibly adding new classes or class hierarchies.

5. Object reuse. This is the real stress test for a class. If someone tries to reuse it in an entirely new situation, they’ll probably discover some shortcomings. As you change a class to adapt to more new programs, the general principles of the class will become clearer, until you have a truly reusable type. However, don’t expect most objects from a system design to be reusable—it is perfectly acceptable for the bulk of your objects to be system-specific. Reusable types tend to be less common, and they must solve more general problems in order to be reusable.

Guidelines for object development

These stages suggest some guidelines when thinking about developing your classes:

  1. Let a specific problem generate a class, then let the class grow and mature during the solution of other problems.
  2. Remember, discovering the classes you need (and their interfaces) is the majority of the system design. If you already had those classes, this would be an easy project.
  3. Don’t force yourself to know everything at the beginning; learn as you go. This will happen anyway.
  4. Start programming; get something working so you can prove or disprove your design. Don’t fear that you’ll end up with procedural-style spaghetti code—classes partition the problem and help control anarchy and entropy. Bad classes do not break good classes.
  5. Always keep it simple. Little clean objects with obvious utility are better than big complicated interfaces. When decision points come up, use an Occam’s Razor approach: Consider the choices and select the one that is simplest, because simple classes are almost always best. Start small and simple, and you can expand the class interface when you understand it better. As time goes on, it’s difficult to remove elements from a class.

Phase 3: Build the core

This is the initial conversion from the rough design into a compiling and executing body of code that can be tested, and especially that will prove or disprove your architecture. This is not a one-pass process, but rather the beginning of a series of steps that will iteratively build the system, as you’ll see in Phase 4.

Your goal is to find the core of your system architecture that needs to be implemented in order to generate a running system, no matter how incomplete that system is in this initial pass. You’re creating a framework that you can build on with further iterations. You’re also performing the first of many system integrations and tests, and giving the stakeholders feedback about what their system will look like and how it is progressing. Ideally, you are also exposing some of the critical risks. You’ll probably also discover changes and improvements that can be made to your original architecture—things you would not have learned without implementing the system.

Part of building the system is the reality check that you get from testing against your requirements analysis and system specification (in whatever form they exist). Make sure that your tests verify the requirements and use cases. When the core of the system is stable, you’re ready to move on and add more functionality.

Phase 4: Iterate the use cases

Once the core framework is running, each feature set you add is a small project in itself. You add a feature set during an iteration, a reasonably short period of development.

How big is an iteration? Ideally, each iteration lasts one to three weeks (this can vary based on the implementation language). At the end of that period, you have an integrated, tested system with more functionality than it had before. But what’s particularly interesting is the basis for the iteration: a single use case. Each use case is a package of related functionality that you build into the system all at once, during one iteration. Not only does this give you a better idea of what the scope of a use case should be, but it also gives more validation to the idea of a use case, since the concept isn’t discarded after analysis and design, but instead it is a fundamental unit of development throughout the software-building process.

You stop iterating when you achieve target functionality or an external deadline arrives and the customer can be satisfied with the current version. (Remember, software is a subscription business.) Because the process is iterative, you have many opportunities to ship a product rather than a single endpoint; open-source projects work exclusively in an iterative, high-feedback environment, which is precisely what makes them successful.

An iterative development process is valuable for many reasons. You can reveal and resolve critical risks early, the customers have ample opportunity to change their minds, programmer satisfaction is higher, and the project can be steered with more precision. But an additional important benefit is the feedback to the stakeholders, who can see by the current state of the product exactly where everything lies. This may reduce or eliminate the need for mind-numbing status meetings and increase the confidence and support from the stakeholders.

Phase 5: Evolution

This is the point in the development cycle that has traditionally been called “maintenance,” a catch-all term that can mean everything from “getting it to work the way it was really supposed to in the first place” to “adding features that the customer forgot to mention” to the more traditional “fixing the bugs that show up” and “adding new features as the need arises.” So many misconceptions have been applied to the term “maintenance” that it has taken on a slightly deceiving quality, partly because it suggests that you’ve actually built a pristine program and all you need to do is change parts, oil it, and keep it from rusting. Perhaps there’s a better term to describe what’s going on.

I’ll use the term evolution [15] . That is, “You won’t get it right the first time, so give yourself the latitude to learn and to go back and make changes.” You might need to make a lot of changes as you learn and understand the problem more deeply. The elegance you’ll produce if you evolve until you get it right will pay off, both in the short and the long term. Evolution is where your program goes from good to great, and where those issues that you didn’t really understand in the first pass become clear. It’s also where your classes can evolve from single-project usage to reusable resources.

What it means to “get it right” isn’t just that the program works according to the requirements and the use cases. It also means that the internal structure of the code makes sense to you, and feels like it fits together well, with no awkward syntax, oversized objects, or ungainly exposed bits of code. In addition, you must have some sense that the program structure will survive the changes that it will inevitably go through during its lifetime, and that those changes can be made easily and cleanly. This is no small feat. You must not only understand what you’re building, but also how the program will evolve (what I call the vector of change [16] ). Fortunately, object-oriented programming languages are particularly adept at supporting this kind of continuing modification—the boundaries created by the objects are what tend to keep the structure from breaking down. They also allow you to make changes—ones that would seem drastic in a procedural program—without causing earthquakes throughout your code. In fact, support for evolution might be the most important benefit of OOP.

With evolution, you create something that at least approximates what you think you’re building, and then you kick the tires, compare it to your requirements, and see where it falls short. Then you can go back and fix it by redesigning and reimplementing the portions of the program that didn’t work right [17] . You might actually need to solve the problem, or an aspect of the problem, several times before you hit on the right solution. (A study of Design Patterns is usually helpful here. You can find information in Thinking in Patterns with Java, downloadable at www.BruceEckel.com.)

Evolution also occurs when you build a system, see that it matches your requirements, and then discover it wasn’t actually what you wanted. When you see the system in operation, you find that you really wanted to solve a different problem. If you think this kind of evolution is going to happen, then you owe it to yourself to build your first version as quickly as possible so you can find out if it is indeed what you want.

Perhaps the most important thing to remember is that by default—by definition, really—if you modify a class, its super- and subclasses will still function. You need not fear modification (especially if you have a built-in set of unit tests to verify the correctness of your modifications). Modification won’t necessarily break the program, and any change in the outcome will be limited to subclasses and/or specific collaborators of the class you change.

Plans pay off

Of course you wouldn’t build a house without a lot of carefully drawn plans. If you build a deck or a dog house your plans won’t be so elaborate, but you’ll probably still start with some kind of sketches to guide you on your way. Software development has gone to extremes. For a long time, people didn’t have much structure in their development, but then big projects began failing. In reaction, we ended up with methodologies that had an intimidating amount of structure and detail, primarily intended for those big projects. These methodologies were too scary to use—it looked like you’d spend all your time writing documents and no time programming. (This was often the case.) I hope that what I’ve shown you here suggests a middle path—a sliding scale. Use an approach that fits your needs (and your personality). No matter how minimal you choose to make it, some kind of plan will make a big improvement in your project as opposed to no plan at all. Remember that, by most estimates, over 50 percent of projects fail (some estimates go up to 70 percent!).

By following a plan—preferably one that is simple and brief—and coming up with design structure before coding, you’ll discover that things fall together far more easily than if you dive in and start hacking. You’ll also realize a great deal of satisfaction. It’s my experience that coming up with an elegant solution is deeply satisfying at an entirely different level; it feels closer to art than technology. And elegance always pays off; it’s not a frivolous pursuit. Not only does it give you a program that’s easier to build and debug, but it’s also easier to understand and maintain, and that’s where the financial value lies.

Extreme programming

I have studied analysis and design techniques, on and off, since I was in graduate school. The concept of Extreme Programming (XP) is the most radical, and delightful, that I’ve seen. You can find it chronicled in Extreme Programming Explained by Kent Beck (Addison-Wesley, 2000) and on the Web at www.xprogramming.com.

XP is both a philosophy about programming work and a set of guidelines to do it. Some of these guidelines are reflected in other recent methodologies, but the two most important and distinct contributions, in my opinion, are “write tests first” and “pair programming.” Although he argues strongly for the whole process, Beck points out that if you adopt only these two practices you’ll greatly improve your productivity and reliability.

Write tests first

Testing has traditionally been relegated to the last part of a project, after you’ve “gotten everything working, but just to be sure.” It’s implicitly had a low priority, and people who specialize in it have not been given a lot of status and have often even been cordoned off in a basement, away from the “real programmers.” Test teams have responded in kind, going so far as to wear black clothing and cackling with glee whenever they break something (to be honest, I’ve had this feeling myself when breaking compilers).

XP completely revolutionizes the concept of testing by giving it equal (or even greater) priority than the code. In fact, you write the tests before you write the code that will be tested, and the tests stay with the code forever. The tests must be executed successfully every time you do an integration of the project (which is often, sometimes more than once a day).

Writing tests first has two extremely important effects.

First, it forces a clear definition of the interface of a class. I’ve often suggested that people “imagine the perfect class to solve a particular problem” as a tool when trying to design the system. The XP testing strategy goes further than that—it specifies exactly what the class must look like, to the consumer of that class, and exactly how the class must behave. In no uncertain terms. You can write all the prose, or create all the diagrams you want, describing how a class should behave and what it looks like, but nothing is as real as a set of tests. The former is a wish list, but the tests are a contract that is enforced by the compiler and the running program. It’s hard to imagine a more concrete description of a class than the tests.

While creating the tests, you are forced to completely think out the class and will often discover needed functionality that might be missed during the thought experiments of UML diagrams, CRC cards, use cases, etc.

The second important effect of writing the tests first comes from running the tests every time you do a build of your software. This activity gives you the other half of the testing that’s performed by the compiler. If you look at the evolution of programming languages from this perspective, you’ll see that the real improvements in the technology have actually revolved around testing. Assembly language checked only for syntax, but C imposed some semantic restrictions, and these prevented you from making certain types of mistakes. OOP languages impose even more semantic restrictions, which if you think about it are actually forms of testing. “Is this data type being used properly?” and “Is this function being called properly?” are the kinds of tests that are being performed by the compiler or run-time system. We’ve seen the results of having these tests built into the language: people have been able to write more complex systems, and get them to work, with much less time and effort. I’ve puzzled over why this is, but now I realize it’s the tests: you do something wrong, and the safety net of the built-in tests tells you there’s a problem and points you to where it is.

But the built-in testing afforded by the design of the language can only go so far. At some point, you must step in and add the rest of the tests that produce a full suite (in cooperation with the compiler and run-time system) that verifies all of your program. And, just like having a compiler watching over your shoulder, wouldn’t you want these tests helping you right from the beginning? That’s why you write them first, and run them automatically with every build of your system. Your tests become an extension of the safety net provided by the language.

One of the things that I’ve discovered about the use of more and more powerful programming languages is that I am emboldened to try more brazen experiments, because I know that the language will keep me from wasting my time chasing bugs. The XP test scheme does the same thing for your entire project. Because you know your tests will always catch any problems that you introduce (and you regularly add any new tests as you think of them), you can make big changes when you need to without worrying that you’ll throw the whole project into complete disarray. This is incredibly powerful.

Pair programming

Pair programming goes against the rugged individualism that we’ve been indoctrinated into from the beginning, through school (where we succeed or fail on our own, and working with our neighbors is considered “cheating”), and media, especially Hollywood movies in which the hero is usually fighting against mindless conformity [18] . Programmers, too, are considered paragons of individuality—“cowboy coders” as Larry Constantine likes to say. And yet XP, which is itself battling against conventional thinking, says that code should be written with two people per workstation. And that this should be done in an area with a group of workstations, without the barriers that the facilities-design people are so fond of. In fact, Beck says that the first task of converting to XP is to arrive with screwdrivers and Allen wrenches and take apart everything that gets in the way. [19] (This will require a manager who can deflect the ire of the facilities department.)

The value of pair programming is that one person is actually doing the coding while the other is thinking about it. The thinker keeps the big picture in mind—not only the picture of the problem at hand, but the guidelines of XP. If two people are working, it’s less likely that one of them will get away with saying, “I don’t want to write the tests first,” for example. And if the coder gets stuck, they can swap places. If both of them get stuck, their musings may be overheard by someone else in the work area who can contribute. Working in pairs keeps things flowing and on track. Probably more important, it makes programming a lot more social and fun.

I’ve begun using pair programming during the exercise periods in some of my seminars and it seems to significantly improve everyone’s experience.

Why Java succeeds

The reason Java has been so successful is that the goal was to solve many of the problems facing developers today. The goal of Java is improved productivity. This productivity comes in many ways, but the language is designed to aid you as much as possible, while hindering you as little as possible with arbitrary rules or any requirement that you use a particular set of features. Java is designed to be practical; Java language design decisions were based on providing the maximum benefits to the programmer.

Systems are easier
to express and understand

Classes designed to fit the problem tend to express it better. This means that when you write the code, you’re describing your solution in the terms of the problem space (“Put the grommet in the bin”) rather than the terms of the computer, which is the solution space (“Set the bit in the chip that means that the relay will close”). You deal with higher-level concepts and can do much more with a single line of code.

The other benefit of this ease of expression is maintenance, which (if reports can be believed) takes a huge portion of the cost over a program’s lifetime. If a program is easier to understand, then it’s easier to maintain. This can also reduce the cost of creating and maintaining the documentation.

Maximal leverage with libraries

The fastest way to create a program is to use code that’s already written: a library. A major goal in Java is to make library use easier. This is accomplished by casting libraries into new data types (classes), so that bringing in a library means adding new types to the language. Because the Java compiler takes care of how the library is used—guaranteeing proper initialization and cleanup, and ensuring that functions are called properly—you can focus on what you want the library to do, not how you have to do it.

Error handling

Error handling in C is a notorious problem, and one that is often ignored—finger-crossing is usually involved. If you’re building a large, complex program, there’s nothing worse than having an error buried somewhere with no clue as to where it came from. Java exception handling is a way to guarantee that an error is noticed, and that something happens as a result.

Programming in the large

Many traditional languages have built-in limitations to program size and complexity. BASIC, for example, can be great for pulling together quick solutions for certain classes of problems, but if the program gets more than a few pages long, or ventures out of the normal problem domain of that language, it’s like trying to swim through an ever-more viscous fluid. There’s no clear line that tells you when your language is failing you, and even if there were, you’d ignore it. You don’t say, “My BASIC program just got too big; I’ll have to rewrite it in C!” Instead, you try to shoehorn a few more lines in to add that one new feature. So the extra costs come creeping up on you.

Java is designed to aid programming in the large—that is, to erase those creeping-complexity boundaries between a small program and a large one. You certainly don’t need to use OOP when you’re writing a “hello world” style utility program, but the features are there when you need them. And the compiler is aggressive about ferreting out bug-producing errors for small and large programs alike.

Strategies for transition

If you buy into OOP, your next question is probably, “How can I get my manager/colleagues/department/peers to start using objects?” Think about how you—one independent programmer—would go about learning to use a new language and a new programming paradigm. You’ve done it before. First comes education and examples; then comes a trial project to give you a feel for the basics without doing anything too confusing. Then comes a “real world” project that actually does something useful. Throughout your first projects you continue your education by reading, asking questions of experts, and trading hints with friends. This is the approach many experienced programmers suggest for the switch to Java. Switching an entire company will of course introduce certain group dynamics, but it will help at each step to remember how one person would do it.

Guidelines

Here are some guidelines to consider when making the transition to OOP and Java:

1. Training

The first step is some form of education. Remember the company’s investment in code, and try not to throw everything into disarray for six to nine months while everyone puzzles over how interfaces work. Pick a small group for indoctrination, preferably one composed of people who are curious, work well together, and can function as their own support network while they’re learning Java.

An alternative approach that is sometimes suggested is the education of all company levels at once, including overview courses for strategic managers as well as design and programming courses for project builders. This is especially good for smaller companies making fundamental shifts in the way they do things, or at the division level of larger companies. Because the cost is higher, however, some may choose to start with project-level training, do a pilot project (possibly with an outside mentor), and let the project team become the teachers for the rest of the company.

2. Low-risk project

Try a low-risk project first and allow for mistakes. Once you’ve gained some experience, you can either seed other projects from members of this first team or use the team members as an OOP technical support staff. This first project may not work right the first time, so it should not be mission-critical for the company. It should be simple, self-contained, and instructive; this means that it should involve creating classes that will be meaningful to the other programmers in the company when they get their turn to learn Java.

3. Model from success

Seek out examples of good object-oriented design before starting from scratch. There’s a good probability that someone has solved your problem already, and if they haven’t solved it exactly you can probably apply what you’ve learned about abstraction to modify an existing design to fit your needs. This is the general concept of design patterns, covered in Thinking in Patterns with Java, downloadable at www.BruceEckel.com.

4. Use existing class libraries

The primary economic motivation for switching to OOP is the easy use of existing code in the form of class libraries (in particular, the Standard Java libraries, which are covered throughout this book). The shortest application development cycle will result when you can create and use objects from off-the-shelf libraries. However, some new programmers don’t understand this, are unaware of existing class libraries, or, through fascination with the language, desire to write classes that may already exist. Your success with OOP and Java will be optimized if you make an effort to seek out and reuse other people’s code early in the transition process.

5. Don’t rewrite existing code in Java

It is not usually the best use of your time to take existing, functional code and rewrite it in Java. (If you must turn it into objects, you can interface to the C or C++ code using the Java Native Interface, described in Appendix B.) There are incremental benefits, especially if the code is slated for reuse. But chances are you aren’t going to see the dramatic increases in productivity that you hope for in your first few projects unless that project is a new one. Java and OOP shine best when taking a project from concept to reality.

Management obstacles

If you’re a manager, your job is to acquire resources for your team, to overcome barriers to your team’s success, and in general to try to provide the most productive and enjoyable environment so your team is most likely to perform those miracles that are always being asked of you. Moving to Java falls in all three of these categories, and it would be wonderful if it didn’t cost you anything as well. Although moving to Java may be cheaper—depending on your constraints—than the OOP alternatives for a team of C programmers (and probably for programmers in other procedural languages), it isn’t free, and there are obstacles you should be aware of before trying to sell the move to Java within your company and embarking on the move itself.

Startup costs

The cost of moving to Java is more than just the acquisition of Java compilers (the Sun Java compiler is free, so this is hardly an obstacle). Your medium- and long-term costs will be minimized if you invest in training (and possibly mentoring for your first project) and also if you identify and purchase class libraries that solve your problem rather than trying to build those libraries yourself. These are hard-money costs that must be factored into a realistic proposal. In addition, there are the hidden costs in loss of productivity while learning a new language and possibly a new programming environment. Training and mentoring can certainly minimize these, but team members must overcome their own struggles to understand the new technology. During this process they will make more mistakes (this is a feature, because acknowledged mistakes are the fastest path to learning) and be less productive. Even then, with some types of programming problems, the right classes, and the right development environment, it’s possible to be more productive while you’re learning Java (even considering that you’re making more mistakes and writing fewer lines of code per day) than if you’d stayed with C.

Performance issues

A common question is, “Doesn’t OOP automatically make my programs a lot bigger and slower?” The answer is, “It depends.” The extra safety features in Java have traditionally extracted a performance penalty over a language like C++. Technologies such as “hotspot” and compilation technologies have improved the speed significantly in most cases, and efforts continue toward higher performance.

When your focus is on rapid prototyping, you can throw together components as fast as possible while ignoring efficiency issues. If you’re using any third-party libraries, these are usually already optimized by their vendors; in any case it’s not an issue while you’re in rapid-development mode. When you have a system that you like, if it’s small and fast enough, then you’re done. If not, you begin tuning with a profiling tool, looking first for speedups that can be done by rewriting small portions of code. If that doesn’t help, you look for modifications that can be made in the underlying implementation so no code that uses a particular class needs to be changed. Only if nothing else solves the problem do you need to change the design. The fact that performance is so critical in that portion of the design is an indicator that it must be part of the primary design criteria. You have the benefit of finding this out early using rapid development.

If you find a function that is a particular bottleneck, you can rewrite it in C/C++ using Java’s native methods, the subject of Appendix B.

Common design errors

When starting your team into OOP and Java, programmers will typically go through a series of common design errors. This often happens due to insufficient feedback from experts during the design and implementation of early projects, because no experts have been developed within the company, and because there may be resistance to retaining consultants. It’s easy to feel that you understand OOP too early in the cycle and go off on a bad tangent. Something that’s obvious to someone experienced with the language may be a subject of great internal debate for a novice. Much of this trauma can be skipped by using an experienced outside expert for training and mentoring.

Java vs. C++?

Java looks a lot like C++, and so naturally it would seem that C++ will be replaced by Java. But I’m starting to question this logic. For one thing, C++ still has some features that Java doesn’t, and although there have been a lot of promises about Java someday being as fast or faster than C++, we’ve seen steady improvements but no dramatic breakthroughs. Also, there seems to be a continuing interest in C++, so I don’t think that language is going away any time soon. (Languages seem to hang around. Speaking at one of my “Intermediate/Advanced Java Seminars,” Allen Holub asserted that the two most commonly used languages are Rexx and COBOL, in that order.)

I’m beginning to think that the strength of Java lies in a slightly different arena than that of C++. C++ is a language that doesn’t try to fit a mold. Certainly it has been adapted in a number of ways to solve particular problems. Some C++ tools combine libraries, component models, and code-generation tools to solve the problem of developing windowed end-user applications (for Microsoft Windows). And yet, what do the vast majority of Windows developers use? Microsoft’s Visual Basic (VB). This despite the fact that VB produces the kind of code that becomes unmanageable when the program is only a few pages long (and syntax that can be positively mystifying). As successful and popular as VB is, it’s not a very good example of language design. It would be nice to have the ease and power of VB without the resulting unmanageable code. And that’s where I think Java will shine: as the “next VB.” You may or may not shudder to hear this, but think about it: so much of Java is intended to make it easy for the programmer to solve application-level problems like networking and cross-platform UI, and yet it has a language design that allows the creation of very large and flexible bodies of code. Add to this the fact that Java has the most robust type checking and error handling systems I’ve ever seen in a language and you have the makings of a significant leap forward in programming productivity.

Should you use Java instead of C++ for your project? Other than Web applets, there are two issues to consider. First, if you want to use a lot of existing C++ libraries (and you’ll certainly get a lot of productivity gains there), or if you have an existing C or C++ code base, Java might slow your development down rather than speeding it up.

If you’re developing all your code primarily from scratch, then the simplicity of Java over C++ will significantly shorten your development time—the anecdotal evidence (stories from C++ teams that I’ve talked to who have switched to Java) suggests a doubling of development speed over C++. If Java performance doesn’t matter or you can somehow compensate for it, sheer time-to-market issues make it difficult to choose C++ over Java.

The biggest issue is performance. Interpreted Java has been slow, even 20 to 50 times slower than C in the original Java interpreters. This has improved greatly over time, but it will still remain an important number. Computers are about speed; if it wasn’t significantly faster to do something on a computer then you’d do it by hand. (I’ve even heard it suggested that you start with Java, to gain the short development time, then use a tool and support libraries to translate your code to C++, if you need faster execution speed.)

The key to making Java feasible for most development projects is the appearance of speed improvements like so-called “just-in time” (JIT) compilers, Sun’s own “hotspot” technology, and even native code compilers. Of course, native code compilers will eliminate the touted cross-platform execution of the compiled programs, but they will also bring the speed of the executable closer to that of C and C++. And cross-compiling a program in Java should be a lot easier than doing so in C or C++. (In theory, you just recompile, but that promise has been made before for other languages.)

You can find comparisons of Java and C++ and observations about Java realities in the appendices of the first edition of this book (Available on this book’s accompanying CD ROM, as well as at www.BruceEckel.com).

Summary

This chapter attempts to give you a feel for the broad issues of object-oriented programming and Java, including why OOP is different, and why Java in particular is different, concepts of OOP methodologies, and finally the kinds of issues you will encounter when moving your own company to OOP and Java.

OOP and Java may not be for everyone. It’s important to evaluate your own needs and decide whether Java will optimally satisfy those needs, or if you might be better off with another programming system (including the one you’re currently using). If you know that your needs will be very specialized for the foreseeable future and if you have specific constraints that may not be satisfied by Java, then you owe it to yourself to investigate the alternatives [20] . Even if you eventually choose Java as your language, you’ll at least understand what the options were and have a clear vision of why you took that direction.

You know what a procedural program looks like: data definitions and function calls. To find the meaning of such a program you have to work a little, looking through the function calls and low-level concepts to create a model in your mind. This is the reason we need intermediate representations when designing procedural programs—by themselves, these programs tend to be confusing because the terms of expression are oriented more toward the computer than to the problem you’re solving.

Because Java adds many new concepts on top of what you find in a procedural language, your natural assumption may be that the main( ) in a Java program will be far more complicated than for the equivalent C program. Here, you’ll be pleasantly surprised: A well-written Java program is generally far simpler and much easier to understand than the equivalent C program. What you’ll see are the definitions of the objects that represent concepts in your problem space (rather than the issues of the computer representation) and messages sent to those objects to represent the activities in that space. One of the delights of object-oriented programming is that, with a well-designed program, it’s easy to understand the code by reading it. Usually there’s a lot less code as well, because many of your problems will be solved by reusing existing library code.

[2] See Multiparadigm Programming in Leda by Timothy Budd (Addison-Wesley 1995).

[3] Some people make a distinction, stating that type determines the interface while class is a particular implementation of that interface.

[4] I’m indebted to my friend Scott Meyers for this term.

[5] This is usually enough detail for most diagrams, and you don’t need to get specific about whether you’re using aggregation or composition.

[6] My term.

[7] Primitive types, which you’ll learn about later, are a special case.

[8] An excellent example of this is UML Distilled, by Martin Fowler (Addison-Wesley 2000), which reduces the sometimes-overwhelming UML process to a manageable subset.

[9] My rule of thumb for estimating such projects: If there’s more than one wild card, don’t even try to plan how long it’s going to take or how much it will cost until you’ve created a working prototype. There are too many degrees of freedom.

[10] Thanks for help from James H Jarrett.

[11] More information on use cases can be found in Applying Use Cases by Schneider & Winters (Addison-Wesley 1998) and Use Case Driven Object Modeling with UML by Rosenberg (Addison-Wesley 1999).

[12] My personal take on this has changed lately. Doubling and adding 10 percent will give you a reasonably accurate estimate (assuming there are not too many wild-card factors), but you still have to work quite diligently to finish in that time. If you want time to really make it elegant and to enjoy yourself in the process, the correct multiplier is more like three or four times, I believe.

[13] For starters, I recommend the aforementioned UML Distilled.

[14] Python (www.Python.org) is often used as “executable pseudocode.”

[15] At least one aspect of evolution is covered in Martin Fowler’s book Refactoring: improving the design of existing code (Addison-Wesley 1999), which uses Java examples exclusively.

[16] This term is explored in Chapter 16: “Design Patterns”.

[17] This is something like “rapid prototyping,” where you were supposed to build a quick-and-dirty version so that you could learn about the system, and then throw away your prototype and build it right. The trouble with rapid prototyping is that people didn’t throw away the prototype, but instead built upon it. Combined with the lack of structure in procedural programming, this often leads to messy systems that are expensive to maintain.

[18] Although this may be a more American perspective, the stories of Hollywood reach everywhere.

[19] Including (especially) the PA system. I once worked in a company that insisted on broadcasting every phone call that arrived for every executive, and it constantly interrupted our productivity (but the managers couldn’t begin to conceive of stifling such an important service as the PA). Finally, when no one was looking I started snipping speaker wires.

[20] In particular, I recommend looking at Python (http://www.Python.org).


2: Everything is an Object

2: Everything
is an Object

Although it is based on C++, Java is more of a “pure” object-oriented language.

Both C++ and Java are hybrid languages, but in Java the designers felt that the hybridization was not as important as it was in C++. A hybrid language allows multiple programming styles; the reason C++ is hybrid is to support backward compatibility with the C language. Because C++ is a superset of the C language, it includes many of that language’s undesirable features, which can make some aspects of C++ overly complicated.

The Java language assumes that you want to do only object-oriented programming. This means that before you can begin you must shift your mindset into an object-oriented world (unless it’s already there). The benefit of this initial effort is the ability to program in a language that is simpler to learn and to use than many other OOP languages. In this chapter we’ll see the basic components of a Java program and we’ll learn that everything in Java is an object, even a Java program.

You manipulate objects
with references

Each programming language has its own means of manipulating data. Sometimes the programmer must be constantly aware of what type of manipulation is going on. Are you manipulating the object directly, or are you dealing with some kind of indirect representation (a pointer in C or C++) that must be treated with a special syntax?

All this is simplified in Java. You treat everything as an object, so there is a single consistent syntax that you use everywhere. Although you treat everything as an object, the identifier you manipulate is actually a “reference” to an object [21] . You might imagine this scene as a television (the object) with your remote control (the reference). As long as you’re holding this reference, you have a connection to the television, but when someone says “change the channel” or “lower the volume,” what you’re manipulating is the reference, which in turn modifies the object. If you want to move around the room and still control the television, you take the remote/reference with you, not the television.

Also, the remote control can stand on its own, with no television. That is, just because you have a reference doesn’t mean there’s necessarily an object connected to it. So if you want to hold a word or sentence, you create a String reference:

String s;

But here you’ve created only the reference, not an object. If you decided to send a message to s at this point, you’ll get an error (at run-time) because s isn’t actually attached to anything (there’s no television). A safer practice, then, is always to initialize a reference when you create it:

String s = "asdf";

However, this uses a special Java feature: strings can be initialized with quoted text. Normally, you must use a more general type of initialization for objects.

You must create
all the objects

When you create a reference, you want to connect it with a new object. You do so, in general, with the new keyword. new says, “Make me a new one of these objects.” So in the above example, you can say:

String s = new String("asdf");

Not only does this mean “Make me a new String,” but it also gives information about how to make the String by supplying an initial character string.

Of course, String is not the only type that exists. Java comes with a plethora of ready-made types. What’s more important is that you can create your own types. In fact, that’s the fundamental activity in Java programming, and it’s what you’ll be learning about in the rest of this book.

Where storage lives

It’s useful to visualize some aspects of how things are laid out while the program is running, in particular how memory is arranged. There are six different places to store data:

  1. Registers. This is the fastest storage because it exists in a place different from that of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated by the compiler according to its needs. You don’t have direct control, nor do you see any evidence in your programs that registers even exist.
  2. The stack. This lives in the general RAM (random-access memory) area, but has direct support from the processor via its stack pointer. The stack pointer is moved down to create new memory and moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java compiler must know, while it is creating the program, the exact size and lifetime of all the data that is stored on the stack, because it must generate the code to move the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack—in particular, object references—Java objects themselves are not placed on the stack.
  3. The heap. This is a general-purpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesn’t need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus, there’s a great deal of flexibility in using storage on the heap. Whenever you need to create an object, you simply write the code to create it using new, and the storage is allocated on the heap when that code is executed. Of course there’s a price you pay for this flexibility: it takes more time to allocate heap storage than it does to allocate stack storage (that is, if you even could create objects on the stack in Java, as you can in C++).
  4. Static storage. “Static” is used here in the sense of “in a fixed location” (although it’s also in RAM). Static storage contains data that is available for the entire time a program is running. You can use the static keyword to specify that a particular element of an object is static, but Java objects themselves are never placed in static storage.
  5. Constant storage. Constant values are often placed directly in the program code, which is safe since they can never change. Sometimes constants are cordoned off by themselves so that they can be optionally placed in read-only memory (ROM).
  6. Non-RAM storage. If data lives completely outside a program it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects, in which objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects, in which the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAM-based object when necessary. Java provides support for lightweight persistence, and future versions of Java might provide more complete solutions for persistence.

Special case: primitive types

There is a group of types that gets special treatment; you can think of these as “primitive” types that you use quite often in your programming. The reason for the special treatment is that to create an object with new—especially a small, simple variable—isn’t very efficient because new places objects on the heap. For these types Java falls back on the approach taken by C and C++. That is, instead of creating the variable using new, an “automatic” variable is created that is not a reference. The variable holds the value, and it’s placed on the stack so it’s much more efficient.

Java determines the size of each primitive type. These sizes don’t change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are so portable.

Primitive type | Size | Minimum | Maximum | Wrapper type
boolean | 1-bit | – | – | Boolean
char | 16-bit | Unicode 0 | Unicode 216- 1 | Character
byte | 8-bit | -128 | +127 | Byte
short | 16-bit | -215 | +215—1 | Short
int | 32-bit | -231 | +231—1 | Integer
long | 64-bit | -263 | +263—1 | Long
float | 32-bit | IEEE754 | IEEE754 | Float
double | 64-bit | IEEE754 | IEEE754 | Double
void | – | – | – | Void

All numeric types are signed, so don’t go looking for unsigned types.

The primitive data types also have “wrapper” classes for them. That means that if you want to make a nonprimitive object on the heap to represent that primitive type, you use the associated wrapper. For example:

char c = 'x';
Character C = new Character(c);

Or you could also use:

Character C = new Character('x');

The reasons for doing this will be shown in a later chapter.

High-precision numbers

Java includes two classes for performing high-precision arithmetic: BigInteger and BigDecimal. Although these approximately fit into the same category as the “wrapper” classes, neither one has a primitive analogue.

Both classes have methods that provide analogues for the operations that you perform on primitive types. That is, you can do anything with a BigInteger or BigDecimal that you can with an int or float, it’s just that you must use method calls instead of operators. Also, since there’s more involved, the operations will be slower. You’re exchanging speed for accuracy.

BigInteger supports arbitrary-precision integers. This means that you can accurately represent integral values of any size without losing any information during operations.

BigDecimal is for arbitrary-precision fixed-point numbers; you can use these for accurate monetary calculations, for example.

Consult your online documentation for details about the constructors and methods you can call for these two classes.

Arrays in Java

Virtually all programming languages support arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory. If a program accesses the array outside of its memory block or uses the memory before initialization (common programming errors) there will be unpredictable results.

One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot be accessed outside of its range. The range checking comes at the price of having a small amount of memory overhead on each array as well as verifying the index at run-time, but the assumption is that the safety and increased productivity is worth the expense.

When you create an array of objects, you are really creating an array of references, and each of those references is automatically initialized to a special value with its own keyword: null. When Java sees null, it recognizes that the reference in question isn’t pointing to an object. You must assign an object to each reference before you use it, and if you try to use a reference that’s still null, the problem will be reported at run-time. Thus, typical array errors are prevented in Java.

You can also create an array of primitives. Again, the compiler guarantees initialization because it zeroes the memory for that array.

Arrays will be covered in detail in later chapters.

You never need to
destroy an object

In most programming languages, the concept of the lifetime of a variable occupies a significant portion of the programming effort. How long does the variable last? If you are supposed to destroy it, when should you? Confusion over variable lifetimes can lead to a lot of bugs, and this section shows how Java greatly simplifies the issue by doing all the cleanup work for you.

Scoping

Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++, and Java, scope is determined by the placement of curly braces {}. So for example:

{
  int x = 12;
  /* only x available */
  {
    int q = 96;
    /* both x & q available */
  }
  /* only x available */
  /* q “out of scope” */
}

A variable defined within a scope is available only to the end of that scope.

Indentation makes Java code easier to read. Since Java is a free-form language, the extra spaces, tabs, and carriage returns do not affect the resulting program.

Note that you cannot do the following, even though it is legal in C and C++:

{
  int x = 12;
  {
    int x = 96; /* illegal */
  }
}

The compiler will announce that the variable x has already been defined. Thus the C and C++ ability to “hide” a variable in a larger scope is not allowed because the Java designers thought that it led to confusing programs.

Scope of objects

Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you use:

{
  String s = new String("a string");
} /* end of scope */

the reference s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object because the only reference to it is out of scope. In later chapters you’ll see how the reference to the object can be passed around and duplicated during the course of a program.

It turns out that because objects created with new stay around for as long as you want them, a whole slew of C++ programming problems simply vanish in Java. The hardest problems seem to occur in C++ because you don’t get any help from the language in making sure that the objects are available when they’re needed. And more important, in C++ you must make sure that you destroy the objects when you’re done with them.

That brings up an interesting question. If Java leaves the objects lying around, what keeps them from filling up memory and halting your program? This is exactly the kind of problem that would occur in C++. This is where a bit of magic happens. Java has a garbage collector, which looks at all the objects that were created with new and figures out which ones are not being referenced anymore. Then it releases the memory for those objects, so the memory can be used for new objects. This means that you never need to worry about reclaiming memory yourself. You simply create objects, and when you no longer need them they will go away by themselves. This eliminates a certain class of programming problem: the so-called “memory leak,” in which a programmer forgets to release memory.

Creating new
data types: class

If everything is an object, what determines how a particular class of object looks and behaves? Put another way, what establishes the type of an object? You might expect there to be a keyword called “type,” and that certainly would have made sense. Historically, however, most object-oriented languages have used the keyword class to mean “I’m about to tell you what a new type of object looks like.” The class keyword (which is so common that it will not be emboldened throughout this book) is followed by the name of the new type. For example:

class ATypeName { /* class body goes here */ }

This introduces a new type, so you can now create an object of this type using new:

ATypeName a = new ATypeName();

In ATypeName, the class body consists only of a comment (the stars and slashes and what is inside, which will be discussed later in this chapter), so there is not too much that you can do with it. In fact, you cannot tell it to do much of anything (that is, you cannot send it any interesting messages) until you define some methods for it.

Fields and methods

When you define a class (and all you do in Java is define classes, make objects of those classes, and send messages to those objects), you can put two types of elements in your class: data members (sometimes called fields), and member functions (typically called methods). A data member is an object of any type that you can communicate with via its reference. It can also be one of the primitive types (which isn’t a reference). If it is a reference to an object, you must initialize that reference to connect it to an actual object (using new, as seen earlier) in a special function called a constructor (described fully in Chapter 4). If it is a primitive type you can initialize it directly at the point of definition in the class. (As you’ll see later, references can also be initialized at the point of definition.)

Each object keeps its own storage for its data members; the data members are not shared among objects. Here is an example of a class with some data members:
class DataOnly {
  int i;
  float f;
  boolean b;
}

This class doesn’t do anything, but you can create an object:

DataOnly d = new DataOnly();

You can assign values to the data members, but you must first know how to refer to a member of an object. This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object:

objectReference.member

For example:

d.i = 47;
d.f = 1.1f;
d.b = false;

It is also possible that your object might contain other objects that contain data you’d like to modify. For this, you just keep “connecting the dots.” For example:

myPlane.leftTank.capacity = 100;

The DataOnly class cannot do much of anything except hold data, because it has no member functions (methods). To understand how those work, you must first understand arguments and return values, which will be described shortly.

Default values for primitive members

When a primitive data type is a member of a class, it is guaranteed to get a default value if you do not initialize it:

Primitive type | Default
boolean | false
char | ‘\u0000’ (null)
byte | (byte)0
short | (short)0
int | 0
long | 0L
float | 0.0f
double | 0.0d

Note carefully that the default values are what Java guarantees when the variable is used as a member of a class. This ensures that member variables of primitive types will always be initialized (something C++ doesn’t do), reducing a source of bugs. However, this initial value may not be correct or even legal for the program you are writing. It’s best to always explicitly initialize your variables.

This guarantee doesn’t apply to “local” variables—those that are not fields of a class. Thus, if within a function definition you have:

int x;

Then x will get some arbitrary value (as in C and C++); it will not automatically be initialized to zero. You are responsible for assigning an appropriate value before you use x. If you forget, Java definitely improves on C++: you get a compile-time error telling you the variable might not have been initialized. (Many C++ compilers will warn you about uninitialized variables, but in Java these are errors.)

Methods, arguments,
and return values

Up until now, the term function has been used to describe a named subroutine. The term that is more commonly used in Java is method, as in “a way to do something.” If you want, you can continue thinking in terms of functions. It’s really only a syntactic difference, but from now on “method” will be used in this book rather than “function.”

Methods in Java determine the messages an object can receive. In this section you will learn how simple it is to define a method.

The fundamental parts of a method are the name, the arguments, the return type, and the body. Here is the basic form:

returnType methodName( /* argument list */ ) {
  /* Method body */
}

The return type is the type of the value that pops out of the method after you call it. The argument list gives the types and names for the information you want to pass into the method. The method name and argument list together uniquely identify the method.

Methods in Java can be created only as part of a class. A method can be called only for an object, [22] and that object must be able to perform that method call. If you try to call the wrong method for an object, you’ll get an error message at compile-time. You call a method for an object by naming the object followed by a period (dot), followed by the name of the method and its argument list, like this: objectName.methodName(arg1, arg2, arg3). For example, suppose you have a method f( ) that takes no arguments and returns a value of type int. Then, if you have an object called a for which f( ) can be called, you can say this:

int x = a.f();

The type of the return value must be compatible with the type of x.

This act of calling a method is commonly referred to as sending a message to an object. In the above example, the message is f( ) and the object is a. Object-oriented programming is often summarized as simply “sending messages to objects.”

The argument list

The method argument list specifies what information you pass into the method. As you might guess, this information—like everything else in Java—takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing references [23] . The type of the reference must be correct, however. If the argument is supposed to be a String, what you pass in must be a string.

Consider a method that takes a String as its argument. Here is the definition, which must be placed within a class definition for it to be compiled:

int storage(String s) {
  return s.length() * 2;
}

This method tells you how many bytes are required to hold the information in a particular String. (Each char in a String is 16 bits, or two bytes, long, to support Unicode characters.) The argument is of type String and is called s. Once s is passed into the method, you can treat it just like any other object. (You can send messages to it.) Here, the length( ) method is called, which is one of the methods for Strings; it returns the number of characters in a string.

You can also see the use of the return keyword, which does two things. First, it means “leave the method, I’m done.” Second, if the method produces a value, that value is placed right after the return statement. In this case, the return value is produced by evaluating the expression s.length( ) * 2.

You can return any type you want, but if you don’t want to return anything at all, you do so by indicating that the method returns void. Here are some examples:

boolean flag() { return true; }
float naturalLogBase() { return 2.718f; }
void nothing() { return; }
void nothing2() {}

When the return type is void, then the return keyword is used only to exit the method, and is therefore unnecessary when you reach the end of the method. You can return from a method at any point, but if you’ve given a non-void return type then the compiler will force you (with error messages) to return the appropriate type of value regardless of where you return.

At this point, it can look like a program is just a bunch of objects with methods that take other objects as arguments and send messages to those other objects. That is indeed much of what goes on, but in the following chapter you’ll learn how to do the detailed low-level work by making decisions within a method. For this chapter, sending messages will suffice.

Building a Java program

There are several other issues you must understand before seeing your first Java program.

Name visibility

A problem in any programming language is the control of names. If you use a name in one module of the program, and another programmer uses the same name in another module, how do you distinguish one name from another and prevent the two names from “clashing?” In C this is a particular problem because a program is often an unmanageable sea of names. C++ classes (on which Java classes are based) nest functions within classes so they cannot clash with function names nested within other classes. However, C++ still allowed global data and global functions, so clashing was still possible. To solve this problem, C++ introduced namespaces using additional keywords.

Java was able to avoid all of this by taking a fresh approach. To produce an unambiguous name for a library, the specifier used is not unlike an Internet domain name. In fact, the Java creators want you to use your Internet domain name in reverse since those are guaranteed to be unique. Since my domain name is BruceEckel.com, my utility library of foibles would be named com.bruceeckel.utility.foibles. After your reversed domain name, the dots are intended to represent subdirectories.

In Java 1.0 and Java 1.1 the domain extensions com, edu, org, net, etc., were capitalized by convention, so the library would appear: COM.bruceeckel.utility.foibles. Partway through the development of Java 2, however, it was discovered that this caused problems, and so now the entire package name is lowercase.

This mechanism means that all of your files automatically live in their own namespaces, and each class within a file must have a unique identifier. So you do not need to learn special language features to solve this problem—the language takes care of it for you.

Using other components

Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source code file that it’s being called from. In that case, you simply use the class—even if the class doesn’t get defined until later in the file. Java eliminates the “forward referencing” problem so you don’t need to think about it.

What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. Imagine that you want to use a class of a particular name, but more than one definition for that class exists (presumably these are different definitions). Or worse, imagine that you’re writing a program, and as you’re building it you add a new class to your library that conflicts with the name of an existing class.

To solve this problem, you must eliminate all potential ambiguities. This is accomplished by telling the Java compiler exactly what classes you want using the import keyword. import tells the compiler to bring in a package, which is a library of classes. (In other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class.)

Most of the time you’ll be using components from the standard Java libraries that come with your compiler. With these, you don’t need to worry about long, reversed domain names; you just say, for example:

import java.util.ArrayList;

to tell the compiler that you want to use Java’s ArrayList class. However, util contains a number of classes and you might want to use several of them without declaring them all explicitly. This is easily accomplished by using ‘*’ to indicate a wild card:

import java.util.*;

It is more common to import a collection of classes in this manner than to import classes individually.

The static keyword

Ordinarily, when you create a class you are describing how objects of that class look and how they will behave. You don’t actually get anything until you create an object of that class with new, and at that point data storage is created and methods become available.

But there are two situations in which this approach is not sufficient. One is if you want to have only one piece of storage for a particular piece of data, regardless of how many objects are created, or even if no objects are created. The other is if you need a method that isn’t associated with any particular object of this class. That is, you need a method that you can call even if no objects are created. You can achieve both of these effects with the static keyword. When you say something is static, it means that data or method is not tied to any particular object instance of that class. So even if you’ve never created an object of that class you can call a static method or access a piece of static data. With ordinary, non-static data and methods you must create an object and use that object to access the data or method, since non-static data and methods must know the particular object they are working with. Of course, since static methods don’t need any objects to be created before they are used, they cannot directly access non-static members or methods by simply calling those other members without referring to a named object (since non-static members and methods must be tied to a particular object).

Some object-oriented languages use the terms class data and class methods, meaning that the data and methods exist only for the class as a whole, and not for any particular objects of the class. Sometimes the Java literature uses these terms too.

To make a data member or method static, you simply place the keyword before the definition. For example, the following produces a static data member and initializes it:

class StaticTest {
    static int i = 47;
}

Now even if you make two StaticTest objects, there will still be only one piece of storage for StaticTest.i. Both objects will share the same i. Consider:

StaticTest st1 = new StaticTest();
StaticTest st2 = new StaticTest();

At this point, both st1.i and st2.i have the same value of 47 since they refer to the same piece of memory.

There are two ways to refer to a static variable. As indicated above, you can name it via an object, by saying, for example, st2.i. You can also refer to it directly through its class name, something you cannot do with a non-static member. (This is the preferred way to refer to a static variable since it emphasizes that variable’s static nature.)

StaticTest.i++;

The ++ operator increments the variable. At this point, both st1.i and st2.i will have the value 48.

Similar logic applies to static methods. You can refer to a static method either through an object as you can with any method, or with the special additional syntax ClassName.method( ). You define a static method in a similar way:

class StaticFun {
  static void incr() { StaticTest.i++; }
}

You can see that the StaticFun method incr( ) increments the static data i. You can call incr( ) in the typical way, through an object:

StaticFun sf = new StaticFun();
sf.incr();

Or, because incr( ) is a static method, you can call it directly through its class:

StaticFun.incr();

While static, when applied to a data member, definitely changes the way the data is created (one for each class vs. the non-static one for each object), when applied to a method it’s not so dramatic. An important use of static for methods is to allow you to call that method without creating an object. This is essential, as we will see, in defining the main( ) method that is the entry point for running an application.

Like any method, a static method can create or use named objects of its type, so a static method is often used as a “shepherd” for a flock of instances of its own type.

Your first Java program

Finally, here’s the program. [24] It starts by printing a string, and then the date, using the Date class from the Java standard library. Note that an additional style of comment is introduced here: the ‘//’, which is a comment until the end of the line:

// HelloDate.java
import java.util.*;

public class HelloDate {
  public static void main(String[] args) {
    System.out.println("Hello, it's: ");
    System.out.println(new Date());
  }
}

At the beginning of each program file, you must place the import statement to bring in any extra classes you’ll need for the code in that file. Note that I say “extra;” that’s because there’s a certain library of classes that are automatically brought into every Java file: java.lang. Start up your Web browser and look at the documentation from Sun. (If you haven’t downloaded it from java.sun.com or otherwise installed the Java documentation, do so now). If you look at the list of the packages, you’ll see all the different class libraries that come with Java. Select java.lang. This will bring up a list of all the classes that are part of that library. Since java.lang is implicitly included in every Java code file, these classes are automatically available. There’s no Date class listed in java.lang, which means you must import another library to use that. If you don’t know the library where a particular class is, or if you want to see all of the classes, you can select “Tree” in the Java documentation. Now you can find every single class that comes with Java. Then you can use the browser’s “find” function to find Date. When you do you’ll see it listed as java.util.Date, which lets you know that it’s in the util library and that you must import java.util.* in order to use Date.

If you go back to the beginning, select java.lang and then System, you’ll see that the System class has several fields, and if you select out you’ll discover that it’s a static PrintStream object. Since it’s static you don’t need to create anything. The out object is always there and you can just use it. What you can do with this out object is determined by the type it is: a PrintStream. Conveniently, PrintStream is shown in the description as a hyperlink, so if you click on that you’ll see a list of all the methods you can call for PrintStream. There are quite a few and these will be covered later in this book. For now all we’re interested in is println( ), which in effect means “print what I’m giving you out to the console and end with a new line.” Thus, in any Java program you write you can say System.out.println(“things”) whenever you want to print something to the console.

The name of the class is the same as the name of the file. When you’re creating a stand-alone program such as this one, one of the classes in the file must have the same name as the file. (The compiler complains if you don’t do this.) That class must contain a method called main( ) with the signature shown:

public static void main(String[] args) {

The public keyword means that the method is available to the outside world (described in detail in Chapter 5). The argument to main( ) is an array of String objects. The args won’t be used in this program, but the Java compiler insists that they be there because they hold the arguments invoked on the command line.

The line that prints the date is quite interesting:

System.out.println(new Date());

Consider the argument: a Date object is being created just to send its value to println( ). As soon as this statement is finished, that Date is unnecessary, and the garbage collector can come along and get it anytime. We don’t need to worry about cleaning it up.

Compiling and running

To compile and run this program, and all the other programs in this book, you must first have a Java programming environment. There are a number of third-party development environments, but in this book we will assume that you are using the JDK from Sun, which is free. If you are using another development system, you will need to look in the documentation for that system to determine how to compile and run programs.

Get on the Internet and go to http://java.sun.com. There you will find information and links that will lead you through the process of downloading and installing the JDK for your particular platform.

Once the JDK is installed, and you’ve set up your computer’s path information so that it will find javac and java, download and unpack the source code for this book (you can find it on the CD ROM that’s bound in with this book, or at www.BruceEckel.com). This will create a subdirectory for each chapter in this book. Move to subdirectory c02 and type:

javac HelloDate.java

This command should produce no response. If you get any kind of an error message it means you haven’t installed the JDK properly and you need to investigate those problems.

On the other hand, if you just get your command prompt back, you can type:

java HelloDate

and you’ll get the message and the date as output.

This is the process you can use to compile and run each of the programs in this book. However, you will see that the source code for this book also has a file called makefile in each chapter, and this contains “make” commands for automatically building the files for that chapter. See this book’s Web page at www.BruceEckel.com for details on how to use the makefiles.

Comments and embedded documentation

There are two types of comments in Java. The first is the traditional C-style comment that was inherited by C++. These comments begin with a /* and continue, possibly across many lines, until a */. Note that many programmers will begin each line of a continued comment with a *, so you’ll often see:

/* This is a comment
*  that continues
*  across lines
*/

Remember, however, that everything inside the /* and */ is ignored, so there’s no difference in saying:

/* This is a comment that
continues across lines */

The second form of comment comes from C++. It is the single-line comment, which starts at a // and continues until the end of the line. This type of comment is convenient and commonly used because it’s easy. You don’t need to hunt on the keyboard to find / and then * (instead, you just press the same key twice), and you don’t need to close the comment. So you will often see:

// this is a one-line comment

Comment documentation

One of the thoughtful parts of the Java language is that the designers didn’t consider writing code to be the only important activity—they also thought about documenting it. Possibly the biggest problem with documenting code has been maintaining that documentation. If the documentation and the code are separate, it becomes a hassle to change the documentation every time you change the code. The solution seems simple: link the code to the documentation. The easiest way to do this is to put everything in the same file. To complete the picture, however, you need a special comment syntax to mark special documentation, and a tool to extract those comments and put them in a useful form. This is what Java has done.

The tool to extract the comments is called javadoc. It uses some of the technology from the Java compiler to look for special comment tags you put in your programs. It not only extracts the information marked by these tags, but it also pulls out the class name or method name that adjoins the comment. This way you can get away with the minimal amount of work to generate decent program documentation.

The output of javadoc is an HTML file that you can view with your Web browser. This tool allows you to create and maintain a single source file and automatically generate useful documentation. Because of javadoc we have a standard for creating documentation, and it’s easy enough that we can expect or even demand documentation with all Java libraries.

Syntax

All of the javadoc commands occur only within /** comments. The comments end with */ as usual. There are two primary ways to use javadoc: embed HTML, or use “doc tags.” Doc tags are commands that start with a ‘@’ and are placed at the beginning of a comment line. (A leading ‘*’, however, is ignored.)

There are three “types” of comment documentation, which correspond to the element the comment precedes: class, variable, or method. That is, a class comment appears right before the definition of a class; a variable comment appears right in front of the definition of a variable, and a method comment appears right in front of the definition of a method. As a simple example:

/** A class comment */
public class docTest {
  /** A variable comment */
  public int i;
  /** A method comment */
  public void f() {}
}

Note that javadoc will process comment documentation for only public and protected members. Comments for private and “friendly” members (see Chapter 5) are ignored and you’ll see no output. (However, you can use the -private flag to include private members as well.) This makes sense, since only public and protected members are available outside the file, which is the client programmer’s perspective. However, all class comments are included in the output.

The output for the above code is an HTML file that has the same standard format as all the rest of the Java documentation, so users will be comfortable with the format and can easily navigate your classes. It’s worth entering the above code, sending it through javadoc and viewing the resulting HTML file to see the results.

Embedded HTML

Javadoc passes HTML commands through to the generated HTML document. This allows you full use of HTML; however, the primary motive is to let you format code, such as:

/**
* <pre>
* System.out.println(new Date());
* </pre>
*/

You can also use HTML just as you would in any other Web document to format the regular text in your descriptions:

/**
* You can <em>even</em> insert a list:
* <ol>
* <li> Item one
* <li> Item two
* <li> Item three
* </ol>
*/

Note that within the documentation comment, asterisks at the beginning of a line are thrown away by javadoc, along with leading spaces. Javadoc reformats everything so that it conforms to the standard documentation appearance. Don’t use headings such as <h1> or <hr> as embedded HTML because javadoc inserts its own headings and yours will interfere with them.

All types of comment documentation—class, variable, and method—can support embedded HTML.

@see: referring to other classes

All three types of comment documentation (class, variable, and method) can contain @see tags, which allow you to refer to the documentation in other classes. Javadoc will generate HTML with the @see tags hyperlinked to the other documentation. The forms are:

@see classname
@see fully-qualified-classname
@see fully-qualified-classname#method-name

Each one adds a hyperlinked “See Also” entry to the generated documentation. Javadoc will not check the hyperlinks you give it to make sure they are valid.

Class documentation tags

Along with embedded HTML and @see references, class documentation can include tags for version information and the author’s name. Class documentation can also be used for interfaces (see Chapter 8).

@version

This is of the form:

@version version-information

in which version-information is any significant information you see fit to include. When the -version flag is placed on the javadoc command line, the version information will be called out specially in the generated HTML documentation.

@author

This is of the form:

@author author-information

in which author-information is, presumably, your name, but it could also include your email address or any other appropriate information. When the -author flag is placed on the javadoc command line, the author information will be called out specially in the generated HTML documentation.

You can have multiple author tags for a list of authors, but they must be placed consecutively. All the author information will be lumped together into a single paragraph in the generated HTML.

@since

This tag allows you to indicate the version of this code that began using a particular feature. You’ll see it appearing in the HTML Java documentation to indicate what version of the JDK is used.

Variable documentation tags

Variable documentation can include only embedded HTML and @see references.

Method documentation tags

As well as embedded documentation and @see references, methods allow documentation tags for parameters, return values, and exceptions.

@param

This is of the form:

@param parameter-name description

in which parameter-name is the identifier in the parameter list, and description is text that can continue on subsequent lines. The description is considered finished when a new documentation tag is encountered. You can have any number of these, presumably one for each parameter.

@return

This is of the form:

@return description

in which description gives you the meaning of the return value. It can continue on subsequent lines.

@throws

Exceptions will be demonstrated in Chapter 10, but briefly they are objects that can be “thrown” out of a method if that method fails. Although only one exception object can emerge when you call a method, a particular method might produce any number of different types of exceptions, all of which need descriptions. So the form for the exception tag is:

@throws fully-qualified-class-name description

in which fully-qualified-class-name gives an unambiguous name of an exception class that’s defined somewhere, and description (which can continue on subsequent lines) tells you why this particular type of exception can emerge from the method call.

@deprecated

This is used to tag features that were superseded by an improved feature. The deprecated tag is a suggestion that you no longer use this particular feature, since sometime in the future it is likely to be removed. A method that is marked @deprecated causes the compiler to issue a warning if it is used.

Documentation example

Here is the first Java program again, this time with documentation comments added:

//: c02:HelloDate.java
import java.util.*;

/** The first Thinking in Java example program.
 * Displays a string and today's date.
 * @author Bruce Eckel
 * @author http://www.BruceEckel.com
 * @version 2.0
*/
public class HelloDate {
  /** Sole entry point to class & application
   * @param args array of string arguments
   * @return No return value
   * @exception exceptions No exceptions thrown
  */
  public static void main(String[] args) {
    System.out.println("Hello, it's: ");
    System.out.println(new Date());
  }
} ///:~

The first line of the file uses my own technique of putting a ‘:’ as a special marker for the comment line containing the source file name. That line contains the path information to the file (in this case, c02 indicates Chapter 2) followed by the file name [25] . The last line also finishes with a comment, and this one indicates the end of the source code listing, which allows it to be automatically extracted from the text of this book and checked with a compiler.

Coding style

The unofficial standard in Java is to capitalize the first letter of a class name. If the class name consists of several words, they are run together (that is, you don’t use underscores to separate the names), and the first letter of each embedded word is capitalized, such as:

class AllTheColorsOfTheRainbow { // ...

For almost everything else: methods, fields (member variables), and object reference names, the accepted style is just as it is for classes except that the first letter of the identifier is lowercase. For example:

class AllTheColorsOfTheRainbow {
  int anIntegerRepresentingColors;
  void changeTheHueOfTheColor(int newHue) {
    // ...
  }
  // ...
}

Of course, you should remember that the user must also type all these long names, and so be merciful.

The Java code you will see in the Sun libraries also follows the placement of open-and-close curly braces that you see used in this book.

Summary

In this chapter you have seen enough of Java programming to understand how to write a simple program, and you have gotten an overview of the language and some of its basic ideas. However, the examples so far have all been of the form “do this, then do that, then do something else.” What if you want the program to make choices, such as “if the result of doing this is red, do that; if not, then do something else”? The support in Java for this fundamental programming activity will be covered in the next chapter.

Exercises
  1. Following the HelloDate.java example in this chapter, create a “hello, world” program that simply prints out that statement. You need only a single method in your class (the “main” one that gets executed when the program starts). Remember to make it static and to include the argument list, even though you don’t use the argument list. Compile the program with javac and run it using java. If you are using a different development environment than the JDK, learn how to compile and run programs in that environment.
  2. Find the code fragments involving ATypeName and turn them into a program that compiles and runs.
  3. Turn the DataOnly code fragments into a program that compiles and runs.
  4. Modify Exercise 3 so that the values of the data in DataOnly are assigned to and printed in main( ).
  5. Write a program that includes and calls the storage( ) method defined as a code fragment in this chapter.
  6. Turn the StaticFun code fragments into a working program.
  7. Write a program that prints three arguments taken from the command line. To do this, you’ll need to index into the command-line array of Strings.
  8. Turn the AllTheColorsOfTheRainbow example into a program that compiles and runs.
  9. Find the code for the second version of HelloDate.java, which is the simple comment documentation example. Execute javadoc on the file and view the results with your Web browser.
  10. Turn docTest into a file that compiles and then run it through javadoc. Verify the resulting documentation with your Web browser.
  11. Add an HTML list of items to the documentation in Exercise 10.
  12. Take the program in Exercise 1 and add comment documentation to it. Extract this comment documentation into an HTML file using javadoc and view it with your Web

    [21] This can be a flashpoint. There are those who say “clearly, it’s a pointer,” but this presumes an underlying implementation. Also, Java references are much more akin to C++ references than pointers in their syntax. In the first edition of this book, I choose to invent a new term, “handle,” because C++ references and Java references have some important differences. I was coming out of C++ and did not want to confuse the C++ programmers whom I assumed would be the largest audience for Java. In the 2nd edition, I decided that “reference” was the more commonly used term, and that anyone changing from C++ would have a lot more to cope with than the terminology of references, so they might as well jump in with both feet. However, there are people who disagree even with the term “reference.” I read in one book where it was “completely wrong to say that Java supports pass by reference,” because Java object identifiers (according to that author) are actually “object references.” And (he goes on) everything is actually pass by value. So you’re not passing by reference, you’re “passing an object reference by value.” One could argue for the precision of such convoluted explanations, but I think my approach simplifies the understanding of the concept without hurting anything (well, the language lawyers may claim that I’m lying to you, but I’ll say that I’m providing an appropriate abstraction.)

    [22] static methods, which you’ll learn about soon, can be called for the class, without an object.

    [23] With the usual exception of the aforementioned “special” data types boolean, char, byte, short, int, long, float, and double. In general, though, you pass objects, which really means you pass references to objects.

    [24] Some programming environments will flash programs up on the screen and close them before you've had a chance to see the results. You can put in the following bit of code at the end of main( ) to pause the output:

        try {
      System.in.read();
    } catch(Exception e) {}

    This will pause the output until you press “Enter” (or any other key). This code involves concepts that will not be introduced until much later in the book, so you won’t understand it until then, but it will do the trick.

    [25] A tool that I created using Python (see www.Python.org) uses this information to extract the code files, put them in appropriate subdirectories, and create makefiles.

    browser.


3: Controlling Program Flow

Like a sentient creature, a program must manipulate its world and make choices during execution.

In Java you manipulate objects and data using operators, and you make choices with execution control statements. Java was inherited from C++, so most of these statements and operators will be familiar to C and C++ programmers. Java has also added some improvements and simplifications.

If you find yourself floundering a bit in this chapter, make sure you go through the multimedia CD ROM bound into this book: Thinking in C: Foundations for Java and C++. It contains audio lectures, slides, exercises, and solutions specifically designed to bring you up to speed with the C syntax necessary to learn Java.

Using Java operators

An operator takes one or more arguments and produces a new value. The arguments are in a different form than ordinary method calls, but the effect is the same. You should be reasonably comfortable with the general concept of operators from your previous programming experience. Addition (+), subtraction and unary minus (-), multiplication (*), division (/), and assignment (=) all work much the same in any programming language.

All operators produce a value from their operands. In addition, an operator can change the value of an operand. This is called a side effect. The most common use for operators that modify their operands is to generate the side effect, but you should keep in mind that the value produced is available for your use just as in operators without side effects.

Almost all operators work only with primitives. The exceptions are ‘=’, ‘==’ and ‘!=’, which work with all objects (and are a point of confusion for objects). In addition, the String class supports ‘+’ and ‘+=’.

Precedence

Operator precedence defines how an expression evaluates when several operators are present. Java has specific rules that determine the order of evaluation. The easiest one to remember is that multiplication and division happen before addition and subtraction. Programmers often forget the other precedence rules, so you should use parentheses to make the order of evaluation explicit. For example:

A = X + Y - 2/2 + Z;

has a very different meaning from the same statement with a particular grouping of parentheses:

A = X + (Y - 2)/(2 + Z);

Assignment

Assignment is performed with the operator =. It means “take the value of the right-hand side (often called the rvalue) and copy it into the left-hand side (often called the lvalue). An rvalue is any constant, variable or expression that can produce a value, but an lvalue must be a distinct, named variable. (That is, there must be a physical space to store a value.) For instance, you can assign a constant value to a variable (A = 4;), but you cannot assign anything to constant value—it cannot be an lvalue. (You can’t say 4 = A;.)

Assignment of primitives is quite straightforward. Since the primitive holds the actual value and not a reference to an object, when you assign primitives you copy the contents from one place to another. For example, if you say A = B for primitives, then the contents of B are copied into A. If you then go on to modify A, B is naturally unaffected by this modification. As a programmer, this is what you’ve come to expect for most situations.

When you assign objects, however, things change. Whenever you manipulate an object, what you’re manipulating is the reference, so when you assign “from one object to another” you’re actually copying a reference from one place to another. This means that if you say C = D for objects, you end up with both C and D pointing to the object that, originally, only D pointed to. The following example will demonstrate this.

Here’s the example:

//: c03:Assignment.java
// Assignment with objects is a bit tricky.

class Number {
  int i;
}

public class Assignment {
  public static void main(String[] args) {
    Number n1 = new Number();
    Number n2 = new Number();
    n1.i = 9;
    n2.i = 47;
    System.out.println("1: n1.i: " + n1.i +
      ", n2.i: " + n2.i);
    n1 = n2;
    System.out.println("2: n1.i: " + n1.i +
      ", n2.i: " + n2.i);
    n1.i = 27;
    System.out.println("3: n1.i: " + n1.i +
      ", n2.i: " + n2.i);
  }
} ///:~

The Number class is simple, and two instances of it (n1 and n2) are created within main( ). The i value within each Number is given a different value, and then n2 is assigned to n1, and n1 is changed. In many programming languages you would expect n1 and n2 to be independent at all times, but because you’ve assigned a reference here’s the output you’ll see:

1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 27

Changing the n1 object appears to change the n2 object as well! This is because both n1 and n2 contain the same reference, which is pointing to the same object. (The original reference that was in n1 that pointed to the object holding a value of 9 was overwritten during the assignment and effectively lost; its object will be cleaned up by the garbage collector.)

This phenomenon is often called aliasing and it’s a fundamental way that Java works with objects. But what if you don’t want aliasing to occur in this case? You could forego the assignment and say:

n1.i = n2.i;

This retains the two separate objects instead of tossing one and tying n1 and n2 to the same object, but you’ll soon realize that manipulating the fields within objects is messy and goes against good object-oriented design principles. This is a nontrivial topic, so it is left for Appendix A, which is devoted to aliasing. In the meantime, you should keep in mind that assignment for objects can add surprises.

Aliasing during method calls

Aliasing will also occur when you pass an object into a method:

//: c03:PassObject.java
// Passing objects to methods may not be what
// you're used to.

class Letter {
  char c;
}

public class PassObject {
  static void f(Letter y) {
    y.c = 'z';
  }
  public static void main(String[] args) {
    Letter x = new Letter();
    x.c = 'a';
    System.out.println("1: x.c: " + x.c);
    f(x);
    System.out.println("2: x.c: " + x.c);
  }
} ///:~

In many programming languages, the method f( ) would appear to be making a copy of its argument Letter y inside the scope of the method. But once again a reference is being passed so the line

y.c = 'z';

is actually changing the object outside of f( ). The output shows this:

1: x.c: a
2: x.c: z

Aliasing and its solution is a complex issue and, although you must wait until Appendix A for all the answers, you should be aware of it at this point so you can watch for pitfalls.

Mathematical operators

The basic mathematical operators are the same as the ones available in most programming languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%, which produces the remainder from integer division). Integer division truncates, rather than rounds, the result.

Java also uses a shorthand notation to perform an operation and an assignment at the same time. This is denoted by an operator followed by an equal sign, and is consistent with all the operators in the language (whenever it makes sense). For example, to add 4 to the variable x and assign the result to x, use: x += 4.

This example shows the use of the mathematical operators:

//: c03:MathOps.java
// Demonstrates the mathematical operators.
import java.util.*;

public class MathOps {
  // Create a shorthand to save typing:
  static void prt(String s) {
    System.out.println(s);
  }
  // shorthand to print a string and an int:
  static void pInt(String s, int i) {
    prt(s + " = " + i);
  }
  // shorthand to print a string and a float:
  static void pFlt(String s, float f) {
    prt(s + " = " + f);
  }
  public static void main(String[] args) {
    // Create a random number generator,
    // seeds with current time by default:
    Random rand = new Random();
    int i, j, k;
    // '%' limits maximum value to 99:
    j = rand.nextInt() % 100;
    k = rand.nextInt() % 100;
    pInt("j",j);  pInt("k",k);
    i = j + k; pInt("j + k", i);
    i = j - k; pInt("j - k", i);
    i = k / j; pInt("k / j", i);
    i = k * j; pInt("k * j", i);
    i = k % j; pInt("k % j", i);
    j %= k; pInt("j %= k", j);
    // Floating-point number tests:
    float u,v,w;  // applies to doubles, too
    v = rand.nextFloat();
    w = rand.nextFloat();
    pFlt("v", v); pFlt("w", w);
    u = v + w; pFlt("v + w", u);
    u = v - w; pFlt("v - w", u);
    u = v * w; pFlt("v * w", u);
    u = v / w; pFlt("v / w", u);
    // the following also works for
    // char, byte, short, int, long,
    // and double:
    u += v; pFlt("u += v", u);
    u -= v; pFlt("u -= v", u);
    u *= v; pFlt("u *= v", u);
    u /= v; pFlt("u /= v", u);
  }
} ///:~

The first thing you will see are some shorthand methods for printing: the prt( ) method prints a String, the pInt( ) prints a String followed by an int and the pFlt( ) prints a String followed by a float. Of course, they all ultimately end up using System.out.println( ).

To generate numbers, the program first creates a Random object. Because no arguments are passed during creation, Java uses the current time as a seed for the random number generator. The program generates a number of different types of random numbers with the Random object simply by calling different methods: nextInt( ), nextLong( ), nextFloat( ) or nextDouble( ).

The modulus operator, when used with the result of the random number generator, limits the result to an upper bound of the operand minus one (99 in this case).

Unary minus and plus operators

The unary minus (-) and unary plus (+) are the same operators as binary minus and plus. The compiler figures out which use is intended by the way you write the expression. For instance, the statement

x = -a;

has an obvious meaning. The compiler is able to figure out:

x = a * -b;

but the reader might get confused, so it is clearer to say:

x = a * (-b);

The unary minus produces the negative of the value. Unary plus provides symmetry with unary minus, although it doesn’t have any effect.

Auto increment and decrement

Java, like C, is full of shortcuts. Shortcuts can make code much easier to type, and either easier or harder to read.

Two of the nicer shortcuts are the increment and decrement operators (often referred to as the auto-increment and auto-decrement operators). The decrement operator is -- and means “decrease by one unit.” The increment operator is ++ and means “increase by one unit.” If a is an int, for example, the expression ++a is equivalent to (a = a + 1). Increment and decrement operators produce the value of the variable as a result.

There are two versions of each type of operator, often called the prefix and postfix versions. Pre-increment means the ++ operator appears before the variable or expression, and post-increment means the ++ operator appears after the variable or expression. Similarly, pre-decrement means the -- operator appears before the variable or expression, and post-decrement means the -- operator appears after the variable or expression. For pre-increment and pre-decrement, (i.e., ++a or --a), the operation is performed and the value is produced. For post-increment and post-decrement (i.e. a++ or a--), the value is produced, then the operation is performed. As an example:

//: c03:AutoInc.java
// Demonstrates the ++ and -- operators.

public class AutoInc {
  public static void main(String[] args) {
    int i = 1;
    prt("i : " + i);
    prt("++i : " + ++i); // Pre-increment
    prt("i++ : " + i++); // Post-increment
    prt("i : " + i);
    prt("--i : " + --i); // Pre-decrement
    prt("i-- : " + i--); // Post-decrement
    prt("i : " + i);
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

The output for this program is:

i : 1
++i : 2
i++ : 2
i : 3
--i : 2
i-- : 2
i : 1

You can see that for the prefix form you get the value after the operation has been performed, but with the postfix form you get the value before the operation is performed. These are the only operators (other than those involving assignment) that have side effects. (That is, they change the operand rather than using just its value.)

The increment operator is one explanation for the name C++, implying “one step beyond C.” In an early Java speech, Bill Joy (one of the creators), said that “Java=C++--” (C plus plus minus minus), suggesting that Java is C++ with the unnecessary hard parts removed and therefore a much simpler language. As you progress in this book you’ll see that many parts are simpler, and yet Java isn’t that much easier than C++.

Relational operators

Relational operators generate a boolean result. They evaluate the relationship between the values of the operands. A relational expression produces true if the relationship is true, and false if the relationship is untrue. The relational operators are less than (<), greater than (>), less than or equal to (<=), greater than or equal to (>=), equivalent (==) and not equivalent (!=). Equivalence and nonequivalence works with all built-in data types, but the other comparisons won’t work with type boolean.

Testing object equivalence

The relational operators == and != also work with all objects, but their meaning often confuses the first-time Java programmer. Here’s an example:

//: c03:Equivalence.java

public class Equivalence {
  public static void main(String[] args) {
    Integer n1 = new Integer(47);
    Integer n2 = new Integer(47);
    System.out.println(n1 == n2);
    System.out.println(n1 != n2);
  }
} ///:~

The expression System.out.println(n1 == n2) will print the result of the boolean comparison within it. Surely the output should be true and then false, since both Integer objects are the same. But while the contents of the objects are the same, the references are not the same and the operators == and != compare object references. So the output is actually false and then true. Naturally, this surprises people at first.

What if you want to compare the actual contents of an object for equivalence? You must use the special method equals( ) that exists for all objects (not primitives, which work fine with == and !=). Here’s how it’s used:

//: c03:EqualsMethod.java

public class EqualsMethod {
  public static void main(String[] args) {
    Integer n1 = new Integer(47);
    Integer n2 = new Integer(47);
    System.out.println(n1.equals(n2));
  }
} ///:~

The result will be true, as you would expect. Ah, but it’s not as simple as that. If you create your own class, like this:

//: c03:EqualsMethod2.java

class Value {
  int i;
}

public class EqualsMethod2 {
  public static void main(String[] args) {
    Value v1 = new Value();
    Value v2 = new Value();
    v1.i = v2.i = 100;
    System.out.println(v1.equals(v2));
  }
} ///:~

you’re back to square one: the result is false. This is because the default behavior of equals( ) is to compare references. So unless you override equals( ) in your new class you won’t get the desired behavior. Unfortunately, you won’t learn about overriding until Chapter 7, but being aware of the way equals( ) behaves might save you some grief in the meantime.

Most of the Java library classes implement equals( ) so that it compares the contents of objects instead of their references.

Logical operators

The logical operators AND (&&), OR (||) and NOT (!) produce a boolean value of true or false based on the logical relationship of its arguments. This example uses the relational and logical operators:

//: c03:Bool.java
// Relational and logical operators.
import java.util.*;

public class Bool {
  public static void main(String[] args) {
    Random rand = new Random();
    int i = rand.nextInt() % 100;
    int j = rand.nextInt() % 100;
    prt("i = " + i);
    prt("j = " + j);
    prt("i > j is " + (i > j));
    prt("i < j is " + (i < j));
    prt("i >= j is " + (i >= j));
    prt("i <= j is " + (i <= j));
    prt("i == j is " + (i == j));
    prt("i != j is " + (i != j));

    // Treating an int as a boolean is
    // not legal Java
//! prt("i && j is " + (i && j));
//! prt("i || j is " + (i || j));
//! prt("!i is " + !i);

    prt("(i < 10) && (j < 10) is "
       + ((i < 10) && (j < 10)) );
    prt("(i < 10) || (j < 10) is "
       + ((i < 10) || (j < 10)) );
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

You can apply AND, OR, or NOT to boolean values only. You can’t use a non-boolean as if it were a boolean in a logical expression as you can in C and C++. You can see the failed attempts at doing this commented out with a //! comment marker. The subsequent expressions, however, produce boolean values using relational comparisons, then use logical operations on the results.

One output listing looked like this:

i = 85
j = 4
i > j is true
i < j is false
i >= j is true
i <= j is false
i == j is false
i != j is true
(i < 10) && (j < 10) is false
(i < 10) || (j < 10) is true

Note that a boolean value is automatically converted to an appropriate text form if it’s used where a String is expected.

You can replace the definition for int in the above program with any other primitive data type except boolean. Be aware, however, that the comparison of floating-point numbers is very strict. A number that is the tiniest fraction different from another number is still “not equal.” A number that is the tiniest bit above zero is still nonzero.

Short-circuiting

When dealing with logical operators you run into a phenomenon called “short circuiting.” This means that the expression will be evaluated only until the truth or falsehood of the entire expression can be unambiguously determined. As a result, all the parts of a logical expression might not be evaluated. Here’s an example that demonstrates short-circuiting:

//: c03:ShortCircuit.java
// Demonstrates short-circuiting behavior.
// with logical operators.

public class ShortCircuit {
  static boolean test1(int val) {
    System.out.println("test1(" + val + ")");
    System.out.println("result: " + (val < 1));
    return val < 1;
  }
  static boolean test2(int val) {
    System.out.println("test2(" + val + ")");
    System.out.println("result: " + (val < 2));
    return val < 2;
  }
  static boolean test3(int val) {
    System.out.println("test3(" + val + ")");
    System.out.println("result: " + (val < 3));
    return val < 3;
  }
  public static void main(String[] args) {
    if(test1(0) && test2(2) && test3(2))
      System.out.println("expression is true");
    else
      System.out.println("expression is false");
  }
} ///:~

Each test performs a comparison against the argument and returns true or false. It also prints information to show you that it’s being called. The tests are used in the expression:

if(test1(0) && test2(2) && test3(2))

You might naturally think that all three tests would be executed, but the output shows otherwise:

test1(0)
result: true
test2(2)
result: false
expression is false

The first test produced a true result, so the expression evaluation continues. However, the second test produced a false result. Since this means that the whole expression must be false, why continue evaluating the rest of the expression? It could be expensive. The reason for short-circuiting, in fact, is precisely that; you can get a potential performance increase if all the parts of a logical expression do not need to be evaluated.

Bitwise operators

The bitwise operators allow you to manipulate individual bits in an integral primitive data type. Bitwise operators perform boolean algebra on the corresponding bits in the two arguments to produce the result.

The bitwise operators come from C’s low-level orientation; you were often manipulating hardware directly and had to set the bits in hardware registers. Java was originally designed to be embedded in TV set-top boxes, so this low-level orientation still made sense. However, you probably won’t use the bitwise operators much.

The bitwise AND operator (&) produces a one in the output bit if both input bits are one; otherwise it produces a zero. The bitwise OR operator (|) produces a one in the output bit if either input bit is a one and produces a zero only if both input bits are zero. The bitwise EXCLUSIVE OR, or XOR (^), produces a one in the output bit if one or the other input bit is a one, but not both. The bitwise NOT (~, also called the ones complement operator) is a unary operator; it takes only one argument. (All other bitwise operators are binary operators.) Bitwise NOT produces the opposite of the input bit—a one if the input bit is zero, a zero if the input bit is one.

The bitwise operators and logical operators use the same characters, so it is helpful to have a mnemonic device to help you remember the meanings: since bits are “small,” there is only one character in the bitwise operators.

Bitwise operators can be combined with the = sign to unite the operation and assignment: &=, |= and ^= are all legitimate. (Since ~ is a unary operator it cannot be combined with the = sign.)

The boolean type is treated as a one-bit value so it is somewhat different. You can perform a bitwise AND, OR and XOR, but you can’t perform a bitwise NOT (presumably to prevent confusion with the logical NOT). For booleans the bitwise operators have the same effect as the logical operators except that they do not short circuit. Also, bitwise operations on booleans include an XOR logical operator that is not included under the list of “logical” operators. You’re prevented from using booleans in shift expressions, which is described next.

Shift operators

The shift operators also manipulate bits. They can be used solely with primitive, integral types. The left-shift operator (<<) produces the operand to the left of the operator shifted to the left by the number of bits specified after the operator (inserting zeroes at the lower-order bits). The signed right-shift operator (>>) produces the operand to the left of the operator shifted to the right by the number of bits specified after the operator. The signed right shift >> uses sign extension: if the value is positive, zeroes are inserted at the higher-order bits; if the value is negative, ones are inserted at the higher-order bits. Java has also added the unsigned right shift >>>, which uses zero extension: regardless of the sign, zeroes are inserted at the higher-order bits. This operator does not exist in C or C++.

If you shift a char, byte, or short, it will be promoted to int before the shift takes place, and the result will be an int. Only the five low-order bits of the right-hand side will be used. This prevents you from shifting more than the number of bits in an int. If you’re operating on a long, you’ll get a long result. Only the six low-order bits of the right-hand side will be used so you can’t shift more than the number of bits in a long. There is a problem, however, with the unsigned right shift. If you use it with byte or short you don’t get the correct results. Instead, these are promoted to int and right shifted, but the zero extension does not occur, so you get -1 in those cases. The following example demonstrates this:

//: c03:URShift.java
// Test of unsigned right shift.

public class URShift {
  public static void main(String[] args) {
    int i = -1;
    i >>>= 10;
    System.out.println(i);
    long l = -1;
    l >>>= 10;
    System.out.println(l);
    short s = -1;
    s >>>= 10;
    System.out.println(s);
    byte b = -1;
    b >>>= 10;
    System.out.println(b);
  }
} ///:~

Shifts can be combined with the equal sign (<<= or >>= or >>>=). The lvalue is replaced by the lvalue shifted by the rvalue.

Here’s an example that demonstrates the use of all the operators involving bits:

//: c03:BitManipulation.java
// Using the bitwise operators.
import java.util.*;

public class BitManipulation {
  public static void main(String[] args) {
    Random rand = new Random();
    int i = rand.nextInt();
    int j = rand.nextInt();
    pBinInt("-1", -1);
    pBinInt("+1", +1);
    int maxpos = 2147483647;
    pBinInt("maxpos", maxpos);
    int maxneg = -2147483648;
    pBinInt("maxneg", maxneg);
    pBinInt("i", i);
    pBinInt("~i", ~i);
    pBinInt("-i", -i);
    pBinInt("j", j);
    pBinInt("i & j", i & j);
    pBinInt("i | j", i | j);
    pBinInt("i ^ j", i ^ j);
    pBinInt("i << 5", i << 5);
    pBinInt("i >> 5", i >> 5);
    pBinInt("(~i) >> 5", (~i) >> 5);
    pBinInt("i >>> 5", i >>> 5);
    pBinInt("(~i) >>> 5", (~i) >>> 5);

    long l = rand.nextLong();
    long m = rand.nextLong();
    pBinLong("-1L", -1L);
    pBinLong("+1L", +1L);
    long ll = 9223372036854775807L;
    pBinLong("maxpos", ll);
    long lln = -9223372036854775808L;
    pBinLong("maxneg", lln);
    pBinLong("l", l);
    pBinLong("~l", ~l);
    pBinLong("-l", -l);
    pBinLong("m", m);
    pBinLong("l & m", l & m);
    pBinLong("l | m", l | m);
    pBinLong("l ^ m", l ^ m);
    pBinLong("l << 5", l << 5);
    pBinLong("l >> 5", l >> 5);
    pBinLong("(~l) >> 5", (~l) >> 5);
    pBinLong("l >>> 5", l >>> 5);
    pBinLong("(~l) >>> 5", (~l) >>> 5);
  }
  static void pBinInt(String s, int i) {
    System.out.println(
      s + ", int: " + i + ", binary: ");
    System.out.print("   ");
    for(int j = 31; j >=0; j--)
      if(((1 << j) &  i) != 0)
        System.out.print("1");
      else
        System.out.print("0");
    System.out.println();
  }
  static void pBinLong(String s, long l) {
    System.out.println(
      s + ", long: " + l + ", binary: ");
    System.out.print("   ");
    for(int i = 63; i >=0; i--)
      if(((1L << i) & l) != 0)
        System.out.print("1");
      else
        System.out.print("0");
    System.out.println();
  }
} ///:~

The two methods at the end, pBinInt( ) and pBinLong( ) take an int or a long, respectively, and print it out in binary format along with a descriptive string. You can ignore the implementation of these for now.

You’ll note the use of System.out.print( ) instead of System.out.println( ). The print( ) method does not emit a new line, so it allows you to output a line in pieces.

As well as demonstrating the effect of all the bitwise operators for int and long, this example also shows the minimum, maximum, +1 and -1 values for int and long so you can see what they look like. Note that the high bit represents the sign: 0 means positive and 1 means negative. The output for the int portion looks like this:

-1, int: -1, binary:
   11111111111111111111111111111111
+1, int: 1, binary:
   00000000000000000000000000000001
maxpos, int: 2147483647, binary:
   01111111111111111111111111111111
maxneg, int: -2147483648, binary:
   10000000000000000000000000000000
i, int: 59081716, binary:
   00000011100001011000001111110100
~i, int: -59081717, binary:
   11111100011110100111110000001011
-i, int: -59081716, binary:
   11111100011110100111110000001100
j, int: 198850956, binary:
   00001011110110100011100110001100
i & j, int: 58720644, binary:
   00000011100000000000000110000100
i | j, int: 199212028, binary:
   00001011110111111011101111111100
i ^ j, int: 140491384, binary:
   00001000010111111011101001111000
i << 5, int: 1890614912, binary:
   01110000101100000111111010000000
i >> 5, int: 1846303, binary:
   00000000000111000010110000011111
(~i) >> 5, int: -1846304, binary:
   11111111111000111101001111100000
i >>> 5, int: 1846303, binary:
   00000000000111000010110000011111
(~i) >>> 5, int: 132371424, binary:
   00000111111000111101001111100000

The binary representation of the numbers is referred to as signed two’s complement.

Ternary if-else operator

This operator is unusual because it has three operands. It is truly an operator because it produces a value, unlike the ordinary if-else statement that you’ll see in the next section of this chapter. The expression is of the form:

boolean-exp ? value0 : value1

If boolean-exp evaluates to true, value0 is evaluated and its result becomes the value produced by the operator. If boolean-exp is false, value1 is evaluated and its result becomes the value produced by the operator.

Of course, you could use an ordinary if-else statement (described later), but the ternary operator is much terser. Although C (where this operator originated) prides itself on being a terse language, and the ternary operator might have been introduced partly for efficiency, you should be somewhat wary of using it on an everyday basis—it’s easy to produce unreadable code.

The conditional operator can be used for its side effects or for the value it produces, but in general you want the value since that’s what makes the operator distinct from the if-else. Here’s an example:

static int ternary(int i) {
  return i < 10 ? i * 100 : i * 10;
}

You can see that this code is more compact than what you’d need to write without the ternary operator:

static int alternative(int i) {
  if (i < 10)
    return i * 100;
  else
    return i * 10;
}

The second form is easier to understand, and doesn’t require a lot more typing. So be sure to ponder your reasons when choosing the ternary operator.

The comma operator

The comma is used in C and C++ not only as a separator in function argument lists, but also as an operator for sequential evaluation. The sole place that the comma operator is used in Java is in for loops, which will be described later in this chapter.

String operator +

There’s one special usage of an operator in Java: the + operator can be used to concatenate strings, as you’ve already seen. It seems a natural use of the + even though it doesn’t fit with the traditional way that + is used. This capability seemed like a good idea in C++, so operator overloading was added to C++ to allow the C++ programmer to add meanings to almost any operator. Unfortunately, operator overloading combined with some of the other restrictions in C++ turns out to be a fairly complicated feature for programmers to design into their classes. Although operator overloading would have been much simpler to implement in Java than it was in C++, this feature was still considered too complex, so Java programmers cannot implement their own overloaded operators as C++ programmers can.

The use of the String + has some interesting behavior. If an expression begins with a String, then all operands that follow must be Strings (remember that the compiler will turn a quoted sequence of characters into a String):

int x = 0, y = 1, z = 2;
String sString = "x, y, z ";
System.out.println(sString + x + y + z);

Here, the Java compiler will convert x, y, and z into their String representations instead of adding them together first. And if you say:

System.out.println(x + sString);

Java will turn x into a String.

Common pitfalls when using operators

One of the pitfalls when using operators is trying to get away without parentheses when you are even the least bit uncertain about how an expression will evaluate. This is still true in Java.

An extremely common error in C and C++ looks like this:

while(x = y) {
    // ....
}

The programmer was trying to test for equivalence (==) rather than do an assignment. In C and C++ the result of this assignment will always be true if y is nonzero, and you’ll probably get an infinite loop. In Java, the result of this expression is not a boolean, and the compiler expects a boolean and won’t convert from an int, so it will conveniently give you a compile-time error and catch the problem before you ever try to run the program. So the pitfall never happens in Java. (The only time you won’t get a compile-time error is when x and y are boolean, in which case x = y is a legal expression, and in the above case, probably an error.)

A similar problem in C and C++ is using bitwise AND and OR instead of the logical versions. Bitwise AND and OR use one of the characters (& or |) while logical AND and OR use two (&& and ||). Just as with = and ==, it’s easy to type just one character instead of two. In Java, the compiler again prevents this because it won’t let you cavalierly use one type where it doesn’t belong.

Casting operators

The word cast is used in the sense of “casting into a mold.” Java will automatically change one type of data into another when appropriate. For instance, if you assign an integral value to a floating-point variable, the compiler will automatically convert the int to a float. Casting allows you to make this type conversion explicit, or to force it when it wouldn’t normally happen.

To perform a cast, put the desired data type (including all modifiers) inside parentheses to the left of any value. Here’s an example:

void casts() {
  int i = 200;
  long l = (long)i;
  long l2 = (long)200;
}

As you can see, it’s possible to perform a cast on a numeric value as well as on a variable. In both casts shown here, however, the cast is superfluous, since the compiler will automatically promote an int value to a long when necessary. However, you allowed use superfluous casts in to make a point or to make your code more clear. In other situations, a cast may be essential just to get the code to compile.

In C and C++, casting can cause some headaches. In Java, casting is safe, with the exception that when you perform a so-called narrowing conversion (that is, when you go from a data type that can hold more information to one that doesn’t hold as much) you run the risk of losing information. Here the compiler forces you to do a cast, in effect saying “this can be a dangerous thing to do—if you want me to do it anyway you must make the cast explicit.” With a widening conversion an explicit cast is not needed because the new type will more than hold the information from the old type so that no information is ever lost.

Java allows you to cast any primitive type to any other primitive type, except for boolean, which doesn’t allow any casting at all. Class types do not allow casting. To convert one to the other there must be special methods. (String is a special case, and you’ll find out later in this book that objects can be cast within a family of types; an Oak can be cast to a Tree and vice-versa, but not to a foreign type such as a Rock.)

Literals

Ordinarily when you insert a literal value into a program the compiler knows exactly what type to make it. Sometimes, however, the type is ambiguous. When this happens you must guide the compiler by adding some extra information in the form of characters associated with the literal value. The following code shows these characters:

//: c03:Literals.java

class Literals {
  char c = 0xffff; // max char hex value
  byte b = 0x7f; // max byte hex value
  short s = 0x7fff; // max short hex value
  int i1 = 0x2f; // Hexadecimal (lowercase)
  int i2 = 0X2F; // Hexadecimal (uppercase)
  int i3 = 0177; // Octal (leading zero)
  // Hex and Oct also work with long.
  long n1 = 200L; // long suffix
  long n2 = 200l; // long suffix
  long n3 = 200;
  //! long l6(200); // not allowed
  float f1 = 1;
  float f2 = 1F; // float suffix
  float f3 = 1f; // float suffix
  float f4 = 1e-45f; // 10 to the power
  float f5 = 1e+9f; // float suffix
  double d1 = 1d; // double suffix
  double d2 = 1D; // double suffix
  double d3 = 47e47d; // 10 to the power
} ///:~

Hexadecimal (base 16), which works with all the integral data types, is denoted by a leading 0x or 0X followed by 0–9 and a–f either in upper or lowercase. If you try to initialize a variable with a value bigger than it can hold (regardless of the numerical form of the value), the compiler will give you an error message. Notice in the above code the maximum possible hexadecimal values for char, byte, and short. If you exceed these, the compiler will automatically make the value an int and tell you that you need a narrowing cast for the assignment. You’ll know you’ve stepped over the line.

Octal (base 8) is denoted by a leading zero in the number and digits from 0-7. There is no literal representation for binary numbers in C, C++ or Java.

A trailing character after a literal value establishes its type. Upper or lowercase L means long, upper or lowercase F means float and upper or lowercase D means double.

Exponents use a notation that I’ve always found rather dismaying: 1.39 e-47f. In science and engineering, ‘e’ refers to the base of natural logarithms, approximately 2.718. (A more precise double value is available in Java as Math.E.) This is used in exponentiation expressions such as 1.39 x e-47, which means 1.39 x 2.718-47. However, when FORTRAN was invented they decided that e would naturally mean “ten to the power,” which is an odd decision because FORTRAN was designed for science and engineering and one would think its designers would be sensitive about introducing such an ambiguity. [26] At any rate, this custom was followed in C, C++ and now Java. So if you’re used to thinking in terms of e as the base of natural logarithms, you must do a mental translation when you see an expression such as 1.39 e-47f in Java; it means 1.39 x 10-47.

Note that you don’t need to use the trailing character when the compiler can figure out the appropriate type. With

long n3 = 200;

there’s no ambiguity, so an L after the 200 would be superfluous. However, with

float f4 = 1e-47f; // 10 to the power

the compiler normally takes exponential numbers as doubles, so without the trailing f it will give you an error telling you that you must use a cast to convert double to float.

Promotion

You’ll discover that if you perform any mathematical or bitwise operations on primitive data types that are smaller than an int (that is, char, byte, or short), those values will be promoted to int before performing the operations, and the resulting value will be of type int. So if you want to assign back into the smaller type, you must use a cast. (And, since you’re assigning back into a smaller type, you might be losing information.) In general, the largest data type in an expression is the one that determines the size of the result of that expression; if you multiply a float and a double, the result will be double; if you add an int and a long, the result will be long.

Java has no “sizeof”

In C and C++, the sizeof( ) operator satisfies a specific need: it tells you the number of bytes allocated for data items. The most compelling need for sizeof( ) in C and C++ is portability. Different data types might be different sizes on different machines, so the programmer must find out how big those types are when performing operations that are sensitive to size. For example, one computer might store integers in 32 bits, whereas another might store integers as 16 bits. Programs could store larger values in integers on the first machine. As you might imagine, portability is a huge headache for C and C++ programmers.

Java does not need a sizeof( ) operator for this purpose because all the data types are the same size on all machines. You do not need to think about portability on this level—it is designed into the language.

Precedence revisited

Upon hearing me complain about the complexity of remembering operator precedence during one of my seminars, a student suggested a mnemonic that is simultaneously a commentary: “Ulcer Addicts Really Like C A lot.”

Mnemonic | Operator type | Operators
Ulcer | Unary | + - ++— [[ rest...]]
Addicts | Arithmetic (and shift) | * / % + - << >>
Really | Relational | > < >= <= == !=
Like | Logical (and bitwise) | && || & | ^
C | Conditional (ternary) | A > B ? X : Y
A Lot | Assignment | = (and compound assignment like *=)

Of course, with the shift and bitwise operators distributed around the table it is not a perfect mnemonic, but for non-bit operations it works.

A compendium of operators

The following example shows which primitive data types can be used with particular operators. Basically, it is the same example repeated over and over, but using different primitive data types. The file will compile without error because the lines that would cause errors are commented out with a //!.

//: c03:AllOps.java
// Tests all the operators on all the
// primitive data types to show which
// ones are accepted by the Java compiler.

class AllOps {
  // To accept the results of a boolean test:
  void f(boolean b) {}
  void boolTest(boolean x, boolean y) {
    // Arithmetic operators:
    //! x = x * y;
    //! x = x / y;
    //! x = x % y;
    //! x = x + y;
    //! x = x - y;
    //! x++;
    //! x--;
    //! x = +y;
    //! x = -y;
    // Relational and logical:
    //! f(x > y);
    //! f(x >= y);
    //! f(x < y);
    //! f(x <= y);
    f(x == y);
    f(x != y);
    f(!y);
    x = x && y;
    x = x || y;
    // Bitwise operators:
    //! x = ~y;
    x = x & y;
    x = x | y;
    x = x ^ y;
    //! x = x << 1;
    //! x = x >> 1;
    //! x = x >>> 1;
    // Compound assignment:
    //! x += y;
    //! x -= y;
    //! x *= y;
    //! x /= y;
    //! x %= y;
    //! x <<= 1;
    //! x >>= 1;
    //! x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! char c = (char)x;
    //! byte B = (byte)x;
    //! short s = (short)x;
    //! int i = (int)x;
    //! long l = (long)x;
    //! float f = (float)x;
    //! double d = (double)x;
  }
  void charTest(char x, char y) {
    // Arithmetic operators:
    x = (char)(x * y);
    x = (char)(x / y);
    x = (char)(x % y);
    x = (char)(x + y);
    x = (char)(x - y);
    x++;
    x--;
    x = (char)+y;
    x = (char)-y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    x= (char)~y;
    x = (char)(x & y);
    x  = (char)(x | y);
    x = (char)(x ^ y);
    x = (char)(x << 1);
    x = (char)(x >> 1);
    x = (char)(x >>> 1);
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    x <<= 1;
    x >>= 1;
    x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    byte B = (byte)x;
    short s = (short)x;
    int i = (int)x;
    long l = (long)x;
    float f = (float)x;
    double d = (double)x;
  }
  void byteTest(byte x, byte y) {
    // Arithmetic operators:
    x = (byte)(x* y);
    x = (byte)(x / y);
    x = (byte)(x % y);
    x = (byte)(x + y);
    x = (byte)(x - y);
    x++;
    x--;
    x = (byte)+ y;
    x = (byte)- y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    x = (byte)~y;
    x = (byte)(x & y);
    x = (byte)(x | y);
    x = (byte)(x ^ y);
    x = (byte)(x << 1);
    x = (byte)(x >> 1);
    x = (byte)(x >>> 1);
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    x <<= 1;
    x >>= 1;
    x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    short s = (short)x;
    int i = (int)x;
    long l = (long)x;
    float f = (float)x;
    double d = (double)x;
  }
  void shortTest(short x, short y) {
    // Arithmetic operators:
    x = (short)(x * y);
    x = (short)(x / y);
    x = (short)(x % y);
    x = (short)(x + y);
    x = (short)(x - y);
    x++;
    x--;
    x = (short)+y;
    x = (short)-y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    x = (short)~y;
    x = (short)(x & y);
    x = (short)(x | y);
    x = (short)(x ^ y);
    x = (short)(x << 1);
    x = (short)(x >> 1);
    x = (short)(x >>> 1);
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    x <<= 1;
    x >>= 1;
    x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    byte B = (byte)x;
    int i = (int)x;
    long l = (long)x;
    float f = (float)x;
    double d = (double)x;
  }
  void intTest(int x, int y) {
    // Arithmetic operators:
    x = x * y;
    x = x / y;
    x = x % y;
    x = x + y;
    x = x - y;
    x++;
    x--;
    x = +y;
    x = -y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    x = ~y;
    x = x & y;
    x = x | y;
    x = x ^ y;
    x = x << 1;
    x = x >> 1;
    x = x >>> 1;
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    x <<= 1;
    x >>= 1;
    x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    byte B = (byte)x;
    short s = (short)x;
    long l = (long)x;
    float f = (float)x;
    double d = (double)x;
  }
  void longTest(long x, long y) {
    // Arithmetic operators:
    x = x * y;
    x = x / y;
    x = x % y;
    x = x + y;
    x = x - y;
    x++;
    x--;
    x = +y;
    x = -y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    x = ~y;
    x = x & y;
    x = x | y;
    x = x ^ y;
    x = x << 1;
    x = x >> 1;
    x = x >>> 1;
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    x <<= 1;
    x >>= 1;
    x >>>= 1;
    x &= y;
    x ^= y;
    x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    byte B = (byte)x;
    short s = (short)x;
    int i = (int)x;
    float f = (float)x;
    double d = (double)x;
  }
  void floatTest(float x, float y) {
    // Arithmetic operators:
    x = x * y;
    x = x / y;
    x = x % y;
    x = x + y;
    x = x - y;
    x++;
    x--;
    x = +y;
    x = -y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    //! x = ~y;
    //! x = x & y;
    //! x = x | y;
    //! x = x ^ y;
    //! x = x << 1;
    //! x = x >> 1;
    //! x = x >>> 1;
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    //! x <<= 1;
    //! x >>= 1;
    //! x >>>= 1;
    //! x &= y;
    //! x ^= y;
    //! x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    byte B = (byte)x;
    short s = (short)x;
    int i = (int)x;
    long l = (long)x;
    double d = (double)x;
  }
  void doubleTest(double x, double y) {
    // Arithmetic operators:
    x = x * y;
    x = x / y;
    x = x % y;
    x = x + y;
    x = x - y;
    x++;
    x--;
    x = +y;
    x = -y;
    // Relational and logical:
    f(x > y);
    f(x >= y);
    f(x < y);
    f(x <= y);
    f(x == y);
    f(x != y);
    //! f(!x);
    //! f(x && y);
    //! f(x || y);
    // Bitwise operators:
    //! x = ~y;
    //! x = x & y;
    //! x = x | y;
    //! x = x ^ y;
    //! x = x << 1;
    //! x = x >> 1;
    //! x = x >>> 1;
    // Compound assignment:
    x += y;
    x -= y;
    x *= y;
    x /= y;
    x %= y;
    //! x <<= 1;
    //! x >>= 1;
    //! x >>>= 1;
    //! x &= y;
    //! x ^= y;
    //! x |= y;
    // Casting:
    //! boolean b = (boolean)x;
    char c = (char)x;
    byte B = (byte)x;
    short s = (short)x;
    int i = (int)x;
    long l = (long)x;
    float f = (float)x;
  }
} ///:~

Note that boolean is quite limited. You can assign to it the values true and false, and you can test it for truth or falsehood, but you cannot add booleans or perform any other type of operation on them.

In char, byte, and short you can see the effect of promotion with the arithmetic operators. Each arithmetic operation on any of those types results in an int result, which must be explicitly cast back to the original type (a narrowing conversion that might lose information) to assign back to that type. With int values, however, you do not need to cast, because everything is already an int. Don’t be lulled into thinking everything is safe, though. If you multiply two ints that are big enough, you’ll overflow the result. The following example demonstrates this:

//: c03:Overflow.java
// Surprise! Java lets you overflow.

public class Overflow {
  public static void main(String[] args) {
    int big = 0x7fffffff; // max int value
    prt("big = " + big);
    int bigger = big * 4;
    prt("bigger = " + bigger);
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

The output of this is:

big = 2147483647
bigger = -4

and you get no errors or warnings from the compiler, and no exceptions at run-time. Java is good, but it’s not that good.

Compound assignments do not require casts for char, byte, or short, even though they are performing promotions that have the same results as the direct arithmetic operations. On the other hand, the lack of the cast certainly simplifies the code.

You can see that, with the exception of boolean, any primitive type can be cast to any other primitive type. Again, you must be aware of the effect of a narrowing conversion when casting to a smaller type, otherwise you might unknowingly lose information during the cast.

Execution control

Java uses all of C’s execution control statements, so if you’ve programmed with C or C++ then most of what you see will be familiar. Most procedural programming languages have some kind of control statements, and there is often overlap among languages. In Java, the keywords include if-else, while, do-while, for, and a selection statement called switch. Java does not, however, support the much-maligned goto (which can still be the most expedient way to solve certain types of problems). You can still do a goto-like jump, but it is much more constrained than a typical goto.

true and false

All conditional statements use the truth or falsehood of a conditional expression to determine the execution path. An example of a conditional expression is A == B. This uses the conditional operator == to see if the value of A is equivalent to the value of B. The expression returns true or false. Any of the relational operators you’ve seen earlier in this chapter can be used to produce a conditional statement. Note that Java doesn’t allow you to use a number as a boolean, even though it’s allowed in C and C++ (where truth is nonzero and falsehood is zero). If you want to use a non-boolean in a boolean test, such as if(a), you must first convert it to a boolean value using a conditional expression, such as if(a != 0).

if-else

The if-else statement is probably the most basic way to control program flow. The else is optional, so you can use if in two forms:

if(Boolean-expression)
  statement

or

if(Boolean-expression)
  statement
else
  statement

The conditional must produce a boolean result. The statement means either a simple statement terminated by a semicolon or a compound statement, which is a group of simple statements enclosed in braces. Any time the word “statement” is used, it always implies that the statement can be simple or compound.

As an example of if-else, here is a test( ) method that will tell you whether a guess is above, below, or equivalent to a target number:

static int test(int testval) {
  int result = 0;
  if(testval > target)
    result = -1;
  else if(testval < target)
    result = +1;
  else
    result = 0; // match
  return result;
}
It is conventional to indent the body of a control flow statement so the reader might easily determine where it begins and ends.

return

The return keyword has two purposes: it specifies what value a method will return (if it doesn’t have a void return value) and it causes that value to be returned immediately. The test( ) method above can be rewritten to take advantage of this:

static int test2(int testval) {
  if(testval > target)
    return -1;
  if(testval < target)
    return +1;
  return 0; // match
}

There’s no need for else because the method will not continue after executing a return.

Iteration

while, do-while and for control looping and are sometimes classified as iteration statements. A statement repeats until the controlling Boolean-expression evaluates to false. The form for a while loop is

while(Boolean-expression)
  statement

The Boolean-expression is evaluated once at the beginning of the loop and again before each further iteration of the statement.

Here’s a simple example that generates random numbers until a particular condition is met:

//: c03:WhileTest.java
// Demonstrates the while loop.

public class WhileTest {
  public static void main(String[] args) {
    double r = 0;
    while(r < 0.99d) {
      r = Math.random();
      System.out.println(r);
    }
  }
} ///:~

This uses the static method random( ) in the Math library, which generates a double value between 0 and 1. (It includes 0, but not 1.) The conditional expression for the while says “keep doing this loop until the number is 0.99 or greater.” Each time you run this program you’ll get a different-sized list of numbers.

do-while

The form for do-while is

do
  statement
while(Boolean-expression);

The sole difference between while and do-while is that the statement of the do-while always executes at least once, even if the expression evaluates to false the first time. In a while, if the conditional is false the first time the statement never executes. In practice, do-while is less common than while.

for

A for loop performs initialization before the first iteration. Then it performs conditional testing and, at the end of each iteration, some form of “stepping.” The form of the for loop is:

for(initialization; Boolean-expression; step)
  statement

Any of the expressions initialization, Boolean-expression or step can be empty. The expression is tested before each iteration, and as soon as it evaluates to false execution will continue at the line following the for statement. At the end of each loop, the step executes.

for loops are usually used for “counting” tasks:

//: c03:ListCharacters.java
// Demonstrates "for" loop by listing
// all the ASCII characters.

public class ListCharacters {
  public static void main(String[] args) {
  for( char c = 0; c < 128; c++)
    if (c != 26 )  // ANSI Clear screen
      System.out.println(
        "value: " + (int)c +
        " character: " + c);
  }
} ///:~

Note that the variable c is defined at the point where it is used, inside the control expression of the for loop, rather than at the beginning of the block denoted by the open curly brace. The scope of c is the expression controlled by the for.

Traditional procedural languages like C require that all variables be defined at the beginning of a block so when the compiler creates a block it can allocate space for those variables. In Java and C++ you can spread your variable declarations throughout the block, defining them at the point that you need them. This allows a more natural coding style and makes code easier to understand.

You can define multiple variables within a for statement, but they must be of the same type:

for(int i = 0, j = 1;
    i < 10 && j != 11;
    i++, j++)
  /* body of for loop */;

The int definition in the for statement covers both i and j. The ability to define variables in the control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements.

The comma operator

Earlier in this chapter I stated that the comma operator (not the comma separator, which is used to separate function arguments) has only one use in Java: in the control expression of a for loop. In both the initialization and step portions of the control expression you can have a number of statements separated by commas, and those statements will be evaluated sequentially. The previous bit of code uses this ability. Here’s another example:

//: c03:CommaOperator.java
public class CommaOperator {
  public static void main(String[] args) {
    for(int i = 1, j = i + 10; i < 5;
        i++, j = i * 2) {
      System.out.println("i= " + i + " j= " + j);
    }
  }
} ///:~

Here’s the output:

i= 1 j= 11
i= 2 j= 4
i= 3 j= 6
i= 4 j= 8

You can see that in both the initialization and step portions the statements are evaluated in sequential order. Also, the initialization portion can have any number of definitions of one type.

break and continue

Inside the body of any of the iteration statements you can also control the flow of the loop by using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin the next iteration.

This program shows examples of break and continue within for and while loops:

//: c03:BreakAndContinue.java
// Demonstrates break and continue keywords.

public class BreakAndContinue {
  public static void main(String[] args) {
    for(int i = 0; i < 100; i++) {
      if(i == 74) break; // Out of for loop
      if(i % 9 != 0) continue; // Next iteration
      System.out.println(i);
    }
    int i = 0;
    // An "infinite loop":
    while(true) {
      i++;
      int j = i * 27;
      if(j == 1269) break; // Out of loop
      if(i % 10 != 0) continue; // Top of loop
      System.out.println(i);
    }
  }
} ///:~

In the for loop the value of i never gets to 100 because the break statement breaks out of the loop when i is 74. Normally, you’d use a break like this only if you didn’t know when the terminating condition was going to occur. The continue statement causes execution to go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly divisible by 9. When it is, the value is printed.

The second portion shows an “infinite loop” that would, in theory, continue forever. However, inside the loop there is a break statement that will break out of the loop. In addition, you’ll see that the continue moves back to the top of the loop without completing the remainder. (Thus printing happens in the second loop only when the value of i is divisible by 10.) The output is:

0
9
18
27
36
45
54
63
72
10
20
30
40

The value 0 is printed because 0 % 9 produces 0.

A second form of the infinite loop is for(;;). The compiler treats both while(true) and for(;;) in the same way so whichever one you use is a matter of programming taste.

The infamous “goto”

The goto keyword has been present in programming languages from the beginning. Indeed, goto was the genesis of program control in assembly language: “if condition A, then jump here, otherwise jump there.” If you read the assembly code that is ultimately generated by virtually any compiler, you’ll see that program control contains many jumps. However, a goto is a jump at the source-code level, and that’s what brought it into disrepute. If a program will always jump from one point to another, isn’t there some way to reorganize the code so the flow of control is not so jumpy? goto fell into true disfavor with the publication of the famous “Goto considered harmful” paper by Edsger Dijkstra, and since then goto-bashing has been a popular sport, with advocates of the cast-out keyword scurrying for cover.

As is typical in situations like this, the middle ground is the most fruitful. The problem is not the use of goto, but the overuse of goto—in rare situations goto is actually the best way to structure control flow.

Although goto is a reserved word in Java, it is not used in the language; Java has no goto. However, it does have something that looks a bit like a jump tied in with the break and continue keywords. It’s not a jump but rather a way to break out of an iteration statement. The reason it’s often thrown in with discussions of goto is because it uses the same mechanism: a label.

A label is an identifier followed by a colon, like this:

label1:

The only place a label is useful in Java is right before an iteration statement. And that means right before—it does no good to put any other statement between the label and the iteration. And the sole reason to put a label before an iteration is if you’re going to nest another iteration or a switch inside it. That’s because the break and continue keywords will normally interrupt only the current loop, but when used with a label they’ll interrupt the loops up to where the label exists:

label1:
outer-iteration {
  inner-iteration {
    //...
    break; // 1
    //...
    continue;  // 2
    //...
    continue label1; // 3
    //...
    break label1;  // 4
  }
}

In case 1, the break breaks out of the inner iteration and you end up in the outer iteration. In case 2, the continue moves back to the beginning of the inner iteration. But in case 3, the continue label1 breaks out of the inner iteration and the outer iteration, all the way back to label1. Then it does in fact continue the iteration, but starting at the outer iteration. In case 4, the break label1 also breaks all the way out to label1, but it does not re-enter the iteration. It actually does break out of both iterations.

Here is an example using for loops:

//: c03:LabeledFor.java
// Java’s "labeled for" loop.

public class LabeledFor {
  public static void main(String[] args) {
    int i = 0;
    outer: // Can't have statements here
    for(; true ;) { // infinite loop
      inner: // Can't have statements here
      for(; i < 10; i++) {
        prt("i = " + i);
        if(i == 2) {
          prt("continue");
          continue;
        }
        if(i == 3) {
          prt("break");
          i++; // Otherwise i never
               // gets incremented.
          break;
        }
        if(i == 7) {
          prt("continue outer");
          i++; // Otherwise i never
               // gets incremented.
          continue outer;
        }
        if(i == 8) {
          prt("break outer");
          break outer;
        }
        for(int k = 0; k < 5; k++) {
          if(k == 3) {
            prt("continue inner");
            continue inner;
          }
        }
      }
    }
    // Can't break or continue
    // to labels here
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

This uses the prt( ) method that has been defined in the other examples.

Note that break breaks out of the for loop, and that the increment-expression doesn’t occur until the end of the pass through the for loop. Since break skips the increment expression, the increment is performed directly in the case of i == 3. The continue outer statement in the case of i == 7 also goes to the top of the loop and also skips the increment, so it too is incremented directly.

Here is the output:

i = 0
continue inner
i = 1
continue inner
i = 2
continue
i = 3
break
i = 4
continue inner
i = 5
continue inner
i = 6
continue inner
i = 7
continue outer
i = 8
break outer

If not for the break outer statement, there would be no way to get out of the outer loop from within an inner loop, since break by itself can break out of only the innermost loop. (The same is true for continue.)

Of course, in the cases where breaking out of a loop will also exit the method, you can simply use a return.

Here is a demonstration of labeled break and continue statements with while loops:

//: c03:LabeledWhile.java
// Java's "labeled while" loop.

public class LabeledWhile {
  public static void main(String[] args) {
    int i = 0;
    outer:
    while(true) {
      prt("Outer while loop");
      while(true) {
        i++;
        prt("i = " + i);
        if(i == 1) {
          prt("continue");
          continue;
        }
        if(i == 3) {
          prt("continue outer");
          continue outer;
        }
        if(i == 5) {
          prt("break");
          break;
        }
        if(i == 7) {
          prt("break outer");
          break outer;
        }
      }
    }
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

The same rules hold true for while:

  1. A plain continue goes to the top of the innermost loop and continues.
  2. A labeled continue goes to the label and re-enters the loop right after that label.
  3. A break “drops out of the bottom” of the loop.
  4. A labeled break drops out of the bottom of the end of the loop denoted by the label.

The output of this method makes it clear:

Outer while loop
i = 1
continue
i = 2
i = 3
continue outer
Outer while loop
i = 4
i = 5
break
Outer while loop
i = 6
i = 7
break outer

It’s important to remember that the only reason to use labels in Java is when you have nested loops and you want to break or continue through more than one nested level.

In Dijkstra’s “goto considered harmful” paper, what he specifically objected to was the labels, not the goto. He observed that the number of bugs seems to increase with the number of labels in a program. Labels and gotos make programs difficult to analyze statically, since it introduces cycles in the program execution graph. Note that Java labels don’t suffer from this problem, since they are constrained in their placement and can’t be used to transfer control in an ad hoc manner. It’s also interesting to note that this is a case where a language feature is made more useful by restricting the power of the statement.

switch

The switch is sometimes classified as a selection statement. The switch statement selects from among pieces of code based on the value of an integral expression. Its form is:

switch(integral-selector) {
  case integral-value1 : statement; break;
  case integral-value2 : statement; break;
  case integral-value3 : statement; break;
  case integral-value4 : statement; break;
  case integral-value5 : statement; break;
          // ...
  default: statement;
}

Integral-selector is an expression that produces an integral value. The switch compares the result of integral-selector to each integral-value. If it finds a match, the corresponding statement (simple or compound) executes. If no match occurs, the default statement executes.

You will notice in the above definition that each case ends with a break, which causes execution to jump to the end of the switch body. This is the conventional way to build a switch statement, but the break is optional. If it is missing, the code for the following case statements execute until a break is encountered. Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer. Note the last statement, following the default, doesn’t have a break because the execution just falls through to where the break would have taken it anyway. You could put a break at the end of the default statement with no harm if you considered it important for style’s sake.

The switch statement is a clean way to implement multi-way selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value such as int or char. If you want to use, for example, a string or a floating-point number as a selector, it won’t work in a switch statement. For non-integral types, you must use a series of if statements.

Here’s an example that creates letters randomly and determines whether they’re vowels or consonants:

//: c03:VowelsAndConsonants.java
// Demonstrates the switch statement.

public class VowelsAndConsonants {
  public static void main(String[] args) {
    for(int i = 0; i < 100; i++) {
      char c = (char)(Math.random() * 26 + 'a');
      System.out.print(c + ": ");
      switch(c) {
      case 'a':
      case 'e':
      case 'i':
      case 'o':
      case 'u':
                System.out.println("vowel");
                break;
      case 'y':
      case 'w':
                System.out.println(
                  "Sometimes a vowel");
                break;
      default:
                System.out.println("consonant");
      }
    }
  }
} ///:~

Since Math.random( ) generates a value between 0 and 1, you need only multiply it by the upper bound of the range of numbers you want to produce (26 for the letters in the alphabet) and add an offset to establish the lower bound.

Although it appears you’re switching on a character here, the switch statement is actually using the integral value of the character. The singly-quoted characters in the case statements also produce integral values that are used for comparison.

Notice how the cases can be “stacked” on top of each other to provide multiple matches for a particular piece of code. You should also be aware that it’s essential to put the break statement at the end of a particular case, otherwise control will simply drop through and continue processing on the next case.

Calculation details

The statement:
char c = (char)(Math.random() * 26 + 'a');

deserves a closer look. Math.random( ) produces a double, so the value 26 is converted to a double to perform the multiplication, which also produces a double. This means that ‘a’ must be converted to a double to perform the addition. The double result is turned back into a char with a cast.

What does the cast to char do? That is, if you have the value 29.7 and you cast it to a char, is the resulting value 30 or 29? The answer to this can be seen in this example:

//: c03:CastingNumbers.java
// What happens when you cast a float
// or double to an integral value?

public class CastingNumbers {
  public static void main(String[] args) {
    double
      above = 0.7,
      below = 0.4;
    System.out.println("above: " + above);
    System.out.println("below: " + below);
    System.out.println(
      "(int)above: " + (int)above);
    System.out.println(
      "(int)below: " + (int)below);
    System.out.println(
      "(char)('a' + above): " +
      (char)('a' + above));
    System.out.println(
      "(char)('a' + below): " +
      (char)('a' + below));
  }
} ///:~

The output is:

above: 0.7
below: 0.4
(int)above: 0
(int)below: 0
(char)('a' + above): a
(char)('a' + below): a

So the answer is that casting from a float or double to an integral value always truncates.

A second question concerns Math.random( ). Does it produce a value from zero to one, inclusive or exclusive of the value ‘1’? In math lingo, is it (0,1), or [0,1], or (0,1] or [0,1)? (The square bracket means “includes” whereas the parenthesis means “doesn’t include.”) Again, a test program might provide the answer:

//: c03:RandomBounds.java
// Does Math.random() produce 0.0 and 1.0?

public class RandomBounds {
  static void usage() {
    System.err.println("Usage: \n\t" +
      "RandomBounds lower\n\t" +
      "RandomBounds upper");
    System.exit(1);
  }
  public static void main(String[] args) {
    if(args.length != 1) usage();
    if(args[0].equals("lower")) {
      while(Math.random() != 0.0)
        ; // Keep trying
      System.out.println("Produced 0.0!");
    }
    else if(args[0].equals("upper")) {
      while(Math.random() != 1.0)
        ; // Keep trying
      System.out.println("Produced 1.0!");
    }
    else
      usage();
  }
} ///:~

To run the program, you type a command line of either:

java RandomBounds lower

or

java RandomBounds upper

In both cases you are forced to break out of the program manually, so it would appear that Math.random( ) never produces either 0.0 or 1.0. But this is where such an experiment can be deceiving. If you consider [27] that there are about 262 different double fractions between 0 and 1, the likelihood of reaching any one value experimentally might exceed the lifetime of one computer, or even one experimenter. It turns out that 0.0 is included in the output of Math.random( ). Or, in math lingo, it is [0,1).

Summary

This chapter concludes the study of fundamental features that appear in most programming languages: calculation, operator precedence, type casting, and selection and iteration. Now you’re ready to begin taking steps that move you closer to the world of object-oriented programming. The next chapter will cover the important issues of initialization and cleanup of objects, followed in the subsequent chapter by the essential concept of implementation hiding.

Exercises
  1. There are two expressions in the section labeled “precedence” early in this chapter. Put these expressions into a program and demonstrate that they produce different results.
  2. Put the methods ternary( ) and alternative( ) into a working program.
  3. From the sections labeled “if-else” and “return”, put the methods test( ) and test2( ) into a working program.
  4. Write a program that prints values from one to 100.
  5. Modify Exercise 4 so that the program exits by using the break keyword at value 47. Try using return instead.
  6. Write a function that takes two String arguments, and uses all the Boolean comparisons to compare the two Strings and print the results. For the == and !=, also perform the equals( ) test. In main( ), call your function with some different String objects.
  7. Write a program that generates 25 random int values. For each value, use an if-then-else statement to classify it as greater than, less than or equal to a second randomly-generated value.
  8. Modify Exercise 7 so that your code is surrounded by an “infinite” while loop. It will then run until you interrupt it from the keyboard.
  9. Write a program that uses two nested for loops and the modulus operator (%) to detect and print prime numbers (integral numbers that are not evenly divisible by any other numbers except for themselves and 1).
  10. Create a switch statement that prints a message for each case, and put the switch inside a for loop that tries each case. Put a break after each case and test it, then remove the breaks and see what happens.

[26] John Kirkham writes, “I started computing in 1962 using FORTRAN II on an IBM 1620. At that time, and throughout the 1960s and into the 1970s, FORTRAN was an all uppercase language. This probably started because many of the early input devices were old teletype units that used 5 bit Baudot code, which had no lowercase capability. The ‘E’ in the exponential notation was also always upper case and was never confused with the natural logarithm base ‘e’, which is always lowercase. The ‘E’ simply stood for exponential, which was for the base of the number system used—usually 10. At the time octal was also widely used by programmers. Although I never saw it used, if I had seen an octal number in exponential notation I would have considered it to be base 8. The first time I remember seeing an exponential using a lowercase ‘e’ was in the late 1970s and I also found it confusing. The problem arose as lowercase crept into FORTRAN, not at its beginning. We actually had functions to use if you really wanted to use the natural logarithm base, but they were all uppercase.”

[27] Chuck Allison writes: The total number of numbers in a floating-point number system is
2(M-m+1)b^(p-1) + 1
where b is the base (usually 2), p is the precision (digits in the mantissa), M is the largest exponent, and m is the smallest exponent. IEEE 754 uses:
M = 1023, m = -1022, p = 53, b = 2
so the total number of numbers is
2(1023+1022+1)2^52
= 2((2^10-1) + (2^10-1))2^52
= (2^10-1)2^54
= 2^64 - 2^54
Half of these numbers (corresponding to exponents in the range [-1022, 0]) are less than 1 in magnitude (both positive and negative), so 1/4 of that expression, or 2^62 - 2^52 + 1 (approximately 2^62) is in the range [0,1). See my paper at http://www.freshsources.com/1995006a.htm (last of text).


4: Initialization & Cleanup

4: Initialization
& Cleanup

As the computer revolution progresses, “unsafe” programming has become one of the major culprits that makes programming expensive.

Two of these safety issues are initialization and cleanup. Many C bugs occur when the programmer forgets to initialize a variable. This is especially true with libraries when users don’t know how to initialize a library component, or even that they must. Cleanup is a special problem because it’s easy to forget about an element when you’re done with it, since it no longer concerns you. Thus, the resources used by that element are retained and you can easily end up running out of resources (most notably, memory).

C++ introduced the concept of a constructor, a special method automatically called when an object is created. Java also adopted the constructor, and in addition has a garbage collector that automatically releases memory resources when they’re no longer being used. This chapter examines the issues of initialization and cleanup, and their support in Java.

Guaranteed initialization
with the constructor

You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call the method. In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.

The next challenge is what to name this method. There are two issues. The first is that any name you use could clash with a name you might like to use as a member in the class. The second is that because the compiler is responsible for calling the constructor, it must always know which method to call. The C++ solution seems the easiest and most logical, so it’s also used in Java: the name of the constructor is the same as the name of the class. It makes sense that such a method will be called automatically on initialization.

Here’s a simple class with a constructor:

//: c04:SimpleConstructor.java
// Demonstration of a simple constructor.

class Rock {
  Rock() { // This is the constructor
    System.out.println("Creating Rock");
  }
}

public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock();
  }
} ///:~

Now, when an object is created:

new Rock();

storage is allocated and the constructor is called. It is guaranteed that the object will be properly initialized before you can get your hands on it.

Note that the coding style of making the first letter of all methods lowercase does not apply to constructors, since the name of the constructor must match the name of the class exactly.

Like any method, the constructor can have arguments to allow you to specify how an object is created. The above example can easily be changed so the constructor takes an argument:

//: c04:SimpleConstructor2.java
// Constructors can have arguments.

class Rock2 {
  Rock2(int i) {
    System.out.println(
      "Creating Rock number " + i);
  }
}

public class SimpleConstructor2 {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock2(i);
  }
} ///:~

Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you would create a Tree object like this:

Tree t = new Tree(12);  // 12-foot tree

If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way.

Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from definition. In Java, definition and initialization are unified concepts—you can’t have one without the other.

The constructor is an unusual type of method because it has no return value. This is distinctly different from a void return value, in which the method returns nothing but you still have the option to make it return something else. Constructors return nothing and you don’t have an option. If there was a return value, and if you could select your own, the compiler would somehow need to know what to do with that return value.

Method overloading

One of the important features in any programming language is the use of names. When you create an object, you give a name to a region of storage. A method is a name for an action. By using names to describe your system, you create a program that is easier for people to understand and change. It’s a lot like writing prose—the goal is to communicate with your readers.

You refer to all objects and methods by using names. Well-chosen names make it easier for you and others to understand your code.

A problem arises when mapping the concept of nuance in human language onto a programming language. Often, the same word expresses a number of different meanings—it’s overloaded. This is useful, especially when it comes to trivial differences. You say “wash the shirt,” “wash the car,” and “wash the dog.” It would be silly to be forced to say, “shirtWash the shirt,” “carWash the car,” and “dogWash the dog” just so the listener doesn’t need to make any distinction about the action performed. Most human languages are redundant, so even if you miss a few words, you can still determine the meaning. We don’t need unique identifiers—we can deduce meaning from context.

Most programming languages (C in particular) require you to have a unique identifier for each function. So you could not have one function called print( ) for printing integers and another called print( ) for printing floats—each function requires a unique name.

In Java (and C++), another factor forces the overloading of method names: the constructor. Because the constructor’s name is predetermined by the name of the class, there can be only one constructor name. But what if you want to create an object in more than one way? For example, suppose you build a class that can initialize itself in a standard way or by reading information from a file. You need two constructors, one that takes no arguments (the default constructor, also called the no-arg constructor), and one that takes a String as an argument, which is the name of the file from which to initialize the object. Both are constructors, so they must have the same name—the name of the class. Thus, method overloading is essential to allow the same method name to be used with different argument types. And although method overloading is a must for constructors, it’s a general convenience and can be used with any method.

Here’s an example that shows both overloaded constructors and overloaded ordinary methods:

//: c04:Overloading.java
// Demonstration of both constructor
// and ordinary method overloading.
import java.util.*;

class Tree {
  int height;
  Tree() {
    prt("Planting a seedling");
    height = 0;
  }
  Tree(int i) {
    prt("Creating new Tree that is "
        + i + " feet tall");
    height = i;
  }
  void info() {
    prt("Tree is " + height
        + " feet tall");
  }
  void info(String s) {
    prt(s + ": Tree is "
        + height + " feet tall");
  }
  static void prt(String s) {
    System.out.println(s);
  }
}

public class Overloading {
  public static void main(String[] args) {
    for(int i = 0; i < 5; i++) {
      Tree t = new Tree(i);
      t.info();
      t.info("overloaded method");
    }
    // Overloaded constructor:
    new Tree();
  }
} ///:~

A Tree object can be created either as a seedling, with no argument, or as a plant grown in a nursery, with an existing height. To support this, there are two constructors, one that takes no arguments (we call constructors that take no arguments default constructors [28] ) and one that takes the existing height.

You might also want to call the info( ) method in more than one way. For example, with a String argument if you have an extra message you want printed, and without if you have nothing more to say. It would seem strange to give two separate names to what is obviously the same concept. Fortunately, method overloading allows you to use the same name for both.

Distinguishing overloaded methods

If the methods have the same name, how can Java know which method you mean? There’s a simple rule: each overloaded method must take a unique list of argument types.

If you think about this for a second, it makes sense: how else could a programmer tell the difference between two methods that have the same name, other than by the types of their arguments?

Even differences in the ordering of arguments are sufficient to distinguish two methods: (Although you don’t normally want to take this approach, as it produces difficult-to-maintain code.)

//: c04:OverloadingOrder.java
// Overloading based on the order of
// the arguments.

public class OverloadingOrder {
  static void print(String s, int i) {
    System.out.println(
      "String: " + s +
      ", int: " + i);
  }
  static void print(int i, String s) {
    System.out.println(
      "int: " + i +
      ", String: " + s);
  }
  public static void main(String[] args) {
    print("String first", 11);
    print(99, "Int first");
  }
} ///:~

The two print( ) methods have identical arguments, but the order is different, and that’s what makes them distinct.

Overloading with primitives

A primitive can be automatically promoted from a smaller type to a larger one and this can be slightly confusing in combination with overloading. The following example demonstrates what happens when a primitive is handed to an overloaded method:

//: c04:PrimitiveOverloading.java
// Promotion of primitives and overloading.

public class PrimitiveOverloading {
  // boolean can't be automatically converted
  static void prt(String s) {
    System.out.println(s);
  }

  void f1(char x) { prt("f1(char)"); }
  void f1(byte x) { prt("f1(byte)"); }
  void f1(short x) { prt("f1(short)"); }
  void f1(int x) { prt("f1(int)"); }
  void f1(long x) { prt("f1(long)"); }
  void f1(float x) { prt("f1(float)"); }
  void f1(double x) { prt("f1(double)"); }

  void f2(byte x) { prt("f2(byte)"); }
  void f2(short x) { prt("f2(short)"); }
  void f2(int x) { prt("f2(int)"); }
  void f2(long x) { prt("f2(long)"); }
  void f2(float x) { prt("f2(float)"); }
  void f2(double x) { prt("f2(double)"); }

  void f3(short x) { prt("f3(short)"); }
  void f3(int x) { prt("f3(int)"); }
  void f3(long x) { prt("f3(long)"); }
  void f3(float x) { prt("f3(float)"); }
  void f3(double x) { prt("f3(double)"); }

  void f4(int x) { prt("f4(int)"); }
  void f4(long x) { prt("f4(long)"); }
  void f4(float x) { prt("f4(float)"); }
  void f4(double x) { prt("f4(double)"); }

  void f5(long x) { prt("f5(long)"); }
  void f5(float x) { prt("f5(float)"); }
  void f5(double x) { prt("f5(double)"); }

  void f6(float x) { prt("f6(float)"); }
  void f6(double x) { prt("f6(double)"); }

  void f7(double x) { prt("f7(double)"); }

  void testConstVal() {
    prt("Testing with 5");
    f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5);
  }
  void testChar() {
    char x = 'x';
    prt("char argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testByte() {
    byte x = 0;
    prt("byte argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testShort() {
    short x = 0;
    prt("short argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testInt() {
    int x = 0;
    prt("int argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testLong() {
    long x = 0;
    prt("long argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testFloat() {
    float x = 0;
    prt("float argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  void testDouble() {
    double x = 0;
    prt("double argument:");
    f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
  }
  public static void main(String[] args) {
    PrimitiveOverloading p =
      new PrimitiveOverloading();
    p.testConstVal();
    p.testChar();
    p.testByte();
    p.testShort();
    p.testInt();
    p.testLong();
    p.testFloat();
    p.testDouble();
  }
} ///:~

If you view the output of this program, you’ll see that the constant value 5 is treated as an int, so if an overloaded method is available that takes an int it is used. In all other cases, if you have a data type that is smaller than the argument in the method, that data type is promoted. char produces a slightly different effect, since if it doesn’t find an exact char match, it is promoted to int.

What happens if your argument is bigger than the argument expected by the overloaded method? A modification of the above program gives the answer:

//: c04:Demotion.java
// Demotion of primitives and overloading.

public class Demotion {
  static void prt(String s) {
    System.out.println(s);
  }

  void f1(char x) { prt("f1(char)"); }
  void f1(byte x) { prt("f1(byte)"); }
  void f1(short x) { prt("f1(short)"); }
  void f1(int x) { prt("f1(int)"); }
  void f1(long x) { prt("f1(long)"); }
  void f1(float x) { prt("f1(float)"); }
  void f1(double x) { prt("f1(double)"); }

  void f2(char x) { prt("f2(char)"); }
  void f2(byte x) { prt("f2(byte)"); }
  void f2(short x) { prt("f2(short)"); }
  void f2(int x) { prt("f2(int)"); }
  void f2(long x) { prt("f2(long)"); }
  void f2(float x) { prt("f2(float)"); }

  void f3(char x) { prt("f3(char)"); }
  void f3(byte x) { prt("f3(byte)"); }
  void f3(short x) { prt("f3(short)"); }
  void f3(int x) { prt("f3(int)"); }
  void f3(long x) { prt("f3(long)"); }

  void f4(char x) { prt("f4(char)"); }
  void f4(byte x) { prt("f4(byte)"); }
  void f4(short x) { prt("f4(short)"); }
  void f4(int x) { prt("f4(int)"); }

  void f5(char x) { prt("f5(char)"); }
  void f5(byte x) { prt("f5(byte)"); }
  void f5(short x) { prt("f5(short)"); }

  void f6(char x) { prt("f6(char)"); }
  void f6(byte x) { prt("f6(byte)"); }

  void f7(char x) { prt("f7(char)"); }

  void testDouble() {
    double x = 0;
    prt("double argument:");
    f1(x);f2((float)x);f3((long)x);f4((int)x);
    f5((short)x);f6((byte)x);f7((char)x);
  }
  public static void main(String[] args) {
    Demotion p = new Demotion();
    p.testDouble();
  }
} ///:~

Here, the methods take narrower primitive values. If your argument is wider then you must cast to the necessary type using the type name in parentheses. If you don’t do this, the compiler will issue an error message.

You should be aware that this is a narrowing conversion, which means you might lose information during the cast. This is why the compiler forces you to do it—to flag the narrowing conversion.

Overloading on return values

It is common to wonder “Why only class names and method argument lists? Why not distinguish between methods based on their return values?” For example, these two methods, which have the same name and arguments, are easily distinguished from each other:

void f() {}
int f() {}

This works fine when the compiler can unequivocally determine the meaning from the context, as in int x = f( ). However, you can call a method and ignore the return value; this is often referred to as calling a method for its side effect since you don’t care about the return value but instead want the other effects of the method call. So if you call the method this way:

f();

how can Java determine which f( ) should be called? And how could someone reading the code see it? Because of this sort of problem, you cannot use return value types to distinguish overloaded methods.

Default constructors

As mentioned previously, a default constructor (a.k.a. a “no-arg” constructor) is one without arguments, used to create a “vanilla object.” If you create a class that has no constructors, the compiler will automatically create a default constructor for you. For example:

//: c04:DefaultConstructor.java

class Bird {
  int i;
}

public class DefaultConstructor {
  public static void main(String[] args) {
    Bird nc = new Bird(); // default!
  }
} ///:~

The line

new Bird();

creates a new object and calls the default constructor, even though one was not explicitly defined. Without it we would have no method to call to build our object. However, if you define any constructors (with or without arguments), the compiler will not synthesize one for you:

class Bush {
  Bush(int i) {}
  Bush(double d) {}
}

Now if you say:

new Bush();

the compiler will complain that it cannot find a constructor that matches. It’s as if when you don’t put in any constructors, the compiler says “You are bound to need some constructor, so let me make one for you.” But if you write a constructor, the compiler says “You’ve written a constructor so you know what you’re doing; if you didn’t put in a default it’s because you meant to leave it out.”

The this keyword

If you have two objects of the same type called a and b, you might wonder how it is that you can call a method f( ) for both those objects:

class Banana { void f(int i) { /* ... */ } }
Banana a = new Banana(), b = new Banana();
a.f(1);
b.f(2);

If there’s only one method called f( ), how can that method know whether it’s being called for the object a or b?

To allow you to write the code in a convenient object-oriented syntax in which you “send a message to an object,” the compiler does some undercover work for you. There’s a secret first argument passed to the method f( ), and that argument is the reference to the object that’s being manipulated. So the two method calls above become something like:

Banana.f(a,1);
Banana.f(b,2);

This is internal and you can’t write these expressions and get the compiler to accept them, but it gives you an idea of what’s happening.

Suppose you’re inside a method and you’d like to get the reference to the current object. Since that reference is passed secretly by the compiler, there’s no identifier for it. However, for this purpose there’s a keyword: this. The this keyword—which can be used only inside a method—produces the reference to the object the method has been called for. You can treat this reference just like any other object reference. Keep in mind that if you’re calling a method of your class from within another method of your class, you don’t need to use this; you simply call the method. The current this reference is automatically used for the other method. Thus you can say:

class Apricot {
  void pick() { /* ... */ }
  void pit() { pick(); /* ... */ }
}

Inside pit( ), you could say this.pick( ) but there’s no need to. The compiler does it for you automatically. The this keyword is used only for those special cases in which you need to explicitly use the reference to the current object. For example, it’s often used in return statements when you want to return the reference to the current object:

//: c04:Leaf.java
// Simple use of the "this" keyword.

public class Leaf {
  int i = 0;
  Leaf increment() {
    i++;
    return this;
  }
  void print() {
    System.out.println("i = " + i);
  }
  public static void main(String[] args) {
    Leaf x = new Leaf();
    x.increment().increment().increment().print();
  }
} ///:~

Because increment( ) returns the reference to the current object via the this keyword, multiple operations can easily be performed on the same object.

Calling constructors from constructors

When you write several constructors for a class, there are times when you’d like to call one constructor from another to avoid duplicating code. You can do this using the this keyword.

Normally, when you say this, it is in the sense of “this object” or “the current object,” and by itself it produces the reference to the current object. In a constructor, the this keyword takes on a different meaning when you give it an argument list: it makes an explicit call to the constructor that matches that argument list. Thus you have a straightforward way to call other constructors:

//: c04:Flower.java
// Calling constructors with "this."

public class Flower {
  int petalCount = 0;
  String s = new String("null");
  Flower(int petals) {
    petalCount = petals;
    System.out.println(
      "Constructor w/ int arg only, petalCount= "
      + petalCount);
  }
  Flower(String ss) {
    System.out.println(
      "Constructor w/ String arg only, s=" + ss);
    s = ss;
  }
  Flower(String s, int petals) {
    this(petals);
//!    this(s); // Can't call two!
    this.s = s; // Another use of "this"
    System.out.println("String & int args");
  }
  Flower() {
    this("hi", 47);
    System.out.println(
      "default constructor (no args)");
  }
  void print() {
//!    this(11); // Not inside non-constructor!
    System.out.println(
      "petalCount = " + petalCount + " s = "+ s);
  }
  public static void main(String[] args) {
    Flower x = new Flower();
    x.print();
  }
} ///:~

The constructor Flower(String s, int petals) shows that, while you can call one constructor using this, you cannot call two. In addition, the constructor call must be the first thing you do or you’ll get a compiler error message.

This example also shows another way you’ll see this used. Since the name of the argument s and the name of the member data s are the same, there’s an ambiguity. You can resolve it by saying this.s to refer to the member data. You’ll often see this form used in Java code, and it’s used in numerous places in this book.

In print( ) you can see that the compiler won’t let you call a constructor from inside any method other than a constructor.

The meaning of static

With the this keyword in mind, you can more fully understand what it means to make a method static. It means that there is no this for that particular method. You cannot call non-static methods from inside static methods [29] (although the reverse is possible), and you can call a static method for the class itself, without any object. In fact, that’s primarily what a static method is for. It’s as if you’re creating the equivalent of a global function (from C). Except global functions are not permitted in Java, and putting the static method inside a class allows it access to other static methods and to static fields.

Some people argue that static methods are not object-oriented since they do have the semantics of a global function; with a static method you don’t send a message to an object, since there’s no this. This is probably a fair argument, and if you find yourself using a lot of static methods you should probably rethink your strategy. However, statics are pragmatic and there are times when you genuinely need them, so whether or not they are “proper OOP” should be left to the theoreticians. Indeed, even Smalltalk has the equivalent in its “class methods.”

Cleanup: finalization and
garbage collection

Programmers know about the importance of initialization, but often forget the importance of cleanup. After all, who needs to clean up an int? But with libraries, simply “letting go” of an object once you’re done with it is not always safe. Of course, Java has the garbage collector to reclaim the memory of objects that are no longer used. Now consider a very unusual case. Suppose your object allocates “special” memory without using new. The garbage collector knows only how to release memory allocated with new, so it won’t know how to release the object’s “special” memory. To handle this case, Java provides a method called finalize( ) that you can define for your class. Here’s how it’s supposed to work. When the garbage collector is ready to release the storage used for your object, it will first call finalize( ), and only on the next garbage-collection pass will it reclaim the object’s memory. So if you choose to use finalize( ), it gives you the ability to perform some important cleanup at the time of garbage collection.

This is a potential programming pitfall because some programmers, especially C++ programmers, might initially mistake finalize( ) for the destructor in C++, which is a function that is always called when an object is destroyed. But it is important to distinguish between C++ and Java here, because in C++ objects always get destroyed (in a bug-free program), whereas in Java objects do not always get garbage-collected. Or, put another way:

Garbage collection is not destruction.

If you remember this, you will stay out of trouble. What it means is that if there is some activity that must be performed before you no longer need an object, you must perform that activity yourself. Java has no destructor or similar concept, so you must create an ordinary method to perform this cleanup. For example, suppose in the process of creating your object it draws itself on the screen. If you don’t explicitly erase its image from the screen, it might never get cleaned up. If you put some kind of erasing functionality inside finalize( ), then if an object is garbage-collected, the image will first be removed from the screen, but if it isn’t, the image will remain. So a second point to remember is:

Your objects might not get garbage-collected.

You might find that the storage for an object never gets released because your program never nears the point of running out of storage. If your program completes and the garbage collector never gets around to releasing the storage for any of your objects, that storage will be returned to the operating system en masse as the program exits. This is a good thing, because garbage collection has some overhead, and if you never do it you never incur that expense.

What is finalize( ) for?

You might believe at this point that you should not use finalize( ) as a general-purpose cleanup method. What good is it?

A third point to remember is:

Garbage collection is only about memory.

That is, the sole reason for the existence of the garbage collector is to recover memory that your program is no longer using. So any activity that is associated with garbage collection, most notably your finalize( ) method, must also be only about memory and its deallocation.

Does this mean that if your object contains other objects finalize( ) should explicitly release those objects? Well, no—the garbage collector takes care of the release of all object memory regardless of how the object is created. It turns out that the need for finalize( ) is limited to special cases, in which your object can allocate some storage in some way other than creating an object. But, you might observe, everything in Java is an object so how can this be?

It would seem that finalize( ) is in place because of the possibility that you’ll do something C-like by allocating memory using a mechanism other than the normal one in Java. This can happen primarily through native methods, which are a way to call non-Java code from Java. (Native methods are discussed in Appendix B.) C and C++ are the only languages currently supported by native methods, but since they can call subprograms in other languages, you can effectively call anything. Inside the non-Java code, C’s malloc( ) family of functions might be called to allocate storage, and unless you call free( ) that storage will not be released, causing a memory leak. Of course, free( ) is a C and C++ function, so you’d need to call it in a native method inside your finalize( ).

After reading this, you probably get the idea that you won’t use finalize( ) much. You’re correct; it is not the appropriate place for normal cleanup to occur. So where should normal cleanup be performed?

You must perform cleanup

To clean up an object, the user of that object must call a cleanup method at the point the cleanup is desired. This sounds pretty straightforward, but it collides a bit with the C++ concept of the destructor. In C++, all objects are destroyed. Or rather, all objects should be destroyed. If the C++ object is created as a local (i.e., on the stack—not possible in Java), then the destruction happens at the closing curly brace of the scope in which the object was created. If the object was created using new (like in Java) the destructor is called when the programmer calls the C++ operator delete (which doesn’t exist in Java). If the C++ programmer forgets to call delete, the destructor is never called and you have a memory leak, plus the other parts of the object never get cleaned up. This kind of bug can be very difficult to track down.

In contrast, Java doesn’t allow you to create local objects—you must always use new. But in Java, there’s no “delete” to call to release the object since the garbage collector releases the storage for you. So from a simplistic standpoint you could say that because of garbage collection, Java has no destructor. You’ll see as this book progresses, however, that the presence of a garbage collector does not remove the need for or utility of destructors. (And you should never call finalize( ) directly, so that’s not an appropriate avenue for a solution.) If you want some kind of cleanup performed other than storage release you must still explicitly call an appropriate method in Java, which is the equivalent of a C++ destructor without the convenience.

One of the things finalize( ) can be useful for is observing the process of garbage collection. The following example shows you what’s going on and summarizes the previous descriptions of garbage collection:

//: c04:Garbage.java
// Demonstration of the garbage
// collector and finalization

class Chair {
  static boolean gcrun = false;
  static boolean f = false;
  static int created = 0;
  static int finalized = 0;
  int i;
  Chair() {
    i = ++created;
    if(created == 47)
      System.out.println("Created 47");
  }
  public void finalize() {
    if(!gcrun) {
      // The first time finalize() is called:
      gcrun = true;
      System.out.println(
        "Beginning to finalize after " +
        created + " Chairs have been created");
    }
    if(i == 47) {
      System.out.println(
        "Finalizing Chair #47, " +
        "Setting flag to stop Chair creation");
      f = true;
    }
    finalized++;
    if(finalized >= created)
      System.out.println(
        "All " + finalized + " finalized");
  }
}

public class Garbage {
  public static void main(String[] args) {
    // As long as the flag hasn't been set,
    // make Chairs and Strings:
    while(!Chair.f) {
      new Chair();
      new String("To take up space");
    }
    System.out.println(
      "After all Chairs have been created:\n" +
      "total created = " + Chair.created +
      ", total finalized = " + Chair.finalized);
    // Optional arguments force garbage
    // collection & finalization:
    if(args.length > 0) {
      if(args[0].equals("gc") ||
         args[0].equals("all")) {
        System.out.println("gc():");
        System.gc();
      }
      if(args[0].equals("finalize") ||
         args[0].equals("all")) {
        System.out.println("runFinalization():");
        System.runFinalization();
      }
    }
    System.out.println("bye!");
  }
} ///:~

The above program creates many Chair objects, and at some point after the garbage collector begins running, the program stops creating Chairs. Since the garbage collector can run at any time, you don’t know exactly when it will start up, so there’s a flag called gcrun to indicate whether the garbage collector has started running yet. A second flag f is a way for Chair to tell the main( ) loop that it should stop making objects. Both of these flags are set within finalize( ), which is called during garbage collection.

Two other static variables, created and finalized, keep track of the number of objs created versus the number that get finalized by the garbage collector. Finally, each Chair has its own (non-static) int i so it can keep track of what number it is. When Chair number 47 is finalized, the flag is set to true to bring the process of Chair creation to a stop.

All this happens in main( ), in the loop

    while(!Chair.f) {
      new Chair();
      new String("To take up space");
    }

You might wonder how this loop could ever finish, since there’s nothing inside the loop that changes the value of Chair.f. However, the finalize( ) process will, eventually, when it finalizes number 47.

The creation of a String object during each iteration is simply extra storage being allocated to encourage the garbage collector to kick in, which it will do when it starts to get nervous about the amount of memory available.

When you run the program, you provide a command-line argument of “gc,” “finalize,” or “all.” The “gc” argument will call the System.gc( ) method (to force execution of the garbage collector). Using the “finalize” argument calls System.runFinalization( ) which—in theory—will cause any unfinalized objects to be finalized. And “all” causes both methods to be called.

The behavior of this program and the version in the first edition of this book shows that the whole issue of garbage collection and finalization has been evolving, with much of the evolution happening behind closed doors. In fact, by the time you read this, the behavior of the program may have changed once again.

If System.gc( ) is called, then finalization happens to all the objects. This was not necessarily the case with previous implementations of the JDK, although the documentation claimed otherwise. In addition, you’ll see that it doesn’t seem to make any difference whether System.runFinalization( ) is called.

However, you will see that only if System.gc( ) is called after all the objects are created and discarded will all the finalizers be called. If you do not call System.gc( ), then only some of the objects will be finalized. In Java 1.1, a method System.runFinalizersOnExit( ) was introduced that caused programs to run all the finalizers as they exited, but the design turned out to be buggy and the method was deprecated. This is yet another clue that the Java designers were thrashing about trying to solve the garbage collection and finalization problem. We can only hope that things have been worked out in Java 2.

The preceding program shows that the promise that finalizers will always be run holds true, but only if you explicitly force it to happen yourself. If you don’t cause System.gc( ) to be called, you’ll get an output like this:

Created 47
Beginning to finalize after 3486 Chairs have been created
Finalizing Chair #47, Setting flag to stop Chair creation
After all Chairs have been created:
total created = 3881, total finalized = 2684
bye!

Thus, not all finalizers get called by the time the program completes. If System.gc( ) is called, it will finalize and destroy all the objects that are no longer in use up to that point.

Remember that neither garbage collection nor finalization is guaranteed. If the Java Virtual Machine (JVM) isn’t close to running out of memory, then it will (wisely) not waste time recovering memory through garbage collection.

The death condition

In general, you can’t rely on finalize( ) being called, and you must create separate “cleanup” functions and call them explicitly. So it appears that finalize( ) is only useful for obscure memory cleanup that most programmers will never use. However, there is a very interesting use of finalize( ) which does not rely on it being called every time. This is the verification of the death condition [30] of an object.

At the point that you’re no longer interested in an object—when it’s ready to be cleaned up—that object should be in a state whereby its memory can be safely released. For example, if the object represents an open file, that file should be closed by the programmer before the object is garbage-collected. If any portions of the object are not properly cleaned up, then you have a bug in your program that could be very difficult to find. The value of finalize( ) is that it can be used to discover this condition, even if it isn’t always called. If one of the finalizations happens to reveal the bug, then you discover the problem, which is all you really care about.

Here’s a simple example of how you might use it:

//: c04:DeathCondition.java
// Using finalize() to detect an object that
// hasn't been properly cleaned up.

class Book {
  boolean checkedOut = false;
  Book(boolean checkOut) {
    checkedOut = checkOut;
  }
  void checkIn() {
    checkedOut = false;
  }
  public void finalize() {
    if(checkedOut)
      System.out.println("Error: checked out");
  }
}

public class DeathCondition {
  public static void main(String[] args) {
    Book novel = new Book(true);
    // Proper cleanup:
    novel.checkIn();
    // Drop the reference, forget to clean up:
    new Book(true);
    // Force garbage collection & finalization:
    System.gc();
  }
} ///:~

The death condition is that all Book objects are supposed to be checked in before they are garbage-collected, but in main( ) a programmer error doesn’t check in one of the books. Without finalize( ) to verify the death condition, this could be a difficult bug to find.

Note that System.gc( ) is used to force finalization (and you should do this during program development to speed debugging). But even if it isn’t, it’s highly probable that the errant Book will eventually be discovered through repeated executions of the program (assuming the program allocates enough storage to cause the garbage collector to execute).

How a garbage collector works

If you come from a programming language where allocating objects on the heap is expensive, you may naturally assume that Java’s scheme of allocating everything (except primitives) on the heap is expensive. However, it turns out that the garbage collector can have a significant impact on increasing the speed of object creation. This might sound a bit odd at first—that storage release affects storage allocation—but it’s the way some JVMs work and it means that allocating storage for heap objects in Java can be nearly as fast as creating storage on the stack in other languages.

For example, you can think of the C++ heap as a yard where each object stakes out its own piece of turf. This real estate can become abandoned sometime later and must be reused. In some JVMs, the Java heap is quite different; it’s more like a conveyor belt that moves forward every time you allocate a new object. This means that object storage allocation is remarkably rapid. The “heap pointer” is simply moved forward into virgin territory, so it’s effectively the same as C++’s stack allocation. (Of course, there’s a little extra overhead for bookkeeping but it’s nothing like searching for storage.)

Now you might observe that the heap isn’t in fact a conveyor belt, and if you treat it that way you’ll eventually start paging memory a lot (which is a big performance hit) and later run out. The trick is that the garbage collector steps in and while it collects the garbage it compacts all the objects in the heap so that you’ve effectively moved the “heap pointer” closer to the beginning of the conveyor belt and further away from a page fault. The garbage collector rearranges things and makes it possible for the high-speed, infinite-free-heap model to be used while allocating storage.

To understand how this works, you need to get a little better idea of the way the different garbage collector (GC) schemes work. A simple but slow GC technique is reference counting. This means that each object contains a reference counter, and every time a reference is attached to an object the reference count is increased. Every time a reference goes out of scope or is set to null, the reference count is decreased. Thus, managing reference counts is a small but constant overhead that happens throughout the lifetime of your program. The garbage collector moves through the entire list of objects and when it finds one with a reference count of zero it releases that storage. The one drawback is that if objects circularly refer to each other they can have nonzero reference counts while still being garbage. Locating such self-referential groups requires significant extra work for the garbage collector. Reference counting is commonly used to explain one kind of garbage collection but it doesn’t seem to be used in any JVM implementations.

In faster schemes, garbage collection is not based on reference counting. Instead, it is based on the idea that any nondead object must ultimately be traceable back to a reference that lives either on the stack or in static storage. The chain might go through several layers of objects. Thus, if you start in the stack and the static storage area and walk through all the references you’ll find all the live objects. For each reference that you find, you must trace into the object that it points to and then follow all the references in that object, tracing into the objects they point to, etc., until you’ve moved through the entire web that originated with the reference on the stack or in static storage. Each object that you move through must still be alive. Note that there is no problem with detached self-referential groups—these are simply not found, and are therefore automatically garbage.

In the approach described here, the JVM uses an adaptive garbage-collection scheme, and what it does with the live objects that it locates depends on the variant currently being used. One of these variants is stop-and-copy. This means that—for reasons that will become apparent—the program is first stopped (this is not a background collection scheme). Then, each live object that is found is copied from one heap to another, leaving behind all the garbage. In addition, as the objects are copied into the new heap they are packed end-to-end, thus compacting the new heap (and allowing new storage to simply be reeled off the end as previously described).

Of course, when an object is moved from one place to another, all references that point at (i.e., that reference) the object must be changed. The reference that goes from the heap or the static storage area to the object can be changed right away, but there can be other references pointing to this object that will be encountered later during the “walk.” These are fixed up as they are found (you could imagine a table that maps old addresses to new ones).

There are two issues that make these so-called “copy collectors” inefficient. The first is the idea that you have two heaps and you slosh all the memory back and forth between these two separate heaps, maintaining twice as much memory as you actually need. Some JVMs deal with this by allocating the heap in chunks as needed and simply copying from one chunk to another.

The second issue is the copying. Once your program becomes stable it might be generating little or no garbage. Despite that, a copy collector will still copy all the memory from one place to another, which is wasteful. To prevent this, some JVMs detect that no new garbage is being generated and switch to a different scheme (this is the “adaptive” part). This other scheme is called mark and sweep, and it’s what earlier versions of Sun’s JVM used all the time. For general use, mark and sweep is fairly slow, but when you know you’re generating little or no garbage it’s fast.

Mark and sweep follows the same logic of starting from the stack and static storage and tracing through all the references to find live objects. However, each time it finds a live object that object is marked by setting a flag in it, but the object isn’t collected yet. Only when the marking process is finished does the sweep occur. During the sweep, the dead objects are released. However, no copying happens, so if the collector chooses to compact a fragmented heap it does so by shuffling objects around.

The “stop-and-copy” refers to the idea that this type of garbage collection is not done in the background; instead, the program is stopped while the GC occurs. In the Sun literature you’ll find many references to garbage collection as a low-priority background process, but it turns out that the GC was not implemented that way, at least in earlier versions of the Sun JVM. Instead, the Sun garbage collector ran when memory got low. In addition, mark-and-sweep requires that the program be stopped.

As previously mentioned, in the JVM described here memory is allocated in big blocks. If you allocate a large object, it gets its own block. Strict stop-and-copy requires copying every live object from the source heap to a new heap before you could free the old one, which translates to lots of memory. With blocks, the GC can typically use dead blocks to copy objects to as it collects. Each block has a generation count to keep track of whether it’s alive. In the normal case, only the blocks created since the last GC are compacted; all other blocks get their generation count bumped if they have been referenced from somewhere. This handles the normal case of lots of short-lived temporary objects. Periodically, a full sweep is made—large objects are still not copied (just get their generation count bumped) and blocks containing small objects are copied and compacted. The JVM monitors the efficiency of GC and if it becomes a waste of time because all objects are long-lived then it switches to mark-and-sweep. Similarly, the JVM keeps track of how successful mark-and-sweep is, and if the heap starts to become fragmented it switches back to stop-and-copy. This is where the “adaptive” part comes in, so you end up with a mouthful: “adaptive generational stop-and-copy mark-and-sweep.”

There are a number of additional speedups possible in a JVM. An especially important one involves the operation of the loader and Just-In-Time (JIT) compiler. When a class must be loaded (typically, the first time you want to create an object of that class), the .class file is located and the byte codes for that class are brought into memory. At this point, one approach is to simply JIT all the code, but this has two drawbacks: it takes a little more time, which, compounded throughout the life of the program, can add up; and it increases the size of the executable (byte codes are significantly more compact than expanded JIT code) and this might cause paging, which definitely slows down a program. An alternative approach is lazy evaluation, which means that the code is not JIT compiled until necessary. Thus, code that never gets executed might never get JIT compiled.

Member initialization

Java goes out of its way to guarantee that variables are properly initialized before they are used. In the case of variables that are defined locally to a method, this guarantee comes in the form of a compile-time error. So if you say:

  void f() {
    int i;
    i++;
  }

you’ll get an error message that says that i might not have been initialized. Of course, the compiler could have given i a default value, but it’s more likely that this is a programmer error and a default value would have covered that up. Forcing the programmer to provide an initialization value is more likely to catch a bug.

If a primitive is a data member of a class, however, things are a bit different. Since any method can initialize or use that data, it might not be practical to force the user to initialize it to its appropriate value before the data is used. However, it’s unsafe to leave it with a garbage value, so each primitive data member of a class is guaranteed to get an initial value. Those values can be seen here:

//: c04:InitialValues.java
// Shows default initial values.

class Measurement {
  boolean t;
  char c;
  byte b;
  short s;
  int i;
  long l;
  float f;
  double d;
  void print() {
    System.out.println(
      "Data type      Initial value\n" +
      "boolean        " + t + "\n" +
      "char           " + c + "\n" +
      "byte           " + b + "\n" +
      "short          " + s + "\n" +
      "int            " + i + "\n" +
      "long           " + l + "\n" +
      "float          " + f + "\n" +
      "double         " + d);
  }
}

public class InitialValues {
  public static void main(String[] args) {
    Measurement d = new Measurement();
    d.print();
    /* In this case you could also say:
    new Measurement().print();
    */
  }
} ///:~

The output of this program is:

Data type      Initial value
boolean        false
char
byte           0
short          0
int            0
long           0
float          0.0
double         0.0

The char value is a zero, which doesn’t print.

You’ll see later that when you define an object reference inside a class without initializing it to a new object, that reference is given a special value of null (which is a Java keyword).

You can see that even though the values are not specified, they automatically get initialized. So at least there’s no threat of working with uninitialized variables.

Specifying initialization

What happens if you want to give a variable an initial value? One direct way to do this is simply to assign the value at the point you define the variable in the class. (Notice you cannot do this in C++, although C++ novices always try.) Here the field definitions in class Measurement are changed to provide initial values:

class Measurement {
  boolean b = true;
  char c = 'x';
  byte B = 47;
  short s = 0xff;
  int i = 999;
  long l = 1;
  float f = 3.14f;
  double d = 3.14159;
  //. . .

You can also initialize nonprimitive objects in this same way. If Depth is a class, you can insert a variable and initialize it like so:

class Measurement {
  Depth o = new Depth();
  boolean b = true;
  // . . .

If you haven’t given o an initial value and you try to use it anyway, you’ll get a run-time error called an exception (covered in Chapter 10).

You can even call a method to provide an initialization value:

class CInit {
  int i = f();
  //...
}

This method can have arguments, of course, but those arguments cannot be other class members that haven’t been initialized yet. Thus, you can do this:

class CInit {
  int i = f();
  int j = g(i);
  //...
}

But you cannot do this:

class CInit {
  int j = g(i);
  int i = f();
  //...
}

This is one place in which the compiler, appropriately, does complain about forward referencing, since this has to do with the order of initialization and not the way the program is compiled.

This approach to initialization is simple and straightforward. It has the limitation that every object of type Measurement will get these same initialization values. Sometimes this is exactly what you need, but at other times you need more flexibility.

Constructor initialization

The constructor can be used to perform initialization, and this gives you greater flexibility in your programming since you can call methods and perform actions at run-time to determine the initial values. There’s one thing to keep in mind, however: you aren’t precluding the automatic initialization, which happens before the constructor is entered. So, for example, if you say:

class Counter {
  int i;
  Counter() { i = 7; }
  // . . .

then i will first be initialized to 0, then to 7. This is true with all the primitive types and with object references, including those that are given explicit initialization at the point of definition. For this reason, the compiler doesn’t try to force you to initialize elements in the constructor at any particular place, or before they are used—initialization is already guaranteed [31] .

Order of initialization

Within a class, the order of initialization is determined by the order that the variables are defined within the class. The variable definitions may be scattered throughout and in between method definitions, but the variables are initialized before any methods can be called—even the constructor. For example:

//: c04:OrderOfInitialization.java
// Demonstrates initialization order.

// When the constructor is called to create a
// Tag object, you'll see a message:
class Tag {
  Tag(int marker) {
    System.out.println("Tag(" + marker + ")");
  }
}

class Card {
  Tag t1 = new Tag(1); // Before constructor
  Card() {
    // Indicate we're in the constructor:
    System.out.println("Card()");
    t3 = new Tag(33); // Reinitialize t3
  }
  Tag t2 = new Tag(2); // After constructor
  void f() {
    System.out.println("f()");
  }
  Tag t3 = new Tag(3); // At end
}

public class OrderOfInitialization {
  public static void main(String[] args) {
    Card t = new Card();
    t.f(); // Shows that construction is done
  }
} ///:~

In Card, the definitions of the Tag objects are intentionally scattered about to prove that they’ll all get initialized before the constructor is entered or anything else can happen. In addition, t3 is reinitialized inside the constructor. The output is:

Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()

Thus, the t3 reference gets initialized twice, once before and once during the constructor call. (The first object is dropped, so it can be garbage-collected later.) This might not seem efficient at first, but it guarantees proper initialization—what would happen if an overloaded constructor were defined that did not initialize t3 and there wasn’t a “default” initialization for t3 in its definition?

Static data initialization

When the data is static the same thing happens; if it’s a primitive and you don’t initialize it, it gets the standard primitive initial values. If it’s a reference to an object, it’s null unless you create a new object and attach your reference to it.

If you want to place initialization at the point of definition, it looks the same as for non-statics. There’s only a single piece of storage for a static, regardless of how many objects are created. But the question arises of when the static storage gets initialized. An example makes this question clear:

//: c04:StaticInitialization.java
// Specifying initial values in a
// class definition.

class Bowl {
  Bowl(int marker) {
    System.out.println("Bowl(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

class Table {
  static Bowl b1 = new Bowl(1);
  Table() {
    System.out.println("Table()");
    b2.f(1);
  }
  void f2(int marker) {
    System.out.println("f2(" + marker + ")");
  }
  static Bowl b2 = new Bowl(2);
}

class Cupboard {
  Bowl b3 = new Bowl(3);
  static Bowl b4 = new Bowl(4);
  Cupboard() {
    System.out.println("Cupboard()");
    b4.f(2);
  }
  void f3(int marker) {
    System.out.println("f3(" + marker + ")");
  }
  static Bowl b5 = new Bowl(5);
}

public class StaticInitialization {
  public static void main(String[] args) {
    System.out.println(
      "Creating new Cupboard() in main");
    new Cupboard();
    System.out.println(
      "Creating new Cupboard() in main");
    new Cupboard();
    t2.f2(1);
    t3.f3(1);
  }
  static Table t2 = new Table();
  static Cupboard t3 = new Cupboard();
} ///:~

Bowl allows you to view the creation of a class, and Table and Cupboard create static members of Bowl scattered through their class definitions. Note that Cupboard creates a non-static Bowl b3 prior to the static definitions. The output shows what happens:

Bowl(1)
Bowl(2)
Table()
f(1)
Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
f2(1)
f3(1)

The static initialization occurs only if it’s necessary. If you don’t create a Table object and you never refer to Table.b1 or Table.b2, the static Bowl b1 and b2 will never be created. However, they are initialized only when the first Table object is created (or the first static access occurs). After that, the static objects are not reinitialized.

The order of initialization is statics first, if they haven’t already been initialized by a previous object creation, and then the non-static objects. You can see the evidence of this in the output.

It’s helpful to summarize the process of creating an object. Consider a class called Dog:

  1. The first time an object of type Dog is created, or the first time a static method or static field of class Dog is accessed, the Java interpreter must locate Dog.class, which it does by searching through the classpath.
  2. As Dog.class is loaded (creating a Class object, which you’ll learn about later), all of its static initializers are run. Thus, static initialization takes place only once, as the Class object is loaded for the first time.
  3. When you create a new Dog( ), the construction process for a Dog object first allocates enough storage for a Dog object on the heap.
  4. This storage is wiped to zero, automatically setting all the primitives in that Dog object to their default values (zero for numbers and the equivalent for boolean and char) and the references to null.
  5. Any initializations that occur at the point of field definition are executed.
  6. Constructors are executed. As you shall see in Chapter 6, this might actually involve a fair amount of activity, especially when inheritance is involved.

Explicit static initialization

Java allows you to group other static initializations inside a special “static construction clause” (sometimes called a static block) in a class. It looks like this:

class Spoon {
  static int i;
  static {
    i = 47;
  }
  // . . .

It appears to be a method, but it’s just the static keyword followed by a method body. This code, like other static initializations, is executed only once, the first time you make an object of that class or the first time you access a static member of that class (even if you never make an object of that class). For example:

//: c04:ExplicitStatic.java
// Explicit static initialization
// with the "static" clause.

class Cup {
  Cup(int marker) {
    System.out.println("Cup(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

class Cups {
  static Cup c1;
  static Cup c2;
  static {
    c1 = new Cup(1);
    c2 = new Cup(2);
  }
  Cups() {
    System.out.println("Cups()");
  }
}

public class ExplicitStatic {
  public static void main(String[] args) {
    System.out.println("Inside main()");
    Cups.c1.f(99);  // (1)
  }
  // static Cups x = new Cups();  // (2)
  // static Cups y = new Cups();  // (2)
} ///:~

The static initializers for Cups run when either the access of the static object c1 occurs on the line marked (1), or if line (1) is commented out and the lines marked (2) are uncommented. If both (1) and (2) are commented out, the static initialization for Cups never occurs. Also, it doesn’t matter if one or both of the lines marked (2) are uncommented; the static initialization only occurs once.

Non-static instance initialization

Java provides a similar syntax for initializing non-static variables for each object. Here’s an example:

//: c04:Mugs.java
// Java "Instance Initialization."

class Mug {
  Mug(int marker) {
    System.out.println("Mug(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

public class Mugs {
  Mug c1;
  Mug c2;
  {
    c1 = new Mug(1);
    c2 = new Mug(2);
    System.out.println("c1 & c2 initialized");
  }
  Mugs() {
    System.out.println("Mugs()");
  }
  public static void main(String[] args) {
    System.out.println("Inside main()");
    Mugs x = new Mugs();
  }
} ///:~

You can see that the instance initialization clause:

  {
    c1 = new Mug(1);
    c2 = new Mug(2);
    System.out.println("c1 & c2 initialized");
  }

looks exactly like the static initialization clause except for the missing static keyword. This syntax is necessary to support the initialization of anonymous inner classes (see Chapter 8).

Array initialization

Initializing arrays in C is error-prone and tedious. C++ uses aggregate initialization to make it much safer [32] . Java has no “aggregates” like C++, since everything is an object in Java. It does have arrays, and these are supported with array initialization.

An array is simply a sequence of either objects or primitives, all the same type and packaged together under one identifier name. Arrays are defined and used with the square-brackets indexing operator [ ]. To define an array you simply follow your type name with empty square brackets:

int[] a1;

You can also put the square brackets after the identifier to produce exactly the same meaning:

int a1[];

This conforms to expectations from C and C++ programmers. The former style, however, is probably a more sensible syntax, since it says that the type is “an int array.” That style will be used in this book.

The compiler doesn’t allow you to tell it how big the array is. This brings us back to that issue of “references.” All that you have at this point is a reference to an array, and there’s been no space allocated for the array. To create storage for the array you must write an initialization expression. For arrays, initialization can appear anywhere in your code, but you can also use a special kind of initialization expression that must occur at the point where the array is created. This special initialization is a set of values surrounded by curly braces. The storage allocation (the equivalent of using new) is taken care of by the compiler in this case. For example:

int[] a1 = { 1, 2, 3, 4, 5 };

So why would you ever define an array reference without an array?

int[] a2;

Well, it’s possible to assign one array to another in Java, so you can say:

a2 = a1;

What you’re really doing is copying a reference, as demonstrated here:

//: c04:Arrays.java
// Arrays of primitives.

public class Arrays {
  public static void main(String[] args) {
    int[] a1 = { 1, 2, 3, 4, 5 };
    int[] a2;
    a2 = a1;
    for(int i = 0; i < a2.length; i++)
      a2[i]++;
    for(int i = 0; i < a1.length; i++)
      System.out.println(
        "a1[" + i + "] = " + a1[i]);
  }
} ///:~

You can see that a1 is given an initialization value while a2 is not; a2 is assigned later—in this case, to another array.

There’s something new here: all arrays have an intrinsic member (whether they’re arrays of objects or arrays of primitives) that you can query—but not change—to tell you how many elements there are in the array. This member is length. Since arrays in Java, like C and C++, start counting from element zero, the largest element you can index is length - 1. If you go out of bounds, C and C++ quietly accept this and allow you to stomp all over your memory, which is the source of many infamous bugs. However, Java protects you against such problems by causing a run-time error (an exception, the subject of Chapter 10) if you step out of bounds. Of course, checking every array access costs time and code and there’s no way to turn it off, which means that array accesses might be a source of inefficiency in your program if they occur at a critical juncture. For Internet security and programmer productivity, the Java designers thought that this was a worthwhile trade-off.

What if you don’t know how many elements you’re going to need in your array while you’re writing the program? You simply use new to create the elements in the array. Here, new works even though it’s creating an array of primitives (new won’t create a nonarray primitive):

//: c04:ArrayNew.java
// Creating arrays with new.
import java.util.*;

public class ArrayNew {
  static Random rand = new Random();
  static int pRand(int mod) {
    return Math.abs(rand.nextInt()) % mod + 1;
  }
  public static void main(String[] args) {
    int[] a;
    a = new int[pRand(20)];
    System.out.println(
      "length of a = " + a.length);
    for(int i = 0; i < a.length; i++)
      System.out.println(
        "a[" + i + "] = " + a[i]);
  }
} ///:~

Since the size of the array is chosen at random (using the pRand( ) method), it’s clear that array creation is actually happening at run-time. In addition, you’ll see from the output of this program that array elements of primitive types are automatically initialized to “empty” values. (For numerics and char, this is zero, and for boolean, it’s false.)

Of course, the array could also have been defined and initialized in the same statement:

int[] a = new int[pRand(20)];

If you’re dealing with an array of nonprimitive objects, you must always use new. Here, the reference issue comes up again because what you create is an array of references. Consider the wrapper type Integer, which is a class and not a primitive:

//: c04:ArrayClassObj.java
// Creating an array of nonprimitive objects.
import java.util.*;

public class ArrayClassObj {
  static Random rand = new Random();
  static int pRand(int mod) {
    return Math.abs(rand.nextInt()) % mod + 1;
  }
  public static void main(String[] args) {
    Integer[] a = new Integer[pRand(20)];
    System.out.println(
      "length of a = " + a.length);
    for(int i = 0; i < a.length; i++) {
      a[i] = new Integer(pRand(500));
      System.out.println(
        "a[" + i + "] = " + a[i]);
    }
  }
} ///:~

Here, even after new is called to create the array:

Integer[] a = new Integer[pRand(20)];

it’s only an array of references, and not until the reference itself is initialized by creating a new Integer object is the initialization complete:

a[i] = new Integer(pRand(500));

If you forget to create the object, however, you’ll get an exception at run-time when you try to read the empty array location.

Take a look at the formation of the String object inside the print statements. You can see that the reference to the Integer object is automatically converted to produce a String representing the value inside the object.

It’s also possible to initialize arrays of objects using the curly-brace-enclosed list. There are two forms:

//: c04:ArrayInit.java
// Array initialization.

public class ArrayInit {
  public static void main(String[] args) {
    Integer[] a = {
      new Integer(1),
      new Integer(2),
      new Integer(3),
    };

    Integer[] b = new Integer[] {
      new Integer(1),
      new Integer(2),
      new Integer(3),
    };
  }
} ///:~

This is useful at times, but it’s more limited since the size of the array is determined at compile-time. The final comma in the list of initializers is optional. (This feature makes for easier maintenance of long lists.)

The second form of array initialization provides a convenient syntax to create and call methods that can produce the same effect as C’s variable argument lists (known as “varargs” in C). These can include unknown quantity of arguments as well as unknown types. Since all classes are ultimately inherited from the common root class Object (a subject you will learn more about as this book progresses), you can create a method that takes an array of Object and call it like this:

//: c04:VarArgs.java
// Using the array syntax to create
// variable argument lists.

class A { int i; }

public class VarArgs {
  static void f(Object[] x) {
    for(int i = 0; i < x.length; i++)
      System.out.println(x[i]);
  }
  public static void main(String[] args) {
    f(new Object[] {
        new Integer(47), new VarArgs(),
        new Float(3.14), new Double(11.11) });
    f(new Object[] {"one", "two", "three" });
    f(new Object[] {new A(), new A(), new A()});
  }
} ///:~

At this point, there’s not much you can do with these unknown objects, and this program uses the automatic String conversion to do something useful with each Object. In Chapter 12, which covers run-time type identification (RTTI), you’ll learn how to discover the exact type of such objects so that you can do something more interesting with them.

Multidimensional arrays

Java allows you to easily create multidimensional arrays:

//: c04:MultiDimArray.java
// Creating multidimensional arrays.
import java.util.*;

public class MultiDimArray {
  static Random rand = new Random();
  static int pRand(int mod) {
    return Math.abs(rand.nextInt()) % mod + 1;
  }
  static void prt(String s) {
    System.out.println(s);
  }
  public static void main(String[] args) {
    int[][] a1 = {
      { 1, 2, 3, },
      { 4, 5, 6, },
    };
    for(int i = 0; i < a1.length; i++)
      for(int j = 0; j < a1[i].length; j++)
        prt("a1[" + i + "][" + j +
            "] = " + a1[i][j]);
    // 3-D array with fixed length:
    int[][][] a2 = new int[2][2][4];
    for(int i = 0; i < a2.length; i++)
      for(int j = 0; j < a2[i].length; j++)
        for(int k = 0; k < a2[i][j].length;
            k++)
          prt("a2[" + i + "][" +
              j + "][" + k +
              "] = " + a2[i][j][k]);
    // 3-D array with varied-length vectors:
    int[][][] a3 = new int[pRand(7)][][];
    for(int i = 0; i < a3.length; i++) {
      a3[i] = new int[pRand(5)][];
      for(int j = 0; j < a3[i].length; j++)
        a3[i][j] = new int[pRand(5)];
    }
    for(int i = 0; i < a3.length; i++)
      for(int j = 0; j < a3[i].length; j++)
        for(int k = 0; k < a3[i][j].length;
            k++)
          prt("a3[" + i + "][" +
              j + "][" + k +
              "] = " + a3[i][j][k]);
    // Array of nonprimitive objects:
    Integer[][] a4 = {
      { new Integer(1), new Integer(2)},
      { new Integer(3), new Integer(4)},
      { new Integer(5), new Integer(6)},
    };
    for(int i = 0; i < a4.length; i++)
      for(int j = 0; j < a4[i].length; j++)
        prt("a4[" + i + "][" + j +
            "] = " + a4[i][j]);
    Integer[][] a5;
    a5 = new Integer[3][];
    for(int i = 0; i < a5.length; i++) {
      a5[i] = new Integer[3];
      for(int j = 0; j < a5[i].length; j++)
        a5[i][j] = new Integer(i*j);
    }
    for(int i = 0; i < a5.length; i++)
      for(int j = 0; j < a5[i].length; j++)
        prt("a5[" + i + "][" + j +
            "] = " + a5[i][j]);
  }
} ///:~

The code used for printing uses length so that it doesn’t depend on fixed array sizes.

The first example shows a multidimensional array of primitives. You delimit each vector in the array with curly braces:

    int[][] a1 = {
      { 1, 2, 3, },
      { 4, 5, 6, },
    };

Each set of square brackets moves you into the next level of the array.

The second example shows a three-dimensional array allocated with new. Here, the whole array is allocated at once:

int[][][] a2 = new int[2][2][4];

But the third example shows that each vector in the arrays that make up the matrix can be of any length:

    int[][][] a3 = new int[pRand(7)][][];
    for(int i = 0; i < a3.length; i++) {
      a3[i] = new int[pRand(5)][];
      for(int j = 0; j < a3[i].length; j++)
        a3[i][j] = new int[pRand(5)];
    }

The first new creates an array with a random-length first element and the rest undetermined. The second new inside the for loop fills out the elements but leaves the third index undetermined until you hit the third new.

You will see from the output that array values are automatically initialized to zero if you don’t give them an explicit initialization value.

You can deal with arrays of nonprimitive objects in a similar fashion, which is shown in the fourth example, demonstrating the ability to collect many new expressions with curly braces:

    Integer[][] a4 = {
      { new Integer(1), new Integer(2)},
      { new Integer(3), new Integer(4)},
      { new Integer(5), new Integer(6)},
    };

The fifth example shows how an array of nonprimitive objects can be built up piece by piece:

    Integer[][] a5;
    a5 = new Integer[3][];
    for(int i = 0; i < a5.length; i++) {
      a5[i] = new Integer[3];
      for(int j = 0; j < a5[i].length; j++)
        a5[i][j] = new Integer(i*j);
    }

The i*j is just to put an interesting value into the Integer.

Summary

This seemingly elaborate mechanism for initialization, the constructor, should give you a strong hint about the critical importance placed on initialization in the language. As Stroustrup was designing C++, one of the first observations he made about productivity in C was that improper initialization of variables causes a significant portion of programming problems. These kinds of bugs are hard to find, and similar issues apply to improper cleanup. Because constructors allow you to guarantee proper initialization and cleanup (the compiler will not allow an object to be created without the proper constructor calls), you get complete control and safety.

In C++, destruction is quite important because objects created with new must be explicitly destroyed. In Java, the garbage collector automatically releases the memory for all objects, so the equivalent cleanup method in Java isn’t necessary much of the time. In cases where you don’t need destructor-like behavior, Java’s garbage collector greatly simplifies programming, and adds much-needed safety in managing memory. Some garbage collectors can even clean up other resources like graphics and file handles. However, the garbage collector does add a run-time cost, the expense of which is difficult to put into perspective because of the overall slowness of Java interpreters at this writing. As this changes, we’ll be able to discover if the overhead of the garbage collector will preclude the use of Java for certain types of programs. (One of the issues is the unpredictability of the garbage collector.)

Because of the guarantee that all objects will be constructed, there’s actually more to the constructor than what is shown here. In particular, when you create new classes using either composition or inheritance the guarantee of construction also holds, and some additional syntax is necessary to support this. You’ll learn about composition, inheritance, and how they affect constructors in future chapters.

Exercises
  1. Create a class with a default constructor (one that takes no arguments) that prints a message. Create an object of this class.
  2. Add an overloaded constructor to Exercise 1 that takes a String argument and prints it along with your message.
  3. Create an array of object references of the class you created in Exercise 2, but don’t actually create objects to assign into the array. When you run the program, notice whether the initialization messages from the constructor calls are printed.
  4. Complete Exercise 3 by creating objects to attach to the array of references.
  5. Create an array of String objects and assign a string to each element. Print the array using a for loop.
  6. Create a class called Dog with an overloaded bark( ) method. This method should be overloaded based on various primitive data types, and print different types of barking, howling, etc., depending on which overloaded version is called. Write a main( ) that calls all the different versions.
  7. Modify Exercise 6 so that two of the overloaded methods have two arguments (of two different types), but in reversed order relative to each other. Verify that this works.
  8. Create a class without a constructor, and then create an object of that class in main( ) to verify that the default constructor is automatically synthesized.
  9. Create a class with two methods. Within the first method, call the second method twice: the first time without using this, and the second time using this.
  10. Create a class with two (overloaded) constructors. Using this, call the second constructor inside the first one.
  11. Create a class with a finalize( ) method that prints a message. In main( ), create an object of your class. Explain the behavior of your program.
  12. Modify Exercise 11 so that your finalize( ) will always be called.
  13. Create a class called Tank that can be filled and emptied, and has a death condition that it must be empty when the object is cleaned up. Write a finalize( ) that verifies this death condition. In main( ), test the possible scenarios that can occur when your Tank is used.
  14. Create a class containing an int and a char that are not initialized, and print their values to verify that Java performs default initialization.
  15. Create a class containing an uninitialized String reference. Demonstrate that this reference is initialized by Java to null.
  16. Create a class with a String field that is initialized at the point of definition, and another one that is initialized by the constructor. What is the difference between the two approaches?
  17. Create a class with a static String field that is initialized at the point of definition, and another one that is initialized by the static block. Add a static method that prints both fields and demonstrates that they are both initialized before they are used.
  18. Create a class with a String that is initialized using “instance initialization.” Describe a use for this feature (other than the one specified in this book).
  19. Write a method that creates and initializes a two-dimensional array of double. The size of the array is determined by the arguments of the method, and the initialization values are a range determined by beginning and ending values that are also arguments of the method. Create a second method that will print the array generated by the first method. In main( ) test the methods by creating and printing several different sizes of arrays.
  20. Repeat Exercise 19 for a three-dimensional array.
  21. Comment the line marked (1) in ExplicitStatic.java and verify that the static initialization clause is not called. Now uncomment one of the lines marked (2) and verify that the static initialization clause is called. Now uncomment the other line marked (2) and verify that static initialization only occurs once.
  22. Experiment with Garbage.java by running the program using the arguments “gc,” “finalize,” or “all.” Repeat the process and see if you detect any patterns in the output. Change the code so that System.runFinalization( ) is called before System.gc( ) and observe the results.

[28] In some of the Java literature from Sun they instead refer to these with the clumsy but descriptive name “no-arg constructors.” The term “default constructor” has been in use for many years and so I will use that.

[29] The one case in which this is possible occurs if you pass a reference to an object into the static method. Then, via the reference (which is now effectively this), you can call non-static methods and access non-static fields. But typically if you want to do something like this you’ll just make an ordinary, non-static method.

[30] A term coined by Bill Venners (www.artima.com) during a seminar that he and I were giving together.

[31] In contrast, C++ has the constructor initializer list that causes initialization to occur before entering the constructor body, and is enforced for objects. See Thinking in C++, 2nd edition (available on this book’s CD ROM and at www.BruceEckel.com).

[32] See Thinking in C++, 2nd edition for a complete description of C++ aggregate initialization.


5: Hiding the Implementation

A primary consideration in object-oriented design is “separating the things that change from the things that stay the same.”

This is particularly important for libraries. The user (client programmer) of that library must be able to rely on the part they use, and know that they won’t need to rewrite code if a new version of the library comes out. On the flip side, the library creator must have the freedom to make modifications and improvements with the certainty that the client programmer’s code won’t be affected by those changes.

This can be achieved through convention. For example, the library programmer must agree to not remove existing methods when modifying a class in the library, since that would break the client programmer’s code. The reverse situation is thornier, however. In the case of a data member, how can the library creator know which data members have been accessed by client programmers? This is also true with methods that are only part of the implementation of a class, and not meant to be used directly by the client programmer. But what if the library creator wants to rip out an old implementation and put in a new one? Changing any of those members might break a client programmer’s code. Thus the library creator is in a strait jacket and can’t change anything.

To solve this problem, Java provides access specifiers to allow the library creator to say what is available to the client programmer and what is not. The levels of access control from “most access” to “least access” are public, “friendly” (which has no keyword), protected, and private. From the previous paragraph you might think that, as a library designer, you’ll want to keep everything as “private” as possible, and expose only the methods that you want the client programmer to use. This is exactly right, even though it’s often counterintuitive for people who program in other languages (especially C) and are used to accessing everything without restriction. By the end of this chapter you should be convinced of the value of access control in Java.

The concept of a library of components and the control over who can access the components of that library is not complete, however. There’s still the question of how the components are bundled together into a cohesive library unit. This is controlled with the package keyword in Java, and the access specifiers are affected by whether a class is in the same package or in a separate package. So to begin this chapter, you’ll learn how library components are placed into packages. Then you’ll be able to understand the complete meaning of the access specifiers.

package: the library unit

A package is what you get when you use the import keyword to bring in an entire library, such as

import java.util.*;

This brings in the entire utility library that’s part of the standard Java distribution. Since, for example, the class ArrayList is in java.util, you can now either specify the full name java.util.ArrayList (which you can do without the import statement), or you can simply say ArrayList (because of the import).

If you want to bring in a single class, you can name that class in the import statement

import java.util.ArrayList;

Now you can use ArrayList with no qualification. However, none of the other classes in java.util are available.

The reason for all this importing is to provide a mechanism to manage “name spaces.” The names of all your class members are insulated from each other. A method f( ) inside a class A will not clash with an f( ) that has the same signature (argument list) in class B. But what about the class names? Suppose you create a stack class that is installed on a machine that already has a stack class that’s written by someone else? With Java on the Internet, this can happen without the user knowing it since classes can get downloaded automatically in the process of running a Java program.

This potential clashing of names is why it’s important to have complete control over the name spaces in Java, and to be able to create a completely unique name regardless of the constraints of the Internet.

So far, most of the examples in this book have existed in a single file and have been designed for local use, and haven’t bothered with package names. (In this case the class name is placed in the “default package.”) This is certainly an option, and for simplicity’s sake this approach will be used whenever possible throughout the rest of this book. However, if you’re planning to create libraries or programs that are friendly to other Java programs on the same machine, you must think about preventing class name clashes.

When you create a source-code file for Java, it’s commonly called a compilation unit (sometimes a translation unit). Each compilation unit must have a name ending in .java, and inside the compilation unit there can be a public class that must have the same name as the file (including capitalization, but excluding the .java filename extension). There can be only one public class in each compilation unit, otherwise the compiler will complain. The rest of the classes in that compilation unit, if there are any, are hidden from the world outside that package because they’re not public, and they comprise “support” classes for the main public class.

When you compile a .java file you get an output file with exactly the same name but an extension of .class for each class in the .java file. Thus you can end up with quite a few .class files from a small number of .java files. If you’ve programmed with a compiled language, you might be used to the compiler spitting out an intermediate form (usually an “obj” file) that is then packaged together with others of its kind using a linker (to create an executable file) or a librarian (to create a library). That’s not how Java works. A working program is a bunch of .class files, which can be packaged and compressed into a JAR file (using Java’s jar archiver). The Java interpreter is responsible for finding, loading and interpreting these files [33] .

A library is also a bunch of these class files. Each file has one class that is public (you’re not forced to have a public class, but it’s typical), so there’s one component for each file. If you want to say that all these components (that are in their own separate .java and .class files) belong together, that’s where the package keyword comes in.

When you say:

package mypackage;

at the beginning of a file, (if you use a package statement, it must appear as the first non-comment in the file) you’re stating that this compilation unit is part of a library named mypackage. Or, put another way, you’re saying that the public class name within this compilation unit is under the umbrella of the name mypackage, and if anyone wants to use the name they must either fully specify the name or use the import keyword in combination with mypackage (using the choices given previously). Note that the convention for Java package names is to use all lowercase letters, even for intermediate words.

For example, suppose the name of the file is MyClass.java. This means there can be one and only one public class in that file, and the name of that class must be MyClass (including the capitalization):

package mypackage;
public class MyClass {
  // . . .

Now, if someone wants to use MyClass or, for that matter, any of the other public classes in mypackage, they must use the import keyword to make the name or names in mypackage available. The alternative is to give the fully-qualified name:

mypackage.MyClass m = new mypackage.MyClass();

The import keyword can make this much cleaner:

import mypackage.*;
// . . .
MyClass m = new MyClass();

It’s worth keeping in mind that what the package and import keywords allow you to do, as a library designer, is to divide up the single global name space so you won’t have clashing names, no matter how many people get on the Internet and start writing classes in Java.

Creating unique package names

You might observe that, since a package never really gets “packaged” into a single file, a package could be made up of many .class files, and things could get a bit cluttered. To prevent this, a logical thing to do is to place all the .class files for a particular package into a single directory; that is, use the hierarchical file structure of the operating system to your advantage. This is one way that Java references the problem of clutter; you’ll see the other way later when the jar utility is introduced.

Collecting the package files into a single subdirectory solves two other problems: creating unique package names and finding those classes that might be buried in a directory structure someplace. This is accomplished, as was introduced in Chapter 2, by encoding the path of the location of the .class file into the name of the package. The compiler enforces this, but by convention, the first part of the package name is the Internet domain name of the creator of the class, reversed. Since Internet domain names are guaranteed to be unique, if you follow this convention it’s guaranteed that your package name will be unique and thus you’ll never have a name clash. (That is, until you lose the domain name to someone else who starts writing Java code with the same path names as you did.) Of course, if you don’t have your own domain name then you must fabricate an unlikely combination (such as your first and last name) to create unique package names. If you’ve decided to start publishing Java code it’s worth the relatively small effort to get a domain name.

The second part of this trick is resolving the package name into a directory on your machine, so when the Java program runs and it needs to load the .class file (which it does dynamically, at the point in the program where it needs to create an object of that particular class, or the first time you access a static member of the class), it can locate the directory where the .class file resides.

The Java interpreter proceeds as follows. First, it finds the environment variable CLASSPATH (set via the operating system, sometimes by the installation program that installs Java or a Java-based tool on your machine). CLASSPATH contains one or more directories that are used as roots for a search for .class files. Starting at that root, the interpreter will take the package name and replace each dot with a slash to generate a path name from the CLASSPATH root (so package foo.bar.baz becomes foo\bar\baz or foo/bar/baz or possibly something else, depending on your operating system). This is then concatenated to the various entries in the CLASSPATH. That’s where it looks for the .class file with the name corresponding to the class you’re trying to create. (It also searches some standard directories relative to where the Java interpreter resides).

To understand this, consider my domain name, which is bruceeckel.com. By reversing this, com.bruceeckel establishes my unique global name for my classes. (The com, edu, org, etc. extension was formerly capitalized in Java packages, but this was changed in Java 2 so the entire package name is lowercase.) I can further subdivide this by deciding that I want to create a library named simple, so I’ll end up with a package name:

package com.bruceeckel.simple;

Now this package name can be used as an umbrella name space for the following two files:

//: com:bruceeckel:simple:Vector.java
// Creating a package.
package com.bruceeckel.simple;

public class Vector {
  public Vector() {
    System.out.println(
      "com.bruceeckel.util.Vector");
  }
} ///:~

When you create your own packages, you’ll discover that the package statement must be the first non-comment code in the file. The second file looks much the same:

//: com:bruceeckel:simple:List.java
// Creating a package.
package com.bruceeckel.simple;

public class List {
  public List() {
    System.out.println(
      "com.bruceeckel.util.List");
  }
} ///:~

Both of these files are placed in the subdirectory on my system:

C:\DOC\JavaT\com\bruceeckel\simple

If you walk back through this, you can see the package name com.bruceeckel.simple, but what about the first portion of the path? That’s taken care of in the CLASSPATH environment variable, which is, on my machine:

CLASSPATH=.;D:\JAVA\LIB;C:\DOC\JavaT

You can see that the CLASSPATH can contain a number of alternative search paths. There’s a variation when using JAR files, however. You must put the name of the JAR file in the classpath, not just the path where it’s located. So for a JAR named grape.jar your classpath would include:

CLASSPATH=.;D:\JAVA\LIB;C:\flavors\grape.jar

Once the classpath is set up properly, the following file can be placed in any directory:

//: c05:LibTest.java
// Uses the library.
import com.bruceeckel.simple.*;

public class LibTest {
  public static void main(String[] args) {
    Vector v = new Vector();
    List l = new List();
  }
} ///:~

When the compiler encounters the import statement, it begins searching at the directories specified by CLASSPATH, looking for subdirectory com\bruceeckel\util, then seeking the compiled files of the appropriate names (Vector.class for Vector and List.class for List). Note that both the classes and the desired methods in Vector and List must be public.

Setting the CLASSPATH has been such a trial for beginning Java users (it was for me, when I started) that Sun made the JDK in Java 2 a bit smarter. You’ll find that, when you install it, even if you don’t set a CLASSPATH you’ll be able to compile and run basic Java programs. To compile and run the source-code package for this book (available on the CD ROM packaged with this book, or at www.BruceEckel.com), however, you will need to make some modifications to your CLASSPATH (these are explained in the source-code package).

Collisions

What happens if two libraries are imported via * and they include the same names? For example, suppose a program does this:

import com.bruceeckel.simple.*;
import java.util.*;

Since java.util.* also contains a Vector class, this causes a potential collision. However, as long as you don’t write the code that actually causes the collision, everything is OK—this is good because otherwise you might end up doing a lot of typing to prevent collisions that would never happen.

The collision does occur if you now try to make a Vector:

Vector v = new Vector();

Which Vector class does this refer to? The compiler can’t know, and the reader can’t know either. So the compiler complains and forces you to be explicit. If I want the standard Java Vector, for example, I must say:

java.util.Vector v = new java.util.Vector();

Since this (along with the CLASSPATH) completely specifies the location of that Vector, there’s no need for the import java.util.* statement unless I’m using something else from java.util.

A custom tool library

With this knowledge, you can now create your own libraries of tools to reduce or eliminate duplicate code. Consider, for example, creating an alias for System.out.println( ) to reduce typing. This can be part of a package called tools:

//: com:bruceeckel:tools:P.java
// The P.rint & P.rintln shorthand.
package com.bruceeckel.tools;

public class P {
  public static void rint(String s) {
    System.out.print(s);
  }
  public static void rintln(String s) {
    System.out.println(s);
  }
} ///:~

You can use this shorthand to print a String either with a newline (P.rintln( )) or without a newline (P.rint( )).

You can guess that the location of this file must be in a directory that starts at one of the CLASSPATH locations, then continues com/bruceeckel/tools. After compiling, the P.class file can be used anywhere on your system with an import statement:

//: c05:ToolTest.java
// Uses the tools library.
import com.bruceeckel.tools.*;

public class ToolTest {
  public static void main(String[] args) {
    P.rintln("Available from now on!");
    P.rintln("" + 100); // Force it to be a String
    P.rintln("" + 100L);
    P.rintln("" + 3.14159);
  }
} ///:~

Notice that all objects can be easily be forced into String representations by putting them in a String expression; in the above case, starting the expression with an empty String does the trick. But this brings up an interesting observation. If you call System.out.println(100), it works without casting it to a String. With some extra overloading, you can get the P class to do this as well (this is an exercise at the end of the chapter).

So from now on, whenever you come up with a useful new utility, you can add it to the tools directory. (Or to your own personal util or tools directory.)

Using imports to change behavior

A feature that is missing from Java is C’s conditional compilation, which allows you to change a switch and get different behavior without changing any other code. The reason such a feature was left out of Java is probably because it is most often used in C to solve cross-platform issues: different portions of the code are compiled depending on the platform that the code is being compiled for. Since Java is intended to be automatically cross-platform, such a feature should not be necessary.

However, there are other valuable uses for conditional compilation. A very common use is for debugging code. The debugging features are enabled during development, and disabled for a shipping product. Allen Holub (www.holub.com) came up with the idea of using packages to mimic conditional compilation. He used this to create a Java version of C’s very useful assertion mechanism, whereby you can say “this should be true” or “this should be false” and if the statement doesn’t agree with your assertion you’ll find out about it. Such a tool is quite helpful during debugging.

Here is the class that you’ll use for debugging:

//: com:bruceeckel:tools:debug:Assert.java
// Assertion tool for debugging.
package com.bruceeckel.tools.debug;

public class Assert {
  private static void perr(String msg) {
    System.err.println(msg);
  }
  public final static void is_true(boolean exp) {
    if(!exp) perr("Assertion failed");
  }
  public final static void is_false(boolean exp){
    if(exp) perr("Assertion failed");
  }
  public final static void
  is_true(boolean exp, String msg) {
    if(!exp) perr("Assertion failed: " + msg);
  }
  public final static void
  is_false(boolean exp, String msg) {
    if(exp) perr("Assertion failed: " + msg);
  }
} ///:~

This class simply encapsulates Boolean tests, which print error messages if they fail. In Chapter 10, you’ll learn about a more sophisticated tool for dealing with errors called exception handling, but the perr( ) method will work fine in the meantime.

When you want to use this class, you add a line in your program:

import com.bruceeckel.tools.debug.*;

To remove the assertions so you can ship the code, a second Assert class is created, but in a different package:

//: com:bruceeckel:tools:Assert.java
// Turning off the assertion output
// so you can ship the program.
package com.bruceeckel.tools;

public class Assert {
  public final static void is_true(boolean exp){}
  public final static void is_false(boolean exp){}
  public final static void
  is_true(boolean exp, String msg) {}
  public final static void
  is_false(boolean exp, String msg) {}
} ///:~

Now if you change the previous import statement to:

import com.bruceeckel.tools.*;

The program will no longer print assertions. Here’s an example:

//: c05:TestAssert.java
// Demonstrating the assertion tool.
// Comment the following, and uncomment the
// subsequent line to change assertion behavior:
import com.bruceeckel.tools.debug.*;
// import com.bruceeckel.tools.*;

public class TestAssert {
  public static void main(String[] args) {
    Assert.is_true((2 + 2) == 5);
    Assert.is_false((1 + 1) == 2);
    Assert.is_true((2 + 2) == 5, "2 + 2 == 5");
    Assert.is_false((1 + 1) == 2, "1 +1 != 2");
  }
} ///:~

By changing the package that’s imported, you change your code from the debug version to the production version. This technique can be used for any kind of conditional code.

Package caveat

It’s worth remembering that anytime you create a package, you implicitly specify a directory structure when you give the package a name. The package must live in the directory indicated by its name, which must be a directory that is searchable starting from the CLASSPATH. Experimenting with the package keyword can be a bit frustrating at first, because unless you adhere to the package-name to directory-path rule, you’ll get a lot of mysterious run-time messages about not being able to find a particular class, even if that class is sitting there in the same directory. If you get a message like this, try commenting out the package statement, and if it runs you’ll know where the problem lies.

Java access specifiers

When used, the Java access specifiers public, protected and private are placed in front of each definition for each member in your class, whether it’s a field or a method. Each access specifier controls the access for only that particular definition. This is a distinct contrast to C++, in which the access specifier controls all the definitions following it until another access specifier comes along.

One way or another, everything has some kind of access specified for it. In the following sections, you’ll learn all about the various types of access, starting with the default access.

“Friendly”

What if you give no access specifier at all, as in all the examples before this chapter? The default access has no keyword, but it is commonly referred to as “friendly.” It means that all the other classes in the current package have access to the friendly member, but to all the classes outside of this package the member appears to be private. Since a compilation unit—a file—can belong only to a single package, all the classes within a single compilation unit are automatically friendly with each other. Thus, friendly elements are also said to have package access.

Friendly access allows you to group related classes together in a package so that they can easily interact with each other. When you put classes together in a package (thus granting mutual access to their friendly members; e.g., making them “friends”) you “own” the code in that package. It makes sense that only code that you own should have friendly access to other code that you own. You could say that friendly access gives a meaning or a reason for grouping classes together in a package. In many languages the way you organize your definitions in files can be willy-nilly, but in Java you’re compelled to organize them in a sensible fashion. In addition, you’ll probably want to exclude classes that shouldn’t have access to the classes being defined in the current package.

The class controls which code has access to its members. There’s no magic way to “break in;” code from another package can’t show up and say, “Hi, I’m a friend of Bob’s!” and expect to see the protected, friendly, and private members of Bob. The only way to grant access to a member is to:

  1. Make the member public. Then everybody, everywhere, can access it.
  2. Make the member friendly by leaving off any access specifier, and put the other classes in the same package. Then the other classes can access the member.
  3. As you’ll see in a later chapter where inheritance is introduced, an inherited class can access a protected member as well as a public member (but not private members). It can access friendly members only if the two classes are in the same package. But don’t worry about that now.
  4. Provide “accessor/mutator” methods (also known as “get/set” methods) that read and change the value. This is the most civilized approach in terms of OOP, and it is fundamental to Java Beans, as you’ll see in Chapter 13.

public: interface access

When you use the public keyword, it means that the member declaration that immediately follows public is available to everyone, in particular to the client programmer who uses the library. Suppose you define a package dessert containing the following compilation unit:

//: c05:dessert:Cookie.java
// Creates a library.
package c05.dessert;

public class Cookie {
  public Cookie() {
   System.out.println("Cookie constructor");
  }
  void bite() { System.out.println("bite"); }
} ///:~

Remember, Cookie.java must reside in a subdirectory called dessert, in a directory under c05 (indicating Chapter 5 of this book) that must be under one of the CLASSPATH directories. Don’t make the mistake of thinking that Java will always look at the current directory as one of the starting points for searching. If you don’t have a ‘.’ as one of the paths in your CLASSPATH, Java won’t look there.

Now if you create a program that uses Cookie:

//: c05:Dinner.java
// Uses the library.
import c05.dessert.*;

public class Dinner {
  public Dinner() {
   System.out.println("Dinner constructor");
  }
  public static void main(String[] args) {
    Cookie x = new Cookie();
    //! x.bite(); // Can't access
  }
} ///:~

You can create a Cookie object, since its constructor is public and the class is public. (We’ll look more at the concept of a public class later.) However, the bite( ) member is inaccessible inside Dinner.java since bite( ) is friendly only within package dessert.

The default package

You might be surprised to discover that the following code compiles, even though it would appear that it breaks the rules:

//: c05:Cake.java
// Accesses a class in a
// separate compilation unit.

class Cake {
  public static void main(String[] args) {
    Pie x = new Pie();
    x.f();
  }
} ///:~

In a second file, in the same directory:

//: c05:Pie.java
// The other class.

class Pie {
  void f() { System.out.println("Pie.f()"); }
} ///:~

You might initially view these as completely foreign files, and yet Cake is able to create a Pie object and call its f( ) method! (Note that you must have ‘.’ in your CLASSPATH in order for the files to compile.) You’d typically think that Pie and f( ) are friendly and therefore not available to Cake. They are friendly—that part is correct. The reason that they are available in Cake.java is because they are in the same directory and have no explicit package name. Java treats files like this as implicitly part of the “default package” for that directory, and therefore friendly to all the other files in that directory.

private: you can’t touch that!

The private keyword means that no one can access that member except that particular class, inside methods of that class. Other classes in the same package cannot access private members, so it’s as if you’re even insulating the class against yourself. On the other hand, it’s not unlikely that a package might be created by several people collaborating together, so private allows you to freely change that member without concern that it will affect another class in the same package.

The default “friendly” package access often provides an adequate amount of hiding; remember, a “friendly” member is inaccessible to the user of the package. This is nice, since the default access is the one that you normally use (and the one that you’ll get if you forget to add any access control). Thus, you’ll typically think about access for the members that you explicitly want to make public for the client programmer, and as a result, you might not initially think you’ll use the private keyword often since it’s tolerable to get away without it. (This is a distinct contrast with C++.) However, it turns out that the consistent use of private is very important, especially where multithreading is concerned. (As you’ll see in Chapter 14.)

Here’s an example of the use of private:

//: c05:IceCream.java
// Demonstrates "private" keyword.

class Sundae {
  private Sundae() {}
  static Sundae makeASundae() {
    return new Sundae();
  }
}

public class IceCream {
  public static void main(String[] args) {
    //! Sundae x = new Sundae();
    Sundae x = Sundae.makeASundae();
  }
} ///:~

This shows an example in which private comes in handy: you might want to control how an object is created and prevent someone from directly accessing a particular constructor (or all of them). In the example above, you cannot create a Sundae object via its constructor; instead you must call the makeASundae( ) method to do it for you [34] .

Any method that you’re certain is only a “helper” method for that class can be made private to ensure that you don’t accidentally use it elsewhere in the package and thus prohibit you from changing or removing the method. Making a method private guarantees that you retain this option.

The same is true for a private field inside a class—unless you must expose the underlying implementation (which is a much rarer situation than you might think) you should make all fields private. However, just because a reference to an object is private inside a class doesn't mean that some other object can't have a public reference to the same object. (See Appendix A for issues about aliasing.)

protected: “sort of friendly”

The protected access specifier requires a jump ahead to understand. First, you should be aware that you don’t need to understand this section to continue through this book up through inheritance (Chapter 6). But for completeness, here is a brief description and example using protected.

The protected keyword deals with a concept called inheritance, which takes an existing class and adds new members to that class without touching the existing class, which we refer to as the base class. You can also change the behavior of existing members of the class. To inherit from an existing class, you say that your new class extends an existing class, like this:

class Foo extends Bar {

The rest of the class definition looks the same.

If you create a new package and you inherit from a class in another package, the only members you have access to are the public members of the original package. (Of course, if you perform the inheritance in the same package, you have the normal package access to all the “friendly” members.) Sometimes the creator of the base class would like to take a particular member and grant access to derived classes but not the world in general. That’s what protected does. If you refer back to the file Cookie.java on page 254, the following class cannot access the “friendly” member:

//: c05:ChocolateChip.java
// Can't access friendly member
// in another class.
import c05.dessert.*;

public class ChocolateChip extends Cookie {
  public ChocolateChip() {
   System.out.println(
     "ChocolateChip constructor");
  }
  public static void main(String[] args) {
    ChocolateChip x = new ChocolateChip();
    //! x.bite(); // Can't access bite
  }
} ///:~

One of the interesting things about inheritance is that if a method bite( ) exists in class Cookie, then it also exists in any class inherited from Cookie. But since bite( ) is “friendly” in a foreign package, it’s unavailable to us in this one. Of course, you could make it public, but then everyone would have access and maybe that’s not what you want. If we change the class Cookie as follows:

public class Cookie {
  public Cookie() {
    System.out.println("Cookie constructor");
  }
  protected void bite() {
    System.out.println("bite");
  }
}

then bite( ) still has “friendly” access within package dessert, but it is also accessible to anyone inheriting from Cookie. However, it is not public.

Interface and implementation

Access control is often referred to as implementation hiding. Wrapping data and methods within classes in combination with implementation hiding is often called encapsulation [35] . The result is a data type with characteristics and behaviors.

Access control puts boundaries within a data type for two important reasons. The first is to establish what the client programmers can and can’t use. You can build your internal mechanisms into the structure without worrying that the client programmers will accidentally treat the internals as part of the interface that they should be using.

This feeds directly into the second reason, which is to separate the interface from the implementation. If the structure is used in a set of programs, but client programmers can’t do anything but send messages to the public interface, then you can change anything that’s not public (e.g., “friendly,” protected, or private) without requiring modifications to client code.

We’re now in the world of object-oriented programming, where a class is actually describing “a class of objects,” as you would describe a class of fishes or a class of birds. Any object belonging to this class will share these characteristics and behaviors. The class is a description of the way all objects of this type will look and act.

In the original OOP language, Simula-67, the keyword class was used to describe a new data type. The same keyword has been used for most object-oriented languages. This is the focal point of the whole language: the creation of new data types that are more than just boxes containing data and methods.

The class is the fundamental OOP concept in Java. It is one of the keywords that will not be set in bold in this book—it becomes annoying with a word repeated as often as “class.”

For clarity, you might prefer a style of creating classes that puts the public members at the beginning, followed by the protected, friendly and private members. The advantage is that the user of the class can then read down from the top and see first what’s important to them (the public members, because they can be accessed outside the file) and stop reading when they encounter the non-public members, which are part of the internal implementation:

public class X {
  public void pub1( ) { /* . . . */ }
  public void pub2( ) { /* . . . */ }
  public void pub3( ) { /* . . . */ }
  private void priv1( ) { /* . . . */ }
  private void priv2( ) { /* . . . */ }
  private void priv3( ) { /* . . . */ }
  private int i;
  // . . .
}

This will make it only partially easier to read because the interface and implementation are still mixed together. That is, you still see the source code—the implementation—because it’s right there in the class. In addition, the comment documentation supported by javadoc (described in Chapter 2) lessens the importance of code readability by the client programmer. Displaying the interface to the consumer of a class is really the job of the class browser, a tool whose job is to look at all the available classes and show you what you can do with them (i.e. what members are available) in a useful fashion. By the time you read this, browsers should be an expected part of any good Java development tool.

Class access

In Java, the access specifiers can also be used to determine which classes within a library will be available to the users of that library. If you want a class to be available to a client programmer, you place the public keyword somewhere before the opening brace of the class body. This controls whether the client programmer can even create an object of the class.

To control the access of a class, the specifier must appear before the keyword class. Thus you can say:

public class Widget {

Now if the name of your library is mylib any client programmer can access Widget by saying

import mylib.Widget;

or

import mylib.*;

However, there’s an extra pair of constraints:

  1. There can be only one public class per compilation unit (file). The idea is that each compilation unit has a single public interface represented by that public class. It can have as many supporting “friendly” classes as you want. If you have more than one public class inside a compilation unit, the compiler will give you an error message.
  2. The name of the public class must exactly match the name of the file containing the compilation unit, including capitalization. So for Widget, the name of the file must be Widget.java, not widget.java or WIDGET.java. Again, you’ll get a compile-time error if they don’t agree.
  3. It is possible, though not typical, to have a compilation unit with no public class at all. In this case, you can name the file whatever you like.

What if you’ve got a class inside mylib that you’re just using to accomplish the tasks performed by Widget or some other public class in mylib? You don’t want to go to the bother of creating documentation for the client programmer, and you think that sometime later you might want to completely change things and rip out your class altogether, substituting a different one. To give you this flexibility, you need to ensure that no client programmers become dependent on your particular implementation details hidden inside mylib. To accomplish this, you just leave the public keyword off the class, in which case it becomes friendly. (That class can be used only within that package.)

Note that a class cannot be private (that would make it accessible to no one but the class), or protected [36] . So you have only two choices for class access: “friendly” or public. If you don’t want anyone else to have access to that class, you can make all the constructors private, thereby preventing anyone but you, inside a static member of the class, from creating an object of that class [37] . Here’s an example:

//: c05:Lunch.java
// Demonstrates class access specifiers.
// Make a class effectively private
// with private constructors:

class Soup {
  private Soup() {}
  // (1) Allow creation via static method:
  public static Soup makeSoup() {
    return new Soup();
  }
  // (2) Create a static object and
  // return a reference upon request.
  // (The "Singleton" pattern):
  private static Soup ps1 = new Soup();
  public static Soup access() {
    return ps1;
  }
  public void f() {}
}

class Sandwich { // Uses Lunch
  void f() { new Lunch(); }
}

// Only one public class allowed per file:
public class Lunch {
  void test() {
    // Can't do this! Private constructor:
    //! Soup priv1 = new Soup();
    Soup priv2 = Soup.makeSoup();
    Sandwich f1 = new Sandwich();
    Soup.access().f();
  }
} ///:~

Up to now, most of the methods have been returning either void or a primitive type so the definition:

  public static Soup access() {
    return ps1;
  }

might look a little confusing at first. The word before the method name (access) tells what the method returns. So far this has most often been void, which means it returns nothing. But you can also return a reference to an object, which is what happens here. This method returns a reference to an object of class Soup.

The class Soup shows how to prevent direct creation of a class by making all the constructors private. Remember that if you don’t explicitly create at least one constructor, the default constructor (a constructor with no arguments) will be created for you. By writing the default constructor, it won’t be created automatically. By making it private, no one can create an object of that class. But now how does anyone use this class? The above example shows two options. First, a static method is created that creates a new Soup and returns a reference to it. This could be useful if you want to do some extra operations on the Soup before returning it, or if you want to keep count of how many Soup objects to create (perhaps to restrict their population).

The second option uses what’s called a design pattern, which is covered in Thinking in Patterns with Java, downloadable at www.BruceEckel.com. This particular pattern is called a “singleton” because it allows only a single object to ever be created. The object of class Soup is created as a static private member of Soup, so there’s one and only one, and you can’t get at it except through the public method access( ).

As previously mentioned, if you don’t put an access specifier for class access it defaults to “friendly.” This means that an object of that class can be created by any other class in the package, but not outside the package. (Remember, all the files within the same directory that don’t have explicit package declarations are implicitly part of the default package for that directory.) However, if a static member of that class is public, the client programmer can still access that static member even though they cannot create an object of that class.

Summary

In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the user of that library—the client programmer—who is another programmer, but one putting together an application or using your library to build a bigger library.

Without rules, client programmers can do anything they want with all the members of a class, even if you might prefer they don’t directly manipulate some of the members. Everything’s naked to the world.

This chapter looked at how classes are built to form libraries; first, the way a group of classes is packaged within a library, and second, the way the class controls access to its members.

It is estimated that a C programming project begins to break down somewhere between 50K and 100K lines of code because C has a single “name space” so names begin to collide, causing an extra management overhead. In Java, the package keyword, the package naming scheme and the import keyword give you complete control over names, so the issue of name collision is easily avoided.

There are two reasons for controlling access to members. The first is to keep users’ hands off tools that they shouldn’t touch; tools that are necessary for the internal machinations of the data type, but not part of the interface that users need to solve their particular problems. So making methods and fields private is a service to users because they can easily see what’s important to them and what they can ignore. It simplifies their understanding of the class.

The second and most important reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. You might build a class one way at first, and then discover that restructuring your code will provide much greater speed. If the interface and implementation are clearly separated and protected, you can accomplish this without forcing the user to rewrite their code.

Access specifiers in Java give valuable control to the creator of a class. The users of the class can clearly see exactly what they can use and what to ignore. More important, though, is the ability to ensure that no user becomes dependent on any part of the underlying implementation of a class. If you know this as the creator of the class, you can change the underlying implementation with the knowledge that no client programmer will be affected by the changes because they can’t access that part of the class.

When you have the ability to change the underlying implementation, you can not only improve your design later, but you also have the freedom to make mistakes. No matter how carefully you plan and design you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn faster and you’ll finish your project sooner.

The public interface to a class is what the user does see, so that is the most important part of the class to get “right” during analysis and design. Even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more methods, as long as you don’t remove any that client programmers have already used in their code.

Exercises
  1. Write a program that creates an ArrayList object without explicitly importing java.util.*.
  2. In the section labeled “package: the library unit,” turn the code fragments concerning mypackage into a compiling and running set of Java files.
  3. In the section labeled “Collisions,” take the code fragments and turn them into a program, and verify that collisions do in fact occur.
  4. Generalize the class P defined in this chapter by adding all the overloaded versions of rint( ) and rintln( ) necessary to handle all the different basic Java types.
  5. Change the import statement in TestAssert.java to enable and disable the assertion mechanism.
  6. Create a class with public, private, protected, and “friendly” data members and method members. Create an object of this class and see what kind of compiler messages you get when you try to access all the class members. Be aware that classes in the same directory are part of the “default” package.
  7. Create a class with protected data. Create a second class in the same file with a method that manipulates the protected data in the first class.
  8. Change the class Cookie as specified in the section labeled “protected: ‘sort of friendly.’” Verify that bite( ) is not public.
  9. In the section titled “Class access” you’ll find code fragments describing mylib and Widget. Create this library, then create a Widget in a class that is not part of the mylib package.
  10. Create a new directory and edit your CLASSPATH to include that new directory. Copy the P.class file (produced by compiling com.bruceeckel.tools.P.java) to your new directory and then change the names of the file, the P class inside and the method names. (You might also want to add additional output to watch how it works.) Create another program in a different directory that uses your new class.
  11. Following the form of the example Lunch.java, create a class called ConnectionManager that manages a fixed array of Connection objects. The client programmer must not be able to explicitly create Connection objects, but can only get them via a static method in ConnectionManager. When the ConnectionManager runs out of objects, it returns a null reference. Test the classes in main( ).
  12. Create the following file in the c05/local directory (presumably in your CLASSPATH):
///: c05:local:PackagedClass.java
package c05.local;
class PackagedClass {
  public PackagedClass() {
    System.out.println(
      "Creating a packaged class");
  }
} ///:~

Then create the following file in a directory other than c05:

///: c05:foreign:Foreign.java
package c05.foreign;
import c05.local.*;
public class Foreign {
   public static void main (String[] args) {
      PackagedClass pc = new PackagedClass();
   }
} ///:~

Explain why the compiler generates an error. Would making the Foreign class part of the c05.local package change anything?

[33] There’s nothing in Java that forces the use of an interpreter. There exist native-code Java compilers that generate a single executable file.

[34] There’s another effect in this case: Since the default constructor is the only one defined, and it’s private, it will prevent inheritance of this class. (A subject that will be introduced in Chapter 6.)

[35] However, people often refer to implementation hiding alone as encapsulation.

[36] Actually, an inner class can be private or protected, but that’s a special case. These will be introduced in Chapter 7.

[37] You can also do it by inheriting (Chapter 6) from that class.


6: Reusing Classes

One of the most compelling features about Java is code reuse. But to be revolutionary, you’ve got to be able to do a lot more than copy code and change it.

That’s the approach used in procedural languages like C, and it hasn’t worked very well. Like everything in Java, the solution revolves around the class. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone has already built and debugged.

The trick is to use the classes without soiling the existing code. In this chapter you’ll see two ways to accomplish this. The first is quite straightforward: You simply create objects of your existing class inside the new class. This is called composition because the new class is composed of objects of existing classes. You’re simply reusing the functionality of the code, not its form.

The second approach is more subtle. It creates a new class as a type of an existing class. You literally take the form of the existing class and add code to it without modifying the existing class. This magical act is called inheritance, and the compiler does most of the work. Inheritance is one of the cornerstones of object-oriented programming and has additional implications that will be explored in the Chapter 7.

It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense because they are both ways of making new types from existing types). In this chapter, you’ll learn about these code reuse mechanisms.

Composition syntax

Until now, composition has been used quite frequently. You simply place object references inside new classes. For example, suppose you’d like an object that holds several String objects, a couple of primitives and an object of another class. For the nonprimitive objects, you put references inside your new class, and for the primitives you define them inside your class:

//: c06:SprinklerSystem.java
// Composition for code reuse.

class WaterSource {
  private String s;
  WaterSource() {
    System.out.println("WaterSource()");
    s = new String("Constructed");
  }
  public String toString() { return s; }
}

public class SprinklerSystem {
  private String valve1, valve2, valve3, valve4;
  WaterSource source;
  int i;
  float f;
  void print() {
    System.out.println("valve1 = " + valve1);
    System.out.println("valve2 = " + valve2);
    System.out.println("valve3 = " + valve3);
    System.out.println("valve4 = " + valve4);
    System.out.println("i = " + i);
    System.out.println("f = " + f);
    System.out.println("source = " + source);
  }
  public static void main(String[] args) {
    SprinklerSystem x = new SprinklerSystem();
    x.print();
  }
} ///:~

One of the methods defined in WaterSource is special: toString( ). You will learn later that every nonprimitive object has a toString( ) method, and it’s called in special situations when the compiler wants a String but it’s got one of these objects. So in the expression:

System.out.println("source = " + source);

the compiler sees you trying to add a String object ("source = ") to a WaterSource. This doesn’t make sense to it, because you can only “add” a String to another String, so it says “I’ll turn source into a String by calling toString( )!” After doing this it can combine the two Strings and pass the resulting String to System.out.println( ). Any time you want to allow this behavior with a class you create you need only write a toString( ) method.

At first glance, you might assume—Java being as safe and careful as it is—that the compiler would automatically construct objects for each of the references in the above code, for example calling the default constructor for WaterSource to initialize source. The output of the print statement is in fact:

valve1 = null
valve2 = null
valve3 = null
valve4 = null
i = 0
f = 0.0
source = null

Primitives that are fields in a class are automatically initialized to zero, as noted in Chapter 2. But the object references are initialized to null, and if you try to call methods for any of them you’ll get an exception. It’s actually pretty good (and useful) that you can still print them out without throwing an exception.

It makes sense that the compiler doesn’t just create a default object for every reference because that would incur unnecessary overhead in many cases. If you want the references initialized, you can do it:

  1. At the point the objects are defined. This means that they’ll always be initialized before the constructor is called.
  2. In the constructor for that class
  3. Right before you actually need to use the object. This is often called lazy initialization. It can reduce overhead in situations where the object doesn’t need to be created every time.

All three approaches are shown here:

//: c06:Bath.java
// Constructor initialization with composition.

class Soap {
  private String s;
  Soap() {
    System.out.println("Soap()");
    s = new String("Constructed");
  }
  public String toString() { return s; }
}

public class Bath {
  private String
    // Initializing at point of definition:
    s1 = new String("Happy"),
    s2 = "Happy",
    s3, s4;
  Soap castille;
  int i;
  float toy;
  Bath() {
    System.out.println("Inside Bath()");
    s3 = new String("Joy");
    i = 47;
    toy = 3.14f;
    castille = new Soap();
  }
  void print() {
    // Delayed initialization:
    if(s4 == null)
      s4 = new String("Joy");
    System.out.println("s1 = " + s1);
    System.out.println("s2 = " + s2);
    System.out.println("s3 = " + s3);
    System.out.println("s4 = " + s4);
    System.out.println("i = " + i);
    System.out.println("toy = " + toy);
    System.out.println("castille = " + castille);
  }
  public static void main(String[] args) {
    Bath b = new Bath();
    b.print();
  }
} ///:~

Note that in the Bath constructor a statement is executed before any of the initializations take place. When you don’t initialize at the point of definition, there’s still no guarantee that you’ll perform any initialization before you send a message to an object reference—except for the inevitable run-time exception.

Here’s the output for the program:

Inside Bath()
Soap()
s1 = Happy
s2 = Happy
s3 = Joy
s4 = Joy
i = 47
toy = 3.14
castille = Constructed

When print( ) is called it fills in s4 so that all the fields are properly initialized by the time they are used.

Inheritance syntax

Inheritance is an integral part of Java (and OOP languages in general). It turns out that you’re always doing inheritance when you create a class, because unless you explicitly inherit from some other class, you implicitly inherit from Java’s standard root class Object.

The syntax for composition is obvious, but to perform inheritance there’s a distinctly different form. When you inherit, you say “This new class is like that old class.” You state this in code by giving the name of the class as usual, but before the opening brace of the class body, put the keyword extends followed by the name of the base class. When you do this, you automatically get all the data members and methods in the base class. Here’s an example:

//: c06:Detergent.java
// Inheritance syntax & properties.

class Cleanser {
  private String s = new String("Cleanser");
  public void append(String a) { s += a; }
  public void dilute() { append(" dilute()"); }
  public void apply() { append(" apply()"); }
  public void scrub() { append(" scrub()"); }
  public void print() { System.out.println(s); }
  public static void main(String[] args) {
    Cleanser x = new Cleanser();
    x.dilute(); x.apply(); x.scrub();
    x.print();
  }
}

public class Detergent extends Cleanser {
  // Change a method:
  public void scrub() {
    append(" Detergent.scrub()");
    super.scrub(); // Call base-class version
  }
  // Add methods to the interface:
  public void foam() { append(" foam()"); }
  // Test the new class:
  public static void main(String[] args) {
    Detergent x = new Detergent();
    x.dilute();
    x.apply();
    x.scrub();
    x.foam();
    x.print();
    System.out.println("Testing base class:");
    Cleanser.main(args);
  }
} ///:~

This demonstrates a number of features. First, in the Cleanser append( ) method, Strings are concatenated to s using the += operator, which is one of the operators (along with ‘+’) that the Java designers “overloaded” to work with Strings.

Second, both Cleanser and Detergent contain a main( ) method. You can create a main( ) for each one of your classes, and it’s often recommended to code this way so that your test code is wrapped in with the class. Even if you have a lot of classes in a program only the main( ) for the class invoked on the command line will be called. (As long as main( ) is public, it doesn’t matter whether the class that it’s part of is public.) So in this case, when you say java Detergent, Detergent.main( ) will be called. But you can also say java Cleanser to invoke Cleanser.main( ), even though Cleanser is not a public class. This technique of putting a main( ) in each class allows easy unit testing for each class. And you don’t need to remove the main( ) when you’re finished testing; you can leave it in for later testing.

Here, you can see that Detergent.main( ) calls Cleanser.main( ) explicitly, passing it the same arguments from the command line (however, you could pass it any String array).

It’s important that all of the methods in Cleanser are public. Remember that if you leave off any access specifier the member defaults to “friendly,” which allows access only to package members. Thus, within this package, anyone could use those methods if there were no access specifier. Detergent would have no trouble, for example. However, if a class from some other package were to inherit from Cleanser it could access only public members. So to plan for inheritance, as a general rule make all fields private and all methods public. (protected members also allow access by derived classes; you’ll learn about this later.) Of course, in particular cases you must make adjustments, but this is a useful guideline.

Note that Cleanser has a set of methods in its interface: append( ), dilute( ), apply( ), scrub( ) and print( ). Because Detergent is derived from Cleanser (via the extends keyword) it automatically gets all these methods in its interface, even though you don’t see them all explicitly defined in Detergent. You can think of inheritance, then, as reusing the interface. (The implementation comes along for free, but that part isn’t the primary point.)

As seen in scrub( ), it’s possible to take a method that’s been defined in the base class and modify it. In this case, you might want to call the method from the base class inside the new version. But inside scrub( ) you cannot simply call scrub( ), since that would produce a recursive call, which isn’t what you want. To solve this problem Java has the keyword super that refers to the “superclass” that the current class has been inherited from. Thus the expression super.scrub( ) calls the base-class version of the method scrub( ).

When inheriting you’re not restricted to using the methods of the base class. You can also add new methods to the derived class exactly the way you put any method in a class: just define it. The method foam( ) is an example of this.

In Detergent.main( ) you can see that for a Detergent object you can call all the methods that are available in Cleanser as well as in Detergent (i.e. foam( )).

Initializing the base class

Since there are now two classes involved—the base class and the derived class—instead of just one, it can be a bit confusing to try to imagine the resulting object produced by a derived class. From the outside, it looks like the new class has the same interface as the base class and maybe some additional methods and fields. But inheritance doesn’t just copy the interface of the base class. When you create an object of the derived class, it contains within it a subobject of the base class. This subobject is the same as if you had created an object of the base class by itself. It’s just that, from the outside, the subobject of the base class is wrapped within the derived-class object.

Of course, it’s essential that the base-class subobject be initialized correctly and there’s only one way to guarantee that: perform the initialization in the constructor, by calling the base-class constructor, which has all the appropriate knowledge and privileges to perform the base-class initialization. Java automatically inserts calls to the base-class constructor in the derived-class constructor. The following example shows this working with three levels of inheritance:

//: c06:Cartoon.java
// Constructor calls during inheritance.

class Art {
  Art() {
    System.out.println("Art constructor");
  }
}

class Drawing extends Art {
  Drawing() {
    System.out.println("Drawing constructor");
  }
}

public class Cartoon extends Drawing {
  Cartoon() {
    System.out.println("Cartoon constructor");
  }
  public static void main(String[] args) {
    Cartoon x = new Cartoon();
  }
} ///:~

The output for this program shows the automatic calls:

Art constructor
Drawing constructor
Cartoon constructor

You can see that the construction happens from the base “outward,” so the base class is initialized before the derived-class constructors can access it.

Even if you don’t create a constructor for Cartoon( ), the compiler will synthesize a default constructor for you that calls the base class constructor.

Constructors with arguments

The above example has default constructors; that is, they don’t have any arguments. It’s easy for the compiler to call these because there’s no question about what arguments to pass. If your class doesn’t have default arguments or if you want to call a base-class constructor that has an argument you must explicitly write the calls to the base-class constructor using the super keyword and the appropriate argument list:

//: c06:Chess.java
// Inheritance, constructors and arguments.

class Game {
  Game(int i) {
    System.out.println("Game constructor");
  }
}

class BoardGame extends Game {
  BoardGame(int i) {
    super(i);
    System.out.println("BoardGame constructor");
  }
}

public class Chess extends BoardGame {
  Chess() {
    super(11);
    System.out.println("Chess constructor");
  }
  public static void main(String[] args) {
    Chess x = new Chess();
  }
} ///:~

If you don’t call the base-class constructor in BoardGame( ), the compiler will complain that it can’t find a constructor of the form Game( ). In addition, the call to the base-class constructor must be the first thing you do in the derived-class constructor. (The compiler will remind you if you get it wrong.)

Catching base constructor exceptions

As just noted, the compiler forces you to place the base-class constructor call first in the body of the derived-class constructor. This means nothing else can appear before it. As you’ll see in Chapter 10, this also prevents a derived-class constructor from catching any exceptions that come from a base class. This can be inconvenient at times.

Combining composition
and inheritance

It is very common to use composition and inheritance together. The following example shows the creation of a more complex class, using both inheritance and composition, along with the necessary constructor initialization:

//: c06:PlaceSetting.java
// Combining composition & inheritance.

class Plate {
  Plate(int i) {
    System.out.println("Plate constructor");
  }
}

class DinnerPlate extends Plate {
  DinnerPlate(int i) {
    super(i);
    System.out.println(
      "DinnerPlate constructor");
  }
}

class Utensil {
  Utensil(int i) {
    System.out.println("Utensil constructor");
  }
}

class Spoon extends Utensil {
  Spoon(int i) {
    super(i);
    System.out.println("Spoon constructor");
  }
}

class Fork extends Utensil {
  Fork(int i) {
    super(i);
    System.out.println("Fork constructor");
  }
}

class Knife extends Utensil {
  Knife(int i) {
    super(i);
    System.out.println("Knife constructor");
  }
}

// A cultural way of doing something:
class Custom {
  Custom(int i) {
    System.out.println("Custom constructor");
  }
}

public class PlaceSetting extends Custom {
  Spoon sp;
  Fork frk;
  Knife kn;
  DinnerPlate pl;
  PlaceSetting(int i) {
    super(i + 1);
    sp = new Spoon(i + 2);
    frk = new Fork(i + 3);
    kn = new Knife(i + 4);
    pl = new DinnerPlate(i + 5);
    System.out.println(
      "PlaceSetting constructor");
  }
  public static void main(String[] args) {
    PlaceSetting x = new PlaceSetting(9);
  }
} ///:~

While the compiler forces you to initialize the base classes, and requires that you do it right at the beginning of the constructor, it doesn’t watch over you to make sure that you initialize the member objects, so you must remember to pay attention to that.

Guaranteeing proper cleanup

Java doesn’t have the C++ concept of a destructor, a method that is automatically called when an object is destroyed. The reason is probably that in Java the practice is simply to forget about objects rather than to destroy them, allowing the garbage collector to reclaim the memory as necessary.

Often this is fine, but there are times when your class might perform some activities during its lifetime that require cleanup. As mentioned in Chapter 4, you can’t know when the garbage collector will be called, or if it will be called. So if you want something cleaned up for a class, you must write a special method to do it explicitly, and make sure that the client programmer knows that they must call this method. On top of this, as described in Chapter 10 (exception handling), you must guard against an exception by putting such cleanup in a finally clause.

Consider an example of a computer-aided design system that draws pictures on the screen:

//: c06:CADSystem.java
// Ensuring proper cleanup.
import java.util.*;

class Shape {
  Shape(int i) {
    System.out.println("Shape constructor");
  }
  void cleanup() {
    System.out.println("Shape cleanup");
  }
}

class Circle extends Shape {
  Circle(int i) {
    super(i);
    System.out.println("Drawing a Circle");
  }
  void cleanup() {
    System.out.println("Erasing a Circle");
    super.cleanup();
  }
}

class Triangle extends Shape {
  Triangle(int i) {
    super(i);
    System.out.println("Drawing a Triangle");
  }
  void cleanup() {
    System.out.println("Erasing a Triangle");
    super.cleanup();
  }
}

class Line extends Shape {
  private int start, end;
  Line(int start, int end) {
    super(start);
    this.start = start;
    this.end = end;
    System.out.println("Drawing a Line: " +
           start + ", " + end);
  }
  void cleanup() {
    System.out.println("Erasing a Line: " +
           start + ", " + end);
    super.cleanup();
  }
}

public class CADSystem extends Shape {
  private Circle c;
  private Triangle t;
  private Line[] lines = new Line[10];
  CADSystem(int i) {
    super(i + 1);
    for(int j = 0; j < 10; j++)
      lines[j] = new Line(j, j*j);
    c = new Circle(1);
    t = new Triangle(1);
    System.out.println("Combined constructor");
  }
  void cleanup() {
    System.out.println("CADSystem.cleanup()");
    // The order of cleanup is the reverse
    // of the order of initialization
    t.cleanup();
    c.cleanup();
    for(int i = lines.length; i >= 0; i--)
      lines[i].cleanup();
    super.cleanup();
  }
  public static void main(String[] args) {
    CADSystem x = new CADSystem(47);
    try {
      // Code and exception handling...
    } finally {
      x.cleanup();
    }
  }
} ///:~

Everything in this system is some kind of Shape (which is itself a kind of Object since it’s implicitly inherited from the root class). Each class redefines Shape’s cleanup( ) method in addition to calling the base-class version of that method using super. The specific Shape classes Circle, Triangle and Line all have constructors that “draw,” although any method called during the lifetime of the object could be responsible for doing something that needs cleanup. Each class has its own cleanup( ) method to restore non-memory things back to the way they were before the object existed.

In main( ), you can see two keywords that are new, and won’t officially be introduced until Chapter 10: try and finally. The try keyword indicates that the block that follows (delimited by curly braces) is a guarded region, which means that it is given special treatment. One of these special treatments is that the code in the finally clause following this guarded region is always executed, no matter how the try block exits. (With exception handling, it’s possible to leave a try block in a number of non-ordinary ways.) Here, the finally clause is saying “always call cleanup( ) for x, no matter what happens.” These keywords will be explained thoroughly in Chapter 10.

Note that in your cleanup method you must also pay attention to the calling order for the base-class and member-object cleanup methods in case one subobject depends on another. In general, you should follow the same form that is imposed by a C++ compiler on its destructors: First perform all of the cleanup work specific to your class, in the reverse order of creation. (In general, this requires that base-class elements still be viable.) Then call the base-class cleanup method, as demonstrated here.

There can be many cases in which the cleanup issue is not a problem; you just let the garbage collector do the work. But when you must do it explicitly, diligence and attention is required.

Order of garbage collection

There’s not much you can rely on when it comes to garbage collection. The garbage collector might never be called. If it is, it can reclaim objects in any order it wants. It’s best to not rely on garbage collection for anything but memory reclamation. If you want cleanup to take place, make your own cleanup methods and don’t rely on finalize( ). (As mentioned in Chapter 4, Java can be forced to call all the finalizers.)

Name hiding

Only C++ programmers might be surprised by name hiding, since it works differently in that language. If a Java base class has a method name that’s overloaded several times, redefining that method name in the derived class will not hide any of the base-class versions. Thus overloading works regardless of whether the method was defined at this level or in a base class:

//: c06:Hide.java
// Overloading a base-class method name
// in a derived class does not hide the
// base-class versions.

class Homer {
  char doh(char c) {
    System.out.println("doh(char)");
    return 'd';
  }
  float doh(float f) {
    System.out.println("doh(float)");
    return 1.0f;
  }
}

class Milhouse {}

class Bart extends Homer {
  void doh(Milhouse m) {}
}

class Hide {
  public static void main(String[] args) {
    Bart b = new Bart();
    b.doh(1); // doh(float) used
    b.doh('x');
    b.doh(1.0f);
    b.doh(new Milhouse());
  }
} ///:~

As you’ll see in the next chapter, it’s far more common to override methods of the same name using exactly the same signature and return type as in the base class. It can be confusing otherwise (which is why C++ disallows it, to prevent you from making what is probably a mistake).

Choosing composition
vs. inheritance

Both composition and inheritance allow you to place subobjects inside your new class. You might wonder about the difference between the two, and when to choose one over the other.

Composition is generally used when you want the features of an existing class inside your new class, but not its interface. That is, you embed an object so that you can use it to implement functionality in your new class, but the user of your new class sees the interface you’ve defined rather for the new class than the interface from the embedded object. For this effect, you embed private objects of existing classes inside your new class.

Sometimes it makes sense to allow the class user to directly access the composition of your new class; that is, to make the member objects public. The member objects use implementation hiding themselves, so this is a safe thing to do and when the user knows you’re assembling a bunch of parts, it makes the interface easier to understand. A car object is a good example:

//: c06:Car.java
// Composition with public objects.

class Engine {
  public void start() {}
  public void rev() {}
  public void stop() {}
}

class Wheel {
  public void inflate(int psi) {}
}

class Window {
  public void rollup() {}
  public void rolldown() {}
}

class Door {
  public Window window = new Window();
  public void open() {}
  public void close() {}
}

public class Car {
  public Engine engine = new Engine();
  public Wheel[] wheel = new Wheel[4];
  public Door left = new Door(),
       right = new Door(); // 2-door
  public Car() {
    for(int i = 0; i < 4; i++)
      wheel[i] = new Wheel();
  }
  public static void main(String[] args) {
    Car car = new Car();
    car.left.window.rollup();
    car.wheel[0].inflate(72);
  }
} ///:~

Because the composition of a car is part of the analysis of the problem (and not simply part of the underlying design), making the members public assists the client programmer’s understanding of how to use the class and requires less code complexity for the creator of the class. However, keep in mind that this is a special case and that in general you should make fields private.

When you inherit, you take an existing class and make a special version of it. In general, this means that you’re taking a general-purpose class and specializing it for a particular need. With a little thought, you’ll see that it would make no sense to compose a car using a vehicle object—a car doesn’t contain a vehicle, it is a vehicle. The is-a relationship is expressed with inheritance, and the has-a relationship is expressed with composition.

protected

Now that you’ve been introduced to inheritance, the keyword protected finally has meaning. In an ideal world, private members would always be hard-and-fast private, but in real projects there are times when you want to make something hidden from the world at large and yet allow access for members of derived classes. The protected keyword is a nod to pragmatism. It says “This is private as far as the class user is concerned, but available to anyone who inherits from this class or anyone else in the same package.” That is, protected in Java is automatically “friendly.”

The best tack to take is to leave the data members private—you should always preserve your right to change the underlying implementation. You can then allow controlled access to inheritors of your class through protected methods:

//: c06:Orc.java
// The protected keyword.
import java.util.*;

class Villain {
  private int i;
  protected int read() { return i; }
  protected void set(int ii) { i = ii; }
  public Villain(int ii) { i = ii; }
  public int value(int m) { return m*i; }
}

public class Orc extends Villain {
  private int j;
  public Orc(int jj) { super(jj); j = jj; }
  public void change(int x) { set(x); }
} ///:~

You can see that change( ) has access to set( ) because it’s protected.

Incremental development

One of the advantages of inheritance is that it supports incremental development by allowing you to introduce new code without causing bugs in existing code. This also isolates new bugs inside the new code. By inheriting from an existing, functional class and adding data members and methods (and redefining existing methods), you leave the existing code—that someone else might still be using—untouched and unbugged. If a bug happens, you know that it’s in your new code, which is much shorter and easier to read than if you had modified the body of existing code.

It’s rather amazing how cleanly the classes are separated. You don’t even need the source code for the methods in order to reuse the code. At most, you just import a package. (This is true for both inheritance and composition.)

It’s important to realize that program development is an incremental process, just like human learning. You can do as much analysis as you want, but you still won’t know all the answers when you set out on a project. You’ll have much more success—and more immediate feedback—if you start out to “grow” your project as an organic, evolutionary creature, rather than constructing it all at once like a glass-box skyscraper.

Although inheritance for experimentation can be a useful technique, at some point after things stabilize you need to take a new look at your class hierarchy with an eye to collapsing it into a sensible structure. Remember that underneath it all, inheritance is meant to express a relationship that says “This new class is a type of that old class.” Your program should not be concerned with pushing bits around, but instead with creating and manipulating objects of various types to express a model in the terms that come from the problem space.

Upcasting

The most important aspect of inheritance is not that it provides methods for the new class. It’s the relationship expressed between the new class and the base class. This relationship can be summarized by saying “The new class is a type of the existing class.”

This description is not just a fanciful way of explaining inheritance—it’s supported directly by the language. As an example, consider a base class called Instrument that represents musical instruments and a derived class called Wind. Because inheritance means that all of the methods in the base class are also available in the derived class, any message you can send to the base class can also be sent to the derived class. If the Instrument class has a play( ) method, so will Wind instruments. This means we can accurately say that a Wind object is also a type of Instrument. The following example shows how the compiler supports this notion:

//: c06:Wind.java
// Inheritance & upcasting.
import java.util.*;

class Instrument {
  public void play() {}
  static void tune(Instrument i) {
    // ...
    i.play();
  }
}

// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
  public static void main(String[] args) {
    Wind flute = new Wind();
    Instrument.tune(flute); // Upcasting
  }
} ///:~

What’s interesting in this example is the tune( ) method, which accepts an Instrument reference. However, in Wind.main( ) the tune( ) method is called by giving it a Wind reference. Given that Java is particular about type checking, it seems strange that a method that accepts one type will readily accept another type, until you realize that a Wind object is also an Instrument object, and there’s no method that tune( ) could call for an Instrument that isn’t also in Wind. Inside tune( ), the code works for Instrument and anything derived from Instrument, and the act of converting a Wind reference into an Instrument reference is called upcasting.

Why “upcasting”?

The reason for the term is historical and is based on the way class inheritance diagrams have traditionally been drawn: with the root at the top of the page, growing downward. (Of course, you can draw your diagrams any way you find helpful.) The inheritance diagram for Wind.java is then:

Casting from derived to base moves up on the inheritance diagram, so it’s commonly referred to as upcasting. Upcasting is always safe because you’re going from a more specific type to a more general type. That is, the derived class is a superset of the base class. It might contain more methods than the base class, but it must contain at least the methods in the base class. The only thing that can occur to the class interface during the upcast is that it can lose methods, not gain them. This is why the compiler allows upcasting without any explicit casts or other special notation.

You can also perform the reverse of upcasting, called downcasting, but this involves a dilemma that is the subject of Chapter 12.

Composition vs. inheritance revisited

In object-oriented programming, the most likely way that you’ll create and use code is by simply packaging data and methods together into a class, and using objects of that class. You’ll also use existing classes to build new classes with composition. Less frequently, you’ll use inheritance. So although inheritance gets a lot of emphasis while learning OOP, it doesn’t mean that you should use it everywhere you possibly can. On the contrary, you should use it sparingly, only when it’s clear that inheritance is useful. One of the clearest ways to determine whether you should use composition or inheritance is to ask whether you’ll ever need to upcast from your new class to the base class. If you must upcast, then inheritance is necessary, but if you don’t need to upcast, then you should look closely at whether you need inheritance. The next chapter (polymorphism) provides one of the most compelling reasons for upcasting, but if you remember to ask “Do I need to upcast?”, you’ll have a good tool for deciding between composition and inheritance.

The final keyword

Java’s final keyword has slightly different meanings depending on the context, but in general it says “This cannot be changed.” You might want to prevent changes for two reasons: design or efficiency. Because these two reasons are quite different, it’s possible to misuse the final keyword.

The following sections discuss the three places where final can be used: for data, methods, and classes.

Final data

Many programming languages have a way to tell the compiler that a piece of data is “constant.” A constant is useful for two reasons:

  1. It can be a compile-time constant that won’t ever change.
  2. It can be a value initialized at run-time that you don’t want changed.

In the case of a compile-time constant the compiler is allowed to “fold” the constant value into any calculations in which it’s used; that is, the calculation can be performed at compile-time, eliminating some run-time overhead. In Java, these sorts of constants must be primitives and are expressed using the final keyword. A value must be given at the time of definition of such a constant.

A field that is both static and final has only one piece of storage that cannot be changed.

When using final with object references rather than primitives the meaning gets a bit confusing. With a primitive, final makes the value a constant, but with an object reference, final makes the reference a constant. Once the reference is initialized to an object, it can never be changed to point to another object. However, the object itself can be modified; Java does not provide a way to make any arbitrary object a constant. (You can, however, write your class so that objects have the effect of being constant.) This restriction includes arrays, which are also objects.

Here’s an example that demonstrates final fields:

//: c06:FinalData.java
// The effect of final on fields.

class Value {
  int i = 1;
}

public class FinalData {
  // Can be compile-time constants
  final int i1 = 9;
  static final int VAL_TWO = 99;
  // Typical public constant:
  public static final int VAL_THREE = 39;
  // Cannot be compile-time constants:
  final int i4 = (int)(Math.random()*20);
  static final int i5 = (int)(Math.random()*20);

  Value v1 = new Value();
  final Value v2 = new Value();
  static final Value v3 = new Value();
  // Arrays:
  final int[] a = { 1, 2, 3, 4, 5, 6 };

  public void print(String id) {
    System.out.println(
      id + ": " + "i4 = " + i4 +
      ", i5 = " + i5);
  }
  public static void main(String[] args) {
    FinalData fd1 = new FinalData();
    //! fd1.i1++; // Error: can't change value
    fd1.v2.i++; // Object isn't constant!
    fd1.v1 = new Value(); // OK -- not final
    for(int i = 0; i < fd1.a.length; i++)
      fd1.a[i]++; // Object isn't constant!
    //! fd1.v2 = new Value(); // Error: Can't
    //! fd1.v3 = new Value(); // change reference
    //! fd1.a = new int[3];

    fd1.print("fd1");
    System.out.println("Creating new FinalData");
    FinalData fd2 = new FinalData();
    fd1.print("fd1");
    fd2.print("fd2");
  }
} ///:~

Since i1 and VAL_TWO are final primitives with compile-time values, they can both be used as compile-time constants and are not different in any important way. VAL_THREE is the more typical way you’ll see such constants defined: public so they’re usable outside the package, static to emphasize that there’s only one, and final to say that it’s a constant. Note that final static primitives with constant initial values (that is, compile-time constants) are named with all capitals by convention, with words separated by underscores (this is just like C constants, which is where the convention originated). Also note that i5 cannot be known at compile-time, so it is not capitalized.

Just because something is final doesn’t mean that its value is known at compile-time. This is demonstrated by initializing i4 and i5 at run-time using randomly generated numbers. This portion of the example also shows the difference between making a final value static or non-static. This difference shows up only when the values are initialized at run-time, since the compile-time values are treated the same by the compiler. (And presumably optimized out of existence.) The difference is shown in the output from one run:

fd1: i4 = 15, i5 = 9
Creating new FinalData
fd1: i4 = 15, i5 = 9
fd2: i4 = 10, i5 = 9

Note that the values of i4 for fd1 and fd2 are unique, but the value for i5 is not changed by creating the second FinalData object. That’s because it’s static and is initialized once upon loading and not each time a new object is created.

The variables v1 through v4 demonstrate the meaning of a final reference. As you can see in main( ), just because v2 is final doesn’t mean that you can’t change its value. However, you cannot rebind v2 to a new object, precisely because it’s final. That’s what final means for a reference. You can also see the same meaning holds true for an array, which is just another kind of reference. (There is no way that I know of to make the array references themselves final.) Making references final seems less useful than making primitives final.

Blank finals

Java allows the creation of blank finals, which are fields that are declared as final but are not given an initialization value. In all cases, the blank final must be initialized before it is used, and the compiler ensures this. However, blank finals provide much more flexibility in the use of the final keyword since, for example, a final field inside a class can now be different for each object and yet it retains its immutable quality. Here’s an example:

//: c06:BlankFinal.java
// "Blank" final data members.

class Poppet { }

class BlankFinal {
  final int i = 0; // Initialized final
  final int j; // Blank final
  final Poppet p; // Blank final reference
  // Blank finals MUST be initialized
  // in the constructor:
  BlankFinal() {
    j = 1; // Initialize blank final
    p = new Poppet();
  }
  BlankFinal(int x) {
    j = x; // Initialize blank final
    p = new Poppet();
  }
  public static void main(String[] args) {
    BlankFinal bf = new BlankFinal();
  }
} ///:~

You’re forced to perform assignments to finals either with an expression at the point of definition of the field or in every constructor. This way it’s guaranteed that the final field is always initialized before use.

Final arguments

Java allows you to make arguments final by declaring them as such in the argument list. This means that inside the method you cannot change what the argument reference points to:

//: c06:FinalArguments.java
// Using "final" with method arguments.

class Gizmo {
  public void spin() {}
}

public class FinalArguments {
  void with(final Gizmo g) {
    //! g = new Gizmo(); // Illegal -- g is final
    g.spin();
  }
  void without(Gizmo g) {
    g = new Gizmo(); // OK -- g not final
    g.spin();
  }
  // void f(final int i) { i++; } // Can't change
  // You can only read from a final primitive:
  int g(final int i) { return i + 1; }
  public static void main(String[] args) {
    FinalArguments bf = new FinalArguments();
    bf.without(null);
    bf.with(null);
  }
} ///:~

Note that you can still assign a null reference to an argument that’s final without the compiler catching it, just like you can with a non-final argument.

The methods f( ) and g( ) show what happens when primitive arguments are final: you can read the argument, but you can't change it.

Final methods

There are two reasons for final methods. The first is to put a “lock” on the method to prevent any inheriting class from changing its meaning. This is done for design reasons when you want to make sure that a method’s behavior is retained during inheritance and cannot be overridden.

The second reason for final methods is efficiency. If you make a method final, you are allowing the compiler to turn any calls to that method into inline calls. When the compiler sees a final method call it can (at its discretion) skip the normal approach of inserting code to perform the method call mechanism (push arguments on the stack, hop over to the method code and execute it, hop back and clean off the stack arguments, and deal with the return value) and instead replace the method call with a copy of the actual code in the method body. This eliminates the overhead of the method call. Of course, if a method is big, then your code begins to bloat and you probably won’t see any performance gains from inlining since any improvements will be dwarfed by the amount of time spent inside the method. It is implied that the Java compiler is able to detect these situations and choose wisely whether to inline a final method. However, it’s better to not trust that the compiler is able to do this and make a method final only if it’s quite small or if you want to explicitly prevent overriding.

final and private

Any private methods in a class are implicitly final. Because you can’t access a private method, you can’t override it (even though the compiler doesn’t give an error message if you try to override it, you haven’t overridden the method, you’ve just created a new method). You can add the final specifier to a private method but it doesn’t give that method any extra meaning.

This issue can cause confusion, because if you try to override a private method (which is implicitly final) it seems to work:

//: c06:FinalOverridingIllusion.java
// It only looks like you can override
// a private or private final method.

class WithFinals {
  // Identical to "private" alone:
  private final void f() {
    System.out.println("WithFinals.f()");
  }
  // Also automatically "final":
  private void g() {
    System.out.println("WithFinals.g()");
  }
}

class OverridingPrivate extends WithFinals {
  private final void f() {
    System.out.println("OverridingPrivate.f()");
  }
  private void g() {
    System.out.println("OverridingPrivate.g()");
  }
}

class OverridingPrivate2
  extends OverridingPrivate {
  public final void f() {
    System.out.println("OverridingPrivate2.f()");
  }
  public void g() {
    System.out.println("OverridingPrivate2.g()");
  }
}

public class FinalOverridingIllusion {
  public static void main(String[] args) {
    OverridingPrivate2 op2 =
      new OverridingPrivate2();
    op2.f();
    op2.g();
    // You can upcast:
    OverridingPrivate op = op2;
    // But you can't call the methods:
    //! op.f();
    //! op.g();
    // Same here:
    WithFinals wf = op2;
    //! wf.f();
    //! wf.g();
  }
} ///:~

“Overriding” can only occur if something is part of the base-class interface. That is, you must be able to upcast an object to its base type and call the same method (the point of this will become clear in the next chapter). If a method is private, it isn’t part of the base-class interface. It is just some code that’s hidden away inside the class, and it just happens to have that name, but if you create a public, protected or “friendly” method in the derived class, there’s no connection to the method that might happen to have that name in the base class. Since a private method is unreachable and effectively invisible, it doesn’t factor into anything except for the code organization of the class for which it was defined.

Final classes

When you say that an entire class is final (by preceding its definition with the final keyword), you state that you don’t want to inherit from this class or allow anyone else to do so. In other words, for some reason the design of your class is such that there is never a need to make any changes, or for safety or security reasons you don’t want subclassing. Alternatively, you might be dealing with an efficiency issue and you want to make sure that any activity involved with objects of this class is as efficient as possible.

//: c06:Jurassic.java
// Making an entire class final.

class SmallBrain {}

final class Dinosaur {
  int i = 7;
  int j = 1;
  SmallBrain x = new SmallBrain();
  void f() {}
}

//! class Further extends Dinosaur {}
// error: Cannot extend final class 'Dinosaur'

public class Jurassic {
  public static void main(String[] args) {
    Dinosaur n = new Dinosaur();
    n.f();
    n.i = 40;
    n.j++;
  }
} ///:~

Note that the data members can be final or not, as you choose. The same rules apply to final for data members regardless of whether the class is defined as final. Defining the class as final simply prevents inheritance—nothing more. However, because it prevents inheritance all methods in a final class are implicitly final, since there’s no way to override them. So the compiler has the same efficiency options as it does if you explicitly declare a method final.

You can add the final specifier to a method in a final class, but it doesn’t add any meaning.

Final caution

It can seem to be sensible to make a method final while you’re designing a class. You might feel that efficiency is very important when using your class and that no one could possibly want to override your methods anyway. Sometimes this is true.

But be careful with your assumptions. In general, it’s difficult to anticipate how a class can be reused, especially a general-purpose class. If you define a method as final you might prevent the possibility of reusing your class through inheritance in some other programmer’s project simply because you couldn’t imagine it being used that way.

The standard Java library is a good example of this. In particular, the Java 1.0/1.1 Vector class was commonly used and might have been even more useful if, in the name of efficiency, all the methods hadn’t been made final. It’s easily conceivable that you might want to inherit and override with such a fundamentally useful class, but the designers somehow decided this wasn’t appropriate. This is ironic for two reasons. First, Stack is inherited from Vector, which says that a Stack is a Vector, which isn’t really true from a logical standpoint. Second, many of the most important methods of Vector, such as addElement( ) and elementAt( ) are synchronized, which as you will see in Chapter 14 incurs a significant performance overhead that probably wipes out any gains provided by final. This lends credence to the theory that programmers are consistently bad at guessing where optimizations should occur. It’s just too bad that such a clumsy design made it into the standard library where we must all cope with it. (Fortunately, the Java 2 container library replaces Vector with ArrayList, which behaves much more civilly. Unfortunately, there’s still plenty of new code being written that uses the old container library.)

It’s also interesting to note that Hashtable, another important standard library class, does not have any final methods. As mentioned elsewhere in this book, it’s quite obvious that some classes were designed by completely different people than others. (You’ll see that the method names in Hashtable are much briefer compared to those in Vector, another piece of evidence.) This is precisely the sort of thing that should not be obvious to consumers of a class library. When things are inconsistent it just makes more work for the user. Yet another paean to the value of design and code walkthroughs. (Note that the Java 2 container library replaces Hashtable with HashMap.)

Initialization and
class loading

In more traditional languages, programs are loaded all at once as part of the startup process. This is followed by initialization, and then the program begins. The process of initialization in these languages must be carefully controlled so that the order of initialization of statics doesn’t cause trouble. C++, for example, has problems if one static expects another static to be valid before the second one has been initialized.

Java doesn’t have this problem because it takes a different approach to loading. Because everything in Java is an object, many activities become easier, and this is one of them. As you will learn more fully in the next chapter, the compiled code for each class exists in its own separate file. That file isn’t loaded until the code is needed. In general, you can say that “Class code is loaded at the point of first use.” This is often not until the first object of that class is constructed, but loading also occurs when a static field or static method is accessed.

The point of first use is also where the static initialization takes place. All the static objects and the static code block will be initialized in textual order (that is, the order that you write them down in the class definition) at the point of loading. The statics, of course, are initialized only once.

Initialization with inheritance

It’s helpful to look at the whole initialization process, including inheritance, to get a full picture of what happens. Consider the following code:

//: c06:Beetle.java
// The full process of initialization.

class Insect {
  int i = 9;
  int j;
  Insect() {
    prt("i = " + i + ", j = " + j);
    j = 39;
  }
  static int x1 =
    prt("static Insect.x1 initialized");
  static int prt(String s) {
    System.out.println(s);
    return 47;
  }
}

public class Beetle extends Insect {
  int k = prt("Beetle.k initialized");
  Beetle() {
    prt("k = " + k);
    prt("j = " + j);
  }
  static int x2 =
    prt("static Beetle.x2 initialized");
  public static void main(String[] args) {
    prt("Beetle constructor");
    Beetle b = new Beetle();
  }
} ///:~

The output for this program is:

static Insect.x1 initialized
static Beetle.x2 initialized
Beetle constructor
i = 9, j = 0
Beetle.k initialized
k = 47
j = 39

The first thing that happens when you run Java on Beetle is that you try to access Beetle.main( ) (a static method), so the loader goes out and finds the compiled code for the Beetle class (this happens to be in a file called Beetle.class). In the process of loading it, the loader notices that it has a base class (that’s what the extends keyword says), which it then loads. This will happen whether or not you’re going to make an object of that base class. (Try commenting out the object creation to prove it to yourself.)

If the base class has a base class, that second base class would then be loaded, and so on. Next, the static initialization in the root base class (in this case, Insect) is performed, and then the next derived class, and so on. This is important because the derived-class static initialization might depend on the base class member being initialized properly.

At this point, the necessary classes have all been loaded so the object can be created. First, all the primitives in this object are set to their default values and the object references are set to null—this happens in one fell swoop by setting the memory in the object to binary zero. Then the base-class constructor will be called. In this case the call is automatic, but you can also specify the base-class constructor call (as the first operation in the Beetle( ) constructor) using super. The base class construction goes through the same process in the same order as the derived-class constructor. After the base-class constructor completes, the instance variables are initialized in textual order. Finally, the rest of the body of the constructor is executed.

Summary

Both inheritance and composition allow you to create a new type from existing types. Typically, however, you use composition to reuse existing types as part of the underlying implementation of the new type and inheritance when you want to reuse the interface. Since the derived class has the base-class interface, it can be upcast to the base, which is critical for polymorphism, as you’ll see in the next chapter.

Despite the strong emphasis on inheritance in object-oriented programming, when you start a design you should generally prefer composition during the first cut and use inheritance only when it is clearly necessary. Composition tends to be more flexible. In addition, by using the added artifice of inheritance with your member type, you can change the exact type, and thus the behavior, of those member objects at run-time. Therefore, you can change the behavior of the composed object at run-time.

Although code reuse through composition and inheritance is helpful for rapid project development, you’ll generally want to redesign your class hierarchy before allowing other programmers to become dependent on it. Your goal is a hierarchy in which each class has a specific use and is neither too big (encompassing so much functionality that it’s unwieldy to reuse) nor annoyingly small (you can’t use it by itself or without adding functionality).

Exercises
  1. Create two classes, A and B, with default constructors (empty argument lists) that announce themselves. Inherit a new class called C from A, and create a member of class B inside C. Do not create a constructor for C. Create an object of class C and observe the results.
  2. Modify Exercise 1 so that A and B have constructors with arguments instead of default constructors. Write a constructor for C and perform all initialization within C’s constructor.
  3. Create a simple class. Inside a second class, define a field for an object of the first class. Use lazy initialization to instantiate this object.
  4. Inherit a new class from class Detergent. Override scrub( ) and add anew method called sterilize( ).
  5. Take the file Cartoon.java and comment out the constructor for the Cartoon class. Explain what happens.
  6. Take the file Chess.java and comment out the constructor for the Chess class. Explain what happens.
  7. Prove that default constructors are created for you by the compiler.
  8. Prove that the base-class constructors are (a) always called, and (b) called before derived-class constructors.
  9. Create a base class with only a non-default constructor, and a derived class with both a default and non-default constructor. In the derived-class constructors, call the base-class constructor.
  10. Create a class called Root which contains an instance of each of classes (that you also create) named Component1, Component2, and Component3. Derive a class Stem from Root which also contains an instance of each “component.” All classes should have default constructors that print a message about that class.
  11. Modify Exercise 10 so that each class only has non-default constructors.
  12. Add a proper hierarchy of cleanup( ) methods to all the classes in Exercise 11.
  13. Create a class with a method that is overloaded three times. Inherit a new class, add a new overloading of the method and show that all four methods are available in the derived class.
  14. In Car.java add a service( ) method to Engine and call this method in main( ).
  15. Create a class inside a package. Your class should contain a protected method. Outside of the package, try to call the protected method and explain the results. Now inherit from your class and call the protected method from inside a method of your derived class.
  16. Create a class called Amphibian. From this, inherit a class called Frog. Put appropriate methods in the base class. In main( ), create a Frog and upcast it to Amphibian, and demonstrate that all the methods still work.
  17. Modify Exercise 16 so that Frog overrides the method definitions from the base class (provides new definitions using the same method signatures). Note what happens in main( ).
  18. Create a class with a static final field and a final field and demonstrate the difference between the two.
  19. Create a class with a blank final reference to an object. Perform the initialization of the blank final inside a method (not the constructor) right before you use it. Demonstrate the guarantee that the final must be initialized before use, and that it cannot be changed once initialized.
  20. Create a class with a final method. Inherit from that class and attempt to override that method.
  21. Create a final class and attempt to inherit from it.
  22. Prove that class loading takes place only once. Prove that loading may be caused by either the creation of the first instance of that class, or the access of a static member.
  23. In Beetle.java, inherit a specific type of beetle from class Beetle, following the same format as the existing classes. Trace and explain the output.


7: Polymorphism

Polymorphism is the third essential feature of an object-oriented programming language, after data abstraction and inheritance.

It provides another dimension of separation of interface from implementation, to decouple what from how. Polymorphism allows improved code organization and readability as well as the creation of extensible programs that can be “grown” not only during the original creation of the project but also when new features are desired.

Encapsulation creates new data types by combining characteristics and behaviors. Implementation hiding separates the interface from the implementation by making the details private. This sort of mechanical organization makes ready sense to someone with a procedural programming background. But polymorphism deals with decoupling in terms of types. In the last chapter, you saw how inheritance allows the treatment of an object as its own type or its base type. This ability is critical because it allows many types (derived from the same base type) to be treated as if they were one type, and a single piece of code to work on all those different types equally. The polymorphic method call allows one type to express its distinction from another, similar type, as long as they’re both derived from the same base type. This distinction is expressed through differences in behavior of the methods that you can call through the base class.

In this chapter, you’ll learn about polymorphism (also called dynamic binding or late binding or run-time binding) starting from the basics, with simple examples that strip away everything but the polymorphic behavior of the program.

Upcasting

In Chapter 6 you saw how an object can be used as its own type or as an object of its base type. Taking an object reference and treating it as a reference to its base type is called upcasting because of the way inheritance trees are drawn with the base class at the top.

You also saw a problem arise, which is embodied in the following:

//: c07:music:Music.java
// Inheritance & upcasting.

class Note {
  private int value;
  private Note(int val) { value = val; }
  public static final Note
    MIDDLE_C = new Note(0),
    C_SHARP  = new Note(1),
    B_FLAT   = new Note(2);
} // Etc.

class Instrument {
  public void play(Note n) {
    System.out.println("Instrument.play()");
  }
}

// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
  // Redefine interface method:
  public void play(Note n) {
    System.out.println("Wind.play()");
  }
}

public class Music {
  public static void tune(Instrument i) {
    // ...
    i.play(Note.MIDDLE_C);
  }
  public static void main(String[] args) {
    Wind flute = new Wind();
    tune(flute); // Upcasting
  }
} ///:~

The method Music.tune( ) accepts an Instrument reference, but also anything derived from Instrument. In main( ), you can see this happening as a Wind reference is passed to tune( ), with no cast necessary. This is acceptable; the interface in Instrument must exist in Wind, because Wind is inherited from Instrument. Upcasting from Wind to Instrument may “narrow” that interface, but it cannot make it anything less than the full interface to Instrument.

Why upcast?

This program might seem strange to you. Why should anyone intentionally forget the type of an object? This is what happens when you upcast, and it seems like it could be much more straightforward if tune( ) simply takes a Wind reference as its argument. This brings up an essential point: If you did that, you’d need to write a new tune( ) for every type of Instrument in your system. Suppose we follow this reasoning and add Stringed and Brass instruments:

//: c07:music2:Music2.java
// Overloading instead of upcasting.

class Note {
  private int value;
  private Note(int val) { value = val; }
  public static final Note
    MIDDLE_C = new Note(0),
    C_SHARP = new Note(1),
    B_FLAT = new Note(2);
} // Etc.

class Instrument {
  public void play(Note n) {
    System.out.println("Instrument.play()");
  }
}

class Wind extends Instrument {
  public void play(Note n) {
    System.out.println("Wind.play()");
  }
}

class Stringed extends Instrument {
  public void play(Note n) {
    System.out.println("Stringed.play()");
  }
}

class Brass extends Instrument {
  public void play(Note n) {
    System.out.println("Brass.play()");
  }
}

public class Music2 {
  public static void tune(Wind i) {
    i.play(Note.MIDDLE_C);
  }
  public static void tune(Stringed i) {
    i.play(Note.MIDDLE_C);
  }
  public static void tune(Brass i) {
    i.play(Note.MIDDLE_C);
  }
  public static void main(String[] args) {
    Wind flute = new Wind();
    Stringed violin = new Stringed();
    Brass frenchHorn = new Brass();
    tune(flute); // No upcasting
    tune(violin);
    tune(frenchHorn);
  }
} ///:~

This works, but there’s a major drawback: You must write type-specific methods for each new Instrument class you add. This means more programming in the first place, but it also means that if you want to add a new method like tune( ) or a new type of Instrument, you’ve got a lot of work to do. Add the fact that the compiler won’t give you any error messages if you forget to overload one of your methods and the whole process of working with types becomes unmanageable.

Wouldn’t it be much nicer if you could just write a single method that takes the base class as its argument, and not any of the specific derived classes? That is, wouldn’t it be nice if you could forget that there are derived classes, and write your code to talk only to the base class?

That’s exactly what polymorphism allows you to do. However, most programmers (who come from a procedural programming background) have a bit of trouble with the way polymorphism works.

The twist

The difficulty with Music.java can be seen by running the program. The output is Wind.play( ). This is clearly the desired output, but it doesn’t seem to make sense that it would work that way. Look at the tune( ) method:

  public static void tune(Instrument i) {
    // ...
    i.play(Note.MIDDLE_C);
  }

It receives an Instrument reference. So how can the compiler possibly know that this Instrument reference points to a Wind in this case and not a Brass or Stringed? The compiler can’t. To get a deeper understanding of the issue, it’s helpful to examine the subject of binding.

Method call binding

Connecting a method call to a method body is called binding. When binding is performed before the program is run (by the compiler and linker, if there is one), it’s called early binding. You might not have heard the term before because it has never been an option with procedural languages. C compilers have only one kind of method call, and that’s early binding.

The confusing part of the above program revolves around early binding because the compiler cannot know the correct method to call when it has only an Instrument reference.

The solution is called late binding, which means that the binding occurs at run-time based on the type of object. Late binding is also called dynamic binding or run-time binding. When a language implements late binding, there must be some mechanism to determine the type of the object at run-time and to call the appropriate method. That is, the compiler still doesn’t know the object type, but the method-call mechanism finds out and calls the correct method body. The late-binding mechanism varies from language to language, but you can imagine that some sort of type information must be installed in the objects.

All method binding in Java uses late binding unless a method has been declared final. This means that ordinarily you don’t need to make any decisions about whether late binding will occur—it happens automatically.

Why would you declare a method final? As noted in the last chapter, it prevents anyone from overriding that method. Perhaps more important, it effectively “turns off” dynamic binding, or rather it tells the compiler that dynamic binding isn’t necessary. This allows the compiler to generate slightly more efficient code for final method calls. However, in most cases it won’t make any overall performance difference in your program, so it’s best to only use final as a design decision, and not as an attempt to improve performance.

Producing the right behavior

Once you know that all method binding in Java happens polymorphically via late binding, you can write your code to talk to the base-class and know that all the derived-class cases will work correctly using the same code. Or to put it another way, you “send a message to an object and let the object figure out the right thing to do.”

The classic example in OOP is the “shape” example. This is commonly used because it is easy to visualize, but unfortunately it can confuse novice programmers into thinking that OOP is just for graphics programming, which is of course not the case.

The shape example has a base class called Shape and various derived types: Circle, Square, Triangle, etc. The reason the example works so well is that it’s easy to say “a circle is a type of shape” and be understood. The inheritance diagram shows the relationships:

The upcast could occur in a statement as simple as:

Shape s = new Circle();

Here, a Circle object is created and the resulting reference is immediately assigned to a Shape, which would seem to be an error (assigning one type to another) and yet it’s fine because a Circle is a Shape by inheritance. So the compiler agrees with the statement and doesn’t issue an error message.

Suppose you call one of the base class methods (that have been overridden in the derived classes):

s.draw();

Again, you might expect that Shape’s draw( ) is called because this is, after all, a Shape reference, so how could the compiler know to do anything else? And yet the proper Circle.draw( ) is called because of late binding (polymorphism).

The following example puts it a slightly different way:

//: c07:Shapes.java
// Polymorphism in Java.

class Shape {
  void draw() {}
  void erase() {}
}

class Circle extends Shape {
  void draw() {
    System.out.println("Circle.draw()");
  }
  void erase() {
    System.out.println("Circle.erase()");
  }
}

class Square extends Shape {
  void draw() {
    System.out.println("Square.draw()");
  }
  void erase() {
    System.out.println("Square.erase()");
  }
}

class Triangle extends Shape {
  void draw() {
    System.out.println("Triangle.draw()");
  }
  void erase() {
    System.out.println("Triangle.erase()");
  }
}

public class Shapes {
  public static Shape randShape() {
    switch((int)(Math.random() * 3)) {
      default:
      case 0: return new Circle();
      case 1: return new Square();
      case 2: return new Triangle();
    }
  }
  public static void main(String[] args) {
    Shape[] s = new Shape[9];
    // Fill up the array with shapes:
    for(int i = 0; i < s.length; i++)
      s[i] = randShape();
    // Make polymorphic method calls:
    for(int i = 0; i < s.length; i++)
      s[i].draw();
  }
} ///:~

The base class Shape establishes the common interface to anything inherited from Shape—that is, all shapes can be drawn and erased. The derived classes override these definitions to provide unique behavior for each specific type of shape.

The main class Shapes contains a static method randShape( ) that produces a reference to a randomly-selected Shape object each time you call it. Note that the upcasting happens in each of the return statements, which take a reference to a Circle, Square, or Triangle and sends it out of the method as the return type, Shape. So whenever you call this method you never get a chance to see what specific type it is, since you always get back a plain Shape reference.

main( ) contains an array of Shape references filled through calls to randShape( ). At this point you know you have Shapes, but you don’t know anything more specific than that (and neither does the compiler). However, when you step through this array and call draw( ) for each one, the correct type-specific behavior magically occurs, as you can see from one output example:

Circle.draw()
Triangle.draw()
Circle.draw()
Circle.draw()
Circle.draw()
Square.draw()
Triangle.draw()
Square.draw()
Square.draw()

Of course, since the shapes are all chosen randomly each time, your runs will have different results. The point of choosing the shapes randomly is to drive home the understanding that the compiler can have no special knowledge that allows it to make the correct calls at compile-time. All the calls to draw( ) are made through dynamic binding.

Extensibility

Now let’s return to the musical instrument example. Because of polymorphism, you can add as many new types as you want to the system without changing the tune( ) method. In a well-designed OOP program, most or all of your methods will follow the model of tune( ) and communicate only with the base-class interface. Such a program is extensible because you can add new functionality by inheriting new data types from the common base class. The methods that manipulate the base-class interface will not need to be changed at all to accommodate the new classes.

Consider what happens if you take the instrument example and add more methods in the base class and a number of new classes. Here’s the diagram:

All these new classes work correctly with the old, unchanged tune( ) method. Even if tune( ) is in a separate file and new methods are added to the interface of Instrument, tune( ) works correctly without recompilation. Here is the implementation of the above diagram:

//: c07:music3:Music3.java
// An extensible program.
import java.util.*;

class Instrument {
  public void play() {
    System.out.println("Instrument.play()");
  }
  public String what() {
    return "Instrument";
  }
  public void adjust() {}
}

class Wind extends Instrument {
  public void play() {
    System.out.println("Wind.play()");
  }
  public String what() { return "Wind"; }
  public void adjust() {}
}

class Percussion extends Instrument {
  public void play() {
    System.out.println("Percussion.play()");
  }
  public String what() { return "Percussion"; }
  public void adjust() {}
}

class Stringed extends Instrument {
  public void play() {
    System.out.println("Stringed.play()");
  }
  public String what() { return "Stringed"; }
  public void adjust() {}
}

class Brass extends Wind {
  public void play() {
    System.out.println("Brass.play()");
  }
  public void adjust() {
    System.out.println("Brass.adjust()");
  }
}

class Woodwind extends Wind {
  public void play() {
    System.out.println("Woodwind.play()");
  }
  public String what() { return "Woodwind"; }
}

public class Music3 {
  // Doesn't care about type, so new types
  // added to the system still work right:
  static void tune(Instrument i) {
    // ...
    i.play();
  }
  static void tuneAll(Instrument[] e) {
    for(int i = 0; i < e.length; i++)
      tune(e[i]);
  }
  public static void main(String[] args) {
    Instrument[] orchestra = new Instrument[5];
    int i = 0;
    // Upcasting during addition to the array:
    orchestra[i++] = new Wind();
    orchestra[i++] = new Percussion();
    orchestra[i++] = new Stringed();
    orchestra[i++] = new Brass();
    orchestra[i++] = new Woodwind();
    tuneAll(orchestra);
  }
} ///:~

The new methods are what( ), which returns a String reference with a description of the class, and adjust( ), which provides some way to adjust each instrument.

In main( ), when you place something inside the Instrument array you automatically upcast to Instrument.

You can see that the tune( ) method is blissfully ignorant of all the code changes that have happened around it, and yet it works correctly. This is exactly what polymorphism is supposed to provide. Your code changes don’t cause damage to parts of the program that should not be affected. Put another way, polymorphism is one of the most important techniques that allow the programmer to “separate the things that change from the things that stay the same.”

Overriding vs. overloading

Let’s take a different look at the first example in this chapter. In the following program, the interface of the method play( ) is changed in the process of overriding it, which means that you haven’t overridden the method, but instead overloaded it. The compiler allows you to overload methods so it gives no complaint. But the behavior is probably not what you want. Here’s the example:

//: c07:WindError.java
// Accidentally changing the interface.

class NoteX {
  public static final int
    MIDDLE_C = 0, C_SHARP = 1, C_FLAT = 2;
}

class InstrumentX {
  public void play(int NoteX) {
    System.out.println("InstrumentX.play()");
  }
}

class WindX extends InstrumentX {
  // OOPS! Changes the method interface:
  public void play(NoteX n) {
    System.out.println("WindX.play(NoteX n)");
  }
}

public class WindError {
  public static void tune(InstrumentX i) {
    // ...
    i.play(NoteX.MIDDLE_C);
  }
  public static void main(String[] args) {
    WindX flute = new WindX();
    tune(flute); // Not the desired behavior!
  }
} ///:~

There’s another confusing aspect thrown in here. In InstrumentX, the play( ) method takes an int that has the identifier NoteX. That is, even though NoteX is a class name, it can also be used as an identifier without complaint. But in WindX, play( ) takes a NoteX reference that has an identifier n. (Although you could even say play(NoteX NoteX) without an error.) Thus it appears that the programmer intended to override play( ) but mistyped the method a bit. The compiler, however, assumed that an overload and not an override was intended. Note that if you follow the standard Java naming convention, the argument identifier would be noteX (lowercase ‘n’), which would distinguish it from the class name.

In tune, the InstrumentX i is sent the play( ) message, with one of NoteX’s members (MIDDLE_C) as an argument. Since NoteX contains int definitions, this means that the int version of the now-overloaded play( ) method is called, and since that has not been overridden the base-class version is used.

The output is:

InstrumentX.play()

This certainly doesn’t appear to be a polymorphic method call. Once you understand what’s happening, you can fix the problem fairly easily, but imagine how difficult it might be to find the bug if it’s buried in a program of significant size.

Abstract classes
and methods

In all the instrument examples, the methods in the base class Instrument were always “dummy” methods. If these methods are ever called, you’ve done something wrong. That’s because the intent of Instrument is to create a common interface for all the classes derived from it.

The only reason to establish this common interface is so it can be expressed differently for each different subtype. It establishes a basic form, so you can say what’s in common with all the derived classes. Another way of saying this is to call Instrument an abstract base class (or simply an abstract class). You create an abstract class when you want to manipulate a set of classes through this common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. (However, as seen in the last section, if the method’s name is the same as the base class but the arguments are different, you’ve got overloading, which probably isn’t what you want.)

If you have an abstract class like Instrument, objects of that class almost always have no meaning. That is, Instrument is meant to express only the interface, and not a particular implementation, so creating an Instrument object makes no sense, and you’ll probably want to prevent the user from doing it. This can be accomplished by making all the methods in Instrument print error messages, but that delays the information until run-time and requires reliable exhaustive testing on the user’s part. It’s always better to catch problems at compile-time.

Java provides a mechanism for doing this called the abstract method [38] . This is a method that is incomplete; it has only a declaration and no method body. Here is the syntax for an abstract method declaration:

abstract void f();

A class containing abstract methods is called an abstract class. If a class contains one or more abstract methods, the class must be qualified as abstract. (Otherwise, the compiler gives you an error message.)

If an abstract class is incomplete, what is the compiler supposed to do when someone tries to make an object of that class? It cannot safely create an object of an abstract class, so you get an error message from the compiler. This way the compiler ensures the purity of the abstract class, and you don’t need to worry about misusing it.

If you inherit from an abstract class and you want to make objects of the new type, you must provide method definitions for all the abstract methods in the base class. If you don’t (and you may choose not to), then the derived class is also abstract and the compiler will force you to qualify that class with the abstract keyword.

It’s possible to create a class as abstract without including any abstract methods. This is useful when you’ve got a class in which it doesn’t make sense to have any abstract methods, and yet you want to prevent any instances of that class.

The Instrument class can easily be turned into an abstract class. Only some of the methods will be abstract, since making a class abstract doesn’t force you to make all the methods abstract. Here’s what it looks like:

Here’s the orchestra example modified to use abstract classes and methods:

//: c07:music4:Music4.java
// Abstract classes and methods.
import java.util.*;

abstract class Instrument {
  int i; // storage allocated for each
  public abstract void play();
  public String what() {
    return "Instrument";
  }
  public abstract void adjust();
}

class Wind extends Instrument {
  public void play() {
    System.out.println("Wind.play()");
  }
  public String what() { return "Wind"; }
  public void adjust() {}
}

class Percussion extends Instrument {
  public void play() {
    System.out.println("Percussion.play()");
  }
  public String what() { return "Percussion"; }
  public void adjust() {}
}

class Stringed extends Instrument {
  public void play() {
    System.out.println("Stringed.play()");
  }
  public String what() { return "Stringed"; }
  public void adjust() {}
}

class Brass extends Wind {
  public void play() {
    System.out.println("Brass.play()");
  }
  public void adjust() {
    System.out.println("Brass.adjust()");
  }
}

class Woodwind extends Wind {
  public void play() {
    System.out.println("Woodwind.play()");
  }
  public String what() { return "Woodwind"; }
}

public class Music4 {
  // Doesn't care about type, so new types
  // added to the system still work right:
  static void tune(Instrument i) {
    // ...
    i.play();
  }
  static void tuneAll(Instrument[] e) {
    for(int i = 0; i < e.length; i++)
      tune(e[i]);
  }
  public static void main(String[] args) {
    Instrument[] orchestra = new Instrument[5];
    int i = 0;
    // Upcasting during addition to the array:
    orchestra[i++] = new Wind();
    orchestra[i++] = new Percussion();
    orchestra[i++] = new Stringed();
    orchestra[i++] = new Brass();
    orchestra[i++] = new Woodwind();
    tuneAll(orchestra);
  }
} ///:~

You can see that there’s really no change except in the base class.

It’s helpful to create abstract classes and methods because they make the abstractness of a class explicit and tell both the user and the compiler how it was intended to be used.

Constructors and polymorphism

As usual, constructors are different from other kinds of methods. This is also true when polymorphism is involved. Even though constructors are not polymorphic (although you can have a kind of “virtual constructor,” as you will see in Chapter 12), it’s important to understand the way constructors work in complex hierarchies and with polymorphism. This understanding will help you avoid unpleasant entanglements.

Order of constructor calls

The order of constructor calls was briefly discussed in Chapter 4 and again in Chapter 7, but that was before polymorphism was introduced.

A constructor for the base class is always called in the constructor for a derived class, chaining up the inheritance hierarchy so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically private). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore, it’s essential that all constructors get called, otherwise the entire object wouldn’t be constructed. That’s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don’t explicitly call a base-class constructor in the derived-class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors, the compiler will automatically synthesize a default constructor.)

Let’s take a look at an example that shows the effects of composition, inheritance, and polymorphism on the order of construction:

//: c07:Sandwich.java
// Order of constructor calls.

class Meal {
  Meal() { System.out.println("Meal()"); }
}

class Bread {
  Bread() { System.out.println("Bread()"); }
}

class Cheese {
  Cheese() { System.out.println("Cheese()"); }
}

class Lettuce {
  Lettuce() { System.out.println("Lettuce()"); }
}

class Lunch extends Meal {
  Lunch() { System.out.println("Lunch()");}
}

class PortableLunch extends Lunch {
  PortableLunch() {
    System.out.println("PortableLunch()");
  }
}

class Sandwich extends PortableLunch {
  Bread b = new Bread();
  Cheese c = new Cheese();
  Lettuce l = new Lettuce();
  Sandwich() {
    System.out.println("Sandwich()");
  }
  public static void main(String[] args) {
    new Sandwich();
  }
} ///:~

This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is Sandwich, which reflects three levels of inheritance (four, if you count the implicit inheritance from Object) and three member objects. When a Sandwich object is created in main( ), the output is:

Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()

This means that the order of constructor calls for a complex object is as follows:

  1. The base-class constructor is called. This step is repeated recursively such that the root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached.
  2. Member initializers are called in the order of declaration.
  3. The body of the derived-class constructor is called.

The order of the constructor calls is important. When you inherit, you know all about the base class and can access any public and protected members of the base class. This means that you must be able to assume that all the members of the base class are valid when you’re in the derived class. In a normal method, construction has already taken place, so all the members of all parts of the object have been built. Inside the constructor, however, you must be able to assume that all members that you use have been built. The only way to guarantee this is for the base-class constructor to be called first. Then when you’re in the derived-class constructor, all the members you can access in the base class have been initialized. “Knowing that all members are valid” inside the constructor is also the reason that, whenever possible, you should initialize all member objects (that is, objects placed in the class using composition) at their point of definition in the class (e.g.: b, c, and l in the example above). If you follow this practice, you will help ensure that all base class members and member objects of the current object have been initialized. Unfortunately, this doesn’t handle every case, as you will see in the next section.

Inheritance and finalize( )

When you use composition to create a new class, you never worry about finalizing the member objects of that class. Each member is an independent object and thus is garbage collected and finalized regardless of whether it happens to be a member of your class. With inheritance, however, you must override finalize( ) in the derived class if you have any special cleanup that must happen as part of garbage collection. When you override finalize( ) in an inherited class, it’s important to remember to call the base-class version of finalize( ), since otherwise the base-class finalization will not happen. The following example proves this:

//: c07:Frog.java
// Testing finalize with inheritance.

class DoBaseFinalization {
  public static boolean flag = false;
}

class Characteristic {
  String s;
  Characteristic(String c) {
    s = c;
    System.out.println(
      "Creating Characteristic " + s);
  }
  protected void finalize() {
    System.out.println(
      "finalizing Characteristic " + s);
  }
}

class LivingCreature {
  Characteristic p =
    new Characteristic("is alive");
  LivingCreature() {
    System.out.println("LivingCreature()");
  }
  protected void finalize() {
    System.out.println(
      "LivingCreature finalize");
    // Call base-class version LAST!
    if(DoBaseFinalization.flag)
      try {
        super.finalize();
      } catch(Throwable t) {}
  }
}

class Animal extends LivingCreature {
  Characteristic p =
    new Characteristic("has heart");
  Animal() {
    System.out.println("Animal()");
  }
  protected void finalize() {
    System.out.println("Animal finalize");
    if(DoBaseFinalization.flag)
      try {
        super.finalize();
      } catch(Throwable t) {}
  }
}

class Amphibian extends Animal {
  Characteristic p =
    new Characteristic("can live in water");
  Amphibian() {
    System.out.println("Amphibian()");
  }
  protected void finalize() {
    System.out.println("Amphibian finalize");
    if(DoBaseFinalization.flag)
      try {
        super.finalize();
      } catch(Throwable t) {}
  }
}

public class Frog extends Amphibian {
  Frog() {
    System.out.println("Frog()");
  }
  protected void finalize() {
    System.out.println("Frog finalize");
    if(DoBaseFinalization.flag)
      try {
        super.finalize();
      } catch(Throwable t) {}
  }
  public static void main(String[] args) {
    if(args.length != 0 &&
       args[0].equals("finalize"))
       DoBaseFinalization.flag = true;
    else
      System.out.println("not finalizing bases");
    new Frog(); // Instantly becomes garbage
    System.out.println("bye!");
    // Force finalizers to be called:
    System.gc();
  }
} ///:~

The class DoBaseFinalization simply holds a flag that indicates to each class in the hierarchy whether to call super.finalize( ). This flag is set based on a command-line argument, so you can view the behavior with and without base-class finalization.

Each class in the hierarchy also contains a member object of class Characteristic. You will see that regardless of whether the base class finalizers are called, the Characteristic member objects are always finalized.

Each overridden finalize( ) must have access to at least protected members since the finalize( ) method in class Object is protected and the compiler will not allow you to reduce the access during inheritance. (“Friendly” is less accessible than protected.)

In Frog.main( ), the DoBaseFinalization flag is configured and a single Frog object is created. Remember that garbage collection and in particular finalization might not happen for any particular object so to enforce this, so System.gc( ) forces garbage collection, and thus finalization. Without base-class finalization, the output is:

not finalizing bases
Creating Characteristic is alive
LivingCreature()
Creating Characteristic has heart
Animal()
Creating Characteristic can live in water
Amphibian()
Frog()
bye!
Frog finalize
finalizing Characteristic is alive
finalizing Characteristic has heart
finalizing Characteristic can live in water

You can see that, indeed, no finalizers are called for the base classes of Frog (the member objects are finalized, as you would expect). But if you add the “finalize” argument on the command line, you get:

Creating Characteristic is alive
LivingCreature()
Creating Characteristic has heart
Animal()
Creating Characteristic can live in water
Amphibian()
Frog()
bye!
Frog finalize
Amphibian finalize
Animal finalize
LivingCreature finalize
finalizing Characteristic is alive
finalizing Characteristic has heart
finalizing Characteristic can live in water

Although the order the member objects are finalized is the same order that they are created, technically the order of finalization of objects is unspecified. With base classes, however, you have control over the order of finalization. The best order to use is the one that’s shown here, which is the reverse of the order of initialization. Following the form that’s used in C++ for destructors, you should perform the derived-class finalization first, then the base-class finalization. That’s because the derived-class finalization could call some methods in the base class that require that the base-class components are still alive, so you must not destroy them prematurely.

Behavior of polymorphic methods
inside constructors

The hierarchy of constructor calls brings up an interesting dilemma. What happens if you’re inside a constructor and you call a dynamically-bound method of the object being constructed? Inside an ordinary method you can imagine what will happen—the dynamically-bound call is resolved at run-time because the object cannot know whether it belongs to the class the method is in or some class derived from it. For consistency, you might think this is what should happen inside constructors.

This is not exactly the case. If you call a dynamically-bound method inside a constructor, the overridden definition for that method is used. However, the effect can be rather unexpected, and can conceal some difficult-to-find bugs.

Conceptually, the constructor’s job is to bring the object into existence (which is hardly an ordinary feat). Inside any constructor, the entire object might be only partially formed—you can know only that the base-class objects have been initialized, but you cannot know which classes are inherited from you. A dynamically-bound method call, however, reaches “outward” into the inheritance hierarchy. It calls a method in a derived class. If you do this inside a constructor, you call a method that might manipulate members that haven’t been initialized yet—a sure recipe for disaster.

You can see the problem in the following example:

//: c07:PolyConstructors.java
// Constructors and polymorphism
// don't produce what you might expect.

abstract class Glyph {
  abstract void draw();
  Glyph() {
    System.out.println("Glyph() before draw()");
    draw();
    System.out.println("Glyph() after draw()");
  }
}

class RoundGlyph extends Glyph {
  int radius = 1;
  RoundGlyph(int r) {
    radius = r;
    System.out.println(
      "RoundGlyph.RoundGlyph(), radius = "
      + radius);
  }
  void draw() {
    System.out.println(
      "RoundGlyph.draw(), radius = " + radius);
  }
}

public class PolyConstructors {
  public static void main(String[] args) {
    new RoundGlyph(5);
  }
} ///:~

In Glyph, the draw( ) method is abstract, so it is designed to be overridden. Indeed, you are forced to override it in RoundGlyph. But the Glyph constructor calls this method, and the call ends up in RoundGlyph.draw( ), which would seem to be the intent. But look at the output:

Glyph() before draw()
RoundGlyph.draw(), radius = 0
Glyph() after draw()
RoundGlyph.RoundGlyph(), radius = 5

When Glyph’s constructor calls draw( ), the value of radius isn’t even the default initial value 1. It’s zero. This would probably result in either a dot or nothing at all being drawn on the screen, and you’d be staring, trying to figure out why the program won’t work.

The order of initialization described in the previous section isn’t quite complete, and that’s the key to solving the mystery. The actual process of initialization is:

  1. The storage allocated for the object is initialized to binary zero before anything else happens.
  2. The base-class constructors are called as described previously. At this point, the overridden draw( ) method is called, (yes, before the RoundGlyph constructor is called), which discovers a radius value of zero, due to step one.
  3. Member initializers are called in the order of declaration.
  4. The body of the derived-class constructor is called.

There’s an upside to this, which is that everything is at least initialized to zero (or whatever zero means for that particular data type) and not just left as garbage. This includes object references that are embedded inside a class via composition, which become null. So if you forget to initialize that reference you’ll get an exception at run-time. Everything else gets zero, which is usually a telltale value when looking at output.

On the other hand, you should be pretty horrified at the outcome of this program. You’ve done a perfectly logical thing and yet the behavior is mysteriously wrong, with no complaints from the compiler. (C++ produces more rational behavior in this situation.) Bugs like this could easily be buried and take a long time to discover.

As a result, a good guideline for constructors is, “Do as little as possible to set the object into a good state, and if you can possibly avoid it, don’t call any methods.” The only safe methods to call inside a constructor are those that are final in the base class. (This also applies to private methods, which are automatically final.) These cannot be overridden and thus cannot produce this kind of surprise.

Designing with inheritance

Once you learn about polymorphism, it can seem that everything ought to be inherited because polymorphism is such a clever tool. This can burden your designs; in fact if you choose inheritance first when you’re using an existing class to make a new class, things can become needlessly complicated.

A better approach is to choose composition first, when it’s not obvious which one you should use. Composition does not force a design into an inheritance hierarchy. But composition is also more flexible since it’s possible to dynamically choose a type (and thus behavior) when using composition, whereas inheritance requires an exact type to be known at compile-time. The following example illustrates this:

//: c07:Transmogrify.java
// Dynamically changing the behavior of
// an object via composition.

abstract class Actor {
  abstract void act();
}

class HappyActor extends Actor {
  public void act() {
    System.out.println("HappyActor");
  }
}

class SadActor extends Actor {
  public void act() {
    System.out.println("SadActor");
  }
}

class Stage {
  Actor a = new HappyActor();
  void change() { a = new SadActor(); }
  void go() { a.act(); }
}

public class Transmogrify {
  public static void main(String[] args) {
    Stage s = new Stage();
    s.go(); // Prints "HappyActor"
    s.change();
    s.go(); // Prints "SadActor"
  }
} ///:~

A Stage object contains a reference to an Actor, which is initialized to a HappyActor object. This means go( ) produces a particular behavior. But since a reference can be rebound to a different object at run-time, a reference for a SadActor object can be substituted in a and then the behavior produced by go( ) changes. Thus you gain dynamic flexibility at run-time. (This is also called the State Pattern. See Thinking in Patterns with Java, downloadable at www.BruceEckel.com.) In contrast, you can’t decide to inherit differently at run-time; that must be completely determined at compile-time.

A general guideline is “Use inheritance to express differences in behavior, and fields to express variations in state.” In the above example, both are used: two different classes are inherited to express the difference in the act( ) method, and Stage uses composition to allow its state to be changed. In this case, that change in state happens to produce a change in behavior.

Pure inheritance vs. extension

When studying inheritance, it would seem that the cleanest way to create an inheritance hierarchy is to take the “pure” approach. That is, only methods that have been established in the base class or interface are to be overridden in the derived class, as seen in this diagram:

This can be termed a pure “is-a” relationship because the interface of a class establishes what it is. Inheritance guarantees that any derived class will have the interface of the base class and nothing less. If you follow the above diagram, derived classes will also have no more than the base class interface.

This can be thought of as pure substitution, because derived class objects can be perfectly substituted for the base class, and you never need to know any extra information about the subclasses when you’re using them:

That is, the base class can receive any message you can send to the derived class because the two have exactly the same interface. All you need to do is upcast from the derived class and never look back to see what exact type of object you’re dealing with. Everything is handled through polymorphism.

When you see it this way, it seems like a pure “is-a” relationship is the only sensible way to do things, and any other design indicates muddled thinking and is by definition broken. This too is a trap. As soon as you start thinking this way, you’ll turn around and discover that extending the interface (which, unfortunately, the keyword extends seems to encourage) is the perfect solution to a particular problem. This could be termed an “is-like-a” relationship because the derived class is like the base class—it has the same fundamental interface—but it has other features that require additional methods to implement:

While this is also a useful and sensible approach (depending on the situation) it has a drawback. The extended part of the interface in the derived class is not available from the base class, so once you upcast you can’t call the new methods:

If you’re not upcasting in this case, it won’t bother you, but often you’ll get into a situation in which you need to rediscover the exact type of the object so you can access the extended methods of that type. The following section shows how this is done.

Downcasting and run-time
type identification

Since you lose the specific type information via an upcast (moving up the inheritance hierarchy), it makes sense that to retrieve the type information—that is, to move back down the inheritance hierarchy—you use a downcast. However, you know an upcast is always safe; the base class cannot have a bigger interface than the derived class, therefore every message you send through the base class interface is guaranteed to be accepted. But with a downcast, you don’t really know that a shape (for example) is actually a circle. It could instead be a triangle or square or some other type.

To solve this problem there must be some way to guarantee that a downcast is correct, so you won’t accidentally cast to the wrong type and then send a message that the object can’t accept. This would be quite unsafe.

In some languages (like C++) you must perform a special operation in order to get a type-safe downcast, but in Java every cast is checked! So even though it looks like you’re just performing an ordinary parenthesized cast, at run-time this cast is checked to ensure that it is in fact the type you think it is. If it isn’t, you get a ClassCastException. This act of checking types at run-time is called run-time type identification (RTTI). The following example demonstrates the behavior of RTTI:

//: c07:RTTI.java
// Downcasting & Run-time Type
// Identification (RTTI).
import java.util.*;

class Useful {
  public void f() {}
  public void g() {}
}

class MoreUseful extends Useful {
  public void f() {}
  public void g() {}
  public void u() {}
  public void v() {}
  public void w() {}
}

public class RTTI {
  public static void main(String[] args) {
    Useful[] x = {
      new Useful(),
      new MoreUseful()
    };
    x[0].f();
    x[1].g();
    // Compile-time: method not found in Useful:
    //! x[1].u();
    ((MoreUseful)x[1]).u(); // Downcast/RTTI
    ((MoreUseful)x[0]).u(); // Exception thrown
  }
} ///:~

As in the diagram, MoreUseful extends the interface of Useful. But since it’s inherited, it can also be upcast to a Useful. You can see this happening in the initialization of the array x in main( ). Since both objects in the array are of class Useful, you can send the f( ) and g( ) methods to both, and if you try to call u( ) (which exists only in MoreUseful) you’ll get a compile-time error message.

If you want to access the extended interface of a MoreUseful object, you can try to downcast. If it’s the correct type, it will be successful. Otherwise, you’ll get a ClassCastException. You don’t need to write any special code for this exception, since it indicates a programmer error that could happen anywhere in a program.

There’s more to RTTI than a simple cast. For example, there’s a way to see what type you’re dealing with before you try to downcast it. All of Chapter 12 is devoted to the study of different aspects of Java run-time type identification.

Summary

Polymorphism means “different forms.” In object-oriented programming, you have the same face (the common interface in the base class) and different forms using that face: the different versions of the dynamically-bound methods.

You’ve seen in this chapter that it’s impossible to understand, or even create, an example of polymorphism without using data abstraction and inheritance. Polymorphism is a feature that cannot be viewed in isolation (like a switch statement can, for example), but instead works only in concert, as part of a “big picture” of class relationships. People are often confused by other, non-object-oriented features of Java, like method overloading, which are sometimes presented as object-oriented. Don’t be fooled: If it isn’t late binding, it isn’t polymorphism.

To use polymorphism—and thus object-oriented techniques—effectively in your programs you must expand your view of programming to include not just members and messages of an individual class, but also the commonality among classes and their relationships with each other. Although this requires significant effort, it’s a worthy struggle, because the results are faster program development, better code organization, extensible programs, and easier code maintenance.

Exercises
  1. Add a new method in the base class of Shapes.java that prints a message, but don’t override it in the derived classes. Explain what happens. Now override it in one of the derived classes but not the others, and see what happens. Finally, override it in all the derived classes.
  2. Add a new type of Shape to Shapes.java and verify in main( ) that polymorphism works for your new type as it does in the old types.
  3. Change Music3.java so that what( ) becomes the root Object method toString( ). Try printing the Instrument objects using System.out.println( ) (without any casting).
  4. Add a new type of Instrument to Music3.java and verify that polymorphism works for your new type.
  5. Modify Music3.java so that it randomly creates Instrument objects the way Shapes.java does.
  6. Create an inheritance hierarchy of Rodent: Mouse, Gerbil, Hamster, etc. In the base class, provide methods that are common to all Rodents, and override these in the derived classes to perform different behaviors depending on the specific type of Rodent. Create an array of Rodent, fill it with different specific types of Rodents, and call your base-class methods to see what happens.
  7. Modify Exercise 6 so that Rodent is an abstract class. Make the methods of Rodent abstract whenever possible.
  8. Create a class as abstract without including any abstract methods, and verify that you cannot create any instances of that class.
  9. Add class Pickle to Sandwich.java.
  10. Modify Exercise 6 so that it demonstrates the order of initialization of the base classes and derived classes. Now add member objects to both the base and derived classes, and show the order in which their initialization occurs during construction.
  11. Create a 3-level inheritance hierarchy. Each class in the hierarchy should have a finalize( ) method, and it should properly call the base-class version of finalize( ). Demonstrate that your hierarchy works properly.
  12. Create a base class with two methods. In the first method, call the second method. Inherit a class and override the second method. Create an object of the derived class, upcast it to the base type, and call the first method. Explain what happens.
  13. Create a base class with an abstract print( ) method that is overridden in a derived class. The overridden version of the method prints the value of an int variable defined in the derived class. At the point of definition of this variable, give it a non-zero value. In the base-class constructor, call this method. In main( ), create an object of the derived type, and then call its print( ) method. Explain the results.
  14. Following the example in Transmogrify.java, create a Starship class that has an AlertStatus reference which can indicate three different states. Include methods to change the states.
  15. Create an abstract class with no methods. Derive a class and add a method. Create a static method that takes a reference to the base class, downcasts it to the derived class, and calls the method. In main( ), demonstrate that it works. Now put the abstract declaration for the method in the base class, thus eliminating the need for the downcast.

[38] For C++ programmers, this is the analogue of C++’s pure virtual function.


8: Interfaces & Inner Classes

Interfaces and inner classes provide more sophisticated ways to organize and control the objects in your system.

C++, for example, does not contain such mechanisms, although the clever programmer may simulate them. The fact that they exist in Java indicates that they were considered important enough to provide direct support through language keywords.

In Chapter 7, you learned about the abstract keyword, which allows you to create one or more methods in a class that have no definitions—you provide part of the interface without providing a corresponding implementation, which is created by inheritors. The interface keyword produces a completely abstract class, one that provides no implementation at all. You’ll learn that the interface is more than just an abstract class taken to the extreme, since it allows you to perform a variation on C++’s “multiple inheritance,” by creating a class that can be upcast to more than one base type.

At first, inner classes look like a simple code hiding mechanism: you place classes inside other classes. You’ll learn, however, that the inner class does more than that—it knows about and can communicate with the surrounding class—and that the kind of code you can write with inner classes is more elegant and clear, although it is a new concept to most so it takes some time to become comfortable with design using inner classes.

Interfaces

The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but these are implicitly static and final. An interface provides only a form, but no implementation.

An interface says: “This is what all classes that implement this particular interface will look like.” Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes. (Some object-oriented programming languages have a keyword called protocol to do the same thing.)

To create an interface, use the interface keyword instead of the class keyword. Like a class, you can add the public keyword before the interface keyword (but only if that interface is defined in a file of the same name) or leave it off to give “friendly” status so that it is only usable within the same package.

To make a class that conforms to a particular interface (or group of interfaces) use the implements keyword. You’re saying “The interface is what it looks like but now I’m going to say how it works.” Other than that, it looks like inheritance. The diagram for the instrument example shows this:

Once you’ve implemented an interface, that implementation becomes an ordinary class that can be extended in the regular way.

You can choose to explicitly declare the method declarations in an interface as public. But they are public even if you don’t say it. So when you implement an interface, the methods from the interface must be defined as public. Otherwise they would default to “friendly” and you’d be reducing the accessibility of a method during inheritance, which is not allowed by the Java compiler.

You can see this in the modified version of the Instrument example. Note that every method in the interface is strictly a declaration, which is the only thing the compiler allows. In addition, none of the methods in Instrument are declared as public, but they’re automatically public anyway:

//: c08:music5:Music5.java
// Interfaces.
import java.util.*;

interface Instrument {
  // Compile-time constant:
  int i = 5; // static & final
  // Cannot have method definitions:
  void play(); // Automatically public
  String what();
  void adjust();
}

class Wind implements Instrument {
  public void play() {
    System.out.println("Wind.play()");
  }
  public String what() { return "Wind"; }
  public void adjust() {}
}

class Percussion implements Instrument {
  public void play() {
    System.out.println("Percussion.play()");
  }
  public String what() { return "Percussion"; }
  public void adjust() {}
}

class Stringed implements Instrument {
  public void play() {
    System.out.println("Stringed.play()");
  }
  public String what() { return "Stringed"; }
  public void adjust() {}
}

class Brass extends Wind {
  public void play() {
    System.out.println("Brass.play()");
  }
  public void adjust() {
    System.out.println("Brass.adjust()");
  }
}

class Woodwind extends Wind {
  public void play() {
    System.out.println("Woodwind.play()");
  }
  public String what() { return "Woodwind"; }
}

public class Music5 {
  // Doesn't care about type, so new types
  // added to the system still work right:
  static void tune(Instrument i) {
    // ...
    i.play();
  }
  static void tuneAll(Instrument[] e) {
    for(int i = 0; i < e.length; i++)
      tune(e[i]);
  }
  public static void main(String[] args) {
    Instrument[] orchestra = new Instrument[5];
    int i = 0;
    // Upcasting during addition to the array:
    orchestra[i++] = new Wind();
    orchestra[i++] = new Percussion();
    orchestra[i++] = new Stringed();
    orchestra[i++] = new Brass();
    orchestra[i++] = new Woodwind();
    tuneAll(orchestra);
  }
} ///:~

The rest of the code works the same. It doesn’t matter if you are upcasting to a “regular” class called Instrument, an abstract class called Instrument, or to an interface called Instrument. The behavior is the same. In fact, you can see in the tune( ) method that there isn’t any evidence about whether Instrument is a “regular” class, an abstract class or an interface. This is the intent: Each approach gives the programmer different control over the way objects are created and used.

“Multiple inheritance” in Java

The interface isn’t simply a “more pure” form of abstract class. It has a higher purpose than that. Because an interface has no implementation at all—that is, there is no storage associated with an interface—there’s nothing to prevent many interfaces from being combined. This is valuable because there are times when you need to say “An x is an a and a b and a c.” In C++, this act of combining multiple class interfaces is called multiple inheritance, and it carries some rather sticky baggage because each class can have an implementation. In Java, you can perform the same act, but only one of the classes can have an implementation, so the problems seen in C++ do not occur with Java when combining multiple interfaces:

In a derived class, you aren’t forced to have a base class that is either an abstract or “concrete” (one with no abstract methods). If you do inherit from a non-interface, you can inherit from only one. All the rest of the base elements must be interfaces. You place all the interface names after the implements keyword and separate them with commas. You can have as many interfaces as you want—each one becomes an independent type that you can upcast to. The following example shows a concrete class combined with several interfaces to produce a new class:

//: c08:Adventure.java
// Multiple interfaces.
import java.util.*;

interface CanFight {
  void fight();
}

interface CanSwim {
  void swim();
}

interface CanFly {
  void fly();
}

class ActionCharacter {
  public void fight() {}
}

class Hero extends ActionCharacter
    implements CanFight, CanSwim, CanFly {
  public void swim() {}
  public void fly() {}
}

public class Adventure {
  static void t(CanFight x) { x.fight(); }
  static void u(CanSwim x) { x.swim(); }
  static void v(CanFly x) { x.fly(); }
  static void w(ActionCharacter x) { x.fight(); }
  public static void main(String[] args) {
    Hero i = new Hero();
    t(i); // Treat it as a CanFight
    u(i); // Treat it as a CanSwim
    v(i); // Treat it as a CanFly
    w(i); // Treat it as an ActionCharacter
  }
} ///:~

You can see that Hero combines the concrete class ActionCharacter with the interfaces CanFight, CanSwim, and CanFly. When you combine a concrete class with interfaces this way, the concrete class must come first, then the interfaces. (The compiler gives an error otherwise.)

Note that the signature for fight( ) is the same in the interface CanFight and the class ActionCharacter, and that fight( ) is not provided with a definition in Hero. The rule for an interface is that you can inherit from it (as you will see shortly), but then you’ve got another interface. If you want to create an object of the new type, it must be a class with all definitions provided. Even though Hero does not explicitly provide a definition for fight( ), the definition comes along with ActionCharacter so it is automatically provided and it’s possible to create objects of Hero.

In class Adventure, you can see that there are four methods that take as arguments the various interfaces and the concrete class. When a Hero object is created, it can be passed to any of these methods, which means it is being upcast to each interface in turn. Because of the way interfaces are designed in Java, this works without a hitch and without any particular effort on the part of the programmer.

Keep in mind that the core reason for interfaces is shown in the above example: to be able to upcast to more than one base type. However, a second reason for using interfaces is the same as using an abstract base class: to prevent the client programmer from making an object of this class and to establish that it is only an interface. This brings up a question: Should you use an interface or an abstract class? An interface gives you the benefits of an abstract class and the benefits of an interface, so if it’s possible to create your base class without any method definitions or member variables you should always prefer interfaces to abstract classes. In fact, if you know something is going to be a base class, your first choice should be to make it an interface, and only if you’re forced to have method definitions or member variables should you change to an abstract class, or if necessary a concrete class.

Name collisions when combining interfaces

You can encounter a small pitfall when implementing multiple interfaces. In the above example, both CanFight and ActionCharacter have an identical void fight( ) method. This is no problem because the method is identical in both cases, but what if it’s not? Here’s an example:

//: c08:InterfaceCollision.java

interface I1 { void f(); }
interface I2 { int f(int i); }
interface I3 { int f(); }
class C { public int f() { return 1; } }

class C2 implements I1, I2 {
  public void f() {}
  public int f(int i) { return 1; } // overloaded
}

class C3 extends C implements I2 {
  public int f(int i) { return 1; } // overloaded
}

class C4 extends C implements I3 {
  // Identical, no problem:
  public int f() { return 1; }
}

// Methods differ only by return type:
//! class C5 extends C implements I1 {}
//! interface I4 extends I1, I3 {} ///:~

The difficulty occurs because overriding, implementation and overloading get unpleasantly mixed together, and overloaded functions cannot differ only by return type. When the last two lines are uncommented, the error messages say it all:

InterfaceCollision.java:23: f() in C cannot
implement f() in I1; attempting to use
incompatible return type
found   : int
required: void
InterfaceCollision.java:24: interfaces I3 and I1 are incompatible; both define f
(), but with different return type

Using the same method names in different interfaces that are intended to be combined generally causes confusion in the readability of the code, as well. Strive to avoid it.

Extending an interface
with inheritance

You can easily add new method declarations to an interface using inheritance, and you can also combine several interfaces into a new interface with inheritance. In both cases you get a new interface, as seen in this example:

//: c08:HorrorShow.java
// Extending an interface with inheritance.

interface Monster {
  void menace();
}

interface DangerousMonster extends Monster {
  void destroy();
}

interface Lethal {
  void kill();
}

class DragonZilla implements DangerousMonster {
  public void menace() {}
  public void destroy() {}
}

interface Vampire
    extends DangerousMonster, Lethal {
  void drinkBlood();
}

class HorrorShow {
  static void u(Monster b) { b.menace(); }
  static void v(DangerousMonster d) {
    d.menace();
    d.destroy();
  }
  public static void main(String[] args) {
    DragonZilla if2 = new DragonZilla();
    u(if2);
    v(if2);
  }
} ///:~

DangerousMonster is a simple extension to Monster that produces a new interface. This is implemented in DragonZilla.

The syntax used in Vampire works only when inheriting interfaces. Normally, you can use extends with only a single class, but since an interface can be made from multiple other interfaces, extends can refer to multiple base interfaces when building a new interface. As you can see, the interface names are simply separated with commas.

Grouping constants

Because any fields you put into an interface are automatically static and final, the interface is a convenient tool for creating groups of constant values, much as you would with an enum in C or C++. For example:

//: c08:Months.java
// Using interfaces to create groups of constants.
package c08;

public interface Months {
  int
    JANUARY = 1, FEBRUARY = 2, MARCH = 3,
    APRIL = 4, MAY = 5, JUNE = 6, JULY = 7,
    AUGUST = 8, SEPTEMBER = 9, OCTOBER = 10,
    NOVEMBER = 11, DECEMBER = 12;
} ///:~

Notice the Java style of using all uppercase letters (with underscores to separate multiple words in a single identifier) for static finals that have constant initializers.

The fields in an interface are automatically public, so it’s unnecessary to specify that.

Now you can use the constants from outside the package by importing c08.* or c08.Months just as you would with any other package, and referencing the values with expressions like Months.JANUARY. Of course, what you get is just an int so there isn’t the extra type safety that C++’s enum has, but this (commonly used) technique is certainly an improvement over hard-coding numbers into your programs. (That approach is often referred to as using “magic numbers” and it produces very difficult-to-maintain code.)

If you do want extra type safety, you can build a class like this: [39]

//: c08:Month2.java
// A more robust enumeration system.
package c08;

public final class Month2 {
  private String name;
  private Month2(String nm) { name = nm; }
  public String toString() { return name; }
  public final static Month2
    JAN = new Month2("January"),
    FEB = new Month2("February"),
    MAR = new Month2("March"),
    APR = new Month2("April"),
    MAY = new Month2("May"),
    JUN = new Month2("June"),
    JUL = new Month2("July"),
    AUG = new Month2("August"),
    SEP = new Month2("September"),
    OCT = new Month2("October"),
    NOV = new Month2("November"),
    DEC = new Month2("December");
  public final static Month2[] month =  {
    JAN, JAN, FEB, MAR, APR, MAY, JUN,
    JUL, AUG, SEP, OCT, NOV, DEC
  };
  public static void main(String[] args) {
    Month2 m = Month2.JAN;
    System.out.println(m);
    m = Month2.month[12];
    System.out.println(m);
    System.out.println(m == Month2.DEC);
    System.out.println(m.equals(Month2.DEC));
  }
} ///:~

The class is called Month2 since there’s already a Month in the standard Java library. It’s a final class with a private constructor so no one can inherit from it or make any instances of it. The only instances are the final static ones created in the class itself: JAN, FEB, MAR, etc. These objects are also used in the array month, which lets you choose months by number instead of by name. (Notice the extra JAN in the array to provide an offset by one, so that December is month 12.) In main( ) you can see the type safety: m is a Month2 object so it can be assigned only to a Month2. The previous example Months.java provided only int values, so an int variable intended to represent a month could actually be given any integer value, which wasn’t too safe.

This approach also allows you to use == or equals( ) interchangeably, as shown at the end of main( ).

Initializing fields in interfaces

Fields defined in interfaces are automatically static and final. These cannot be “blank finals,” but they can be initialized with non-constant expressions. For example:

//: c08:RandVals.java
// Initializing interface fields with
// non-constant initializers.
import java.util.*;

public interface RandVals {
  int rint = (int)(Math.random() * 10);
  long rlong = (long)(Math.random() * 10);
  float rfloat = (float)(Math.random() * 10);
  double rdouble = Math.random() * 10;
} ///:~

Since the fields are static, they are initialized when the class is first loaded, which happens when any of the fields are accessed for the first time. Here’s a simple test:

//: c08:TestRandVals.java

public class TestRandVals {
  public static void main(String[] args) {
    System.out.println(RandVals.rint);
    System.out.println(RandVals.rlong);
    System.out.println(RandVals.rfloat);
    System.out.println(RandVals.rdouble);
  }
} ///:~

The fields, of course, are not part of the interface but instead are stored in the static storage area for that interface.

Inner classes

It’s possible to place a class definition within another class definition. This is called an inner class. The inner class is a useful feature because it allows you to group classes that logically belong together and to control the visibility of one within the other. However, it’s important to understand that inner classes are distinctly different from composition.

Often, while you’re learning about them, the need for inner classes isn’t immediately obvious. At the end of this section, after all of the syntax and semantics of inner classes have been described, you’ll find examples that should make clear the benefits of inner classes.

You create an inner class just as you’d expect: by placing the class definition inside a surrounding class:

//: c08:Parcel1.java
// Creating inner classes.

public class Parcel1 {
  class Contents {
    private int i = 11;
    public int value() { return i; }
  }
  class Destination {
    private String label;
    Destination(String whereTo) {
      label = whereTo;
    }
    String readLabel() { return label; }
  }
  // Using inner classes looks just like
  // using any other class, within Parcel1:
  public void ship(String dest) {
    Contents c = new Contents();
    Destination d = new Destination(dest);
    System.out.println(d.readLabel());
  }
  public static void main(String[] args) {
    Parcel1 p = new Parcel1();
    p.ship("Tanzania");
  }
} ///:~

The inner classes, when used inside ship( ), look just like the use of any other classes. Here, the only practical difference is that the names are nested within Parcel1. You’ll see in a while that this isn’t the only difference.

More typically, an outer class will have a method that returns a reference to an inner class, like this:

//: c08:Parcel2.java
// Returning a reference to an inner class.

public class Parcel2 {
  class Contents {
    private int i = 11;
    public int value() { return i; }
  }
  class Destination {
    private String label;
    Destination(String whereTo) {
      label = whereTo;
    }
    String readLabel() { return label; }
  }
  public Destination to(String s) {
    return new Destination(s);
  }
  public Contents cont() {
    return new Contents();
  }
  public void ship(String dest) {
    Contents c = cont();
    Destination d = to(dest);
    System.out.println(d.readLabel());
  }
  public static void main(String[] args) {
    Parcel2 p = new Parcel2();
    p.ship("Tanzania");
    Parcel2 q = new Parcel2();
    // Defining references to inner classes:
    Parcel2.Contents c = q.cont();
    Parcel2.Destination d = q.to("Borneo");
  }
} ///:~

If you want to make an object of the inner class anywhere except from within a non-static method of the outer class, you must specify the type of that object as OuterClassName.InnerClassName, as seen in main( ).

Inner classes and upcasting

So far, inner classes don’t seem that dramatic. After all, if it’s hiding you’re after, Java already has a perfectly good hiding mechanism—just allow the class to be “friendly” (visible only within a package) rather than creating it as an inner class.

However, inner classes really come into their own when you start upcasting to a base class, and in particular to an interface. (The effect of producing an interface reference from an object that implements it is essentially the same as upcasting to a base class.) That’s because the inner class—the implementation of the interface—can then be completely unseen and unavailable to anyone, which is convenient for hiding the implementation. All you get back is a reference to the base class or the interface.

First, the common interfaces will be defined in their own files so they can be used in all the examples:

//: c08:Destination.java
public interface Destination {
  String readLabel();
} ///:~
//: c08:Contents.java
public interface Contents {
  int value();
} ///:~

Now Contents and Destination represent interfaces available to the client programmer. (The interface, remember, automatically makes all of its members public.)

When you get back a reference to the base class or the interface, it’s possible that you can’t even find out the exact type, as shown here:

//: c08:Parcel3.java
// Returning a reference to an inner class.

public class Parcel3 {
  private class PContents implements Contents {
    private int i = 11;
    public int value() { return i; }
  }
  protected class PDestination
      implements Destination {
    private String label;
    private PDestination(String whereTo) {
      label = whereTo;
    }
    public String readLabel() { return label; }
  }
  public Destination dest(String s) {
    return new PDestination(s);
  }
  public Contents cont() {
    return new PContents();
  }
}

class Test {
  public static void main(String[] args) {
    Parcel3 p = new Parcel3();
    Contents c = p.cont();
    Destination d = p.dest("Tanzania");
    // Illegal -- can't access private class:
    //! Parcel3.PContents pc = p.new PContents();
  }
} ///:~

Note: since main( ) is in Test, when you want to run this program you don’t execute Parcel3, but instead:

java Test

In the example, main( ) must be in a separate class in order to demonstrate the privateness of the inner class PContents.

In Parcel3, something new has been added: the inner class PContents is private so no one but Parcel3 can access it. PDestination is protected, so no one but Parcel3, classes in the Parcel3 package (since protected also gives package access—that is, protected is also “friendly”), and the inheritors of Parcel3 can access PDestination. This means that the client programmer has restricted knowledge and access to these members. In fact, you can’t even downcast to a private inner class (or a protected inner class unless you’re an inheritor), because you can’t access the name, as you can see in class Test. Thus, the private inner class provides a way for the class designer to completely prevent any type-coding dependencies and to completely hide details about implementation. In addition, extension of an interface is useless from the client programmer’s perspective since the client programmer cannot access any additional methods that aren’t part of the public interface class. This also provides an opportunity for the Java compiler to generate more efficient code.

Normal (non-inner) classes cannot be made private or protected—only public or “friendly.”

Inner classes
in methods and scopes

What you’ve seen so far encompasses the typical use for inner classes. In general, the code that you’ll write and read involving inner classes will be “plain” inner classes that are simple and easy to understand. However, the design for inner classes is quite complete and there are a number of other, more obscure, ways that you can use them if you choose: inner classes can be created within a method or even an arbitrary scope. There are two reasons for doing this:

  1. As shown previously, you’re implementing an interface of some kind so that you can create and return a reference.
  2. You’re solving a complicated problem and you want to create a class to aid in your solution, but you don’t want it publicly available.

In the following examples, the previous code will be modified to use:

  1. A class defined within a method
  2. A class defined within a scope inside a method
  3. An anonymous class implementing an interface
  4. An anonymous class extending a class that has a non-default constructor
  5. An anonymous class that performs field initialization
  6. An anonymous class that performs construction using instance initialization (anonymous inner classes cannot have constructors)

Although it’s an ordinary class with an implementation, Wrapping is also being used as a common “interface” to its derived classes:

//: c08:Wrapping.java
public class Wrapping {
  private int i;
  public Wrapping(int x) { i = x; }
  public int value() { return i; }
} ///:~

You’ll notice above that Wrapping has a constructor that requires an argument, to make things a bit more interesting.

The first example shows the creation of an entire class within the scope of a method (instead of the scope of another class):

//: c08:Parcel4.java
// Nesting a class within a method.

public class Parcel4 {
  public Destination dest(String s) {
    class PDestination
        implements Destination {
      private String label;
      private PDestination(String whereTo) {
        label = whereTo;
      }
      public String readLabel() { return label; }
    }
    return new PDestination(s);
  }
  public static void main(String[] args) {
    Parcel4 p = new Parcel4();
    Destination d = p.dest("Tanzania");
  }
} ///:~

The class PDestination is part of dest( ) rather than being part of Parcel4. (Also notice that you could use the class identifier PDestination for an inner class inside each class in the same subdirectory without a name clash.) Therefore, PDestination cannot be accessed outside of dest( ). Notice the upcasting that occurs in the return statement—nothing comes out of dest( ) except a reference to Destination, the base class. Of course, the fact that the name of the class PDestination is placed inside dest( ) doesn’t mean that PDestination is not a valid object once dest( ) returns.

The next example shows how you can nest an inner class within any arbitrary scope:

//: c08:Parcel5.java
// Nesting a class within a scope.

public class Parcel5 {
  private void internalTracking(boolean b) {
    if(b) {
      class TrackingSlip {
        private String id;
        TrackingSlip(String s) {
          id = s;
        }
        String getSlip() { return id; }
      }
      TrackingSlip ts = new TrackingSlip("slip");
      String s = ts.getSlip();
    }
    // Can't use it here! Out of scope:
    //! TrackingSlip ts = new TrackingSlip("x");
  }
  public void track() { internalTracking(true); }
  public static void main(String[] args) {
    Parcel5 p = new Parcel5();
    p.track();
  }
} ///:~

The class TrackingSlip is nested inside the scope of an if statement. This does not mean that the class is conditionally created—it gets compiled along with everything else. However, it’s not available outside the scope in which it is defined. Other than that, it looks just like an ordinary class.

Anonymous inner classes

The next example looks a little strange:

//: c08:Parcel6.java
// A method that returns an anonymous inner class.

public class Parcel6 {
  public Contents cont() {
    return new Contents() {
      private int i = 11;
      public int value() { return i; }
    }; // Semicolon required in this case
  }
  public static void main(String[] args) {
    Parcel6 p = new Parcel6();
    Contents c = p.cont();
  }
} ///:~

The cont( ) method combines the creation of the return value with the definition of the class that represents that return value! In addition, the class is anonymous—it has no name. To make matters a bit worse, it looks like you’re starting out to create a Contents object:

return new Contents()

but then, before you get to the semicolon, you say, “But wait, I think I’ll slip in a class definition”:

return new Contents() {
  private int i = 11;
  public int value() { return i; }
};

What this strange syntax means is “create an object of an anonymous class that’s inherited from Contents.” The reference returned by the new expression is automatically upcast to a Contents reference. The anonymous inner class syntax is a shorthand for:

class MyContents implements Contents {
  private int i = 11;
  public int value() { return i; }
}
return new MyContents();

In the anonymous inner class, Contents is created using a default constructor. The following code shows what to do if your base class needs a constructor with an argument:

//: c08:Parcel7.java
// An anonymous inner class that calls
// the base-class constructor.

public class Parcel7 {
  public Wrapping wrap(int x) {
    // Base constructor call:
    return new Wrapping(x) {
      public int value() {
        return super.value() * 47;
      }
    }; // Semicolon required
  }
  public static void main(String[] args) {
    Parcel7 p = new Parcel7();
    Wrapping w = p.wrap(10);
  }
} ///:~

That is, you simply pass the appropriate argument to the base-class constructor, seen here as the x passed in new Wrapping(x). An anonymous class cannot have a constructor where you would normally call super( ).

In both of the previous examples, the semicolon doesn’t mark the end of the class body (as it does in C++). Instead, it marks the end of the expression that happens to contain the anonymous class. Thus, it’s identical to the use of the semicolon everywhere else.

What happens if you need to perform some kind of initialization for an object of an anonymous inner class? Since it’s anonymous, there’s no name to give the constructor—so you can’t have a constructor. You can, however, perform initialization at the point of definition of your fields:

//: c08:Parcel8.java
// An anonymous inner class that performs
// initialization. A briefer version
// of Parcel5.java.

public class Parcel8 {
  // Argument must be final to use inside
  // anonymous inner class:
  public Destination dest(final String dest) {
    return new Destination() {
      private String label = dest;
      public String readLabel() { return label; }
    };
  }
  public static void main(String[] args) {
    Parcel8 p = new Parcel8();
    Destination d = p.dest("Tanzania");
  }
} ///:~

If you’re defining an anonymous inner class and want to use an object that’s defined outside the anonymous inner class, the compiler requires that the outside object be final. This is why the argument to dest( ) is final. If you forget, you’ll get a compile-time error message.

As long as you’re simply assigning a field, the above approach is fine. But what if you need to perform some constructor-like activity? With instance initialization, you can, in effect, create a constructor for an anonymous inner class:

//: c08:Parcel9.java
// Using "instance initialization" to perform
// construction on an anonymous inner class.

public class Parcel9 {
  public Destination
  dest(final String dest, final float price) {
    return new Destination() {
      private int cost;
      // Instance initialization for each object:
      {
        cost = Math.round(price);
        if(cost > 100)
          System.out.println("Over budget!");
      }
      private String label = dest;
      public String readLabel() { return label; }
    };
  }
  public static void main(String[] args) {
    Parcel9 p = new Parcel9();
    Destination d = p.dest("Tanzania", 101.395F);
  }
} ///:~

Inside the instance initializer you can see code that couldn’t be executed as part of a field initializer (that is, the if statement). So in effect, an instance initializer is the constructor for an anonymous inner class. Of course, it’s limited; you can’t overload instance initializers so you can have only one of these constructors.

The link to the outer class

So far, it appears that inner classes are just a name-hiding and code-organization scheme, which is helpful but not totally compelling. However, there’s another twist. When you create an inner class, an object of that inner class has a link to the enclosing object that made it, and so it can access the members of that enclosing object—without any special qualifications. In addition, inner classes have access rights to all the elements in the enclosing class [40] . The following example demonstrates this:

//: c08:Sequence.java
// Holds a sequence of Objects.

interface Selector {
  boolean end();
  Object current();
  void next();
}

public class Sequence {
  private Object[] obs;
  private int next = 0;
  public Sequence(int size) {
    obs = new Object[size];
  }
  public void add(Object x) {
    if(next < obs.length) {
      obs[next] = x;
      next++;
    }
  }
  private class SSelector implements Selector {
    int i = 0;
    public boolean end() {
      return i == obs.length;
    }
    public Object current() {
      return obs[i];
    }
    public void next() {
      if(i < obs.length) i++;
    }
  }
  public Selector getSelector() {
    return new SSelector();
  }
  public static void main(String[] args) {
    Sequence s = new Sequence(10);
    for(int i = 0; i < 10; i++)
      s.add(Integer.toString(i));
    Selector sl = s.getSelector();
    while(!sl.end()) {
      System.out.println(sl.current());
      sl.next();
    }
  }
} ///:~

The Sequence is simply a fixed-sized array of Object with a class wrapped around it. You call add( ) to add a new Object to the end of the sequence (if there’s room left). To fetch each of the objects in a Sequence, there’s an interface called Selector, which allows you to see if you’re at the end( ), to look at the current( ) Object, and to move to the next( ) Object in the Sequence. Because Selector is an interface, many other classes can implement the interface in their own ways, and many methods can take the interface as an argument, in order to create generic code.

Here, the SSelector is a private class that provides Selector functionality. In main( ), you can see the creation of a Sequence, followed by the addition of a number of String objects. Then, a Selector is produced with a call to getSelector( ) and this is used to move through the Sequence and select each item.

At first, the creation of SSelector looks like just another inner class. But examine it closely. Note that each of the methods end( ), current( ), and next( ) refer to obs, which is a reference that isn’t part of SSelector, but is instead a private field in the enclosing class. However, the inner class can access methods and fields from the enclosing class as if they owned them. This turns out to be very convenient, as you can see in the above example.

So an inner class has automatic access to the members of the enclosing class. How can this happen? The inner class must keep a reference to the particular object of the enclosing class that was responsible for creating it. Then when you refer to a member of the enclosing class, that (hidden) reference is used to select that member. Fortunately, the compiler takes care of all these details for you, but you can also understand now that an object of an inner class can be created only in association with an object of the enclosing class. Construction of the inner class object requires the reference to the object of the enclosing class, and the compiler will complain if it cannot access that reference. Most of the time this occurs without any intervention on the part of the programmer.

static inner classes

If you don’t need a connection between the inner class object and the outer class object, then you can make the inner class static. To understand the meaning of static when applied to inner classes, you must remember that the object of an ordinary inner class implicitly keeps a reference to the object of the enclosing class that created it. This is not true, however, when you say an inner class is static. A static inner class means:

  1. You don’t need an outer-class object in order to create an object of a static inner class.
  2. You can’t access an outer-class object from an object of a static inner class.

static inner classes are different than non-static inner classes in another way, as well. Fields and methods in non-static inner classes can only be at the outer level of a class, so non-static inner classes cannot have static data, static fields, or static inner classes. However, static inner classes can have all of these:

//: c08:Parcel10.java
// Static inner classes.

public class Parcel10 {
  private static class PContents
  implements Contents {
    private int i = 11;
    public int value() { return i; }
  }
  protected static class PDestination
      implements Destination {
    private String label;
    private PDestination(String whereTo) {
      label = whereTo;
    }
    public String readLabel() { return label; }
    // Static inner classes can contain
    // other static elements:
    public static void f() {}
    static int x = 10;
    static class AnotherLevel {
      public static void f() {}
      static int x = 10;
    }
  }
  public static Destination dest(String s) {
    return new PDestination(s);
  }
  public static Contents cont() {
    return new PContents();
  }
  public static void main(String[] args) {
    Contents c = cont();
    Destination d = dest("Tanzania");
  }
} ///:~

In main( ), no object of Parcel10 is necessary; instead you use the normal syntax for selecting a static member to call the methods that return references to Contents and Destination.

As you will see shortly, in an ordinary (non-static) inner class, the link to the outer class object is achieved with a special this reference. A static inner class does not have this special this reference, which makes it analogous to a static method.

Normally you can’t put any code inside an interface, but a static inner class can be part of an interface. Since the class is static it doesn’t violate the rules for interfaces—the static inner class is only placed inside the namespace of the interface:

//: c08:IInterface.java
// Static inner classes inside interfaces.

interface IInterface {
  static class Inner {
    int i, j, k;
    public Inner() {}
    void f() {}
  }
} ///:~

Earlier in this book I suggested putting a main( ) in every class to act as a test bed for that class. One drawback to this is the amount of extra compiled code you must carry around. If this is a problem, you can use a static inner class to hold your test code:

//: c08:TestBed.java
// Putting test code in a static inner class.

class TestBed {
  TestBed() {}
  void f() { System.out.println("f()"); }
  public static class Tester {
    public static void main(String[] args) {
      TestBed t = new TestBed();
      t.f();
    }
  }
} ///:~

This generates a separate class called TestBed$Tester (to run the program you say java TestBed$Tester). You can use this class for testing, but you don’t need to include it in your shipping product.

Referring to the outer class object

If you need to produce the reference to the outer class object, you name the outer class followed by a dot and this. For example, in the class Sequence.SSelector, any of its methods can produce the stored reference to the outer class Sequence by saying Sequence.this. The resulting reference is automatically the correct type. (This is known and checked at compile-time, so there is no run-time overhead.)

Sometimes you want to tell some other object to create an object of one of its inner classes. To do this you must provide a reference to the other outer class object in the new expression, like this:

//: c08:Parcel11.java
// Creating instances of inner classes.

public class Parcel11 {
  class Contents {
    private int i = 11;
    public int value() { return i; }
  }
  class Destination {
    private String label;
    Destination(String whereTo) {
      label = whereTo;
    }
    String readLabel() { return label; }
  }
  public static void main(String[] args) {
    Parcel11 p = new Parcel11();
    // Must use instance of outer class
    // to create an instances of the inner class:
    Parcel11.Contents c = p.new Contents();
    Parcel11.Destination d =
      p.new Destination("Tanzania");
  }
} ///:~

To create an object of the inner class directly, you don’t follow the same form and refer to the outer class name Parcel11 as you might expect, but instead you must use an object of the outer class to make an object of the inner class:

Parcel11.Contents c = p.new Contents();

Thus, it’s not possible to create an object of the inner class unless you already have an object of the outer class. This is because the object of the inner class is quietly connected to the object of the outer class that it was made from. However, if you make a static inner class, then it doesn’t need a reference to the outer class object.

Inheriting from inner classes

Because the inner class constructor must attach to a reference of the enclosing class object, things are slightly complicated when you inherit from an inner class. The problem is that the “secret” reference to the enclosing class object must be initialized, and yet in the derived class there’s no longer a default object to attach to. The answer is to use a syntax provided to make the association explicit:

//: c08:InheritInner.java
// Inheriting an inner class.

class WithInner {
  class Inner {}
}

public class InheritInner
    extends WithInner.Inner {
  //! InheritInner() {} // Won't compile
  InheritInner(WithInner wi) {
    wi.super();
  }
  public static void main(String[] args) {
    WithInner wi = new WithInner();
    InheritInner ii = new InheritInner(wi);
  }
} ///:~

You can see that InheritInner is extending only the inner class, not the outer one. But when it comes time to create a constructor, the default one is no good and you can’t just pass a reference to an enclosing object. In addition, you must use the syntax

enclosingClassReference.super();

inside the constructor. This provides the necessary reference and the program will then compile.

Can inner classes be overridden?

What happens when you create an inner class, then inherit from the enclosing class and redefine the inner class? That is, is it possible to override an inner class? This seems like it would be a powerful concept, but “overriding” an inner class as if it were another method of the outer class doesn’t really do anything:

//: c08:BigEgg.java
// An inner class cannot be overriden
// like a method.

class Egg {
  protected class Yolk {
    public Yolk() {
      System.out.println("Egg.Yolk()");
    }
  }
  private Yolk y;
  public Egg() {
    System.out.println("New Egg()");
    y = new Yolk();
  }
}

public class BigEgg extends Egg {
  public class Yolk {
    public Yolk() {
      System.out.println("BigEgg.Yolk()");
    }
  }
  public static void main(String[] args) {
    new BigEgg();
  }
} ///:~

The default constructor is synthesized automatically by the compiler, and this calls the base-class default constructor. You might think that since a BigEgg is being created, the “overridden” version of Yolk would be used, but this is not the case. The output is:

New Egg()
Egg.Yolk()

This example simply shows that there isn’t any extra inner class magic going on when you inherit from the outer class. The two inner classes are completely separate entities, each in their own namespace. However, it’s still possible to explicitly inherit from the inner class:

//: c08:BigEgg2.java
// Proper inheritance of an inner class.

class Egg2 {
  protected class Yolk {
    public Yolk() {
      System.out.println("Egg2.Yolk()");
    }
    public void f() {
      System.out.println("Egg2.Yolk.f()");
    }
  }
  private Yolk y = new Yolk();
  public Egg2() {
    System.out.println("New Egg2()");
  }
  public void insertYolk(Yolk yy) { y = yy; }
  public void g() { y.f(); }
}

public class BigEgg2 extends Egg2 {
  public class Yolk extends Egg2.Yolk {
    public Yolk() {
      System.out.println("BigEgg2.Yolk()");
    }
    public void f() {
      System.out.println("BigEgg2.Yolk.f()");
    }
  }
  public BigEgg2() { insertYolk(new Yolk()); }
  public static void main(String[] args) {
    Egg2 e2 = new BigEgg2();
    e2.g();
  }
} ///:~

Now BigEgg2.Yolk explicitly extends Egg2.Yolk and overrides its methods. The method insertYolk( ) allows BigEgg2 to upcast one of its own Yolk objects into the y reference in Egg2, so when g( ) calls y.f( ) the overridden version of f( ) is used. The output is:

Egg2.Yolk()
New Egg2()
Egg2.Yolk()
BigEgg2.Yolk()
BigEgg2.Yolk.f()

The second call to Egg2.Yolk( ) is the base-class constructor call of the BigEgg2.Yolk constructor. You can see that the overridden version of f( ) is used when g( ) is called.

Inner class identifiers

Since every class produces a .class file that holds all the information about how to create objects of this type (this information produces a “meta-class” called the Class object), you might guess that inner classes must also produce .class files to contain the information for their Class objects. The names of these files/classes have a strict formula: the name of the enclosing class, followed by a ‘$’, followed by the name of the inner class. For example, the .class files created by InheritInner.java include:

InheritInner.class
WithInner$Inner.class
WithInner.class

If inner classes are anonymous, the compiler simply starts generating numbers as inner class identifiers. If inner classes are nested within inner classes, their names are simply appended after a ‘$’ and the outer class identifier(s).

Although this scheme of generating internal names is simple and straightforward, it’s also robust and handles most situations [41] . Since it is the standard naming scheme for Java, the generated files are automatically platform-independent. (Note that the Java compiler is changing your inner classes in all sorts of other ways in order to make them work.)

Why inner classes?

At this point you’ve seen a lot of syntax and semantics describing the way inner classes work, but this doesn’t answer the question of why they exist. Why did Sun go to so much trouble to add this fundamental language feature?

Typically, the inner class inherits from a class or implements an interface, and the code in the inner class manipulates the outer class object that it was created within. So you could say that an inner class provides a kind of window into the outer class.

A question that cuts to the heart of inner classes is this: if I just need a reference to an interface, why don’t I just make the outer class implement that interface? The answer is “If that’s all you need, then that’s how you should do it.” So what is it that distinguishes an inner class implementing an interface from an outer class implementing the same interface? The answer is that you can’t always have the convenience of interfaces—sometimes you’re working with implementations. So the most compelling reason for inner classes is:

Each inner class can independently inherit from an implementation. Thus, the inner class is not limited by whether the outer class is already inheriting from an implementation.

Without the ability that inner classes provide to inherit—in effect –from more than one concrete or abstract class, some design and programming problems would be intractable. So one way to look at the inner class is as the completion of the solution of the multiple-inheritance problem. Interfaces solve part of the problem, but inner classes effectively allow “multiple implementation inheritance.” That is, inner classes effectively allow you to inherit from more than one non-interface.

To see this in more detail, consider a situation where you have two interfaces that must somehow be implemented within a class. Because of the flexibility of interfaces, you have two choices: a single class or an inner class:

//: c08:MultiInterfaces.java
// Two ways that a class can
// implement multiple interfaces.

interface A {}
interface B {}

class X implements A, B {}

class Y implements A {
  B makeB() {
    // Anonymous inner class:
    return new B() {};
  }
}

public class MultiInterfaces {
  static void takesA(A a) {}
  static void takesB(B b) {}
  public static void main(String[] args) {
    X x = new X();
    Y y = new Y();
    takesA(x);
    takesA(y);
    takesB(x);
    takesB(y.makeB());
  }
} ///:~

Of course, this assumes that the structure of your code makes logical sense either way. However, you’ll ordinarily have some kind of guidance from the nature of the problem about whether to use a single class or an inner class. But without any other constraints, in the above example the approach you take doesn’t really make much difference from an implementation standpoint. Both of them work.

However, if you have abstract or concrete classes instead of interfaces, you are suddenly limited to using inner classes if your class must somehow implement both of the others:

//: c08:MultiImplementation.java
// With concrete or abstract classes, inner
// classes are the only way to produce the effect
// of "multiple implementation inheritance."

class C {}
abstract class D {}

class Z extends C {
  D makeD() { return new D() {}; }
}

public class MultiImplementation {
  static void takesC(C c) {}
  static void takesD(D d) {}
  public static void main(String[] args) {
    Z z = new Z();
    takesC(z);
    takesD(z.makeD());
  }
} ///:~

If you didn’t need to solve the “multiple implementation inheritance” problem, you could conceivably code around everything else without the need for inner classes. But with inner classes you have these additional features:

  1. The inner class can have multiple instances, each with its own state information that is independent of the information in the outer class object.
  2. In a single outer class you can have several inner classes, each of which implement the same interface or inherit from the same class in a different way. An example of this will be shown shortly.
  3. The point of creation of the inner class object is not tied to the creation of the outer class object.
  4. There is no potentially confusing “is-a” relationship with the inner class; it’s a separate entity.

As an example, if Sequence.java did not use inner classes, you’d have to say “a Sequence is a Selector,” and you’d only be able to have one Selector in existence for a particular Sequence. Also, you can have a second method, getRSelector( ), that produces a Selector that moves backwards through the sequence. This kind of flexibility is only available with inner classes.

Closures & Callbacks

A closure is a callable object that retains information from the scope in which it was created. From this definition, you can see that an inner class is a object-oriented closure, because it doesn’t just contain each piece of information from the outer class object (“the scope in which it was created”), but it automatically holds a reference back to the whole outer class object, where it has permission to manipulate all the members, even private ones.

One of the most compelling arguments made to include some kind of pointer mechanism in Java was to allow callbacks. With a callback, some other object is given a piece of information that allows it to call back into the originating object at some later point. This is a very powerful concept, as you will see in Chapters 13 and 16. If a callback is implemented using a pointer, however, you must rely on the programmer to behave and not misuse the pointer. As you’ve seen by now, Java tends to be more careful than that so pointers were not included in the language.

The closure provided by the inner class is a perfect solution; more flexible and far safer than a pointer. Here’s a simple example:

//: c08:Callbacks.java
// Using inner classes for callbacks

interface Incrementable {
  void increment();
}

class Callee {
  private int i = 0;
  private void incr() {
    i++;
    System.out.println(i);
  }
  private class Closure implements Incrementable {
    public void increment() { incr(); }
  }
  Incrementable getCallbackReference() {
    return new Closure();
  }
}

class Caller {
  private Incrementable callbackReference;
  Caller(Incrementable cbh) {
    callbackReference = cbh;
  }
  void go() {
    callbackReference.increment();
  }
}

public class Callbacks {
  public static void main(String[] args) {
    Callee c = new Callee();
    Caller cc =
      new Caller(c.getCallbackReference());
    cc.go();
    cc.go();
  }
} ///:~

Notice that everything except getCallbackReference( ) in Callee is private. To allow any connection to the outside world, the interface Incrementable is essential. Here you can see how interfaces allow for a complete separation of interface from implementation.

The inner class Closure simply implements Incrementable to provide a hook back into Callee—but a safe hook. Whoever gets the Incrementable reference can, of course, only call increment( ) and has no other abilities (unlike a pointer, which would allow you to run wild).

Caller takes an Incrementable reference in its constructor (although the capturing of the callback reference could happen at any time) and then, sometime latter, uses the reference to “call back” into the Callee class.

The value of the callback is in its flexibility—you can dynamically decide what functions will be called at run-time. The benefit of this will become more evident in Chapter 13, where callbacks are used everywhere to implement graphical user interface (GUI) functionality.

Inner classes & control frameworks

A more concrete example of the use of inner classes can be found in something that I will refer to here as a control framework.

An application framework is a class or a set of classes that’s designed to solve a particular type of problem. To apply an application framework, you inherit from one or more classes and override some of the methods. The code you write in the overridden methods customizes the general solution provided by that application framework, in order to solve your specific problem. The control framework is a particular type of application framework dominated by the need to respond to events; a system that primarily responds to events is called an event-driven system. One of the most important problems in application programming is the graphical user interface (GUI), which is almost entirely event-driven. As you will see in Chapter 13, the Java Swing library is a control framework that elegantly solves the GUI problem and that heavily uses inner classes.

To see how inner classes allow the simple creation and use of control frameworks, consider a control framework whose job is to execute events whenever those events are “ready.” Although “ready” could mean anything, in this case the default will be based on clock time. What follows is a control framework that contains no specific information about what it’s controlling. First, here is the interface that describes any control event. It’s an abstract class instead of an actual interface because the default behavior is to perform the control based on time, so some of the implementation can be included here:

//: c08:controller:Event.java
// The common methods for any control event.
package c08.controller;

abstract public class Event {
  private long evtTime;
  public Event(long eventTime) {
    evtTime = eventTime;
  }
  public boolean ready() {
    return System.currentTimeMillis() >= evtTime;
  }
  abstract public void action();
  abstract public String description();
} ///:~

The constructor simply captures the time when you want the Event to run, while ready( ) tells you when it’s time to run it. Of course, ready( ) could be overridden in a derived class to base the Event on something other than time.

action( ) is the method that’s called when the Event is ready( ), and description( ) gives textual information about the Event.

The following file contains the actual control framework that manages and fires events. The first class is really just a “helper” class whose job is to hold Event objects. You can replace it with any appropriate container, and in Chapter 9 you’ll discover other containers that will do the trick without requiring you to write this extra code:

//: c08:controller:Controller.java
// Along with Event, the generic
// framework for all control systems:
package c08.controller;

// This is just a way to hold Event objects.
class EventSet {
  private Event[] events = new Event[100];
  private int index = 0;
  private int next = 0;
  public void add(Event e) {
    if(index >= events.length)
      return; // (In real life, throw exception)
    events[index++] = e;
  }
  public Event getNext() {
    boolean looped = false;
    int start = next;
    do {
      next = (next + 1) % events.length;
      // See if it has looped to the beginning:
      if(start == next) looped = true;
      // If it loops past start, the list
      // is empty:
      if((next == (start + 1) % events.length)
         && looped)
        return null;
    } while(events[next] == null);
    return events[next];
  }
  public void removeCurrent() {
    events[next] = null;
  }
}

public class Controller {
  private EventSet es = new EventSet();
  public void addEvent(Event c) { es.add(c); }
  public void run() {
    Event e;
    while((e = es.getNext()) != null) {
      if(e.ready()) {
        e.action();
        System.out.println(e.description());
        es.removeCurrent();
      }
    }
  }
} ///:~

EventSet arbitrarily holds 100 Events. (If a “real” container from Chapter 9 is used here you don’t need to worry about its maximum size, since it will resize itself). The index is used to keep track of the next available space, and next is used when you’re looking for the next Event in the list, to see whether you’ve looped around. This is important during a call to getNext( ), because Event objects are removed from the list (using removeCurrent( )) once they’re run, so getNext( ) will encounter holes in the list as it moves through it.

Note that removeCurrent( ) doesn’t just set some flag indicating that the object is no longer in use. Instead, it sets the reference to null. This is important because if the garbage collector sees a reference that’s still in use then it can’t clean up the object. If you think your references might hang around (as they would here), then it’s a good idea to set them to null to give the garbage collector permission to clean them up.

Controller is where the actual work goes on. It uses an EventSet to hold its Event objects, and addEvent( ) allows you to add new events to this list. But the important method is run( ). This method loops through the EventSet, hunting for an Event object that’s ready( ) to run. For each one it finds ready( ), it calls the action( ) method, prints out the description( ), and then removes the Event from the list.

Note that so far in this design you know nothing about exactly what an Event does. And this is the crux of the design; how it “separates the things that change from the things that stay the same.” Or, to use my term, the “vector of change” is the different actions of the various kinds of Event objects, and you express different actions by creating different Event subclasses.

This is where inner classes come into play. They allow two things:

  1. To create the entire implementation of a control-framework application in a single class, thereby encapsulating everything that’s unique about that implementation. Inner classes are used to express the many different kinds of action( ) necessary to solve the problem. In addition, the following example uses private inner classes so the implementation is completely hidden and can be changed with impunity.
  2. Inner classes keep this implementation from becoming awkward, since you’re able to easily access any of the members in the outer class. Without this ability the code might become unpleasant enough that you’d end up seeking an alternative.

Consider a particular implementation of the control framework designed to control greenhouse functions [42] . Each action is entirely different: turning lights, water, and thermostats on and off, ringing bells, and restarting the system. But the control framework is designed to easily isolate this different code. Inner classes allow you to have multiple derived versions of the same base class, Event, within a single class. For each type of action you inherit a new Event inner class, and write the control code inside of action( ).

As is typical with an application framework, the class GreenhouseControls is inherited from Controller:

//: c08:GreenhouseControls.java
// This produces a specific application of the
// control system, all in a single class. Inner
// classes allow you to encapsulate different
// functionality for each type of event.
import c08.controller.*;

public class GreenhouseControls
    extends Controller {
  private boolean light = false;
  private boolean water = false;
  private String thermostat = "Day";
  private class LightOn extends Event {
    public LightOn(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here to
      // physically turn on the light.
      light = true;
    }
    public String description() {
      return "Light is on";
    }
  }
  private class LightOff extends Event {
    public LightOff(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here to
      // physically turn off the light.
      light = false;
    }
    public String description() {
      return "Light is off";
    }
  }
  private class WaterOn extends Event {
    public WaterOn(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here
      water = true;
    }
    public String description() {
      return "Greenhouse water is on";
    }
  }
  private class WaterOff extends Event {
    public WaterOff(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here
      water = false;
    }
    public String description() {
      return "Greenhouse water is off";
    }
  }
  private class ThermostatNight extends Event {
    public ThermostatNight(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here
      thermostat = "Night";
    }
    public String description() {
      return "Thermostat on night setting";
    }
  }
  private class ThermostatDay extends Event {
    public ThermostatDay(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Put hardware control code here
      thermostat = "Day";
    }
    public String description() {
      return "Thermostat on day setting";
    }
  }
  // An example of an action() that inserts a
  // new one of itself into the event list:
  private int rings;
  private class Bell extends Event {
    public Bell(long eventTime) {
      super(eventTime);
    }
    public void action() {
      // Ring every 2 seconds, 'rings' times:
      System.out.println("Bing!");
      if(--rings > 0)
        addEvent(new Bell(
          System.currentTimeMillis() + 2000));
    }
    public String description() {
      return "Ring bell";
    }
  }
  private class Restart extends Event {
    public Restart(long eventTime) {
      super(eventTime);
    }
    public void action() {
      long tm = System.currentTimeMillis();
      // Instead of hard-wiring, you could parse
      // configuration information from a text
      // file here:
      rings = 5;
      addEvent(new ThermostatNight(tm));
      addEvent(new LightOn(tm + 1000));
      addEvent(new LightOff(tm + 2000));
      addEvent(new WaterOn(tm + 3000));
      addEvent(new WaterOff(tm + 8000));
      addEvent(new Bell(tm + 9000));
      addEvent(new ThermostatDay(tm + 10000));
      // Can even add a Restart object!
      addEvent(new Restart(tm + 20000));
    }
    public String description() {
      return "Restarting system";
    }
  }
  public static void main(String[] args) {
    GreenhouseControls gc =
      new GreenhouseControls();
    long tm = System.currentTimeMillis();
    gc.addEvent(gc.new Restart(tm));
    gc.run();
  }
} ///:~

Note that light, water, thermostat, and rings all belong to the outer class GreenhouseControls, and yet the inner classes can access those fields without qualification or special permission. Also, most of the action( ) methods involve some sort of hardware control, which would most likely involve calls to non-Java code.

Most of the Event classes look similar, but Bell and Restart are special. Bell rings, and if it hasn’t yet rung enough times it adds a new Bell object to the event list, so it will ring again later. Notice how inner classes almost look like multiple inheritance: Bell has all the methods of Event and it also appears to have all the methods of the outer class GreenhouseControls.

Restart is responsible for initializing the system, so it adds all the appropriate events. Of course, a more flexible way to accomplish this is to avoid hard-coding the events and instead read them from a file. (An exercise in Chapter 11 asks you to modify this example to do just that.) Since Restart( ) is just another Event object, you can also add a Restart object within Restart.action( ) so that the system regularly restarts itself. And all you need to do in main( ) is create a GreenhouseControls object and add a Restart object to get it going.

This example should move you a long way toward appreciating the value of inner classes, especially when used within a control framework. However, in Chapter 13 you’ll see how elegantly inner classes are used to describe the actions of a graphical user interface. By the time you finish that chapter you should be fully convinced."_Toc477589539">

Summary

Interfaces and inner classes are more sophisticated concepts than what you’ll find in many OOP languages. For example, there’s nothing like them in C++. Together, they solve the same problem that C++ attempts to solve with its multiple inheritance (MI) feature. However, MI in C++ turns out to be rather difficult to use, while Java interfaces and inner classes are, by comparison, much more accessible.

Although the features themselves are reasonably straightforward, the use of these features is a design issue, much the same as polymorphism. Over time, you’ll become better at recognizing situations where you should use an interface, or an inner class, or both. But at this point in this book you should at least be comfortable with the syntax and semantics. As you see these language features in use you’ll eventually internalize them.

Exercises
  1. Prove that the fields in an interface are implicitly static and final.
  2. Create an interface containing three methods, in its own package. Implement the interface in a different package.
  3. Prove that all the methods in an interface are automatically public.
  4. In c07:Sandwich.java, create an interface called FastFood (with appropriate methods) and change Sandwich so that it also implements FastFood.
  5. Create three interfaces, each with two methods. Inherit a new interface from the three, adding a new method. Create a class by implementing the new interface and also inheriting from a concrete class. Now write four methods, each of which takes one of the four interfaces as an argument. In main( ), create an object of your class and pass it to each of the methods.
  6. Modify Exercise 5 by creating an abstract class and inheriting that into the derived class.
  7. Modify Music5.java by adding a Playable interface. Remove the play( ) declaration from Instrument. Add Playable to the derived classes by including it in the implements list. Change tune( ) so that it takes a Playable instead of an Instrument.
  8. Change Exercise 6 in Chapter 7 so that Rodent is an interface.
  9. In Adventure.java, add an interface called CanClimb, following the form of the other interfaces.
  10. Write a program that imports and uses Month2.java.
  11. Following the example given in Month2.java, create an enumeration of days of the week.
  12. Create an interface with at least one method, in its own package. Create a class in a separate package. Add a protected inner class that implements the interface. In a third package, inherit from your class and, inside a method, return an object of the protected inner class, upcasting to the interface during the return.
  13. Create an interface with at least one method, and implement that interface by defining an inner class within a method, which returns a reference to your interface.
  14. Repeat Exercise 13 but define the inner class within a scope within a method.
  15. Repeat Exercise 13 using an anonymous inner class.
  16. Create a private inner class that implements a public interface. Write a method that returns a reference to an instance of the private inner class, upcast to the interface. Show that the inner class is completely hidden by trying to downcast to it.
  17. Create a class with a non-default constructor and no default constructor. Create a second class that has a method which returns a reference to the first class. Create the object to return by making an anonymous inner class that inherits from the first class.
  18. Create a class with a private field and a private method. Create an inner class with a method that modifies the outer class field and calls the outer class method. In a second outer class method, create an object of the inner class and call it’s method, then show the effect on the outer class object.
  19. Repeat Exercise 18 using an anonymous inner class.
  20. Create a class containing a static inner class. In main( ), create an instance of the inner class.
  21. Create an interface containing a static inner class. Implement this interface and create an instance of the inner class.
  22. Create a class containing an inner class that itself contains an inner class. Repeat this using static inner classes. Note the names of the .class files produced by the compiler.
  23. Create a class with an inner class. In a separate class, make an instance of the inner class.
  24. Create a class with an inner class that has a non-default constructor. Create a second class with an inner class that inherits from the first inner class.
  25. Repair the problem in WindError.java.
  26. Modify Sequence.java by adding a method getRSelector( ) that produces a different implementation of the Selector interface that moves backwards through the sequence from the end to the beginning.
  27. Create an interface U with 3 methods. Create a class A with a method that produces a reference to a U by building an anonymous inner class. Create a second class B that contains an array of U. B should have one method that accepts and stores a reference to a U in the array, a second method that sets a reference in the array (specified by the method argument) to null and a third method that moves through the array and calls the methods in U. In main( ), create a group of A objects and a single B. Fill the B with U references produced by the A objects. Use the B to call back into all the A objects. Remove some of the U references from the B.
  28. In GreenhouseControls.java, add Event inner classes that turn fans on and

    [39] This approach was inspired by an e-mail from Rich Hoffarth.

    [40] This is very different from the design of nested classes in C++, which is simply a name-hiding mechanism. There is no link to an enclosing object and no implied permissions in C++.

    [41] On the other hand, ‘$’ is a meta-character to the Unix shell and so you’ll sometimes have trouble when listing the .class files. This is a bit strange coming from Sun, a Unix-based company. My guess is that they weren’t considering this issue, but instead thought you’d naturally focus on the source-code files.

    [42] For some reason this has always been a pleasing problem for me to solve; it came from my earlier book C++ Inside & Out, but Java allows a much more elegant solution.

    off.


9: Holding Your Objects

9: Holding
Your Objects

It’s a fairly simple program that has only a fixed quantity of objects with known lifetimes.

In general, your programs will always be creating new objects based on some criteria that will be known only at the time the program is running. You won’t know until run-time the quantity or even the exact type of the objects you need. To solve the general programming problem, you need to be able to create any number of objects, anytime, anywhere. So you can’t rely on creating a named reference to hold each one of your objects:

MyObject myReference;

since you’ll never know how many of these you’ll actually need.

To solve this rather essential problem, Java has several ways to hold objects (or rather, references to objects). The built-in type is the array, which has been discussed before and will get additional coverage in this chapter. Also, the Java utilities library has a reasonably complete set of container classes (also known as collection classes, but because the Java 2 libraries use the name Collection to refer to a particular subset of the library, I shall use the more inclusive term “container”). Containers provide sophisticated ways to hold and even manipulate your objects, and will comprise the remainder of this chapter.

Arrays

Most of the necessary introduction to arrays is in the last section of Chapter 4, which shows how you define and initialize an array. Holding objects is the focus of this chapter, and an array is just one way to hold objects. But there are a number of other ways to hold objects, so what makes an array special?

There are two issues that distinguish arrays from other types of containers: efficiency and type. The array is the most efficient way that Java provides to store and randomly-access a sequence of objects (actually, object references). The array is a simple linear sequence, which makes element access fast, but you pay for this speed: when you create an array object, its size is fixed and cannot be changed for the lifetime of that array object. You might suggest creating an array of a particular size and then, if you run out of space, creating a new one and moving all the references from the old one to the new one. This is the behavior of the ArrayList class, which will be studied later in the chapter. However, because of the overhead of this size flexibility, an ArrayList is measurably less efficient than an array.

The vector container class in C++ does know the type of objects it holds, but it has a different drawback when compared with arrays in Java: the C++ vector’s operator[] doesn’t do bounds checking, so you can run past the end [43] . In Java, you get bounds checking regardless of whether you’re using an array or a container—you’ll get a RuntimeException if you exceed the bounds. As you’ll learn in Chapter 10, this type of exception indicates a programmer error and thus you don’t need to check for it in your code. As an aside, the reason the C++ vector doesn’t check bounds with every access is speed—in Java you have the constant performance overhead of bounds checking all the time for both arrays and containers.

The other generic container classes that will be studied in this chapter, List, Set, and Map, all deal with objects as if they had no specific type. That is, they treat them as type Object, the root class of all classes in Java. This works fine from one standpoint: you need to build only one container, and any Java object will go into that container. (Except for primitives—these can be placed in containers as constants using the Java primitive wrapper classes, or as changeable values by wrapping in your own class.) This is the second place where an array is superior to the generic containers: when you create an array, you create it to hold a specific type. This means that you get compile-time type checking to prevent you from putting the wrong type in, or mistaking the type that you’re extracting. Of course, Java will prevent you from sending an inappropriate message to an object, either at compile-time or at run-time. So it’s not much riskier one way or the other, it’s just nicer if the compiler points it out to you, faster at run-time, and there’s less likelihood that the end user will get surprised by an exception.

For efficiency and type checking it’s always worth trying to use an array if you can. However, when you’re trying to solve a more general problem arrays can be too restrictive. After looking at arrays, the rest of this chapter will be devoted to the container classes provided by Java.

Arrays are first-class objects

Regardless of what type of array you’re working with, the array identifier is actually a reference to a true object that’s created on the heap. This is the object that holds the references to the other objects, and it can be created either implicitly, as part of the array initialization syntax, or explicitly with a new expression. Part of the array object (in fact, the only field or method you can access) is the read-only length member that tells you how many elements can be stored in that array object. The ‘[]’ syntax is the only other access that you have to the array object.

The following example shows the various ways that an array can be initialized, and how the array references can be assigned to different array objects. It also shows that arrays of objects and arrays of primitives are almost identical in their use. The only difference is that arrays of objects hold references while arrays of primitives hold the primitive values directly.

//: c09:ArraySize.java
// Initialization & re-assignment of arrays.

class Weeble {} // A small mythical creature

public class ArraySize {
  public static void main(String[] args) {
    // Arrays of objects:
    Weeble[] a; // Null reference
    Weeble[] b = new Weeble[5]; // Null references
    Weeble[] c = new Weeble[4];
    for(int i = 0; i < c.length; i++)
      c[i] = new Weeble();
    // Aggregate initialization:
    Weeble[] d = {
      new Weeble(), new Weeble(), new Weeble()
    };
    // Dynamic aggregate initialization:
    a = new Weeble[] {
      new Weeble(), new Weeble()
    };
    // Compile error: variable a not initialized:
    //!System.out.println("a.length=" + a.length);
    System.out.println("b.length = " + b.length);
    // The references inside the array are
    // automatically initialized to null:
    for(int i = 0; i < b.length; i++)
      System.out.println("b[" + i + "]=" + b[i]);
    System.out.println("c.length = " + c.length);
    System.out.println("d.length = " + d.length);
    a = d;
    System.out.println("a.length = " + a.length);
    System.out.println("a.length = " + a.length);

    // Arrays of primitives:
    int[] e; // Null reference
    int[] f = new int[5];
    int[] g = new int[4];
    for(int i = 0; i < g.length; i++)
      g[i] = i*i;
    int[] h = { 11, 47, 93 };
    // Compile error: variable e not initialized:
    //!System.out.println("e.length=" + e.length);
    System.out.println("f.length = " + f.length);
    // The primitives inside the array are
    // automatically initialized to zero:
    for(int i = 0; i < f.length; i++)
      System.out.println("f[" + i + "]=" + f[i]);
    System.out.println("g.length = " + g.length);
    System.out.println("h.length = " + h.length);
    e = h;
    System.out.println("e.length = " + e.length);
    e = new int[] { 1, 2 };
    System.out.println("e.length = " + e.length);
  }
} ///:~

Here’s the output from the program:

b.length = 5
b[0]=null
b[1]=null
b[2]=null
b[3]=null
b[4]=null
c.length = 4
d.length = 3
a.length = 3
a.length = 2
f.length = 5
f[0]=0
f[1]=0
f[2]=0
f[3]=0
f[4]=0
g.length = 4
h.length = 3
e.length = 3
e.length = 2

The array a is initially just a null reference, and the compiler prevents you from doing anything with this reference until you’ve properly initialized it. The array b is initialized to point to an array of Weeble references, but no actual Weeble objects are ever placed in that array. However, you can still ask what the size of the array is, since b is pointing to a legitimate object. This brings up a slight drawback: you can’t find out how many elements are actually in the array, since length tells you only how many elements can be placed in the array; that is, the size of the array object, not the number of elements it actually holds. However, when an array object is created its references are automatically initialized to null so you can see whether a particular array slot has an object in it by checking to see whether it’s null. Similarly, an array of primitives is automatically initialized to zero for numeric types, (char)0 for char, and false for boolean.

Array c shows the creation of the array object followed by the assignment of Weeble objects to all the slots in the array. Array d shows the “aggregate initialization” syntax that causes the array object to be created (implicitly with new on the heap, just like for array c) and initialized with Weeble objects, all in one statement.

The next array initialization could be thought of as a “dynamic aggregate initialization.” The aggregate initialization used by d must be used at the point of d’s definition, but with the second syntax you can create and initialize an array object anywhere. For example, suppose hide( ) is a method that takes an array of Weeble objects. You could call it by saying:

hide(d);

but you can also dynamically create the array you want to pass as the argument:

hide(new Weeble[] { new Weeble(), new Weeble() });

This new syntax provides a more convenient way to write code in some situations.

The expression

a = d;

shows how you can take a reference that’s attached to one array object and assign it to another array object, just as you can do with any other type of object reference. Now both a and d are pointing to the same array object on the heap.

The second part of ArraySize.java shows that primitive arrays work just like object arrays except that primitive arrays hold the primitive values directly.

Containers of primitives

Container classes can hold only references to objects. An array, however, can be created to hold primitives directly, as well as references to objects. It is possible to use the “wrapper” classes such as Integer, Double, etc. to place primitive values inside a container, but the wrapper classes for primitives can be awkward to use. In addition, it’s much more efficient to create and access an array of primitives than a container of wrapped primitives.

Of course, if you’re using a primitive type and you need the flexibility of a container that automatically expands when more space is needed, the array won’t work and you’re forced to use a container of wrapped primitives. You might think that there should be a specialized type of ArrayList for each of the primitive data types, but Java doesn’t provide this for you. Some sort of templatizing mechanism might someday provide a better way for Java to handle this problem. [44]

Returning an array

Suppose you’re writing a method and you don’t just want to return one thing, but a whole bunch of things. Languages like C and C++ make this difficult because you can’t just return an array, only a pointer to an array. This introduces problems because it becomes messy to control the lifetime of the array, which easily leads to memory leaks.

Java takes a similar approach, but you just “return an array.” Actually, of course, you’re returning a reference to an array, but with Java you never worry about responsibility for that array—it will be around as long as you need it, and the garbage collector will clean it up when you’re done.

As an example, consider returning an array of String:

//: c09:IceCream.java
// Returning arrays from methods.

public class IceCream {
  static String[] flav = {
    "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie"
  };
  static String[] flavorSet(int n) {
    // Force it to be positive & within bounds:
    n = Math.abs(n) % (flav.length + 1);
    String[] results = new String[n];
    boolean[] picked =
      new boolean[flav.length];
    for (int i = 0; i < n; i++) {
      int t;
      do
        t = (int)(Math.random() * flav.length);
      while (picked[t]);
      results[i] = flav[t];
      picked[t] = true;
    }
    return results;
  }
  public static void main(String[] args) {
    for(int i = 0; i < 20; i++) {
      System.out.println(
        "flavorSet(" + i + ") = ");
      String[] fl = flavorSet(flav.length);
      for(int j = 0; j < fl.length; j++)
        System.out.println("\t" + fl[j]);
    }
  }
} ///:~

The method flavorSet( ) creates an array of String called results. The size of this array is n, determined by the argument you pass into the method. Then it proceeds to choose flavors randomly from the array flav and place them into results, which it finally returns. Returning an array is just like returning any other object—it’s a reference. It’s not important that the array was created within flavorSet( ), or that the array was created anyplace else, for that matter. The garbage collector takes care of cleaning up the array when you’re done with it, and the array will persist for as long as you need it.

As an aside, notice that when flavorSet( ) chooses flavors randomly, it ensures that a random choice hasn’t been picked before. This is performed in a do loop that keeps making random choices until it finds one that’s not already in the picked array. (Of course, a String comparison could also have been performed to see if the random choice was already in the results array, but String comparisons are inefficient.) If it’s successful it adds the entry and finds the next one (i gets incremented).

main( ) prints out 20 full sets of flavors, so you can see that flavorSet( ) chooses the flavors in a random order each time. It’s easiest to see this if you redirect the output into a file. And while you’re looking at the file, remember, you just want the ice cream, you don’t need it.

The Arrays class

In java.util, you’ll find the Arrays class, which holds a set of static methods that perform utility functions for arrays. There are four basic functions: equals( ), to compare two arrays for equality; fill( ), to fill an array with a value, sort( ), to sort the array, and binarySearch( ), to find an element in a sorted array. All of these methods are overloaded for all the primitive types and Objects. In addition, there’s a single asList( ) method that takes any array and turns it into a List container—which you’ll learn about later in the chapter.

While useful, the Arrays class stops short of being fully functional. For example, it would be nice to be able to easily print the elements of an array without having to code a for loop by hand every time. And as you’ll see, the fill( ) method only takes a single value and places it in the array, so if you wanted—for example—to fill an array with randomly-generated numbers, fill( ) is no help.

Thus it makes sense to supplement the Arrays class with some additional utilities, which will be placed in the package com.bruceeckel.util for convenience. These will print an array of any type, and fill an array with values or objects that are created by an object called a generator that you can define.

Because code needs to be created for each primitive type as well as Object, there’s a lot of nearly-duplicated code [45] . For example, a “generator” interface is required for each type because the return type of next( ) must be different in each case:

//: com:bruceeckel:util:Generator.java
package com.bruceeckel.util;
public interface Generator {
  Object next();
} ///:~

//: com:bruceeckel:util:BooleanGenerator.java
package com.bruceeckel.util;
public interface BooleanGenerator {
  boolean next();
} ///:~

//: com:bruceeckel:util:ByteGenerator.java
package com.bruceeckel.util;
public interface ByteGenerator {
  byte next();
} ///:~

//: com:bruceeckel:util:CharGenerator.java
package com.bruceeckel.util;
public interface CharGenerator {
  char next();
} ///:~

//: com:bruceeckel:util:ShortGenerator.java
package com.bruceeckel.util;
public interface ShortGenerator {
  short next();
} ///:~

//: com:bruceeckel:util:IntGenerator.java
package com.bruceeckel.util;
public interface IntGenerator {
  int next();
} ///:~

//: com:bruceeckel:util:LongGenerator.java
package com.bruceeckel.util;
public interface LongGenerator {
  long next();
} ///:~

//: com:bruceeckel:util:FloatGenerator.java
package com.bruceeckel.util;
public interface FloatGenerator {
  float next();
} ///:~

//: com:bruceeckel:util:DoubleGenerator.java
package com.bruceeckel.util;
public interface DoubleGenerator {
  double next();
} ///:~

Arrays2 contains a variety of print( ) functions, overloaded for each type. You can simply print an array, you can add a message before the array is printed, or you can print a range of elements within an array. The print( ) code is self-explanatory:

//: com:bruceeckel:util:Arrays2.java
// A supplement to java.util.Arrays, to provide
// additional useful functionality when working
// with arrays. Allows any array to be printed,
// and to be filled via a user-defined
// "generator" object.
package com.bruceeckel.util;
import java.util.*;

public class Arrays2 {
  private static void
  start(int from, int to, int length) {
    if(from != 0 || to != length)
      System.out.print("["+ from +":"+ to +"] ");
    System.out.print("(");
  }
  private static void end() {
    System.out.println(")");
  }
  public static void print(Object[] a) {
    print(a, 0, a.length);
  }
  public static void
  print(String msg, Object[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(Object[] a, int from, int to){
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to -1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(boolean[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, boolean[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(boolean[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to -1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(byte[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, byte[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(byte[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to -1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(char[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, char[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(char[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to -1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(short[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, short[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(short[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to - 1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(int[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, int[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(int[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to - 1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(long[] a) {
    print(a, 0, a.length);
  }
  public static void
  print(String msg, long[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(long[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to - 1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(float[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, float[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(float[] a, int from, int to) {
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to - 1)
        System.out.print(", ");
    }
    end();
  }
  public static void print(double[] a) {
      print(a, 0, a.length);
  }
  public static void
  print(String msg, double[] a) {
    System.out.print(msg + " ");
    print(a, 0, a.length);
  }
  public static void
  print(double[] a, int from, int to){
    start(from, to, a.length);
    for(int i = from; i < to; i++) {
      System.out.print(a[i]);
      if(i < to - 1)
        System.out.print(", ");
    }
    end();
  }
  // Fill an array using a generator:
  public static void
  fill(Object[] a, Generator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(Object[] a, int from, int to,
       Generator gen){
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(boolean[] a, BooleanGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(boolean[] a, int from, int to,
       BooleanGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(byte[] a, ByteGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(byte[] a, int from, int to,
       ByteGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(char[] a, CharGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(char[] a, int from, int to,
       CharGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(short[] a, ShortGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(short[] a, int from, int to,
       ShortGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(int[] a, IntGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(int[] a, int from, int to,
       IntGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(long[] a, LongGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(long[] a, int from, int to,
       LongGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(float[] a, FloatGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(float[] a, int from, int to,
       FloatGenerator gen) {
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  public static void
  fill(double[] a, DoubleGenerator gen) {
      fill(a, 0, a.length, gen);
  }
  public static void
  fill(double[] a, int from, int to,
       DoubleGenerator gen){
    for(int i = from; i < to; i++)
      a[i] = gen.next();
  }
  private static Random r = new Random();
  public static class RandBooleanGenerator
  implements BooleanGenerator {
    public boolean next() {
      return r.nextBoolean();
    }
  }
  public static class RandByteGenerator
  implements ByteGenerator {
    public byte next() {
      return (byte)r.nextInt();
    }
  }
  static String ssource =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
    "abcdefghijklmnopqrstuvwxyz";
  static char[] src = ssource.toCharArray();
  public static class RandCharGenerator
  implements CharGenerator {
    public char next() {
      int pos = Math.abs(r.nextInt());
      return src[pos % src.length];
    }
  }
  public static class RandStringGenerator
  implements Generator {
    private int len;
    private RandCharGenerator cg =
      new RandCharGenerator();
    public RandStringGenerator(int length) {
      len = length;
    }
    public Object next() {
      char[] buf = new char[len];
      for(int i = 0; i < len; i++)
        buf[i] = cg.next();
      return new String(buf);
    }
  }
  public static class RandShortGenerator
  implements ShortGenerator {
    public short next() {
      return (short)r.nextInt();
    }
  }
  public static class RandIntGenerator
  implements IntGenerator {
    private int mod = 10000;
    public RandIntGenerator() {}
    public RandIntGenerator(int modulo) {
      mod = modulo;
    }
    public int next() {
      return r.nextInt() % mod;
    }
  }
  public static class RandLongGenerator
  implements LongGenerator {
    public long next() { return r.nextLong(); }
  }
  public static class RandFloatGenerator
  implements FloatGenerator {
    public float next() { return r.nextFloat(); }
  }
  public static class RandDoubleGenerator
  implements DoubleGenerator {
    public double next() {return r.nextDouble();}
  }
} ///:~

To fill an array of elements using a generator, the fill( ) method takes a reference to an appropriate generator interface, which has a next( ) method that will somehow produce an object of the right type (depending on how the interface is implemented). The fill( ) method simply calls next( ) until the desired range has been filled. Now you can create any generator by implementing the appropriate interface, and use your generate with fill( ).

Random data generators are useful for testing, so a set of inner classes is created to implement all the primitive generator interfaces, as well as a String generator to represent Object. You can see that RandStringGenerator uses RandCharGenerator to fill an array of characters, which is then turned into a String. The size of the array is determined by the constructor argument.

To generate numbers that aren’t too large, RandIntGenerator defaults to a modulus of 10,000, but the overloaded constructor allows you to choose a smaller value.

Here’s a program to test the library and to demonstrate how it is used:

//: c09:TestArrays2.java
// Test and demonstrate Arrays2 utilities
import com.bruceeckel.util.*;

public class TestArrays2 {
  public static void main(String[] args) {
    int size = 6;
    // Or get the size from the command line:
    if(args.length != 0)
      size = Integer.parseInt(args[0]);
    boolean[] a1 = new boolean[size];
    byte[] a2 = new byte[size];
    char[] a3 = new char[size];
    short[] a4 = new short[size];
    int[] a5 = new int[size];
    long[] a6 = new long[size];
    float[] a7 = new float[size];
    double[] a8 = new double[size];
    String[] a9 = new String[size];
    Arrays2.fill(a1,
      new Arrays2.RandBooleanGenerator());
    Arrays2.print(a1);
    Arrays2.print("a1 = ", a1);
    Arrays2.print(a1, size/3, size/3 + size/3);
    Arrays2.fill(a2,
      new Arrays2.RandByteGenerator());
    Arrays2.print(a2);
    Arrays2.print("a2 = ", a2);
    Arrays2.print(a2, size/3, size/3 + size/3);
    Arrays2.fill(a3,
      new Arrays2.RandCharGenerator());
    Arrays2.print(a3);
    Arrays2.print("a3 = ", a3);
    Arrays2.print(a3, size/3, size/3 + size/3);
    Arrays2.fill(a4,
      new Arrays2.RandShortGenerator());
    Arrays2.print(a4);
    Arrays2.print("a4 = ", a4);
    Arrays2.print(a4, size/3, size/3 + size/3);
    Arrays2.fill(a5,
      new Arrays2.RandIntGenerator());
    Arrays2.print(a5);
    Arrays2.print("a5 = ", a5);
    Arrays2.print(a5, size/3, size/3 + size/3);
    Arrays2.fill(a6,
      new Arrays2.RandLongGenerator());
    Arrays2.print(a6);
    Arrays2.print("a6 = ", a6);
    Arrays2.print(a6, size/3, size/3 + size/3);
    Arrays2.fill(a7,
      new Arrays2.RandFloatGenerator());
    Arrays2.print(a7);
    Arrays2.print("a7 = ", a7);
    Arrays2.print(a7, size/3, size/3 + size/3);
    Arrays2.fill(a8,
      new Arrays2.RandDoubleGenerator());
    Arrays2.print(a8);
    Arrays2.print("a8 = ", a8);
    Arrays2.print(a8, size/3, size/3 + size/3);
    Arrays2.fill(a9,
      new Arrays2.RandStringGenerator(7));
    Arrays2.print(a9);
    Arrays2.print("a9 = ", a9);
    Arrays2.print(a9, size/3, size/3 + size/3);
  }
} ///:~

The size parameter has a default value, but you can also set it from the command line.

Filling an array

The Java standard library Arrays also has a fill( ) method, but that is rather trivial—it only duplicates a single value into each location, or in the case of objects, copies the same reference into each location. Using Arrays2.print( ), the Arrays.fill( ) methods can be easily demonstrated:

//: c09:FillingArrays.java
// Using Arrays.fill()
import com.bruceeckel.util.*;
import java.util.*;

public class FillingArrays {
  public static void main(String[] args) {
    int size = 6;
    // Or get the size from the command line:
    if(args.length != 0)
      size = Integer.parseInt(args[0]);
    boolean[] a1 = new boolean[size];
    byte[] a2 = new byte[size];
    char[] a3 = new char[size];
    short[] a4 = new short[size];
    int[] a5 = new int[size];
    long[] a6 = new long[size];
    float[] a7 = new float[size];
    double[] a8 = new double[size];
    String[] a9 = new String[size];
    Arrays.fill(a1, true);
    Arrays2.print("a1 = ", a1);
    Arrays.fill(a2, (byte)11);
    Arrays2.print("a2 = ", a2);
    Arrays.fill(a3, 'x');
    Arrays2.print("a3 = ", a3);
    Arrays.fill(a4, (short)17);
    Arrays2.print("a4 = ", a4);
    Arrays.fill(a5, 19);
    Arrays2.print("a5 = ", a5);
    Arrays.fill(a6, 23);
    Arrays2.print("a6 = ", a6);
    Arrays.fill(a7, 29);
    Arrays2.print("a7 = ", a7);
    Arrays.fill(a8, 47);
    Arrays2.print("a8 = ", a8);
    Arrays.fill(a9, "Hello");
    Arrays2.print("a9 = ", a9);
    // Manipulating ranges:
    Arrays.fill(a9, 3, 5, "World");
    Arrays2.print("a9 = ", a9);
  }
} ///:~

You can either fill the entire array, or as the last two statements show, a range of elements. But since you can only provide a single value to use for filling using Arrays.fill( ), the Arrays2.fill( ) methods produce much more interesting results.

Copying an array

The Java standard library provides a static method, System.arraycopy( ), which can make much faster copies of an array than if you use a for loop to perform the copy by hand. System.arraycopy( ) is overloaded to handle all types. Here’s an example that manipulates arrays of int:

//: c09:CopyingArrays.java
// Using System.arraycopy()
import com.bruceeckel.util.*;
import java.util.*;

public class CopyingArrays {
  public static void main(String[] args) {
    int[] i = new int[25];
    int[] j = new int[25];
    Arrays.fill(i, 47);
    Arrays.fill(j, 99);
    Arrays2.print("i = ", i);
    Arrays2.print("j = ", j);
    System.arraycopy(i, 0, j, 0, i.length);
    Arrays2.print("j = ", j);
    int[] k = new int[10];
    Arrays.fill(k, 103);
    System.arraycopy(i, 0, k, 0, k.length);
    Arrays2.print("k = ", k);
    Arrays.fill(k, 103);
    System.arraycopy(k, 0, i, 0, k.length);
    Arrays2.print("i = ", i);
    // Objects:
    Integer[] u = new Integer[10];
    Integer[] v = new Integer[5];
    Arrays.fill(u, new Integer(47));
    Arrays.fill(v, new Integer(99));
    Arrays2.print("u = ", u);
    Arrays2.print("v = ", v);
    System.arraycopy(v, 0,
      u, u.length/2, v.length);
    Arrays2.print("u = ", u);
  }
} ///:~

The arguments to arraycopy( ) are the source array, the offset into the source array from whence to start copying, the destination array, the offset into the destination array where the copying begins, and the number of elements to copy. Naturally, any violation of the array boundaries will cause an exception.

The example shows that both primitive arrays and object arrays can be copied. However, if you copy arrays of objects then only the references get copied—there’s no duplication of the objects themselves. This is called a shallow copy (see Appendix A).

Comparing arrays

Arrays provides the overloaded method equals( ) to compare entire arrays for equality. Again, these are overloaded for all the primitives, and for Object. To be equal, the arrays must have the same number of elements and each element must be equivalent to each corresponding element in the other array, using the equals( ) for each element (for primitives, that primitive’s wrapper class equals( ) is used; for example, Integer.equals( ) for int). Here’s an example:

//: c09:ComparingArrays.java
// Using Arrays.equals()
import java.util.*;

public class ComparingArrays {
  public static void main(String[] args) {
    int[] a1 = new int[10];
    int[] a2 = new int[10];
    Arrays.fill(a1, 47);
    Arrays.fill(a2, 47);
    System.out.println(Arrays.equals(a1, a2));
    a2[3] = 11;
    System.out.println(Arrays.equals(a1, a2));
    String[] s1 = new String[5];
    Arrays.fill(s1, "Hi");
    String[] s2 = {"Hi", "Hi", "Hi", "Hi", "Hi"};
    System.out.println(Arrays.equals(s1, s2));
  }
} ///:~

Originally, a1 and a2 are exactly equal, so the output is “true,” but then one of the elements is changed so the second line of output is “false.” In the last case, all the elements of s1 point to the same object, but s2 has 5 unique objects. However, array equality is based on contents (via Object.equals( )) and so the result is “true.”

Array element comparisons

One of the missing features in the Java 1.0 and 1.1 libraries is algorithmic operations, even simple sorting. This was a rather confusing situation to someone expecting an adequate standard library. Fortunately, Java 2 remedies the situation, at least for the sorting problem.

A problem with writing generic sorting code is that sorting must perform comparisons based on the actual type of the object. Of course, one approach is to write a different sorting method for every different type, but you should be able to recognize that this does not produce code that is easily reused for new types.

A primary goal of programming design is to “separate things that change from things that stay the same,” and here, the code that stays the same is the general sort algorithm, but the thing that changes from one use to the next is the way objects are compared. So instead of hard-wiring the comparison code into many different sort routines, the technique of the callback is used. With a callback, the part of the code that varies from case to case is encapsulated inside its own class, and the part of the code that’s always the same will call back to the code that changes. That way you can make different objects to express different ways of comparison and feed them to the same sorting code.

In Java 2, there are two ways to provide comparison functionality. The first is with the natural comparison method that is imparted to a class by implementing the java.lang.Comparable interface. This is a very simple interface with a single method, compareTo( ). This method takes another Object as an argument, and produces a negative value if the argument is less than the current object, zero if the argument is equal, and a positive value if the argument is greater than the current object.

Here’s a class that implements Comparable and demonstrates the comparability by using the Java standard library method Arrays.sort( ):

//: c09:CompType.java
// Implementing Comparable in a class.
import com.bruceeckel.util.*;
import java.util.*;

public class CompType implements Comparable {
  int i;
  int j;
  public CompType(int n1, int n2) {
    i = n1;
    j = n2;
  }
  public String toString() {
    return "[i = " + i + ", j = " + j + "]";
  }
  public int compareTo(Object rv) {
    int rvi = ((CompType)rv).i;
    return (i < rvi ? -1 : (i == rvi ? 0 : 1));
  }
  private static Random r = new Random();
  private static int randInt() {
    return Math.abs(r.nextInt()) % 100;
  }
  public static Generator generator() {
    return new Generator() {
      public Object next() {
        return new CompType(randInt(),randInt());
      }
    };
  }
  public static void main(String[] args) {
    CompType[] a = new CompType[10];
    Arrays2.fill(a, generator());
    Arrays2.print("before sorting, a = ", a);
    Arrays.sort(a);
    Arrays2.print("after sorting, a = ", a);
  }
} ///:~

When you define the comparison function, you are responsible for deciding what it means to compare one of your objects to another. Here, only the i values are used in the comparison, and the j values are ignored.

The static randInt( ) method produces positive values between zero and 100, and the generator( ) method produces an object that implements the Generator interface, by creating an anonymous inner class (see Chapter 8). This builds CompType objects by initializing them with random values. In main( ), the generator is used to fill an array of CompType which is then sorted. If Comparable hadn’t been implemented, then you’d get a compile-time error message when you tried to call sort( ).

Now suppose someone hands you a class that doesn’t implement Comparable, or they hand you this class which does implement Comparable, but you decide you don’t like the way it works and would rather have a different comparison function for the type. To do this, you use the second approach for comparing objects, by creating a separate class that implements an interface called Comparator. This has two methods, compare( ) and equals( ). However, you don’t have to implement equals( ) except for special performance needs, because anytime you create a class it is implicitly inherited from Object which has an equals( ). So you can just use the default Object equals( ) and satisfy the contract imposed by the interface.

The Collections class (which we’ll look at more later) contains a single Comparator that reverses the natural sorting order. This can easily be applied to the CompType:

//: c09:Reverse.java
// The Collecions.reverseOrder() Comparator.
import com.bruceeckel.util.*;
import java.util.*;

public class Reverse {
  public static void main(String[] args) {
    CompType[] a = new CompType[10];
    Arrays2.fill(a, CompType.generator());
    Arrays2.print("before sorting, a = ", a);
    Arrays.sort(a, Collections.reverseOrder());
    Arrays2.print("after sorting, a = ", a);
  }
} ///:~

The call to Collections.reverseOrder( ) produces the reference to the Comparator.

As a second example, the following Comparator compares CompType objects based on their j values rather than their i values:

//: c09:ComparatorTest.java
// Implementing a Comparator for a class.
import com.bruceeckel.util.*;
import java.util.*;

class CompTypeComparator implements Comparator {
  public int compare(Object o1, Object o2) {
    int j1 = ((CompType)o1).j;
    int j2 = ((CompType)o2).j;
    return (j1 < j2 ? -1 : (j1 == j2 ? 0 : 1));
  }
}

public class ComparatorTest {
  public static void main(String[] args) {
    CompType[] a = new CompType[10];
    Arrays2.fill(a, CompType.generator());
    Arrays2.print("before sorting, a = ", a);
    Arrays.sort(a, new CompTypeComparator());
    Arrays2.print("after sorting, a = ", a);
  }
} ///:~

The compare( ) method must return a negative integer, zero, or a positive integer if the first argument is less than, equal to, or greater than the second, respectively.

Sorting an array

With the built-in sorting methods, you can sort any array of primitives, and any array of objects that either implements Comparable or has an associated Comparator. This fills a big hole in the Java libraries—believe it or not, there was no support in Java 1.0 or 1.1 for sorting Strings! Here’s an example that generates random String objects and sorts them:

//: c09:StringSorting.java
// Sorting an array of Strings.
import com.bruceeckel.util.*;
import java.util.*;

public class StringSorting {
  public static void main(String[] args) {
    String[] sa = new String[30];
    Arrays2.fill(sa,
      new Arrays2.RandStringGenerator(5));
    Arrays2.print("Before sorting: ", sa);
    Arrays.sort(sa);
    Arrays2.print("After sorting: ", sa);
  }
} ///:~

One thing you’ll notice about the output is that the String sorting algorithm is that it’s lexicographic, so it puts all the words starting with uppercase letters first, followed by all the words starting with lowercase letters. It turns out that even telephone books are typically sorted this way. You may also want to group the words together regardless of case, and you can do this by defining a Comparator class and thereby overriding the default String Comparable behavior. For reuse, this will be added to the “util” package:

//: com:bruceeckel:util:AlphabeticComparator.java
// Keeping upper and lowercase letters together.
package com.bruceeckel.util;
import java.util.*;

public class AlphabeticComparator
implements Comparator{
  public int compare(Object o1, Object o2) {
    String s1 = (String)o1;
    String s2 = (String)o2;
    return s1.toLowerCase().compareTo(
      s2.toLowerCase());
  }
} ///:~

Each String is converted to lowercase before the comparison. String’s built-in compareTo( ) method provides the desired functionality.

Here’s a test using AlphabeticComparator:

//: c09:AlphabeticSorting.java
// Keeping upper and lowercase letters together.
import com.bruceeckel.util.*;
import java.util.*;

public class AlphabeticSorting {
  public static void main(String[] args) {
    String[] sa = new String[30];
    Arrays2.fill(sa,
      new Arrays2.RandStringGenerator(5));
    Arrays2.print("Before sorting: ", sa);
    Arrays.sort(sa, new AlphabeticComparator());
    Arrays2.print("After sorting: ", sa);
  }
} ///:~

The sorting algorithm that’s used in the Java standard library is designed to be optimal for the particular type you’re sorting—a Quicksort for primitives, and a stable merge sort for objects. So you shouldn’t need to spend any time worrying about performance unless your profiling tool points you to the sorting process as a bottleneck.

Searching a sorted array

Once an array is sorted, you can perform a fast search for a particular item using Arrays.binarySearch( ). However, it’s very important that you do not try to use binarySearch( ) on an unsorted array; the results will be unpredictable. The following example uses a RandIntGenerator to fill an array, then to produces values to search for:

//: c09:ArraySearching.java
// Using Arrays.binarySearch().
import com.bruceeckel.util.*;
import java.util.*;

public class ArraySearching {
  public static void main(String[] args) {
    int[] a = new int[100];
    Arrays2.RandIntGenerator gen =
      new Arrays2.RandIntGenerator(1000);
    Arrays2.fill(a, gen);
    Arrays.sort(a);
    Arrays2.print("Sorted array: ", a);
    while(true) {
      int r = gen.next();
      int location = Arrays.binarySearch(a, r);
      if(location >= 0) {
        System.out.println("Location of " + r +
          " is " + location + ", a[" +
          location + "] = " + a[location]);
        break; // Out of while loop
      }
    }
  }
} ///:~

In the while loop, random values are generated as search items, until one of them is found.

Arrays.binarySearch( ) produces a value greater than or equal to zero if the search item is found. Otherwise, it produces a negative value representing the place that the element should be inserted if you are maintaining the sorted array by hand. The value produced is

-(insertion point) - 1

The insertion point is the index of the first element greater than the key, or a.size( ), if all elements in the array are less than the specified key.

If the array contains duplicate elements, there is no guarantee which one will be found. The algorithm is thus not really designed to support duplicate elements, as much as tolerate them. If you need a sorted list of non-duplicated elements, however, use a TreeSet, which will be introduced later in the chapter. This takes care of all the details for you automatically. Only in cases of performance bottlenecks should you replace the TreeSet with a hand-maintained array.

If you have sorted an object array using a Comparator (primitive arrays do not allow sorting with a Comparator), you must include that same Comparator when you perform a binarySearch( ) (using the overloaded version of the function that’s provided). For example, the AlphabeticSorting.java program can be modified to perform a search:

//: c09:AlphabeticSearch.java
// Searching with a Comparator.
import com.bruceeckel.util.*;
import java.util.*;

public class AlphabeticSearch {
  public static void main(String[] args) {
    String[] sa = new String[30];
    Arrays2.fill(sa,
      new Arrays2.RandStringGenerator(5));
    AlphabeticComparator comp =
      new AlphabeticComparator();
    Arrays.sort(sa, comp);
    int index =
      Arrays.binarySearch(sa, sa[10], comp);
    System.out.println("Index = " + index);
  }
} ///:~

The Comparator must be passed to the overloaded binarySearch( ) as the third argument. In the above example, success is guaranteed because the search item is plucked out of the array itself.

Array summary

To summarize what you’ve seen so far, your first, most efficient choice to hold a group of objects should be an array, and you’re forced into this choice if you want to hold a group of primitives. In the remainder of the chapter we’ll look at the more general case, when you don’t know at the time you’re writing the program how many objects you’re going to need, or if you need a more sophisticated way to store your objects. Java provides a library of container classes to solve this problem, the basic types of which are List, Set, and Map. You can solve a surprising number of problems using these tools.

Among their other characteristics—Set, for example, holds only one object of each value, and Map is an associative array that lets you associate any object with any other object—the Java container classes will automatically resize themselves. So unlike arrays, you can put in any number of objects and you don’t need to worry about how big to make the container while you’re writing the program.

Introduction to containers

To me, container classes are one of the most powerful tools for raw development because they significantly increase your programming muscle. The Java 2 containers represent a thorough redesign [46] of the rather poor showings in Java 1.0 and 1.1. Some of the redesign makes things tighter and more sensible. It also fills out the functionality of the containers library, providing the behavior of linked lists, queues, and deques (double-ended queues, pronounced “decks”).

The design of a containers library is difficult (true of most library design problems). In C++, the container classes covered the bases with many different classes. This was better than what was available prior to the C++ container classes (nothing), but it didn’t translate well into Java. On the other extreme, I’ve seen a containers library that consists of a single class, “container,” which acts like both a linear sequence and an associative array at the same time. The Java 2 container library strikes a balance: the full functionality that you expect from a mature container library, but easier to learn and use than the C++ container classes and other similar container libraries. The result can seem a bit odd in places. Unlike some of the decisions made in the early Java libraries, these oddities were not accidents, but carefully considered decisions based on tradeoffs in complexity. It might take you a little while to get comfortable with some aspects of the library, but I think you’ll find yourself rapidly acquiring and using these new tools.

The Java 2 container library takes the issue of “holding your objects” and divides it into two distinct concepts:

  1. Collection: a group of individual elements, often with some rule applied to them. A List must hold the elements in a particular sequence, and a Set cannot have any duplicate elements. (A bag, which is not implemented in the Java container library since Lists provide you with enough of that functionality, has no such rules.)
  2. Map: a group of key-value object pairs. At first glance, this might seem like it ought to be a Collection of pairs, but when you try to implement it that way the design gets awkward, so it’s clearer to make it a separate concept. On the other hand, it’s convenient to look at portions of a Map by creating a Collection to represent that portion. Thus, a Map can return a Set of its keys, a Collection of its values, or a Set of its pairs. Maps, like arrays, can easily be expanded to multiple dimensions without adding new concepts: you simply make a Map whose values are Maps (and the values of those Maps can be Maps, etc.).

We will first look at the general features of containers, then go into details and finally learn why there are different versions of some containers, and how to choose between them.

Printing containers

Unlike arrays, the containers print nicely without any help. Here’s an example that also introduces you to the basic types of containers:

//: c09:PrintingContainers.java
// Containers print themselves automatically.
import java.util.*;

public class PrintingContainers {
  static Collection fill(Collection c) {
    c.add("dog");
    c.add("dog");
    c.add("cat");
    return c;
  }
  static Map mfill(Map m) {
    m.put("dog", "Bosco");
    m.put("dog", "Spot");
    m.put("cat", "Rags");
    return m;
  }
  public static void main(String[] args) {
    System.out.println(fill(new ArrayList()));
    System.out.println(fill(new HashSet()));
    System.out.println(mfill(new HashMap()));
  }
} ///:~

As mentioned before, there are two basic categories in the Java container library. The distinction is based on the number of items that are held in each location of the container. The Collection category only holds one item in each location (the name is a bit misleading since entire container libraries are often called “collections”). It includes the List, which holds a group of items in a specified sequence, and the Set, which only allows the addition of one item of each type. The ArrayList is a type of List, and HashSet is a type of Set. To add items to any Collection, there’s an add( ) method.

The Map holds key-value pairs, rather like a mini database. The above program uses one flavor of Map, the HashMap. If you have a Map that associates states with their capitols and you want to know the capitol of Ohio, you look it up, almost as if you were indexing into an array (maps are also called associative arrays). To add elements to a Map there’s a put( ) method that takes a key and a value as arguments. The above example only shows adding elements and does not look the elements up after they’re added. That will be shown later.

The fill( ) and mfill( ) methods fill Collections and Maps, respectively. If you look at the output, you can see that the default printing behavior (provided via the container’s various toString( ) methods) produces quite readable results, so no additional printing support is necessary as it was with arrays:

[dog, dog, cat]
[cat, dog]
{cat=Rags, dog=Spot}

A Collection is printed surrounded by square braces, with each element separated by a comma. A Map is surrounded by curly braces, with each key and value associated with an equal sign (keys on the left, values on the right).

You can also immediately see the basic behavior of the different containers. The List holds the objects exactly as they are entered, without any reordering or editing. The Set, however, only accepts one of each object and it uses its own internal ordering method (in general, you are only concerned with whether or not something is a member of the Set, not the order in which it appears—for that you’d use a List). And the Map also only accepts one of each type of item, based on the key, and it also has its own internal ordering and does not care about the order in which you enter the items.

Filling containers

Although the problem of printing the containers is taken care of, filling containers suffers from the same deficiency as java.util.Arrays. Just like Arrays, there is a companion class called Collections containing static utility methods including one called fill( ). This fill( ) also just duplicates a single object reference throughout the container, and also only works for List objects and not Sets or Maps:

//: c09:FillingLists.java
// The Collections.fill() method.
import java.util.*;

public class FillingLists {
  public static void main(String[] args) {
    List list = new ArrayList();
    for(int i = 0; i < 10; i++)
      list.add("");
    Collections.fill(list, "Hello");
    System.out.println(list);
  }
} ///:~

This method is made even less useful by the fact that it can only replace elements that are already in the List, and will not add new elements.

To be able to create interesting examples, here is a complementary Collections2 library (part of com.bruceeckel.util for convenience) with a fill( ) method that uses a generator to add elements, and allows you to specify the number of elements you want to add( ). The Generator interface defined previously will work for Collections, but the Map requires its own generator interface since a pair of objects (one key and one value) must be produced by each call to next( ). Here is the Pair class:

//: com:bruceeckel:util:Pair.java
package com.bruceeckel.util;
class Pair {
  public Object key, value;
  Pair(Object k, Object v) {
    key = k;
    value = v;
  }
} ///:~

Next, the generator interface that produces the Pair:

//: com:bruceeckel:util:MapGenerator.java
package com.bruceeckel.util;
public interface MapGenerator {
  Pair next();
} ///:~

With these, a set of utilities for working with the container classes can be developed:

//: com:bruceeckel:util:Collections2.java
// To fill any type of container
// using a generator object.
package com.bruceeckel.util;
import java.util.*;

public class Collections2 {
  // Fill an array using a generator:
  public static void
  fill(Collection c, Generator gen, int count) {
    for(int i = 0; i < count; i++)
      c.add(gen.next());
  }
  public static void
  fill(Map m, MapGenerator gen, int count) {
    for(int i = 0; i < count; i++) {
      Pair p = gen.next();
      m.put(p.key, p.value);
    }
  }
  public static class RandStringPairGenerator
  implements MapGenerator {
    private Arrays2.RandStringGenerator gen;
    public RandStringPairGenerator(int len) {
      gen = new Arrays2.RandStringGenerator(len);
    }
    public Pair next() {
      return new Pair(gen.next(), gen.next());
    }
  }
  // Default object so you don't have
  // to create your own:
  public static RandStringPairGenerator rsp =
    new RandStringPairGenerator(10);
  public static class StringPairGenerator
  implements MapGenerator {
    private int index = -1;
    private String[][] d;
    public StringPairGenerator(String[][] data) {
      d = data;
    }
    public Pair next() {
      // Force the index to wrap:
      index = (index + 1) % d.length;
      return new Pair(d[index][0], d[index][1]);
    }
    public StringPairGenerator reset() {
      index = -1;
      return this;
    }
  }
  // Use a predefined dataset:
  public static StringPairGenerator geography =
    new StringPairGenerator(
      CountryCapitals.pairs);
  // Produce a sequence from a 2D array:
  public static class StringGenerator
  implements Generator {
    private String[][] d;
    private int position;
    private int index = -1;
    public
    StringGenerator(String[][] data, int pos) {
      d = data;
      position = pos;
    }
    public Object next() {
      // Force the index to wrap:
      index = (index + 1) % d.length;
      return d[index][position];
    }
    public StringGenerator reset() {
      index = -1;
      return this;
    }
  }
  // Use a predefined dataset:
  public static StringGenerator countries =
    new StringGenerator(CountryCapitals.pairs,0);
  public static StringGenerator capitals =
    new StringGenerator(CountryCapitals.pairs,1);
} ///:~

Both versions of fill( ) take an argument that determines the number of items to add to the container. In addition, there are two generators for the map: RandStringPairGenerator which creates any number of pairs of gibberish Strings with length determined by the constructor argument, and StringPairGenerator which produces pairs of Strings given a two-dimensional array of String. The StringGenerator also takes a two-dimensional array of String but generates single items rather than Pairs. The static rsp, geography, countries, and capitals objects provide prebuilt generators, the last three using all the countries of the world and their capitals. Note that if you try to create more pairs than are available, the generators will loop around to the beginning, and if you are putting the pairs into a Map, the duplicates will just be ignored.

Here is the predefined dataset, which consists of country names and their capitals. It is set in a small font to prevent taking up unnecessary space:

//: com:bruceeckel:util:CountryCapitals.java
package com.bruceeckel.util;
public class CountryCapitals {
  public static final String[][] pairs = {
    {"ALGERIA","Algiers"}, {"ANGOLA","Luanda"},
    {"BENIN","Porto-Novo"}, {"BOTSWANA","Gaberone"},
    {"BURKINA FASO","Ouagadougou"}, {"BURUNDI","Bujumbura"},
    {"CAMEROON","Yaounde"}, {"CAPE VERDE","Praia"},
    {"CENTRAL AFRICAN REPUBLIC","Bangui"},
    {"CHAD","N'djamena"},  {"COMOROS","Moroni"},
    {"CONGO","Brazzaville"}, {"DJIBOUTI","Dijibouti"},
    {"EGYPT","Cairo"}, {"EQUATORIAL GUINEA","Malabo"},
    {"ERITREA","Asmara"}, {"ETHIOPIA","Addis Ababa"},
    {"GABON","Libreville"}, {"THE GAMBIA","Banjul"},
    {"GHANA","Accra"}, {"GUINEA","Conakry"},
    {"GUINEA","-"}, {"BISSAU","Bissau"},
    {"CETE D'IVOIR (IVORY COAST)","Yamoussoukro"},
    {"KENYA","Nairobi"}, {"LESOTHO","Maseru"},
    {"LIBERIA","Monrovia"}, {"LIBYA","Tripoli"},
    {"MADAGASCAR","Antananarivo"}, {"MALAWI","Lilongwe"},
    {"MALI","Bamako"}, {"MAURITANIA","Nouakchott"},
    {"MAURITIUS","Port Louis"}, {"MOROCCO","Rabat"},
    {"MOZAMBIQUE","Maputo"}, {"NAMIBIA","Windhoek"},
    {"NIGER","Niamey"}, {"NIGERIA","Abuja"},
    {"RWANDA","Kigali"}, {"SAO TOME E PRINCIPE","Sao Tome"},
    {"SENEGAL","Dakar"}, {"SEYCHELLES","Victoria"},
    {"SIERRA LEONE","Freetown"}, {"SOMALIA","Mogadishu"},
    {"SOUTH AFRICA","Pretoria/Cape Town"}, {"SUDAN","Khartoum"},
    {"SWAZILAND","Mbabane"}, {"TANZANIA","Dodoma"},
    {"TOGO","Lome"}, {"TUNISIA","Tunis"},
    {"UGANDA","Kampala"},
    {"DEMOCRATIC REPUBLIC OF THE CONGO (ZAIRE)","Kinshasa"},
    {"ZAMBIA","Lusaka"}, {"ZIMBABWE","Harare"},
    // Asia
    {"AFGHANISTAN","Kabul"}, {"BAHRAIN","Manama"},
    {"BANGLADESH","Dhaka"}, {"BHUTAN","Thimphu"},
    {"BRUNEI","Bandar Seri Begawan"}, {"CAMBODIA","Phnom Penh"},
    {"CHINA","Beijing"}, {"CYPRUS","Nicosia"},
    {"INDIA","New Delhi"}, {"INDONESIA","Jakarta"},
    {"IRAN","Tehran"}, {"IRAQ","Baghdad"},
    {"ISRAEL","Jerusalem"}, {"JAPAN","Tokyo"},
    {"JORDAN","Amman"}, {"KUWAIT","Kuwait City"},
    {"LAOS","Vientiane"}, {"LEBANON","Beirut"},
    {"MALAYSIA","Kuala Lumpur"}, {"THE MALDIVES","Male"},
    {"MONGOLIA","Ulan Bator"}, {"MYANMAR (BURMA)","Rangoon"},
    {"NEPAL","Katmandu"}, {"NORTH KOREA","P'yongyang"},
    {"OMAN","Muscat"}, {"PAKISTAN","Islamabad"},
    {"PHILIPPINES","Manila"}, {"QATAR","Doha"},
    {"SAUDI ARABIA","Riyadh"}, {"SINGAPORE","Singapore"},
    {"SOUTH KOREA","Seoul"}, {"SRI LANKA","Colombo"},
    {"SYRIA","Damascus"}, {"TAIWAN (REPUBLIC OF CHINA)","Taipei"},
    {"THAILAND","Bangkok"}, {"TURKEY","Ankara"},
    {"UNITED ARAB EMIRATES","Abu Dhabi"}, {"VIETNAM","Hanoi"},
    {"YEMEN","Sana'a"},
    // Australia and Oceania
    {"AUSTRALIA","Canberra"}, {"FIJI","Suva"},
    {"KIRIBATI","Bairiki"},
    {"MARSHALL ISLANDS","Dalap-Uliga-Darrit"},
    {"MICRONESIA","Palikir"}, {"NAURU","Yaren"},
    {"NEW ZEALAND","Wellington"}, {"PALAU","Koror"},
    {"PAPUA NEW GUINEA","Port Moresby"},
    {"SOLOMON ISLANDS","Honaira"}, {"TONGA","Nuku'alofa"},
    {"TUVALU","Fongafale"}, {"VANUATU","< Port-Vila"},
    {"WESTERN SAMOA","Apia"},
    // Eastern Europe and former USSR
    {"ARMENIA","Yerevan"}, {"AZERBAIJAN","Baku"},
    {"BELARUS (BYELORUSSIA)","Minsk"}, {"GEORGIA","Tbilisi"},
    {"KAZAKSTAN","Almaty"}, {"KYRGYZSTAN","Alma-Ata"},
    {"MOLDOVA","Chisinau"}, {"RUSSIA","Moscow"},
    {"TAJIKISTAN","Dushanbe"}, {"TURKMENISTAN","Ashkabad"},
    {"UKRAINE","Kyiv"}, {"UZBEKISTAN","Tashkent"},
    // Europe
    {"ALBANIA","Tirana"}, {"ANDORRA","Andorra la Vella"},
    {"AUSTRIA","Vienna"}, {"BELGIUM","Brussels"},
    {"BOSNIA","-"}, {"HERZEGOVINA","Sarajevo"},
    {"CROATIA","Zagreb"}, {"CZECH REPUBLIC","Prague"},
    {"DENMARK","Copenhagen"}, {"ESTONIA","Tallinn"},
    {"FINLAND","Helsinki"}, {"FRANCE","Paris"},
    {"GERMANY","Berlin"}, {"GREECE","Athens"},
    {"HUNGARY","Budapest"}, {"ICELAND","Reykjavik"},
    {"IRELAND","Dublin"}, {"ITALY","Rome"},
    {"LATVIA","Riga"}, {"LIECHTENSTEIN","Vaduz"},
    {"LITHUANIA","Vilnius"}, {"LUXEMBOURG","Luxembourg"},
    {"MACEDONIA","Skopje"}, {"MALTA","Valletta"},
    {"MONACO","Monaco"}, {"MONTENEGRO","Podgorica"},
    {"THE NETHERLANDS","Amsterdam"}, {"NORWAY","Oslo"},
    {"POLAND","Warsaw"}, {"PORTUGAL","Lisbon"},
    {"ROMANIA","Bucharest"}, {"SAN MARINO","San Marino"},
    {"SERBIA","Belgrade"}, {"SLOVAKIA","Bratislava"},
    {"SLOVENIA","Ljujiana"}, {"SPAIN","Madrid"},
    {"SWEDEN","Stockholm"}, {"SWITZERLAND","Berne"},
    {"UNITED KINGDOM","London"}, {"VATICAN CITY","---"},
    // North and Central America
    {"ANTIGUA AND BARBUDA","Saint John's"}, {"BAHAMAS","Nassau"},
    {"BARBADOS","Bridgetown"}, {"BELIZE","Belmopan"},
    {"CANADA","Ottawa"}, {"COSTA RICA","San Jose"},
    {"CUBA","Havana"}, {"DOMINICA","Roseau"},
    {"DOMINICAN REPUBLIC","Santo Domingo"},
    {"EL SALVADOR","San Salvador"}, {"GRENADA","Saint George's"},
    {"GUATEMALA","Guatemala City"}, {"HAITI","Port-au-Prince"},
    {"HONDURAS","Tegucigalpa"}, {"JAMAICA","Kingston"},
    {"MEXICO","Mexico City"}, {"NICARAGUA","Managua"},
    {"PANAMA","Panama City"}, {"ST. KITTS","-"},
    {"NEVIS","Basseterre"}, {"ST. LUCIA","Castries"},
    {"ST. VINCENT AND THE GRENADINES","Kingstown"},
    {"UNITED STATES OF AMERICA","Washington, D.C."},
    // South America
    {"ARGENTINA","Buenos Aires"},
    {"BOLIVIA","Sucre (legal)/La Paz(administrative)"},
    {"BRAZIL","Brasilia"}, {"CHILE","Santiago"},
    {"COLOMBIA","Bogota"}, {"ECUADOR","Quito"},
    {"GUYANA","Georgetown"}, {"PARAGUAY","Asuncion"},
    {"PERU","Lima"}, {"SURINAME","Paramaribo"},
    {"TRINIDAD AND TOBAGO","Port of Spain"},
    {"URUGUAY","Montevideo"}, {"VENEZUELA","Caracas"},
  };
} ///:~

This is simply a two-dimensional array of String data [47] . Here’s a simple test using the fill( ) methods and generators:

//: c09:FillTest.java
import com.bruceeckel.util.*;
import java.util.*;

public class FillTest {
  static Generator sg =
    new Arrays2.RandStringGenerator(7);
  public static void main(String[] args) {
    List list = new ArrayList();
    Collections2.fill(list, sg, 25);
    System.out.println(list + "\n");
    List list2 = new ArrayList();
    Collections2.fill(list2,
      Collections2.capitals, 25);
    System.out.println(list2 + "\n");
    Set set = new HashSet();
    Collections2.fill(set, sg, 25);
    System.out.println(set + "\n");
    Map m = new HashMap();
    Collections2.fill(m, Collections2.rsp, 25);
    System.out.println(m + "\n");
    Map m2 = new HashMap();
    Collections2.fill(m2,
      Collections2.geography, 25);
    System.out.println(m2);
  }
} ///:~

With these tools you can easily test the various containers by filling them with interesting data.

Container disadvantage:
unknown type

The “disadvantage” to using the Java containers is that you lose type information when you put an object into a container. This happens because the programmer of that container class had no idea what specific type you wanted to put in the container, and making the container hold only your type would prevent it from being a general-purpose tool. So instead, the container holds references to Object, which is the root of all the classes so it holds any type. (Of course, this doesn’t include primitive types, since they aren’t inherited from anything.) This is a great solution, except:

  1. Since the type information is thrown away when you put an object reference into a container, there’s no restriction on the type of object that can be put into your container, even if you mean it to hold only, say, cats. Someone could just as easily put a dog into the container.
  2. Since the type information is lost, the only thing the container knows that it holds is a reference to an object. You must perform a cast to the correct type before you use it.

On the up side, Java won’t let you misuse the objects that you put into a container. If you throw a dog into a container of cats and then try to treat everything in the container as a cat, you’ll get a run-time exception when you pull the dog reference out of the cat container and try to cast it to a cat.

Here’s an example using the basic workhorse container, ArrayList. For starters, you can think of ArrayList as “an array that automatically expands itself.” Using an ArrayList is straightforward: create one, put objects in using add( ), and later get them out with get( ) using an index, just like you would with an array but without the square brackets [48] . ArrayList also has a method size( ) to let you know how many elements have been added so you don’t inadvertently run off the end and cause an exception.

First, Cat and Dog classes are created:

//: c09:Cat.java
public class Cat {
  private int catNumber;
  Cat(int i) { catNumber = i; }
  void print() {
    System.out.println("Cat #" + catNumber);
  }
} ///:~

//: c09:Dog.java
public class Dog {
  private int dogNumber;
  Dog(int i) { dogNumber = i; }
  void print() {
    System.out.println("Dog #" + dogNumber);
  }
} ///:~

Cats and Dogs are placed into the container, then pulled out:

//: c09:CatsAndDogs.java
// Simple container example.
import java.util.*;

public class CatsAndDogs {
  public static void main(String[] args) {
    ArrayList cats = new ArrayList();
    for(int i = 0; i < 7; i++)
      cats.add(new Cat(i));
    // Not a problem to add a dog to cats:
    cats.add(new Dog(7));
    for(int i = 0; i < cats.size(); i++)
      ((Cat)cats.get(i)).print();
    // Dog is detected only at run-time
  }
} ///:~

The classes Cat and Dog are distinct—they have nothing in common except that they are Objects. (If you don’t explicitly say what class you’re inheriting from, you automatically inherit from Object.) Since ArrayList holds Objects, you can not only put Cat objects into this container using the ArrayList method add( ), but you can also add Dog objects without complaint at either compile-time or run-time. When you go to fetch out what you think are Cat objects using the ArrayList method get( ), you get back a reference to an object that you must cast to a Cat. Then you need to surround the entire expression with parentheses to force the evaluation of the cast before calling the print( ) method for Cat, otherwise you’ll get a syntax error. Then, at run-time, when you try to cast the Dog object to a Cat, you’ll get an exception.

This is more than just an annoyance. It’s something that can create difficult-to-find bugs. If one part (or several parts) of a program inserts objects into a container, and you discover only in a separate part of the program through an exception that a bad object was placed in the container, then you must find out where the bad insert occurred. On the upside, it’s convenient to start with some standardized container classes for programming, despite the scarcity and awkwardness.

Sometimes it works anyway

It turns out that in some cases things seem to work correctly without casting back to your original type. One case is quite special: the String class has some extra help from the compiler to make it work smoothly. Whenever the compiler expects a String object and it hasn’t got one, it will automatically call the toString( ) method that’s defined in Object and can be overridden by any Java class. This method produces the desired String object, which is then used wherever it was wanted.

Thus, all you need to do to make objects of your class print is to override the toString( ) method, as shown in the following example:

//: c09:Mouse.java
// Overriding toString().
public class Mouse {
  private int mouseNumber;
  Mouse(int i) { mouseNumber = i; }
  // Override Object.toString():
  public String toString() {
    return "This is Mouse #" + mouseNumber;
  }
  public int getNumber() {
    return mouseNumber;
  }
} ///:~

//: c09:WorksAnyway.java
// In special cases, things just
// seem to work correctly.
import java.util.*;

class MouseTrap {
  static void caughtYa(Object m) {
    Mouse mouse = (Mouse)m; // Cast from Object
    System.out.println("Mouse: " +
      mouse.getNumber());
  }
}

public class WorksAnyway {
  public static void main(String[] args) {
    ArrayList mice = new ArrayList();
    for(int i = 0; i < 3; i++)
      mice.add(new Mouse(i));
    for(int i = 0; i < mice.size(); i++) {
      // No cast necessary, automatic
      // call to Object.toString():
      System.out.println(
        "Free mouse: " + mice.get(i));
      MouseTrap.caughtYa(mice.get(i));
    }
  }
} ///:~

You can see toString( ) overridden in Mouse. In the second for loop in main( ) you find the statement:

System.out.println("Free mouse: " + mice.get(i));

After the ‘+’ sign the compiler expects to see a String object. get( ) produces an Object, so to get the desired String the compiler implicitly calls toString( ). Unfortunately, you can work this kind of magic only with String; it isn’t available for any other type.

A second approach to hiding the cast has been placed inside MouseTrap. The caughtYa( ) method accepts not a Mouse, but an Object, which it then casts to a Mouse. This is quite presumptuous, of course, since by accepting an Object anything could be passed to the method. However, if the cast is incorrect—if you passed the wrong type—you’ll get an exception at run-time. This is not as good as compile-time checking but it’s still robust. Note that in the use of this method:

MouseTrap.caughtYa(mice.get(i));

no cast is necessary.

Making a type-conscious ArrayList

You might not want to give up on this issue just yet. A more ironclad solution is to create a new class using the ArrayList, such that it will accept only your type and produce only your type:

//: c09:MouseList.java
// A type-conscious ArrayList.
import java.util.*;

public class MouseList {
  private ArrayList list = new ArrayList();
  public void add(Mouse m) {
    list.add(m);
  }
  public Mouse get(int index) {
    return (Mouse)list.get(index);
  }
  public int size() { return list.size(); }
} ///:~

Here’s a test for the new container:

//: c09:MouseListTest.java
public class MouseListTest {
  public static void main(String[] args) {
    MouseList mice = new MouseList();
    for(int i = 0; i < 3; i++)
      mice.add(new Mouse(i));
    for(int i = 0; i < mice.size(); i++)
      MouseTrap.caughtYa(mice.get(i));
  }
} ///:~

This is similar to the previous example, except that the new MouseList class has a private member of type ArrayList, and methods just like ArrayList. However, it doesn’t accept and produce generic Objects, only Mouse objects.

Note that if MouseList had instead been inherited from ArrayList, the add(Mouse) method would simply overload the existing add(Object) and there would still be no restriction on what type of objects could be added. Thus, the MouseList becomes a surrogate to the ArrayList, performing some activities before passing on the responsibility (see Thinking in Patterns with Java, downloadable at www.BruceEckel.com).

Because a MouseList will accept only a Mouse, if you say:

mice.add(new Pigeon());

you will get an error message at compile-time. This approach, while more tedious from a coding standpoint, will tell you immediately if you’re using a type improperly.

Note that no cast is necessary when using get( )—it’s always a Mouse.

Parameterized types

This kind of problem isn’t isolated—there are numerous cases in which you need to create new types based on other types, and in which it is useful to have specific type information at compile-time. This is the concept of a parameterized type. In C++, this is directly supported by the language with templates. It is likely that a future version of Java will support some variation of parameterized types; the current front-runner automatically creates classes similar to MouseList.

Iterators

In any container class, you must have a way to put things in and a way to get things out. After all, that’s the primary job of a container—to hold things. In the ArrayList, add( ) is the way that you insert objects, and get( ) is one way to get things out. ArrayList is quite flexible—you can select anything at any time, and select multiple elements at once using different indexes.

If you want to start thinking at a higher level, there’s a drawback: you need to know the exact type of the container in order to use it. This might not seem bad at first, but what if you start out using an ArrayList, and later on in your program you discover that because of the way you are using the container it would be much more efficient to use a LinkedList instead? Or suppose you’d like to write a piece of generic code that doesn’t know or care what type of container it’s working with, so that it could be used on different types of containers without rewriting that code?

The concept of an iterator can be used to achieve this abstraction. An iterator is an object whose job is to move through a sequence of objects and select each object in that sequence without the client programmer knowing or caring about the underlying structure of that sequence. In addition, an iterator is usually what’s called a “light-weight” object: one that’s cheap to create. For that reason, you’ll often find seemingly strange constraints for iterators; for example, some iterators can move in only one direction.

The Java Iterator is an example of an iterator with these kinds of constraints. There’s not much you can do with one except:

  1. Ask a container to hand you an Iterator using a method called iterator( ). This Iterator will be ready to return the first element in the sequence on your first call to its next( ) method.
  2. Get the next object in the sequence with next( ).
  3. See if there are any more objects in the sequence with hasNext( ).
  4. Remove the last element returned by the iterator with remove( ).

That’s all. It’s a simple implementation of an iterator, but still powerful (and there’s a more sophisticated ListIterator for Lists). To see how it works, let’s revisit the CatsAndDogs.java program from earlier in the chapter. In the original version, the method get( ) was used to select each element, but in the following modified version an Iterator is used:

//: c09:CatsAndDogs2.java
// Simple container with Iterator.
import java.util.*;

public class CatsAndDogs2 {
  public static void main(String[] args) {
    ArrayList cats = new ArrayList();
    for(int i = 0; i < 7; i++)
      cats.add(new Cat(i));
    cats.add(new Dog(7));
    Iterator e = cats.iterator();
    while(e.hasNext())
      ((Cat)e.next()).print();
  }
} ///:~

You can see that the only change is in the last few lines, where an Iterator is used to step through the sequence instead of a for loop. With the Iterator, you don’t need to worry about the number of elements in the container. That’s taken care of for you by hasNext( ) and next( ).

As another example, consider the creation of a general-purpose printing method:

//: c09:HamsterMaze.java
// Using an Iterator.
import java.util.*;

class Hamster {
  private int hamsterNumber;
  Hamster(int i) { hamsterNumber = i; }
  public String toString() {
    return "This is Hamster #" + hamsterNumber;
  }
}

class Printer {
  static void printAll(Iterator e) {
    while(e.hasNext())
      System.out.println(e.next());
  }
}

public class HamsterMaze {
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 3; i++)
      v.add(new Hamster(i));
    Printer.printAll(v.iterator());
  }
} ///:~

Look closely at printAll( ). Note that there’s no information about the type of sequence. All you have is an Iterator, and that’s all you need to know about the sequence: that you can get the next object, and that you can know when you’re at the end. This idea of taking a container of objects and passing through it to perform an operation on each one is powerful and will be seen throughout this book.

The example is even more generic, since it implicitly uses the Object.toString( ) method. The println( ) method is overloaded for all the primitive types as well as Object; in each case a String is automatically produced by calling the appropriate toString( ) method.

Although it’s unnecessary, you can be more explicit using a cast, which has the effect of calling toString( ):

System.out.println((String)e.next());

In general, however, you’ll want to do something more than call Object methods, so you’ll run up against the type-casting issue again. You must assume you’ve gotten an Iterator to a sequence of the particular type you’re interested in, and cast the resulting objects to that type (getting a run-time exception if you’re wrong).

Unintended recursion

Because (like every other class), the Java standard containers are inherited from Object, they contain a toString( ) method. This has been overridden so that they can produce a String representation of themselves, including the objects they hold. Inside ArrayList, for example, the toString( ) steps through the elements of the ArrayList and calls toString( ) for each one. Suppose you’d like to print the address of your class. It seems to make sense to simply refer to this (in particular, C++ programmers are prone to this approach):

//: c09:InfiniteRecursion.java
// Accidental recursion.
import java.util.*;

public class InfiniteRecursion {
  public String toString() {
    return " InfiniteRecursion address: "
      + this + "\n";
  }
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 10; i++)
      v.add(new InfiniteRecursion());
    System.out.println(v);
  }
} ///:~

If you simply create an InfiniteRecursion object and then print it, you’ll get an endless sequence of exceptions. This is also true if you place the InfiniteRecursion objects in an ArrayList and print that ArrayList as shown here. What’s happening is automatic type conversion for Strings. When you say:

"InfiniteRecursion address: " + this

The compiler sees a String followed by a ‘+’ and something that’s not a String, so it tries to convert this to a String. It does this conversion by calling toString( ), which produces a recursive call.

If you really do want to print the address of the object in this case, the solution is to call the Object toString( ) method, which does just that. So instead of saying this, you’d say super.toString( ). (This only works if you're directly inheriting from Object or if none of your parent classes have overridden the toString( ) method).

Container taxonomy

Collections and Maps may be implemented in different ways, according to your programming needs. It’s helpful to look at a diagram of the Java 2 containers:

This diagram can be a bit overwhelming at first, but you’ll see that there are really only three container components: Map, List, and Set, and only two or three implementations of each one (with, typically, a preferred version). When you see this, the containers are not so daunting.

The dotted boxes represent interfaces, the dashed boxes represent abstract classes, and the solid boxes are regular (concrete) classes. The dotted-line arrows indicate that a particular class is implementing an interface (or in the case of an abstract class, partially implementing that interface). The solid arrows show that a class can produce objects of the class the arrow is pointing to. For example, any Collection can produce an Iterator, while a List can produce a ListIterator (as well as an ordinary Iterator, since List is inherited from Collection).

The interfaces that are concerned with holding objects are Collection, List, Set, and Map. Ideally, you’ll write most of your code to talk to these interfaces, and the only place where you’ll specify the precise type you’re using is at the point of creation. So you can create a List like this:

List x = new LinkedList();

Of course, you can also decide to make x a LinkedList (instead of a generic List) and carry the precise type information around with x. The beauty (and the intent) of using the interface is that if you decide you want to change your implementation, all you need to do is change it at the point of creation, like this:

List x = new ArrayList();

The rest of your code can remain untouched (some of this genericity can also be achieved with iterators).

In the class hierarchy, you can see a number of classes whose names begin with “Abstract,” and these can seem a bit confusing at first. They are simply tools that partially implement a particular interface. If you were making your own Set, for example, you wouldn’t start with the Set interface and implement all the methods, instead you’d inherit from AbstractSet and do the minimal necessary work to make your new class. However, the containers library contains enough functionality to satisfy your needs virtually all the time. So for our purposes, you can ignore any class that begins with “Abstract.”

Therefore, when you look at the diagram, you’re really concerned with only those interfaces at the top of the diagram and the concrete classes (those with solid boxes around them). You’ll typically make an object of a concrete class, upcast it to the corresponding interface, and then use the interface throughout the rest of your code. In addition, you do not need to consider the legacy elements when writing new code. Therefore, the diagram can be greatly simplified to look like this:

Now it only includes the interfaces and classes that you will encounter on a regular basis, and also the elements that we will focus on in this chapter.

Here’s a simple example, which fills a Collection (represented here with an ArrayList) with String objects and then prints each element in the Collection:

//: c09:SimpleCollection.java
// A simple example using Java 2 Collections.
import java.util.*;

public class SimpleCollection {
  public static void main(String[] args) {
    // Upcast because we just want to
    // work with Collection features
    Collection c = new ArrayList();
    for(int i = 0; i < 10; i++)
      c.add(Integer.toString(i));
    Iterator it = c.iterator();
    while(it.hasNext())
      System.out.println(it.next());
  }
} ///:~

The first line in main( ) creates an ArrayList object and then upcasts it to a Collection. Since this example uses only the Collection methods, any object of a class inherited from Collection would work, but ArrayList is the typical workhorse Collection.

The add( ) method, as its name suggests, puts a new element in the Collection. However, the documentation carefully states that add( ) “ensures that this Container contains the specified element.” This is to allow for the meaning of Set, which adds the element only if it isn’t already there. With an ArrayList, or any sort of List, add( ) always means “put it in,” because Lists don’t care if there are duplicates.

All Collections can produce an Iterator via their iterator( ) method. Here, an Iterator is created and used to traverse the Collection, printing each element.

Collection functionality

The following table shows everything you can do with a Collection (not including the methods that automatically come through with Object), and thus, everything you can do with a Set or a List. (List also has additional functionality.) Maps are not inherited from Collection, and will be treated separately.

boolean add(Object) | Ensures that the container holds the argument. Returns false if it doesn’t add the argument. (This is an “optional” method, described later in the chapter.)
boolean addAll(Collection) | Adds all the elements in the argument. Returns true if any elements were added. (“Optional.”)
void clear( ) | Removes all the elements in the container. (“Optional.”)
boolean contains(Object) | true if the container holds the argument.
boolean containsAll(Collection) | true if the container holds all the elements in the argument.
boolean isEmpty( ) | true if the container has no elements.
Iterator iterator( ) | Returns an Iterator that you can use to move through the elements in the container.
boolean remove(Object) | If the argument is in the container, one instance of that element is removed. Returns true if a removal occurred. (“Optional.”)
boolean removeAll(Collection) | Removes all the elements that are contained in the argument. Returns true if any removals occurred. (“Optional.”)
boolean retainAll(Collection) | Retains only elements that are contained in the argument (an “intersection” from set theory). Returns true if any changes occurred. (“Optional.”)
int size( ) | Returns the number of elements in the container.
Object[] toArray( ) | Returns an array containing all the elements in the container.
Object[] toArray(Object[] a) | Returns an array containing all the elements in the container, whose type is that of the array a rather than plain Object (you must cast the array to the right type).

Notice that there’s no get( ) function for random-access element selection. That’s because Collection also includes Set, which maintains its own internal ordering (and thus makes random-access lookup meaningless). Thus, if you want to examine all the elements of a Collection you must use an iterator; that’s the only way to fetch things back.

The following example demonstrates all of these methods. Again, these work with anything that inherits from Collection, but an ArrayList is used as a kind of “least-common denominator”:

//: c09:Collection1.java
// Things you can do with all Collections.
import java.util.*;
import com.bruceeckel.util.*;

public class Collection1 {
  public static void main(String[] args) {
    Collection c = new ArrayList();
    Collections2.fill(c,
      Collections2.countries, 10);
    c.add("ten");
    c.add("eleven");
    System.out.println(c);
    // Make an array from the List:
    Object[] array = c.toArray();
    // Make a String array from the List:
    String[] str =
      (String[])c.toArray(new String[1]);
    // Find max and min elements; this means
    // different things depending on the way
    // the Comparable interface is implemented:
    System.out.println("Collections.max(c) = " +
      Collections.max(c));
    System.out.println("Collections.min(c) = " +
      Collections.min(c));
    // Add a Collection to another Collection
    Collection c2 = new ArrayList();
    Collections2.fill(c2,
      Collections2.countries, 10);
    c.addAll(c2);
    System.out.println(c);
    c.remove(CountryCapitals.pairs[0][0]);
    System.out.println(c);
    c.remove(CountryCapitals.pairs[1][0]);
    System.out.println(c);
    // Remove all components that are in the
    // argument collection:
    c.removeAll(c2);
    System.out.println(c);
    c.addAll(c2);
    System.out.println(c);
    // Is an element in this Collection?
    String val = CountryCapitals.pairs[3][0];
    System.out.println(
      "c.contains(" + val  + ") = "
      + c.contains(val));
    // Is a Collection in this Collection?
    System.out.println(
      "c.containsAll(c2) = "+ c.containsAll(c2));
    Collection c3 = ((List)c).subList(3, 5);
    // Keep all the elements that are in both
    // c2 and c3 (an intersection of sets):
    c2.retainAll(c3);
    System.out.println(c);
    // Throw away all the elements
    // in c2 that also appear in c3:
    c2.removeAll(c3);
    System.out.println("c.isEmpty() = " +
      c.isEmpty());
    c = new ArrayList();
    Collections2.fill(c,
      Collections2.countries, 10);
    System.out.println(c);
    c.clear(); // Remove all elements
    System.out.println("after c.clear():");
    System.out.println(c);
  }
} ///:~

ArrayLists are created containing different sets of data and upcast to Collection objects, so it’s clear that nothing other than the Collection interface is being used. main( ) uses simple exercises to show all of the methods in Collection.

The following sections describe the various implementations of List, Set, and Map and indicate in each case (with an asterisk) which one should be your default choice. You’ll notice that the legacy classes Vector, Stack, and Hashtable are not included because in all cases there are preferred classes within the Java 2 Containers.

List functionality

The basic List is quite simple to use, as you’ve seen so far with ArrayList. Although most of the time you’ll just use add( ) to insert objects, get( ) to get them out one at a time, and iterator( ) to get an Iterator to the sequence, there’s also a set of other methods that can be useful.

In addition, there are actually two types of List: the basic ArrayList which excels at randomly accessing elements, and the much more powerful LinkedList (which is not designed for fast random access, but has a much more general set of methods).

List (interface) | Order is the most important feature of a List; it promises to maintain elements in a particular sequence. List adds a number of methods to Collection that allow insertion and removal of elements in the middle of a List. (This is recommended only for a LinkedList.) A List will produce a ListIterator, and using this you can traverse the List in both directions, as well as insert and remove elements in the middle of the list.
ArrayList* | A List implemented with an array. Allows rapid random access to elements, but is slow when inserting and removing elements from the middle of a list. ListIterator should be used only for back-and-forth traversal of an ArrayList, but not for inserting and removing elements, which is expensive compared to LinkedList.
LinkedList | Provides optimal sequential access, with inexpensive insertions and deletions from the middle of the list. Relatively slow for random access. (Use ArrayList instead.) Also has addFirst( ), addLast( ), getFirst( ), getLast( ), removeFirst( ), and removeLast( ) (which are not defined in any interfaces or base classes) to allow it to be used as a stack, a queue, and a deque.

The methods in the following example each cover a different group of activities: things that every list can do (basicTest( )), moving around with an Iterator (iterMotion( )) versus changing things with an Iterator (iterManipulation( )), seeing the effects of List manipulation (testVisual( )), and operations available only to LinkedLists.

//: c09:List1.java
// Things you can do with Lists.
import java.util.*;
import com.bruceeckel.util.*;

public class List1 {
  public static List fill(List a) {
    Collections2.countries.reset();
    Collections2.fill(a,
      Collections2.countries, 10);
    return a;
  }
  static boolean b;
  static Object o;
  static int i;
  static Iterator it;
  static ListIterator lit;
  public static void basicTest(List a) {
    a.add(1, "x"); // Add at location 1
    a.add("x"); // Add at end
    // Add a collection:
    a.addAll(fill(new ArrayList()));
    // Add a collection starting at location 3:
    a.addAll(3, fill(new ArrayList()));
    b = a.contains("1"); // Is it in there?
    // Is the entire collection in there?
    b = a.containsAll(fill(new ArrayList()));
    // Lists allow random access, which is cheap
    // for ArrayList, expensive for LinkedList:
    o = a.get(1); // Get object at location 1
    i = a.indexOf("1"); // Tell index of object
    b = a.isEmpty(); // Any elements inside?
    it = a.iterator(); // Ordinary Iterator
    lit = a.listIterator(); // ListIterator
    lit = a.listIterator(3); // Start at loc 3
    i = a.lastIndexOf("1"); // Last match
    a.remove(1); // Remove location 1
    a.remove("3"); // Remove this object
    a.set(1, "y"); // Set location 1 to "y"
    // Keep everything that's in the argument
    // (the intersection of the two sets):
    a.retainAll(fill(new ArrayList()));
    // Remove everything that's in the argument:
    a.removeAll(fill(new ArrayList()));
    i = a.size(); // How big is it?
    a.clear(); // Remove all elements
  }
  public static void iterMotion(List a) {
    ListIterator it = a.listIterator();
    b = it.hasNext();
    b = it.hasPrevious();
    o = it.next();
    i = it.nextIndex();
    o = it.previous();
    i = it.previousIndex();
  }
  public static void iterManipulation(List a) {
    ListIterator it = a.listIterator();
    it.add("47");
    // Must move to an element after add():
    it.next();
    // Remove the element that was just produced:
    it.remove();
    // Must move to an element after remove():
    it.next();
    // Change the element that was just produced:
    it.set("47");
  }
  public static void testVisual(List a) {
    System.out.println(a);
    List b = new ArrayList();
    fill(b);
    System.out.print("b = ");
    System.out.println(b);
    a.addAll(b);
    a.addAll(fill(new ArrayList()));
    System.out.println(a);
    // Insert, remove, and replace elements
    // using a ListIterator:
    ListIterator x = a.listIterator(a.size()/2);
    x.add("one");
    System.out.println(a);
    System.out.println(x.next());
    x.remove();
    System.out.println(x.next());
    x.set("47");
    System.out.println(a);
    // Traverse the list backwards:
    x = a.listIterator(a.size());
    while(x.hasPrevious())
      System.out.print(x.previous() + " ");
    System.out.println();
    System.out.println("testVisual finished");
  }
  // There are some things that only
  // LinkedLists can do:
  public static void testLinkedList() {
    LinkedList ll = new LinkedList();
    fill(ll);
    System.out.println(ll);
    // Treat it like a stack, pushing:
    ll.addFirst("one");
    ll.addFirst("two");
    System.out.println(ll);
    // Like "peeking" at the top of a stack:
    System.out.println(ll.getFirst());
    // Like popping a stack:
    System.out.println(ll.removeFirst());
    System.out.println(ll.removeFirst());
    // Treat it like a queue, pulling elements
    // off the tail end:
    System.out.println(ll.removeLast());
    // With the above operations, it's a dequeue!
    System.out.println(ll);
  }
  public static void main(String[] args) {
    // Make and fill a new list each time:
    basicTest(fill(new LinkedList()));
    basicTest(fill(new ArrayList()));
    iterMotion(fill(new LinkedList()));
    iterMotion(fill(new ArrayList()));
    iterManipulation(fill(new LinkedList()));
    iterManipulation(fill(new ArrayList()));
    testVisual(fill(new LinkedList()));
    testLinkedList();
  }
} ///:~

In basicTest( ) and iterMotion( ) the calls are simply made to show the proper syntax, and while the return value is captured, it is not used. In some cases, the return value isn’t captured since it isn’t typically used. You should look up the full usage of each of these methods in the online documentation from http://java.sun.com before you use them.

Making a stack from a LinkedList

A stack is sometimes referred to as a “last-in, first-out” (LIFO) container. That is, whatever you “push” on the stack last is the first item you can “pop” out. Like all of the other containers in Java, what you push and pop are Objects, so you must cast what you pop, unless you’re just using Object behavior.

The LinkedList has methods that directly implement stack functionality, so you can also just use a LinkedList rather than making a stack class. However, a stack class can sometimes tell the story better:

//: c09:StackL.java
// Making a stack from a LinkedList.
import java.util.*;
import com.bruceeckel.util.*;

public class StackL {
  private LinkedList list = new LinkedList();
  public void push(Object v) {
    list.addFirst(v);
  }
  public Object top() { return list.getFirst(); }
  public Object pop() {
    return list.removeFirst();
  }
  public static void main(String[] args) {
    StackL stack = new StackL();
    for(int i = 0; i < 10; i++)
      stack.push(Collections2.countries.next());
    System.out.println(stack.top());
    System.out.println(stack.top());
    System.out.println(stack.pop());
    System.out.println(stack.pop());
    System.out.println(stack.pop());
  }
} ///:~

If you want only stack behavior, inheritance is inappropriate here because it would produce a class with all the rest of the LinkedList methods (you’ll see later that this very mistake was made by the Java 1.0 library designers with Stack).

Making a queue from a LinkedList

A queue is a first-in, first-out (FIFO) container. That is, you put things in at one end, and pull them out at the other. So the order in which you put them in will be the same order that they come out. LinkedList has methods to support queue behavior, so these can be used in a Queue class:

//: c09:Queue.java
// Making a queue from a LinkedList.
import java.util.*;

public class Queue {
  private LinkedList list = new LinkedList();
  public void put(Object v) { list.addFirst(v); }
  public Object get() {
    return list.removeLast();
  }
  public boolean isEmpty() {
    return list.isEmpty();
  }
  public static void main(String[] args) {
    Queue queue = new Queue();
    for(int i = 0; i < 10; i++)
      queue.put(Integer.toString(i));
    while(!queue.isEmpty())
      System.out.println(queue.get());
  }
} ///:~

You can also easily create a deque (double-ended queue) from a LinkedList. This is like a queue, but you can add and remove elements from either end.

Set functionality

Set has exactly the same interface as Collection, so there isn’t any extra functionality like there is with the two different Lists. Instead, the Set is exactly a Collection, it just has different behavior. (This is the ideal use of inheritance and polymorphism: to express different behavior.) A Set refuses to hold more than one instance of each object value (what constitutes the “value” of an object is more complex, as you shall see).

Set (interface) | Each element that you add to the Set must be unique; otherwise the Set doesn’t add the duplicate element. Objects added to a Set must define equals( ) to establish object uniqueness. Set has exactly the same interface as Collection. The Set interface does not guarantee it will maintain its elements in any particular order.
HashSet* | For Sets where fast lookup time is important. Objects must also define hashCode( ).
TreeSet | An ordered Set backed by a tree. This way, you can extract an ordered sequence from a Set.

The following example does not show everything you can do with a Set, since the interface is the same as Collection and so was exercised in the previous example. Instead, this demonstrates the behavior that makes a Set unique:

//: c09:Set1.java
// Things you can do with Sets.
import java.util.*;
import com.bruceeckel.util.*;

public class Set1 {
  static Collections2.StringGenerator gen =
    Collections2.countries;
  public static void testVisual(Set a) {
    Collections2.fill(a, gen.reset(), 10);
    Collections2.fill(a, gen.reset(), 10);
    Collections2.fill(a, gen.reset(), 10);
    System.out.println(a); // No duplicates!
    // Add another set to this one:
    a.addAll(a);
    a.add("one");
    a.add("one");
    a.add("one");
    System.out.println(a);
    // Look something up:
    System.out.println("a.contains(\"one\"): " +
      a.contains("one"));
  }
  public static void main(String[] args) {
    System.out.println("HashSet");
    testVisual(new HashSet());
    System.out.println("TreeSet");
    testVisual(new TreeSet());
  }
} ///:~

Duplicate values are added to the Set, but when it is printed you’ll see the Set has accepted only one instance of each value.

When you run this program you’ll notice that the order maintained by the HashSet is different from TreeSet, since each has a different way of storing elements so they can be located later. (TreeSet keeps them sorted, while HashSet uses a hashing function, which is designed specifically for rapid lookups.) When creating your own types, be aware that a Set needs a way to maintain a storage order, which means you must implement the Comparable interface and define the compareTo( ) method. Here’s an example:

//: c09:Set2.java
// Putting your own type in a Set.
import java.util.*;

class MyType implements Comparable {
  private int i;
  public MyType(int n) { i = n; }
  public boolean equals(Object o) {
    return
      (o instanceof MyType)
      && (i == ((MyType)o).i);
  }
  public int hashCode() { return i; }
  public String toString() { return i + " "; }
  public int compareTo(Object o) {
    int i2 = ((MyType)o).i;
    return (i2 < i ? -1 : (i2 == i ? 0 : 1));
  }
}

public class Set2 {
  public static Set fill(Set a, int size) {
    for(int i = 0; i < size; i++)
      a.add(new MyType(i));
    return a;
  }
  public static void test(Set a) {
    fill(a, 10);
    fill(a, 10); // Try to add duplicates
    fill(a, 10);
    a.addAll(fill(new TreeSet(), 10));
    System.out.println(a);
  }
  public static void main(String[] args) {
    test(new HashSet());
    test(new TreeSet());
  }
} ///:~

The form for the definitions for equals( ) and hashCode( ) will be described later in this chapter. You must define an equals( ) in both cases, but the hashCode( ) is absolutely necessary only if the class will be placed in a HashSet (which is likely, since that should generally be your first choice as a Set implementation). However, as a programming style you should always override hashCode( ) when you override equals( ). This process will be fully detailed later in this chapter.

In the compareTo( ), note that I did not use the “simple and obvious” form return i-i2. Although this is a common programming error, it would only work properly if i and i2 were “unsigned” ints (if Java had an “unsigned” keyword, which it does not). It breaks for Java’s signed int, which is not big enough to represent the difference of two signed ints. If i is a large positive integer and j is a large negative integer, i-j will overflow and return a negative value, which will not work.

SortedSet

If you have a SortedSet (of which TreeSet is the only one available), the elements are guaranteed to be in sorted order which allows additional functionality to be provided with these methods in the SortedSet interface:

Comparator comparator(): Produces the Comparator used for this Set, or null for natural ordering.

Object first(): Produces the lowest element.

Object last(): Produces the highest element.

SortedSet subSet(fromElement, toElement): Produces a view of this Set with elements from fromElement, inclusive, to toElement, exclusive.

SortedSet headSet(toElement): Produces a view of this Set with elements less than toElement.

SortedSet tailSet(fromElement): Produces a view of this Set with elements greater than or equal to fromElement.

Map functionality

An ArrayList allows you to select from a sequence of objects using a number, so in a sense it associates numbers to objects. But what if you’d like to select from a sequence of objects using some other criterion? A stack is an example: its selection criterion is “the last thing pushed on the stack.” A powerful twist on this idea of “selecting from a sequence” is alternately termed a map, a dictionary, or an associative array. Conceptually, it seems like an ArrayList, but instead of looking up objects using a number, you look them up using another object! This is often a key process in a program.

The concept shows up in Java as the Map interface. The put(Object key, Object value) method adds a value (the thing you want), and associates it with a key (the thing you look it up with). get(Object key) produces the value given the corresponding key. You can also test a Map to see if it contains a key or a value with containsKey( ) and containsValue( ).

The standard Java library contains two different types of Maps: HashMap and TreeMap. Both have the same interface (since they both implement Map), but they differ in one distinct way: efficiency. If you look at what must be done for a get( ), it seems pretty slow to search through (for example) an ArrayList for the key. This is where HashMap speeds things up. Instead of a slow search for the key, it uses a special value called a hash code. The hash code is a way to take some information in the object in question and turn it into a “relatively unique” int for that object. All Java objects can produce a hash code, and hashCode( ) is a method in the root class Object. A HashMap takes the hashCode( ) of the object and uses it to quickly hunt for the key. This results in a dramatic performance improvement [49] .

Map (interface) | Maintains key-value associations (pairs), so you can look up a value using a key.
HashMap* | Implementation based on a hash table. (Use this instead of Hashtable.) Provides constant-time performance for inserting and locating pairs. Performance can be adjusted via constructors that allow you to set the capacity and load factor of the hash table.
TreeMap | Implementation based on a red-black tree. When you view the keys or the pairs, they will be in sorted order (determined by Comparable or Comparator, discussed later). The point of a TreeMap is that you get the results in sorted order. TreeMap is the only Map with the subMap( ) method, which allows you to return a portion of the tree.

Sometimes you’ll also need to know the details of how hashing works, so we’ll look at that a little later.

The following example uses the Collections2.fill( ) method and the test data sets that were previously defined:

//: c09:Map1.java
// Things you can do with Maps.
import java.util.*;
import com.bruceeckel.util.*;

public class Map1 {
  static Collections2.StringPairGenerator geo =
    Collections2.geography;
  static Collections2.RandStringPairGenerator
    rsp = Collections2.rsp;
  // Producing a Set of the keys:
  public static void printKeys(Map m) {
    System.out.print("Size = " + m.size() +", ");
    System.out.print("Keys: ");
    System.out.println(m.keySet());
  }
  // Producing a Collection of the values:
  public static void printValues(Map m) {
    System.out.print("Values: ");
    System.out.println(m.values());
  }
  public static void test(Map m) {
    Collections2.fill(m, geo, 25);
    // Map has 'Set' behavior for keys:
    Collections2.fill(m, geo.reset(), 25);
    printKeys(m);
    printValues(m);
    System.out.println(m);
    String key = CountryCapitals.pairs[4][0];
    String value = CountryCapitals.pairs[4][1];
    System.out.println("m.containsKey(\"" + key +
      "\"): " + m.containsKey(key));
    System.out.println("m.get(\"" + key + "\"): "
      + m.get(key));
    System.out.println("m.containsValue(\""
      + value + "\"): " +
      m.containsValue(value));
    Map m2 = new TreeMap();
    Collections2.fill(m2, rsp, 25);
    m.putAll(m2);
    printKeys(m);
    key = m.keySet().iterator().next().toString();
    System.out.println("First key in map: "+key);
    m.remove(key);
    printKeys(m);
    m.clear();
    System.out.println("m.isEmpty(): "
      + m.isEmpty());
    Collections2.fill(m, geo.reset(), 25);
    // Operations on the Set change the Map:
    m.keySet().removeAll(m.keySet());
    System.out.println("m.isEmpty(): "
      + m.isEmpty());
  }
  public static void main(String[] args) {
    System.out.println("Testing HashMap");
    test(new HashMap());
    System.out.println("Testing TreeMap");
    test(new TreeMap());
  }
} ///:~

The printKeys( ) and printValues( ) methods are not only useful utilities, they also demonstrate how to produce Collection views of a Map. The keySet( ) method produces a Set backed by the keys in the Map. Similar treatment is given to values( ), which produces a Collection containing all the values in the Map. (Note that keys must be unique, while values may contain duplicates.) Since these Collections are backed by the Map, any changes in a Collection will be reflected in the associated Map.

The rest of the program provides simple examples of each Map operation, and tests each type of Map.

As an example of the use of a HashMap, consider a program to check the randomness of Java’s Math.random( ) method. Ideally, it would produce a perfect distribution of random numbers, but to test this you need to generate a bunch of random numbers and count the ones that fall in the various ranges. A HashMap is perfect for this, since it associates objects with objects (in this case, the value object contains the number produced by Math.random( ) along with the number of times that number appears):

//: c09:Statistics.java
// Simple demonstration of HashMap.
import java.util.*;

class Counter {
  int i = 1;
  public String toString() {
    return Integer.toString(i);
  }
}

class Statistics {
  public static void main(String[] args) {
    HashMap hm = new HashMap();
    for(int i = 0; i < 10000; i++) {
      // Produce a number between 0 and 20:
      Integer r =
        new Integer((int)(Math.random() * 20));
      if(hm.containsKey(r))
        ((Counter)hm.get(r)).i++;
      else
        hm.put(r, new Counter());
    }
    System.out.println(hm);
  }
} ///:~

In main( ), each time a random number is generated it is wrapped inside an Integer object so that reference can be used with the HashMap. (You can’t use a primitive with a container, only an object reference.) The containsKey( ) method checks to see if this key is already in the container. (That is, has the number been found already?) If so, the get( ) method produces the associated value for the key, which in this case is a Counter object. The value i inside the counter is incremented to indicate that one more of this particular random number has been found.

If the key has not been found yet, the method put( ) will place a new key-value pair into the HashMap. Since Counter automatically initializes its variable i to one when it’s created, it indicates the first occurrence of this particular random number.

To display the HashMap, it is simply printed. The HashMap toString( ) method moves through all the key-value pairs and calls the toString( ) for each one. The Integer.toString( ) is predefined, and you can see the toString( ) for Counter. The output from one run (with some line breaks added) is:

{19=526, 18=533, 17=460, 16=513, 15=521, 14=495,
 13=512, 12=483, 11=488, 10=487, 9=514, 8=523,
 7=497, 6=487, 5=480, 4=489, 3=509, 2=503, 1=475,
 0=505}

You might wonder at the necessity of the class Counter, which seems like it doesn’t even have the functionality of the wrapper class Integer. Why not use int or Integer? Well, you can’t use an int because all of the containers can hold only Object references. After seeing containers the wrapper classes might begin to make a little more sense to you, since you can’t put any of the primitive types in containers. However, the only thing you can do with the Java wrappers is to initialize them to a particular value and read that value. That is, there’s no way to change a value once a wrapper object has been created. This makes the Integer wrapper immediately useless to solve our problem, so we’re forced to create a new class that does satisfy the need.

SortedMap

If you have a SortedMap (of which TreeMap is the only one available), the keys are guaranteed to be in sorted order which allows additional functionality to be provided with these methods in the SortedMap interface:

Comparator comparator(): Produces the comparator used for this Map, or null for natural ordering.

Object firstKey(): Produces the lowest key.

Object lastKey(): Produces the highest key.

SortedMap subMap(fromKey, toKey): Produces a view of this Map with keys from fromKey, inclusive, to toKey, exclusive.

SortedMap headMap(toKey): Produces a view of this Map with keys less than toKey.

SortedMap tailMap(fromKey): Produces a view of this Map with keys greater than or equal to fromKey.

Hashing and hash codes

In the previous example, a standard library class (Integer) was used as a key for the HashMap. It worked fine as a key, because it has all the necessary wiring to make it work correctly as a key. But a common pitfall occurs with HashMaps when you create your own classes to be used as keys. For example, consider a weather predicting system that matches Groundhog objects to Prediction objects. It seems fairly straightforward: you create the two classes and use Groundhog as the key and Prediction as the value:

//: c09:SpringDetector.java
// Looks plausible, but doesn't work.
import java.util.*;

class Groundhog {
  int ghNumber;
  Groundhog(int n) { ghNumber = n; }
}

class Prediction {
  boolean shadow = Math.random() > 0.5;
  public String toString() {
    if(shadow)
      return "Six more weeks of Winter!";
    else
      return "Early Spring!";
  }
}

public class SpringDetector {
  public static void main(String[] args) {
    HashMap hm = new HashMap();
    for(int i = 0; i < 10; i++)
      hm.put(new Groundhog(i), new Prediction());
    System.out.println("hm = " + hm + "\n");
    System.out.println(
      "Looking up prediction for Groundhog #3:");
    Groundhog gh = new Groundhog(3);
    if(hm.containsKey(gh))
      System.out.println((Prediction)hm.get(gh));
    else
      System.out.println("Key not found: " + gh);
  }
} ///:~

Each Groundhog is given an identity number, so you can look up a Prediction in the HashMap by saying “Give me the Prediction associated with Groundhog number 3.” The Prediction class contains a boolean that is initialized using Math.random( ), and a toString( ) that interprets the result for you. In main( ), a HashMap is filled with Groundhogs and their associated Predictions. The HashMap is printed so you can see that it has been filled. Then a Groundhog with an identity number of 3 is used as a key to look up the prediction for Groundhog #3 (which you can see must be in the Map).

It seems simple enough, but it doesn’t work. The problem is that Groundhog is inherited from the common root class Object (which is what happens if you don’t specify a base class, thus all classes are ultimately inherited from Object). It is Object’s hashCode( ) method that is used to generate the hash code for each object, and by default it just uses the address of its object. Thus, the first instance of Groundhog(3) does not produce a hash code equal to the hash code for the second instance of Groundhog(3) that we tried to use as a lookup.

You might think that all you need to do is write an appropriate override for hashCode( ). But it still won’t work until you’ve done one more thing: override the equals( ) that is also part of Object. This method is used by the HashMap when trying to determine if your key is equal to any of the keys in the table. Again, the default Object.equals( ) simply compares object addresses, so one Groundhog(3) is not equal to another Groundhog(3).

Thus, to use your own classes as keys in a HashMap, you must override both hashCode( ) and equals( ), as shown in the following solution to the problem above:

//: c09:SpringDetector2.java
// A class that's used as a key in a HashMap
// must override hashCode() and equals().
import java.util.*;

class Groundhog2 {
  int ghNumber;
  Groundhog2(int n) { ghNumber = n; }
  public int hashCode() { return ghNumber; }
  public boolean equals(Object o) {
    return (o instanceof Groundhog2)
      && (ghNumber == ((Groundhog2)o).ghNumber);
  }
}

public class SpringDetector2 {
  public static void main(String[] args) {
    HashMap hm = new HashMap();
    for(int i = 0; i < 10; i++)
      hm.put(new Groundhog2(i),new Prediction());
    System.out.println("hm = " + hm + "\n");
    System.out.println(
      "Looking up prediction for groundhog #3:");
    Groundhog2 gh = new Groundhog2(3);
    if(hm.containsKey(gh))
      System.out.println((Prediction)hm.get(gh));
  }
} ///:~

Note that this uses the Prediction class from the previous example, so SpringDetector.java must be compiled first or you’ll get a compile-time error when you try to compile SpringDetector2.java.

Groundhog2.hashCode( ) returns the ground hog number as an identifier. In this example, the programmer is responsible for ensuring that no two ground hogs exist with the same ID number. The hashCode( ) is not required to return a unique identifier (something you’ll understand better later in this chapter), but the equals( ) method must be able to strictly determine whether two objects are equivalent.

Even though it appears that the equals( ) method is only checking to see whether the argument is an instance of Groundhog2 (using the instanceof keyword, which is fully explained in Chapter 12), the instanceof actually quietly does a second sanity check, to see if the object is null, since instanceof produces false if the left-hand argument is null. Assuming it’s the correct type and not null, the comparison is based on the actual ghNumbers. This time, when you run the program, you’ll see it produces the correct output. (Many of the Java library classes override the hashCode( ) and equals( ) methods to be based upon their contents.)

When creating your own class to use in a HashSet, you must pay attention to the same issues as when it is used as a key in a HashMap.

Understanding hashCode( )

The above example is only a start toward solving the problem correctly. It shows that if you do not override hashCode( ) and equals( ) for your key, the hashed data structure (HashSet or HashMap) will not be able to deal with your key properly. However, to get a good solution for the problem you need to understand what’s going on inside the hashed data structure.

First, consider the motivation behind hashing: you want to look up an object using another object. But you can accomplish this with a TreeSet or TreeMap, too. It’s also possible to implement your own Map. To do so, the Map.entrySet( ) method must be supplied to produce a set of Map.Entry objects. MPair will be defined as the new type of Map.Entry. In order for it to be placed in a TreeSet it must implement equals( ) and be Comparable:

//: c09:MPair.java
// A Map implemented with ArrayLists.
import java.util.*;

public class MPair
implements Map.Entry, Comparable {
  Object key, value;
  MPair(Object k, Object v) {
    key = k;
    value = v;
  }
  public Object getKey() { return key; }
  public Object getValue() { return value; }
  public Object setValue(Object v){
    Object result = value;
    value = v;
    return result;
  }
  public boolean equals(Object o) {
    return key.equals(((MPair)o).key);
  }
  public int compareTo(Object rv) {
    return ((Comparable)key).compareTo(
      ((MPair)rv).key);
  }
} ///:~
Notice that the comparisons are only interested in the keys, so duplicate values are perfectly acceptable.

The following example implements a Map using a pair of ArrayLists:

//: c09:SlowMap.java
// A Map implemented with ArrayLists.
import java.util.*;
import com.bruceeckel.util.*;

public class SlowMap extends AbstractMap {
  private ArrayList
    keys = new ArrayList(),
    values = new ArrayList();
  public Object put(Object key, Object value) {
    Object result = get(key);
    if(!keys.contains(key)) {
      keys.add(key);
      values.add(value);
    } else
      values.set(keys.indexOf(key), value);
    return result;
  }
  public Object get(Object key) {
    if(!keys.contains(key))
      return null;
    return values.get(keys.indexOf(key));
  }
  public Set entrySet() {
    Set entries = new HashSet();
    Iterator
      ki = keys.iterator(),
      vi = values.iterator();
    while(ki.hasNext())
      entries.add(new MPair(ki.next(), vi.next()));
    return entries;
  }
  public static void main(String[] args) {
    SlowMap m = new SlowMap();
    Collections2.fill(m,
      Collections2.geography, 25);
    System.out.println(m);
  }
} ///:~

The put( ) method simply places the keys and values in corresponding ArrayLists. In main( ), a SlowMap is loaded and then printed to show that it works.

This shows that it’s not that hard to produce a new type of Map. But as the name suggests, a SlowMap isn’t very fast, so you probably wouldn’t use it if you had an alternative available. The problem is in the lookup of the key: there is no order so a simple linear search is used, which is the slowest way to look something up.

The whole point of hashing is speed: hashing allows the lookup to happen quickly. Since the bottleneck is in the speed of the key lookup, one of the solutions to the problem could be by keeping the keys sorted and then using Collections.binarySearch( ) to perform the lookup (an exercise at the end of this chapter will walk you through this process).

Hashing goes further by saying that all you want to do is to store the key somewhere so that it can be quickly found. As you’ve seen in this chapter, the fastest structure in which to store a group of elements is an array, so that will used for representing the key information (note carefully that I said “key information” and not the key itself). Also seen in this chapter was the fact that an array, once allocated, cannot be resized, so we have a problem: we want to be able to store any number of values in the Map, but if the number of keys is fixed by the array size, how can this be?

The answer is that the array will not hold the keys. From the key object, a number will be derived that will index into the array. This number is the hash code, produced by the hashCode( ) method (in computer science parlance, this is the hash function) defined in Object and presumably overridden by your class. To solve the problem of the fixed-size array, more than one key may produce the same index. That is, there may be collisions. Because of this, it doesn’t matter how big the array is because each key object will land somewhere in that array.

So the process of looking up a value starts by computing the hash code and using it to index into the array. If you could guarantee that there were no collisions (which could be possible if you have a fixed number of values) then you’d have a perfect hashing function, but that’s a special case. In all other cases, collisions are handled by external chaining: the array points, not directly to a value, but instead to a list of values. These values are searched in a linear fashion using the equals( ) method. Of course, this aspect of the search is much slower, but if the hash function is good there will only be a few values in each slot, at the most. So instead of searching through the entire list, you quickly jump to a slot where you have to compare a few entries to find the value. This is much faster, which is why the HashMap is so quick.

Knowing the basics of hashing, it’s possible to implement a simple hashed Map:

//: c09:SimpleHashMap.java
// A demonstration hashed Map.
import java.util.*;
import com.bruceeckel.util.*;

public class SimpleHashMap extends AbstractMap {
  // Choose a prime number for the hash table
  // size, to achieve a uniform distribution:
  private final static int SZ = 997;
  private LinkedList[] bucket= new LinkedList[SZ];
  public Object put(Object key, Object value) {
    Object result = null;
    int index = key.hashCode() % SZ;
    if(index < 0) index = -index;
    if(bucket[index] == null)
      bucket[index] = new LinkedList();
    LinkedList pairs = bucket[index];
    MPair pair = new MPair(key, value);
    ListIterator it = pairs.listIterator();
    boolean found = false;
    while(it.hasNext()) {
      Object iPair = it.next();
      if(iPair.equals(pair)) {
        result = ((MPair)iPair).getValue();
        it.set(pair); // Replace old with new
        found = true;
        break;
      }
    }
    if(!found)
      bucket[index].add(pair);
    return result;
  }
  public Object get(Object key) {
    int index = key.hashCode() % SZ;
    if(index < 0) index = -index;
    if(bucket[index] == null) return null;
    LinkedList pairs = bucket[index];
    MPair match = new MPair(key, null);
    ListIterator it = pairs.listIterator();
    while(it.hasNext()) {
      Object iPair = it.next();
      if(iPair.equals(match))
        return ((MPair)iPair).getValue();
    }
    return null;
  }
  public Set entrySet() {
    Set entries = new HashSet();
    for(int i = 0; i < bucket.length; i++) {
      if(bucket[i] == null) continue;
      Iterator it = bucket[i].iterator();
      while(it.hasNext())
        entries.add(it.next());
    }
    return entries;
  }
  public static void main(String[] args) {
    SimpleHashMap m = new SimpleHashMap();
    Collections2.fill(m,
      Collections2.geography, 25);
    System.out.println(m);
  }
} ///:~

Because the “slots” in a hash table are often referred to as buckets, the array that represents the actual table is called bucket. To promote even distribution, the number of buckets is typically a prime number. Notice that it is an array of LinkedList, which automatically provides for collisions—each new item is simply added to the end of the list.

The return value of put( ) is null or, if the key was already in the list, the old value associated with that key. The return value is result, which is initialized to null, but if a key is discovered in the list then result is assigned to that key.

For both put( ) and get( ), the first thing that happens is that the hashCode( ) is called for the key, and the result is forced to a positive number. Then it is forced to fit into the bucket array using the modulus operator and the size of the array. If that location is null, it means there are no elements that hash to that location, so a new LinkedList is created to hold the object that just did. However, the normal process is to look through the list to see if there are duplicates, and if there are, the old value is put into result and the new value replaces the old. The found flag keeps track of whether an old key-value pair was found, and if not the new pair is appended to the end of the list.

In get( ), you’ll see very similar code as that contained in put( ), but simpler. The index is calculated into the bucket array, and if a LinkedList exists it is searched for a match.

entrySet( ) must find and traverse all the lists, adding them to the result Set. Once this method has been created, the Map can be tested by filling it with values and then printing them.

HashMap performance factors

To understand the issues, some terminology is necessary:

Capacity: The number of buckets in the table.

Initial capacity: The number of buckets when the table is created. HashMap and HashSet have constructors that allow you to specify the initial capacity.

Size: The number of entries currently in the table.

Load factor: size/capacity. A load factor of 0 is an empty table, 0.5 is a half-full table, etc. A lightly-loaded table will have few collisions and so is optimal for insertions and lookups (but will slow down the process of traversing with an iterator). HashMap and HashSet have constructors that allow you to specify the load factor, which means that when this load factor is reached the container will automatically increase the capacity (the number of buckets) by roughly doubling it, and will redistribute the existing objects into the new set of buckets (this is called rehashing).

The default load factor used by HashMap is 0.75 (it doesn’t rehash until the table is ¾ full). This seems to be a good tradeoff between time and space costs. A higher load factor decreases the space required by the table but increases the lookup cost, which is important because lookup is what you do most of the time (including both get( ) and put( )).

If you know that you’ll be storing many entries in a HashMap, creating it with an appropriately large initial capacity will prevent the overhead of automatic rehashing.

Overriding hashCode( )

Now that you understand what’s involved in the function of the HashMap, the issues involved in writing a hashCode( ) will make more sense.

First of all, you don’t have control of the creation of the actual value that’s used to index into the array of buckets. That is dependent upon the capacity of the particular HashMap object, and that capacity changes depending on how full the container is, and what the load factor is. The value produced by your hashCode( ) will be further processed in order to create the bucket index (in SimpleHashMap the calculation is just a modulo by the size of the bucket array).

The most important factor in creating a hashCode( ) is that, regardless of when hashCode( ) is called, it produces the same value for a particular object every time it is called. If you end up with an object that produces one hashCode( ) value when it is put( ) into a HashMap and another during a get( ), you won’t be able to retrieve the objects. So if your hashCode( ) depends on mutable data in the object the user must be made aware that changing the data will effectively produce a different key by generating a different hashCode( ).

In addition, you will probably not want to generate a hashCode( ) that is based on unique object information – in particular, the value of this makes a bad hashCode( ) because then you can’t generate a new identical key to the one used to put( ) the original key-value pair. This was the problem that occurred in SpringDetector.java because the default implementation of hashCode( ) does use the object address. So you’ll want to use information in the object that identifies the object in a meaningful way.

One example is found in the String class. Strings have the special characteristic that if a program has several String objects that contain identical character sequences, then those String objects all map to the same memory (the mechanism for this is described in Appendix A). So it makes sense that the hashCode( ) produced by two separate instances of new String(“hello”) should be identical. You can see it by running this program:

//: c09:StringHashCode.java
public class StringHashCode {
  public static void main(String[] args) {
    System.out.println("Hello".hashCode());
    System.out.println("Hello".hashCode());
  }
} ///:~

For this to work, the hashCode( ) for String must be based on the contents of the String.

So for a hashCode( ) to be effective, it must be fast and it must be meaningful: that is, it must generate a value based on the contents of the object. Remember that this value doesn’t have to be unique—you should lean towards speed rather than uniqueness—but between hashCode( ) and equals( ) the identity of the object must be completely resolved.

Because the hashCode( ) is further processed before the bucket index is produced, the range of values is not important; it just needs to generate an int.

There’s one other factor: a good hashCode( ) should result in an even distribution of values. If the values tend to cluster, then the HashMap or HashSet will be more heavily loaded in some areas and will not be as fast as it could be with an evenly-distributed hashing function.

Here’s an example that follows these guidelines:

//: c09:CountedString.java
// Creating a good hashCode().
import java.util.*;

public class CountedString {
  private String s;
  private int id = 0;
  private static ArrayList created =
    new ArrayList();
  public CountedString(String str) {
    s = str;
    created.add(s);
    Iterator it = created.iterator();
    // Id is the total number of instances
    // of this string in use by CountedString:
    while(it.hasNext())
      if(it.next().equals(s))
        id++;
  }
  public String toString() {
    return "String: " + s + " id: " + id +
      " hashCode(): " + hashCode() + "\n";
  }
  public int hashCode() {
    return s.hashCode() * id;
  }
  public boolean equals(Object o) {
    return (o instanceof CountedString)
      && s.equals(((CountedString)o).s)
      && id == ((CountedString)o).id;
  }
  public static void main(String[] args) {
    HashMap m = new HashMap();
    CountedString[] cs = new CountedString[10];
    for(int i = 0; i < cs.length; i++)
      m.put(new Integer(i),
        new CountedString("hi"));
    System.out.println(m);
  }
} ///:~

CountedString includes a String and an id that represents the number of CountedString objects that contain an identical String. The counting is accomplished in the constructor by iterating through the static ArrayList where all the Strings are stored.

Both hashCode( ) and equals( ) produce results based on both fields; if they were just based on the String alone or the id alone there would be duplicate matches for distinct values.

Note how simple the hashCode( ) is: String’s hashCode( ) is multiplied by the id. Smaller is generally better (and faster) for hashCode( ).

In main( ), a bunch of CountedString objects are created, using the same String to show that the duplicates create unique values because of the count id.

Holding references

The java.lang.ref library contains a set of classes that allow greater flexibility in garbage collection, which are especially useful when you have large objects that may cause memory exhaustion. There are three classes inherited from the abstract class Reference: SoftReference, WeakReference, and PhantomReference. Each of these provides a different level of indirection for the garbage collector, if the object in question is only reachable through one of these Reference objects.

If an object is reachable it means that somewhere in your program the object can be found. This could mean that you have an ordinary reference on the stack that goes right to the object, but you might also have a reference to an object that has a reference to the object in question; there could be many intermediate links. If an object is reachable, the garbage collector cannot release it because it’s still in use by your program. If an object isn’t reachable, there’s no way for your program to use it so it’s safe to garbage-collect that object.

You use Reference objects when you want to continue to hold onto a reference to that object—you want to be able to reach that object—but you also want to allow the garbage collector to release that object. Thus, you have a way to go on using the object but if memory exhaustion is imminent you allow that object to be released.

You accomplish this by using a Reference object as an intermediary between you and the ordinary reference, and there must be no ordinary references to the object (ones that are not wrapped inside Reference objects). If the garbage collector discovers that an object is reachable through an ordinary reference, it will not release that object.

In the order SoftReference, WeakReference, and PhantomReference, each one is “weaker” than the last, and corresponds to a different level of reachability. Soft references are for implementing memory-sensitive caches, weak references are for implementing “canonicalizing mappings” (where instances of objects can be simultaneously used in multiple places in a program, to save storage) that do not prevent their keys (or values) from being reclaimed, and phantom references are for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

With SoftReferences and WeakReferences, you have a choice about whether to place them on a ReferenceQueue (the device used for pre-mortem cleanup actions), but a PhantomReference can only be built on a ReferenceQueue. Here’s a simple demonstration:

//: c09:References.java
// Demonstrates Reference objects
import java.lang.ref.*;

class VeryBig {
  static final int SZ = 10000;
  double[] d = new double[SZ];
  String ident;
  public VeryBig(String id) { ident = id; }
  public String toString() { return ident; }
  public void finalize() {
    System.out.println("Finalizing " + ident);
  }
}

public class References {
  static ReferenceQueue rq= new ReferenceQueue();
  public static void checkQueue() {
    Object inq = rq.poll();
    if(inq != null)
      System.out.println("In queue: " +
        (VeryBig)((Reference)inq).get());
  }
  public static void main(String[] args) {
    int size = 10;
    // Or, choose size via the command line:
    if(args.length > 0)
      size = Integer.parseInt(args[0]);
    SoftReference[] sa =
      new SoftReference[size];
    for(int i = 0; i < sa.length; i++) {
      sa[i] = new SoftReference(
        new VeryBig("Soft " + i), rq);
      System.out.println("Just created: " +
        (VeryBig)sa[i].get());
      checkQueue();
    }
    WeakReference[] wa =
      new WeakReference[size];
    for(int i = 0; i < wa.length; i++) {
      wa[i] = new WeakReference(
        new VeryBig("Weak " + i), rq);
      System.out.println("Just created: " +
        (VeryBig)wa[i].get());
      checkQueue();
    }
    SoftReference s = new SoftReference(
      new VeryBig("Soft"));
    WeakReference w = new WeakReference(
      new VeryBig("Weak"));
    System.gc();
    PhantomReference[] pa =
      new PhantomReference[size];
    for(int i = 0; i < pa.length; i++) {
      pa[i] = new PhantomReference(
        new VeryBig("Phantom " + i), rq);
      System.out.println("Just created: " +
        (VeryBig)pa[i].get());
      checkQueue();
    }
  }
} ///:~

When you run this program (you’ll want to pipe the output through a “more” utility so that you can view the output in pages), you’ll see that the objects are garbage collected, even though you still have access to them through the Reference object (to get the actual object reference, you use get( )). You’ll also see that the ReferenceQueue always produces a Reference containing a null object. To make use of this, you can inherit from the particular Reference class you’re interested in and add more useful methods to the new type of Reference.

The WeakHashMap

The containers library has a special Map to hold weak references: the WeakHashMap. This class is designed to make the creation of canonicalized mappings easier. In such a mapping, you are saving storage by making only one instance of a particular value. When the program needs that value, instead of creating one from scratch, it looks up the existing object in the mapping and uses that. The mapping may make the values as part of its initialization, but it’s more likely that the values are made on demand.

Since this is a storage-saving technique, it’s very convenient that the WeakHashMap allows the garbage collector to automatically clean up the keys and values. You don’t have to do anything special to the keys and values you want to place in the WeakHashMap; these are automatically wrapped in WeakReferences by the map. The trigger to allow cleanup is if the key is no longer in use, as demonstrated here:

//: c09:CanonicalMapping.java
// Demonstrates WeakHashMap.
import java.util.*;
import java.lang.ref.*;

class Key {
  String ident;
  public Key(String id) { ident = id; }
  public String toString() { return ident; }
  public int hashCode() {
    return ident.hashCode();
  }
  public boolean equals(Object r) {
    return (r instanceof Key)
      && ident.equals(((Key)r).ident);
  }
  public void finalize() {
    System.out.println("Finalizing Key "+ ident);
  }
}

class Value {
  String ident;
  public Value(String id) { ident = id; }
  public String toString() { return ident; }
  public void finalize() {
    System.out.println("Finalizing Value "+ident);
  }
}

public class CanonicalMapping {
  public static void main(String[] args) {
    int size = 1000;
    // Or, choose size via the command line:
    if(args.length > 0)
      size = Integer.parseInt(args[0]);
    Key[] keys = new Key[size];
    WeakHashMap whm = new WeakHashMap();
    for(int i = 0; i < size; i++) {
      Key k = new Key(Integer.toString(i));
      Value v = new Value(Integer.toString(i));
      if(i % 3 == 0)
        keys[i] = k; // Save as "real" references
      whm.put(k, v);
    }
    System.gc();
  }
} ///:~

The Key class must have a hashCode( ) and an equals( ) since it is being used as a key in a hashed data structure, as described previously in this chapter.

When you run the program you’ll see that the garbage collector will skip every third key, because an ordinary reference to that key has also been placed in the keys array and thus those objects cannot be garbage collected.

Iterators revisited

We can now demonstrate the true power of the Iterator: the ability to separate the operation of traversing a sequence from the underlying structure of that sequence. In the following example, the class PrintData uses an Iterator to move through a sequence and call the toString( ) method for every object. Two different types of containers are created, an ArrayList and a HashMap, and they are each filled with, respectively, Mouse and Hamster objects. (These classes are defined earlier in this chapter.) Because an Iterator hides the structure of the underlying container, PrintData doesn’t know or care what kind of container the Iterator comes from:

//: c09:Iterators2.java
// Revisiting Iterators.
import java.util.*;

class PrintData {
  static void print(Iterator e) {
    while(e.hasNext())
      System.out.println(e.next());
  }
}

class Iterators2 {
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 5; i++)
      v.add(new Mouse(i));
    HashMap m = new HashMap();
    for(int i = 0; i < 5; i++)
      m.put(new Integer(i), new Hamster(i));
    System.out.println("ArrayList");
    PrintData.print(v.iterator());
    System.out.println("HashMap");
    PrintData.print(m.entrySet().iterator());
  }
} ///:~

For the HashMap, the entrySet( ) method produces a Set of Map.entry objects, which contain both the key and the value for each entry, so you see both of them printed.

Note that PrintData.print( ) takes advantage of the fact that the objects in these containers are of class Object so the call toString( ) by System.out.println( ) is automatic. It’s more likely that in your problem, you must make the assumption that your Iterator is walking through a container of some specific type. For example, you might assume that everything in the container is a Shape with a draw( ) method. Then you must downcast from the Object that Iterator.next( ) returns to produce a Shape.

Choosing an implementation

By now you should understand that there are really only three container components: Map, List, and Set, and only two or three implementations of each interface. If you need to use the functionality offered by a particular interface, how do you decide which particular implementation to use?

To understand the answer, you must be aware that each different implementation has its own features, strengths, and weaknesses. For example, you can see in the diagram that the “feature” of Hashtable, Vector, and Stack is that they are legacy classes, so that old code doesn’t break. On the other hand, it’s best if you don’t use those for new (Java 2) code.

The distinction between the other containers often comes down to what they are “backed by;” that is, the data structures that physically implement your desired interface. This means that, for example, ArrayList and LinkedList implement the List interface so your program will produce the same results regardless of the one you use. However, ArrayList is backed by an array, while the LinkedList is implemented in the usual way for a doubly-linked list, as individual objects each containing data along with references to the previous and next elements in the list. Because of this, if you want to do many insertions and removals in the middle of a list a LinkedList is the appropriate choice. (LinkedList also has additional functionality that is established in AbstractSequentialList.) If not, an ArrayList is typically faster.

As another example, a Set can be implemented as either a TreeSet or a HashSet. A TreeSet is backed by a TreeMap and is designed to produce a constantly-sorted set. However, if you’re going to have larger quantities in your Set, the performance of TreeSet insertions will get slow. When you’re writing a program that needs a Set, you should choose HashSet by default, and change to TreeSet when it's more important to have a constantly-sorted set.

Choosing between Lists

The most convincing way to see the differences between the implementations of List is with a performance test. The following code establishes an inner base class to use as a test framework, then creates an array of anonymous inner classes, one for each different test. Each of these inner classes is called by the test( ) method. This approach allows you to easily add and remove new kinds of tests.

//: c09:ListPerformance.java
// Demonstrates performance differences in Lists.
import java.util.*;
import com.bruceeckel.util.*;

public class ListPerformance {
  private abstract static class Tester {
    String name;
    int size; // Test quantity
    Tester(String name, int size) {
      this.name = name;
      this.size = size;
    }
    abstract void test(List a, int reps);
  }
  private static Tester[] tests = {
    new Tester("get", 300) {
      void test(List a, int reps) {
        for(int i = 0; i < reps; i++) {
          for(int j = 0; j < a.size(); j++)
            a.get(j);
        }
      }
    },
    new Tester("iteration", 300) {
      void test(List a, int reps) {
        for(int i = 0; i < reps; i++) {
          Iterator it = a.iterator();
          while(it.hasNext())
            it.next();
        }
      }
    },
    new Tester("insert", 5000) {
      void test(List a, int reps) {
        int half = a.size()/2;
        String s = "test";
        ListIterator it = a.listIterator(half);
        for(int i = 0; i < size * 10; i++)
          it.add(s);
      }
    },
    new Tester("remove", 5000) {
      void test(List a, int reps) {
        ListIterator it = a.listIterator(3);
        while(it.hasNext()) {
          it.next();
          it.remove();
        }
      }
    },
  };
  public static void test(List a, int reps) {
    // A trick to print out the class name:
    System.out.println("Testing " +
      a.getClass().getName());
    for(int i = 0; i < tests.length; i++) {
      Collections2.fill(a,
        Collections2.countries.reset(),
        tests[i].size);
      System.out.print(tests[i].name);
      long t1 = System.currentTimeMillis();
      tests[i].test(a, reps);
      long t2 = System.currentTimeMillis();
      System.out.println(": " + (t2 - t1));
    }
  }
  public static void testArray(int reps) {
    System.out.println("Testing array as List");
    // Can only do first two tests on an array:
    for(int i = 0; i < 2; i++) {
      String[] sa = new String[tests[i].size];
      Arrays2.fill(sa,
        Collections2.countries.reset());
      List a = Arrays.asList(sa);
      System.out.print(tests[i].name);
      long t1 = System.currentTimeMillis();
      tests[i].test(a, reps);
      long t2 = System.currentTimeMillis();
      System.out.println(": " + (t2 - t1));
    }
  }
  public static void main(String[] args) {
    int reps = 50000;
    // Or, choose the number of repetitions
    // via the command line:
    if(args.length > 0)
      reps = Integer.parseInt(args[0]);
    System.out.println(reps + " repetitions");
    testArray(reps);
    test(new ArrayList(), reps);
    test(new LinkedList(), reps);
    test(new Vector(), reps);
  }
} ///:~

The inner class Tester is abstract, to provide a base class for the specific tests. It contains a String to be printed when the test starts, a size parameter to be used by the test for quantity of elements or repetitions of tests, a constructor to initialize the fields, and an abstract method test( ) that does the work. All the different types of tests are collected in one place, the array tests, which is initialized with different anonymous inner classes that inherit from Tester. To add or remove tests, simply add or remove an inner class definition from the array, and everything else happens automatically.

To compare array access to container access (primarily against ArrayList), a special test is created for arrays by wrapping one as a List using Arrays.asList( ). Note that only the first two tests can be performed in this case, because you cannot insert or remove elements from an array.

The List that’s handed to test( ) is first filled with elements, then each test in the tests array is timed. The results will vary from machine to machine; they are intended to give only an order of magnitude comparison between the performance of the different containers. Here is a summary of one run:

Type | Get | Iteration | Insert | Remove
array | 1430 | 3850 | na | na
ArrayList | 3070 | 12200 | 500 | 46850
LinkedList | 16320 | 9110 | 110 | 60
Vector | 4890 | 16250 | 550 | 46850

As expected, arrays are faster than any container for random-access lookups and iteration. You can see that random accesses (get( )) are cheap for ArrayLists and expensive for LinkedLists. (Oddly, iteration is faster for a LinkedList than an ArrayList, which is a bit counter-intuitive.) On the other hand, insertions and removals from the middle of a list are dramatically cheaper for a LinkedList than for an ArrayList—especially removals. Vector is generally not as fast as ArrayList, and it should be avoided; it’s only in the library for legacy code support (the only reason it works in this program is because it was adapted to be a List in Java 2). The best approach is probably to choose an ArrayList as your default and to change to a LinkedList if you discover performance problems because of many insertions and removals from the middle of the list. And of course, if you are working with a fixed-sized group of elements, use an array.

Choosing between Sets

You can choose between a TreeSet and a HashSet, depending on the size of the Set (if you need to produce an ordered sequence from a Set, use TreeSet). The following test program gives an indication of this tradeoff:

//: c09:SetPerformance.java
import java.util.*;
import com.bruceeckel.util.*;

public class SetPerformance {
  private abstract static class Tester {
    String name;
    Tester(String name) { this.name = name; }
    abstract void test(Set s, int size, int reps);
  }
  private static Tester[] tests = {
    new Tester("add") {
      void test(Set s, int size, int reps) {
        for(int i = 0; i < reps; i++) {
          s.clear();
          Collections2.fill(s,
            Collections2.countries.reset(),size);
        }
      }
    },
    new Tester("contains") {
      void test(Set s, int size, int reps) {
        for(int i = 0; i < reps; i++)
          for(int j = 0; j < size; j++)
            s.contains(Integer.toString(j));
      }
    },
    new Tester("iteration") {
      void test(Set s, int size, int reps) {
        for(int i = 0; i < reps * 10; i++) {
          Iterator it = s.iterator();
          while(it.hasNext())
            it.next();
        }
      }
    },
  };
  public static void
  test(Set s, int size, int reps) {
    System.out.println("Testing " +
      s.getClass().getName() + " size " + size);
    Collections2.fill(s,
      Collections2.countries.reset(), size);
    for(int i = 0; i < tests.length; i++) {
      System.out.print(tests[i].name);
      long t1 = System.currentTimeMillis();
      tests[i].test(s, size, reps);
      long t2 = System.currentTimeMillis();
      System.out.println(": " +
        ((double)(t2 - t1)/(double)size));
    }
  }
  public static void main(String[] args) {
    int reps = 50000;
    // Or, choose the number of repetitions
    // via the command line:
    if(args.length > 0)
      reps = Integer.parseInt(args[0]);
    // Small:
    test(new TreeSet(), 10, reps);
    test(new HashSet(), 10, reps);
    // Medium:
    test(new TreeSet(), 100, reps);
    test(new HashSet(), 100, reps);
    // Large:
    test(new TreeSet(), 1000, reps);
    test(new HashSet(), 1000, reps);
  }
} ///:~

The following table shows the results of one run (Of course this will be different according to the computer and JVM you are using; you should run the test yourself as well):

Type | Test size | Add | Contains | Iteration
 | 10 | 138.0 | 115.0 | 187.0
TreeSet | 100 | 189.5 | 151.1 | 206.5
 | 1000 | 150.6 | 177.4 | 40.04
 | 10 | 55.0 | 82.0 | 192.0
HashSet | 100 | 45.6 | 90.0 | 202.2
 | 1000 | 36.14 | 106.5 | 39.39

The performance of HashSet is generally superior to TreeSet for all operations (but in particular addition and lookup, the two most important operations). The only reason TreeSet exists is because it maintains its elements in sorted order, so you only use it when you need a sorted Set.

Choosing between Maps

When choosing between implementations of Map, the size of the Map is what most strongly affects performance, and the following test program gives an indication of this tradeoff:

//: c09:MapPerformance.java
// Demonstrates performance differences in Maps.
import java.util.*;
import com.bruceeckel.util.*;

public class MapPerformance {
  private abstract static class Tester {
    String name;
    Tester(String name) { this.name = name; }
    abstract void test(Map m, int size, int reps);
  }
  private static Tester[] tests = {
    new Tester("put") {
      void test(Map m, int size, int reps) {
        for(int i = 0; i < reps; i++) {
          m.clear();
          Collections2.fill(m,
            Collections2.geography.reset(), size);
        }
      }
    },
    new Tester("get") {
      void test(Map m, int size, int reps) {
        for(int i = 0; i < reps; i++)
          for(int j = 0; j < size; j++)
            m.get(Integer.toString(j));
      }
    },
    new Tester("iteration") {
      void test(Map m, int size, int reps) {
        for(int i = 0; i < reps * 10; i++) {
          Iterator it = m.entrySet().iterator();
          while(it.hasNext())
            it.next();
        }
      }
    },
  };
  public static void
  test(Map m, int size, int reps) {
    System.out.println("Testing " +
      m.getClass().getName() + " size " + size);
    Collections2.fill(m,
      Collections2.geography.reset(), size);
    for(int i = 0; i < tests.length; i++) {
      System.out.print(tests[i].name);
      long t1 = System.currentTimeMillis();
      tests[i].test(m, size, reps);
      long t2 = System.currentTimeMillis();
      System.out.println(": " +
        ((double)(t2 - t1)/(double)size));
    }
  }
  public static void main(String[] args) {
    int reps = 50000;
    // Or, choose the number of repetitions
    // via the command line:
    if(args.length > 0)
      reps = Integer.parseInt(args[0]);
    // Small:
    test(new TreeMap(), 10, reps);
    test(new HashMap(), 10, reps);
    test(new Hashtable(), 10, reps);
    // Medium:
    test(new TreeMap(), 100, reps);
    test(new HashMap(), 100, reps);
    test(new Hashtable(), 100, reps);
    // Large:
    test(new TreeMap(), 1000, reps);
    test(new HashMap(), 1000, reps);
    test(new Hashtable(), 1000, reps);
  }
} ///:~

Because the size of the map is the issue, you’ll see that the timing tests divide the time by the size to normalize each measurement. Here is one set of results. (Yours will probably be different.)

Type | Test size | Put | Get | Iteration
 | 10 | 143.0 | 110.0 | 186.0
TreeMap | 100 | 201.1 | 188.4 | 280.1
 | 1000 | 222.8 | 205.2 | 40.7
 | 10 | 66.0 | 83.0 | 197.0
HashMap | 100 | 80.7 | 135.7 | 278.5
 | 1000 | 48.2 | 105.7 | 41.4
 | 10 | 61.0 | 93.0 | 302.0
Hashtable | 100 | 90.6 | 143.3 | 329.0
 | 1000 | 54.1 | 110.95 | 47.3

As you might expect, Hashtable performance is roughly equivalent to HashMap (you can also see that HashMap is generally a bit faster. HashMap is intended to replace Hashtable). The TreeMap is generally slower than the HashMap, so why would you use it? So you could use it not as a Map, but as a way to create an ordered list. The behavior of a tree is such that it’s always in order and doesn’t have to be specially sorted. Once you fill a TreeMap, you can call keySet( ) to get a Set view of the keys, then toArray( ) to produce an array of those keys. You can then use the static method Arrays.binarySearch( ) (discussed later) to rapidly find objects in your sorted array. Of course, you would probably only do this if, for some reason, the behavior of a HashMap was unacceptable, since HashMap is designed to rapidly find things. Also, you can easily create a HashMap from a TreeMap with a single object creation In the end, when you’re using a Map your first choice should be HashMap, and only if you need a constantly-sorted Map will you need TreeMap.

Sorting and searching Lists

Utilities to perform sorting and searching for Lists have the same names and signatures as those for sorting arrays of objects, but are static methods of Collections instead of Arrays. Here’s an example, modified from ArraySearching.java:

//: c09:ListSortSearch.java
// Sorting and searching Lists with 'Collections.'
import com.bruceeckel.util.*;
import java.util.*;

public class ListSortSearch {
  public static void main(String[] args) {
    List list = new ArrayList();
    Collections2.fill(list,
      Collections2.capitals, 25);
    System.out.println(list + "\n");
    Collections.shuffle(list);
    System.out.println("After shuffling: "+list);
    Collections.sort(list);
    System.out.println(list + "\n");
    Object key = list.get(12);
    int index =
      Collections.binarySearch(list, key);
    System.out.println("Location of " + key +
      " is " + index + ", list.get(" +
      index + ") = " + list.get(index));
    AlphabeticComparator comp =
      new AlphabeticComparator();
    Collections.sort(list, comp);
    System.out.println(list + "\n");
    key = list.get(12);
    index =
      Collections.binarySearch(list, key, comp);
    System.out.println("Location of " + key +
      " is " + index + ", list.get(" +
      index + ") = " + list.get(index));
  }
} ///:~

The use of these methods is identical to the ones in Arrays, but you’re using a List instead of an array. Just like searching and sorting with arrays, if you sort using a Comparator you must binarySearch( ) using the same Comparator.

This program also demonstrates the shuffle( ) method in Collections, which randomizes the order of a List.

Utilities

There are a number of other useful utilities in the Collections class:

enumeration(Collection)

Produces an old-style Enumeration for the argument.

max(Collection)

min(Collection)

Produces the maximum or minimum element in the argument using the natural comparison method of the objects in the Collection.

max(Collection, Comparator)

min(Collection, Comparator)

Produces the maximum or minimum element in the Collection using the Comparator.

reverse( )

Reverses all the elements in place.

copy(List dest, List src)

Copies elements from src to dest.

fill(List list, Object o)

Replaces all the elements of list with o.

nCopies(int n, Object o)

Returns an immutable List of size n whose references all point to o.

Note that min( ) and max( ) work with Collection objects, not with Lists, so you don’t need to worry about whether the Collection should be sorted or not. (As mentioned earlier, you do need to sort( ) a List or an array before performing a binarySearch( ).)

Making a Collection or Map unmodifiable

Often it is convenient to create a read-only version of a Collection or Map. The Collections class allows you to do this by passing the original container into a method that hands back a read-only version. There are four variations on this method, one each for Collection (if you don’t want to treat a Collection as a more specific type), List, Set, and Map. This example shows the proper way to build read-only versions of each:

//: c09:ReadOnly.java
// Using the Collections.unmodifiable methods.
import java.util.*;
import com.bruceeckel.util.*;

public class ReadOnly {
  static Collections2.StringGenerator gen =
    Collections2.countries;
  public static void main(String[] args) {
    Collection c = new ArrayList();
    Collections2.fill(c, gen, 25); // Insert data
    c = Collections.unmodifiableCollection(c);
    System.out.println(c); // Reading is OK
    c.add("one"); // Can't change it

    List a = new ArrayList();
    Collections2.fill(a, gen.reset(), 25);
    a = Collections.unmodifiableList(a);
    ListIterator lit = a.listIterator();
    System.out.println(lit.next()); // Reading OK
    lit.add("one"); // Can't change it

    Set s = new HashSet();
    Collections2.fill(s, gen.reset(), 25);
    s = Collections.unmodifiableSet(s);
    System.out.println(s); // Reading OK
    //! s.add("one"); // Can't change it

    Map m = new HashMap();
    Collections2.fill(m,
      Collections2.geography, 25);
    m = Collections.unmodifiableMap(m);
    System.out.println(m); // Reading OK
    //! m.put("Ralph", "Howdy!");
  }
} ///:~

In each case, you must fill the container with meaningful data before you make it read-only. Once it is loaded, the best approach is to replace the existing reference with the reference that is produced by the “unmodifiable” call. That way, you don’t run the risk of accidentally changing the contents once you’ve made it unmodifiable. On the other hand, this tool also allows you to keep a modifiable container as private within a class and to return a read-only reference to that container from a method call. So you can change it from within the class but everyone else can only read it.

Calling the “unmodifiable” method for a particular type does not cause compile-time checking, but once the transformation has occurred, any calls to methods that modify the contents of a particular container will produce an UnsupportedOperationException.

Synchronizing a Collection or Map

The synchronized keyword is an important part of the subject of multithreading, a more complicated topic that will not be introduced until Chapter 14. Here, I shall note only that the Collections class contains a way to automatically synchronize an entire container. The syntax is similar to the “unmodifiable” methods:

//: c09:Synchronization.java
// Using the Collections.synchronized methods.
import java.util.*;

public class Synchronization {
  public static void main(String[] args) {
    Collection c =
      Collections.synchronizedCollection(
        new ArrayList());
    List list = Collections.synchronizedList(
      new ArrayList());
    Set s = Collections.synchronizedSet(
      new HashSet());
    Map m = Collections.synchronizedMap(
      new HashMap());
  }
} ///:~

In this case, you immediately pass the new container through the appropriate “synchronized” method; that way there’s no chance of accidentally exposing the unsynchronized version.

Fail fast

The Java containers also have a mechanism to prevent more than one process from modifying the contents of a container. The problem occurs if you’re iterating through a container and some other process steps in and inserts, removes, or changes an object in that container. Maybe you’ve already passed that object, maybe it’s ahead of you, maybe the size of the container shrinks after you call size( )—there are many scenarios for disaster. The Java containers library incorporates a fail fast mechanism that looks for any changes to the container other than the ones your process is personally responsible for. If it detects that someone else is modifying the container, it immediately produces a ConcurrentModificationException. This is the “fail-fast” aspect—it doesn’t try to detect a problem later on using a more complex algorithm.

Unsupported operations

It’s possible to turn an array into a List with the Arrays.asList( ) method:

//: c09:Unsupported.java
// Sometimes methods defined in the
// Collection interfaces don't work!
import java.util.*;

public class Unsupported {
  private static String[] s = {
    "one", "two", "three", "four", "five",
    "six", "seven", "eight", "nine", "ten",
  };
  static List a = Arrays.asList(s);
  static List a2 = a.subList(3, 6);
  public static void main(String[] args) {
    System.out.println(a);
    System.out.println(a2);
    System.out.println(
      "a.contains(" + s[0] + ") = " +
      a.contains(s[0]));
    System.out.println(
      "a.containsAll(a2) = " +
      a.containsAll(a2));
    System.out.println("a.isEmpty() = " +
      a.isEmpty());
    System.out.println(
      "a.indexOf(" + s[5] + ") = " +
      a.indexOf(s[5]));
    // Traverse backwards:
    ListIterator lit = a.listIterator(a.size());
    while(lit.hasPrevious())
      System.out.print(lit.previous() + " ");
    System.out.println();
    // Set the elements to different values:
    for(int i = 0; i < a.size(); i++)
      a.set(i, "47");
    System.out.println(a);
    // Compiles, but won't run:
    lit.add("X"); // Unsupported operation
    a.clear(); // Unsupported
    a.add("eleven"); // Unsupported
    a.addAll(a2); // Unsupported
    a.retainAll(a2); // Unsupported
    a.remove(s[0]); // Unsupported
    a.removeAll(a2); // Unsupported
  }
} ///:~

You’ll discover that only a portion of the Collection and List interfaces are actually implemented. The rest of the methods cause the unwelcome appearance of something called an UnsupportedOperationException. You’ll learn all about exceptions in the next chapter, but the short story is that the Collection interface, as well as some of the other interfaces in the Java containers library, contain “optional” methods, which might or might not be “supported” in the concrete class that implements that interface. Calling an unsupported method causes an UnsupportedOperationException to indicate a programming error.

“What?!?” you say, incredulous. “The whole point of interfaces and base classes is that they promise these methods will do something meaningful! This breaks that promise—it says that not only will calling some methods not perform a meaningful behavior, they will stop the program! Type safety was just thrown out the window!” It’s not quite that bad. With a Collection, List, Set, or Map, the compiler still restricts you to calling only the methods in that interface, so it’s not like Smalltalk (in which you can call any method for any object, and find out only when you run the program whether your call does anything). In addition, most methods that take a Collection as an argument only read from that Collection –all the “read” methods of Collection are not optional.

This approach prevents an explosion of interfaces in the design. Other designs for container libraries always seem to end up with a confusing plethora of interfaces to describe each of the variations on the main theme and are thus difficult to learn. It’s not even possible to capture all of the special cases in interfaces, because someone can always invent a new interface. The “unsupported operation” approach achieves an important goal of the Java containers library: the containers are simple to learn and use; unsupported operations are a special case that can be learned later. For this approach to work, however:

  1. The UnsupportedOperationException must be a rare event. That is, for most classes all operations should work, and only in special cases should an operation be unsupported. This is true in the Java containers library, since the classes you’ll use 99 percent of the time—ArrayList, LinkedList, HashSet, and HashMap, as well as the other concrete implementations—support all of the operations. The design does provide a “back door” if you want to create a new Collection without providing meaningful definitions for all the methods in the Collection interface, and yet still fit it into the existing library.
  2. When an operation is unsupported, there should be reasonable likelihood that an UnsupportedOperationException will appear at implementation time, rather than after you’ve shipped the product to the customer. After all, it indicates a programming error: you’ve used an implementation incorrectly. This point is less certain, and is where the experimental nature of this design comes into play. Only over time will we find out how well it works.

In the example above, Arrays.asList( ) produces a List that is backed by a fixed-size array. Therefore it makes sense that the only supported operations are the ones that don’t change the size of the array. If, on the other hand, a new interface were required to express this different kind of behavior (called, perhaps, “FixedSizeList”), it would throw open the door to complexity and soon you wouldn’t know where to start when trying to use the library.

The documentation for a method that takes a Collection, List, Set, or Map as an argument should specify which of the optional methods must be implemented. For example, sorting requires the set( ) and Iterator.set( ) methods but not add( ) and remove( ).

Java 1.0/1.1 containers

Unfortunately, lots of code was written using the Java 1.0/1.1 containers, and even new code is sometimes written using these classes. So although you should never use the old containers when writing new code, you’ll still need to be aware of them. However, the old containers were quite limited so there’s not that much to say (since they are in the past, I will try to refrain from overemphasizing some of the hideous design decisions).

Vector & Enumeration

The only self-expanding sequence in Java 1.0/1.1 was the Vector, and so it saw a lot of use. Its flaws are too numerous to describe here (see the first edition of this book, available on this book’s CD ROM and as a free download from www.BruceEckel.com). Basically, you can think of it as an ArrayList with long, awkward method names. In the Java 2 container library, Vector was adapted so that it could fit as a Collection and a List, so in the following example the Collections2.fill( ) method is successfully used. This turns out to be a bit perverse, as it may confuse some people into thinking that Vector has gotten better, when it is actually included only to support pre-Java-2 code.

The Java 1.0/1.1 version of the iterator chose to invent a new name, “enumeration,” instead of using a term that everyone was already familiar with. The Enumeration interface is smaller than Iterator, with only two methods, and is uses longer method names: boolean hasMoreElements( ) produces true if this enumeration contains more elements, and Object nextElement( ) returns the next element of this enumeration if there are any more (otherwise it throws an exception).

Enumeration is only an interface, not an implementation, and even new libraries sometimes still use the old Enumeration, which is unfortunate but generally harmless. Even though you should always use Iterator when you can in your own code, you must be prepared for libraries that want to hand you an Enumeration.

In addition, you can produce an Enumeration for any Collection by using the Collections.enumeration( ) method, as seen in this example:

//: c09:Enumerations.java
// Java 1.0/1.1 Vector and Enumeration.
import java.util.*;
import com.bruceeckel.util.*;

class Enumerations {
  public static void main(String[] args) {
    Vector v = new Vector();
    Collections2.fill(
      v, Collections2.countries, 100);
    Enumeration e = v.elements();
    while(e.hasMoreElements())
      System.out.println(e.nextElement());
    // Produce an Enumeration from a Collection:
    e = Collections.enumeration(new ArrayList());
  }
} ///:~

The Java 1.0/1.1 Vector has only an addElement( ) method, but fill( ) uses the add( ) method that was pasted on as Vector was turned into a List. To produce an Enumeration, you call elements( ), then you can use it to perform a forward iteration.

The last line creates an ArrayList and uses enumeration( ) to adapt an Enumeration from the ArrayList Iterator. Thus, if you have old code that wants an Enumeration, you can still use the new containers.

Hashtable

As you’ve seen in the performance comparison in this chapter, the basic Hashtable is very similar to the HashMap, even down to the method names. There’s no reason to use Hashtable instead of HashMap in new code.

Stack

The concept of the stack was introduced earlier, with the LinkedList. What’s rather odd about the Java 1.0/1.1 Stack is that instead of using a Vector as a building block, Stack is inherited from Vector. So it has all of the characteristics and behaviors of a Vector plus some extra Stack behaviors. It’s difficult to know whether the designers explicitly decided that this was an especially useful way of doing things, or whether it was just a naïve design.

Here’s a simple demonstration of Stack that pushes each line from a String array:

//: c09:Stacks.java
// Demonstration of Stack Class.
import java.util.*;

public class Stacks {
  static String[] months = {
    "January", "February", "March", "April",
    "May", "June", "July", "August", "September",
    "October", "November", "December" };
  public static void main(String[] args) {
    Stack stk = new Stack();
    for(int i = 0; i < months.length; i++)
      stk.push(months[i] + " ");
    System.out.println("stk = " + stk);
    // Treating a stack as a Vector:
    stk.addElement("The last line");
    System.out.println(
      "element 5 = " + stk.elementAt(5));
    System.out.println("popping elements:");
    while(!stk.empty())
      System.out.println(stk.pop());
  }
} ///:~

Each line in the months array is inserted into the Stack with push( ), and later fetched from the top of the stack with a pop( ). To make a point, Vector operations are also performed on the Stack object. This is possible because, by virtue of inheritance, a Stack is a Vector. Thus, all operations that can be performed on a Vector can also be performed on a Stack, such as elementAt( ).

As mentioned earlier, you should use a LinkedList when you want stack behavior.

BitSet

A BitSet is used if you want to efficiently store a lot of on-off information. It’s efficient only from the standpoint of size; if you’re looking for efficient access, it is slightly slower than using an array of some native type.

In addition, the minimum size of the BitSet is that of a long: 64 bits. This implies that if you’re storing anything smaller, like 8 bits, a BitSet will be wasteful, so you’re better off creating your own class, or just an array, to hold your flags if size is an issue.

A normal container expands as you add more elements, and the BitSet does this as well. The following example shows how the BitSet works:

//: c09:Bits.java
// Demonstration of BitSet.
import java.util.*;

public class Bits {
  static void printBitSet(BitSet b) {
    System.out.println("bits: " + b);
    String bbits = new String();
    for(int j = 0; j < b.size() ; j++)
      bbits += (b.get(j) ? "1" : "0");
    System.out.println("bit pattern: " + bbits);
  }
  public static void main(String[] args) {
    Random rand = new Random();
    // Take the LSB of nextInt():
    byte bt = (byte)rand.nextInt();
    BitSet bb = new BitSet();
    for(int i = 7; i >=0; i--)
      if(((1 << i) &  bt) != 0)
        bb.set(i);
      else
        bb.clear(i);
    System.out.println("byte value: " + bt);
    printBitSet(bb);

    short st = (short)rand.nextInt();
    BitSet bs = new BitSet();
    for(int i = 15; i >=0; i--)
      if(((1 << i) &  st) != 0)
        bs.set(i);
      else
        bs.clear(i);
    System.out.println("short value: " + st);
    printBitSet(bs);

    int it = rand.nextInt();
    BitSet bi = new BitSet();
    for(int i = 31; i >=0; i--)
      if(((1 << i) &  it) != 0)
        bi.set(i);
      else
        bi.clear(i);
    System.out.println("int value: " + it);
    printBitSet(bi);

    // Test bitsets >= 64 bits:
    BitSet b127 = new BitSet();
    b127.set(127);
    System.out.println("set bit 127: " + b127);
    BitSet b255 = new BitSet(65);
    b255.set(255);
    System.out.println("set bit 255: " + b255);
    BitSet b1023 = new BitSet(512);
    b1023.set(1023);
    b1023.set(1024);
    System.out.println("set bit 1023: " + b1023);
  }
} ///:~

The random number generator is used to create a random byte, short, and int, and each one is transformed into a corresponding bit pattern in a BitSet. This works fine because a BitSet is 64 bits, so none of these cause it to increase in size. Then a BitSet of 512 bits is created. The constructor allocates storage for twice that number of bits. However, you can still set bit 1024 or greater.

Summary

To review the containers provided in the standard Java library:

  1. An array associates numerical indices to objects. It holds objects of a known type so that you don’t have to cast the result when you’re looking up an object. It can be multidimensional, and it can hold primitives. However, its size cannot be changed once you create it.
  2. A Collection holds single elements, while a Map holds associated pairs.
  3. Like an array, a List also associates numerical indices to objects—you can think of arrays and Lists as ordered containers. The List automatically resizes itself as you add more elements. But a List can hold only Object references, so it won’t hold primitives and you must always cast the result when you pull an Object reference out of a container.
  4. Use an ArrayList if you’re doing a lot of random accesses, and a LinkedList if you will be doing a lot of insertions and removals in the middle of the list.
  5. The behavior of queues, deques, and stacks is provided via the LinkedList.
  6. A Map is a way to associate, not numbers, but objects with other objects. The design of a HashMap is focused on rapid access, while a TreeMap keeps its keys in sorted order, and thus is not as fast as a HashMap.
  7. A Set only accepts one of each type of object. HashSets provide maximally-fast lookups, while TreeSets keep the elements in sorted order.
  8. There’s no need to use the legacy classes Vector, Hashtable and Stack in new code.

The containers are tools that you can use on a day-to-day basis to make your programs simpler, more powerful and more effective.

Exercises
  1. Create an array of double and fill( ) it using RandDoubleGenerator. Print the results.
  2. Create a new class called Gerbil with an int gerbilNumber that’s initialized in the constructor (similar to the Mouse example in this chapter). Give it a method called hop( ) that prints out which gerbil number this is and that it’s hopping. Create an ArrayList and add a bunch of Gerbil objects to the List. Now use the get( ) method to move through the List and call hop( ) for each Gerbil.
  3. Modify Exercise 2 so you use an Iterator to move through the List while calling hop( ).
  4. Take the Gerbil class in Exercise 2 and put it into a Map instead, associating the name of the Gerbil as a String (the key) for each Gerbil (the value) you put in the table. Get an Iterator for the keySet( ) and use it to move through the Map, looking up the Gerbil for each key and printing out the key and telling the gerbil to hop( ).
  5. Create a List (try both ArrayList and LinkedList) and fill it using Collections2.countries. Sort the list and print it, then apply Collections.shuffle( ) to the list repeatedly, printing it each time so that you can see how the shuffle( ) method randomizes the list differently each time.
  6. Demonstrate that you can’t add anything but a Mouse to a MouseList.
  7. Modify MouseList.java so that it inherits from ArrayList instead of using composition. Demonstrate the problem with this approach.
  8. Repair CatsAndDogs.java by creating a Cats container (utilizing ArrayList) that will only accept and retrieve Cat objects.
  9. Create a container that encapsulates an array of String, and that only adds Strings and gets Strings, so that there are no casting issues during use. If the internal array isn’t big enough for the next add, your container should automatically resize it. In main( ), compare the performance of your container with an ArrayList holding Strings.
  10. Repeat Exercise 9 for a container of int, and compare the performance to an ArrayList holding Integer objects. In your performance comparison, include the process of incrementing each object in the container.
  11. Using the utilities in com.bruceeckel.util, create an array of each primitive type and of String, then fill each array using an appropriate generator, and print each array using the appropriate print( ) method.
  12. Create a generator that produces character names from your favorite movies (you can use Snow White or Star Wars as a fallback), and loops around to the beginning when it runs out of names. Use the utilities in com.bruceeckel.util to fill an array, an ArrayList, a LinkedList and both types of Set, then print each container.
  13. Create a class containing two String objects, and make it Comparable so that the comparison only cares about the first String. Fill an array and an ArrayList with objects of your class, using the geography generator. Demonstrate that sorting works properly. Now make a Comparator that only cares about the second String and demonstrate that sorting works properly; also perform a binary search using your Comparator.
  14. Modify Exercise 13 so that an alphabetic sort is used.
  15. Use Arrays2.RandStringGenerator to fill a TreeSet but using alphabetic ordering. Print the TreeSet to verify the sort order.
  16. Create both an ArrayList and a LinkedList, and fill each using the Collections2.capitals generator. Print each list using an ordinary Iterator, then insert one list into the other using a ListIterator, inserting at every other location. Now perform the insertion starting at the end of the first list and moving backwards.
  17. Write a method that uses an Iterator to step through a Collection and print the hashCode( ) of each object in the container. Fill all the different types of Collections with objects and apply your method to each container.
  18. Repair the problem in InfiniteRecursion.java.
  19. Create a class, then make an initialized array of objects of your class. Fill a List from your array. Create a subset of your List using subList( ), and then remove this subset from your List using removeAll( ).
  20. Change Exercise 6 in Chapter 7 to use an ArrayList to hold the Rodents and an Iterator to move through the sequence of Rodents. Remember that an ArrayList holds only Objects so you must use a cast when accessing individual Rodents.
  21. Following the Queue.java example, create a Deque class and test it.
  22. Use a TreeMap in Statistics.java. Now add code that tests the performance difference between HashMap and TreeMap in that program.
  23. Produce a Map and a Set containing all the countries that begin with ‘A.’
  24. Using Collections2.countries, fill a Set multiple times with the same data and verify that the Set ends up with only one of each instance. Try this with both kinds of Set.
  25. Starting with Statistics.java, create a program that runs the test repeatedly and looks to see if any one number tends to appear more than the others in the results.
  26. Rewrite Statistics.java using a HashSet of Counter objects (you’ll have to modify Counter so that it will work in the HashSet). Which approach seems better?
  27. Modify the class in Exercise 13 so that it will work with HashSets and as a key in HashMaps.
  28. Using SlowMap.java for inspiration, create a SlowSet.
  29. Apply the tests in Map1.java to SlowMap to verify that it works. Fix anything in SlowMap that doesn’t work correctly.
  30. Implement the rest of the Map interface for SlowMap.
  31. Modify MapPerformance.java to include tests of SlowMap.
  32. Modify SlowMap so that instead of two ArrayLists, it holds a single ArrayList of MPair objects. Verify that the modified version works correctly. Using MapPerformance.java, test the speed of your new Map. Now change the put( ) method so that it performs a sort( ) after each pair is entered, and modify get( ) to use Collections.binarySearch( ) to look up the key. Compare the performance of the new version with the old ones.
  33. Add a char field to CountedString that is also initialized in the constructor, and modify the hashCode( ) and equals( ) methods to include the value of this char.
  34. Modify SimpleHashMap so that it reports collisions, and test this by adding the same data set twice so that you see collisions.
  35. Modify SimpleHashMap so that it reports the number of “probes” necessary when collisions occur. That is, how many calls to next( ) must be made on the Iterators that walk the LinkedLists looking for matches?
  36. Implement the clear( ) and remove( ) methods for SimpleHashMap.
  37. Implement the rest of the Map interface for SimpleHashMap.
  38. Add a private rehash( ) method to SimpleHashMap, which is invoked when the load factor exceeds 0.75. During rehashing, double the number of buckets, then search for the first prime number greater than that to determine the new number of buckets.
  39. Following the example in SimpleHashMap.java, create and test a SimpleHashSet.
  40. Modify SimpleHashMap to use ArrayLists instead of LinkedLists. Modify MapPerformance.java to compare the performance of the two implementations.
  41. Using the HTML documentation for the JDK (downloadable from http://java.sun.com), look up the HashMap class. Create a HashMap, fill it with elements, and determine the load factor. Test the lookup speed with this map, then attempt to increase the speed by making a new HashMap with a larger initial capacity and copying the old map into the new one, running your lookup speed test again on the new map.
  42. In Chapter 8, locate the GreenhouseControls.java example, which consists of three files. In Controller.java, the class EventSet is just a container. Change the code to use a LinkedList instead of an EventSet. This will require more than just replacing EventSet with LinkedList; you’ll also need to use an Iterator to cycle through the set of events.
  43. (Challenging). Write your own hashed map class, customized for a particular key type: String for this example. Do not inherit it from Map. Instead, duplicate the methods so that the put( ) and get( ) methods specifically take String objects, not Objects, as keys. Everything that involves keys should not use generic types, but instead work with Strings, to avoid the cost of upcasting and downcasting. Your goal is to make the fastest possible custom implementation. Modify MapPerformance.java to test your implementation vs. a HashMap.
  44. (Challenging). Find the source code for List in the Java source code library that comes with all Java distributions. Copy this code and make a special version called intList that holds only ints. Consider what it would take to make a special version of List for all the primitive types. Now consider what happens if you want to make a linked list class that works with all the primitive types. If parameterized types are ever implemented in Java, they will provide a way to do this work for you automatically (as well as many other benefits).

[43] It’s possible, however, to ask how big the vector is, and the at( ) method does perform bounds checking.

[44] This is one of the places where C++ is distinctly superior to Java, since C++ supports parameterized types with the template keyword.

[45] The C++ programmer will note how much the code could be collapsed with the use of default arguments and templates. The Python programmer will note that this entire library would be largely unnecessary in that language.

[46] By Joshua Bloch at Sun.

[47] This data was found on the Internet, then processed by creating a Python program (see www.Python.org).

[48] This is a place where operator overloading would be nice.

[49] If these speedups still don’t meet your performance needs, you can further accelerate table lookup by writing your own Map and customizing it to your particular types to avoid delays due to casting to and from Objects. To reach even higher levels of performance, speed enthusiasts can use Donald Knuth’s The Art of Computer Programming, Volume 3: Sorting and Searching, Second Edition to replace overflow bucket lists with arrays that have two additional benefits: they can be optimized for disk storage characteristics and they can save most of the time of creating and garbage collecting individual records.


10: Error Handling with Exceptions

10: Error Handling
with Exceptions

The basic philosophy of Java is that “badly formed code will not be run.”

The ideal time to catch an error is at compile-time, before you even try to run the program. However, not all errors can be detected at compile-time. The rest of the problems must be handled at run-time through some formality that allows the originator of the error to pass appropriate information to a recipient who will know how to handle the difficulty properly.

In C and other earlier languages, there could be several of these formalities, and they were generally established by convention and not as part of the programming language. Typically, you returned a special value or set a flag, and the recipient was supposed to look at the value or the flag and determine that something was amiss. However, as the years passed, it was discovered that programmers who use a library tend to think of themselves as invincible, as in, “Yes, errors might happen to others, but not in my code.” So, not too surprisingly, they wouldn’t check for the error conditions (and sometimes the error conditions were too silly to check for [50] ). If you were thorough enough to check for an error every time you called a method, your code could turn into an unreadable nightmare. Because programmers could still coax systems out of these languages they were resistant to admitting the truth: This approach to handling errors was a major limitation to creating large, robust, maintainable programs.

The solution is to take the casual nature out of error handling and to enforce formality. This actually has a long history, since implementations of exception handling go back to operating systems in the 1960s and even to BASIC’s “on error goto.” But C++ exception handling was based on Ada, and Java’s is based primarily on C++ (although it looks even more like Object Pascal).

The word “exception” is meant in the sense of “I take exception to that.” At the point where the problem occurs you might not know what to do with it, but you do know that you can’t just continue on merrily; you must stop and somebody, somewhere, must figure out what to do. But you don’t have enough information in the current context to fix the problem. So you hand the problem out to a higher context where someone is qualified to make the proper decision (much like a chain of command).

The other rather significant benefit of exceptions is that they clean up error handling code. Instead of checking for a particular error and dealing with it at multiple places in your program, you no longer need to check at the point of the method call (since the exception will guarantee that someone catches it). And, you need to handle the problem in only one place, the so-called exception handler. This saves you code and it separates the code that describes what you want to do from the code that is executed when things go awry. In general, reading, writing, and debugging code becomes much clearer with exceptions than when using the old way of error handling.

Because exception handling is enforced by the Java compiler, there are only so many examples that can be written in this book without learning about exception handling. This chapter introduces you to the code you need to write to properly handle exceptions, and the way you can generate your own exceptions if one of your methods gets into trouble.

Basic exceptions

An exceptional condition is a problem that prevents the continuation of the method or scope that you’re in. It’s important to distinguish an exceptional condition from a normal problem, in which you have enough information in the current context to somehow cope with the difficulty. With an exceptional condition, you cannot continue processing because you don’t have the information necessary to deal with the problem in the current context. All you can do is jump out of the current context and relegate that problem to a higher context. This is what happens when you throw an exception.

A simple example is a divide. If you’re about to divide by zero, it’s worth checking to make sure you don’t go ahead and perform the divide. But what does it mean that the denominator is zero? Maybe you know, in the context of the problem you’re trying to solve in that particular method, how to deal with a zero denominator. But if it’s an unexpected value, you can’t deal with it and so must throw an exception rather than continuing along that path.

When you throw an exception, several things happen. First, the exception object is created in the same way that any Java object is created: on the heap, with new. Then the current path of execution (the one you couldn’t continue) is stopped and the reference for the exception object is ejected from the current context. At this point the exception handling mechanism takes over and begins to look for an appropriate place to continue executing the program. This appropriate place is the exception handler, whose job is to recover from the problem so the program can either try another tack or just continue.

As a simple example of throwing an exception, consider an object reference called t. It’s possible that you might be passed a reference that hasn’t been initialized, so you might want to check before trying to call a method using that object reference. You can send information about the error into a larger context by creating an object representing your information and “throwing” it out of your current context. This is called throwing an exception. Here’s what it looks like:

if(t == null)
  throw new NullPointerException();

This throws the exception, which allows you—in the current context—to abdicate responsibility for thinking about the issue further. It’s just magically handled somewhere else. Precisely where will be shown shortly.

Exception arguments

Like any object in Java, you always create exceptions on the heap using new, which allocates storage and calls a constructor. There are two constructors in all standard exceptions: the first is the default constructor, and the second takes a string argument so you can place pertinent information in the exception:

if(t == null)
  throw new NullPointerException("t = null");

This string can later be extracted using various methods, as will be shown later.

The keyword throw causes a number of relatively magical things to happen. Typically, you’ll first use new to create an object that represents the error condition. You give the resulting reference to throw. The object is, in effect, “returned” from the method, even though that object type isn’t normally what the method is designed to return. A simplistic way to think about exception handling is as an alternate return mechanism, although you get into trouble if you take that analogy too far. You can also exit from ordinary scopes by throwing an exception. But a value is returned, and the method or scope exits.

Any similarity to an ordinary return from a method ends here, because where you return is someplace completely different from where you return for a normal method call. (You end up in an appropriate exception handler that might be miles away—many levels lower on the call stack—from where the exception was thrown.)

In addition, you can throw any type of Throwable object that you want. Typically, you’ll throw a different class of exception for each different type of error. The information about the error is represented both inside the exception object and implicitly in the type of exception object chosen, so someone in the bigger context can figure out what to do with your exception. (Often, the only information is the type of exception object, and nothing meaningful is stored within the exception object.)

Catching an exception

If a method throws an exception, it must assume that exception is “caught” and dealt with. One of the advantages of Java exception handling is that it allows you to concentrate on the problem you’re trying to solve in one place, and then deal with the errors from that code in another place.

To see how an exception is caught, you must first understand the concept of a guarded region, which is a section of code that might produce exceptions, and which is followed by the code to handle those exceptions.

The try block

If you’re inside a method and you throw an exception (or another method you call within this method throws an exception), that method will exit in the process of throwing. If you don’t want a throw to exit the method, you can set up a special block within that method to capture the exception. This is called the try block because you “try” your various method calls there. The try block is an ordinary scope, preceded by the keyword try:

try {
  // Code that might generate exceptions
}

If you were checking for errors carefully in a programming language that didn’t support exception handling, you’d have to surround every method call with setup and error testing code, even if you call the same method several times. With exception handling, you put everything in a try block and capture all the exceptions in one place. This means your code is a lot easier to write and easier to read because the goal of the code is not confused with the error checking.

Exception handlers

Of course, the thrown exception must end up someplace. This “place” is the exception handler, and there’s one for every exception type you want to catch. Exception handlers immediately follow the try block and are denoted by the keyword catch:

try {
  // Code that might generate exceptions
} catch(Type1 id1) {
  // Handle exceptions of Type1
} catch(Type2 id2) {
  // Handle exceptions of Type2
} catch(Type3 id3) {
  // Handle exceptions of Type3
}

// etc...

Each catch clause (exception handler) is like a little method that takes one and only one argument of a particular type. The identifier (id1, id2, and so on) can be used inside the handler, just like a method argument. Sometimes you never use the identifier because the type of the exception gives you enough information to deal with the exception, but the identifier must still be there.

The handlers must appear directly after the try block. If an exception is thrown, the exception handling mechanism goes hunting for the first handler with an argument that matches the type of the exception. Then it enters that catch clause, and the exception is considered handled. The search for handlers stops once the catch clause is finished. Only the matching catch clause executes; it’s not like a switch statement in which you need a break after each case to prevent the remaining ones from executing.

Note that, within the try block, a number of different method calls might generate the same exception, but you need only one handler.

Termination vs. resumption

There are two basic models in exception handling theory. In termination (which is what Java and C++ support), you assume the error is so critical that there’s no way to get back to where the exception occurred. Whoever threw the exception decided that there was no way to salvage the situation, and they don’t want to come back.

The alternative is called resumption. It means that the exception handler is expected to do something to rectify the situation, and then the faulting method is retried, presuming success the second time. If you want resumption, it means you still hope to continue execution after the exception is handled. In this case, your exception is more like a method call—which is how you should set up situations in Java in which you want resumption-like behavior. (That is, don’t throw an exception; call a method that fixes the problem.) Alternatively, place your try block inside a while loop that keeps reentering the try block until the result is satisfactory.

Historically, programmers using operating systems that supported resumptive exception handling eventually ended up using termination-like code and skipping resumption. So although resumption sounds attractive at first, it seems it isn’t quite so useful in practice. The dominant reason is probably the coupling that results: your handler must often be aware of where the exception is thrown from and contain non-generic code specific to the throwing location. This makes the code difficult to write and maintain, especially for large systems where the exception can be generated from many points.

The exception specification

In Java, you’re required to inform the client programmer, who calls your method, of the exceptions that might be thrown from your method. This is civilized because the caller can know exactly what code to write to catch all potential exceptions. Of course, if source code is available, the client programmer could hunt through and look for throw statements, but often a library doesn’t come with sources. To prevent this from being a problem, Java provides syntax (and forces you to use that syntax) to allow you to politely tell the client programmer what exceptions this method throws, so the client programmer can handle them. This is the exception specification and it’s part of the method declaration, appearing after the argument list.

The exception specification uses an additional keyword, throws, followed by a list of all the potential exception types. So your method definition might look like this:

void f() throws tooBig, tooSmall, divZero { //...

If you say

void f() { // ...

it means that no exceptions are thrown from the method. (Except for the exceptions of type RuntimeException, which can reasonably be thrown anywhere—this will be described later.)

You can’t lie about an exception specification—if your method causes exceptions and doesn’t handle them, the compiler will detect this and tell you that you must either handle the exception or indicate with an exception specification that it may be thrown from your method. By enforcing exception specifications from top to bottom, Java guarantees that exception correctness can be ensured at compile-time [51] .

There is one place you can lie: you can claim to throw an exception that you don’t. The compiler takes your word for it and forces the users of your method to treat it as if it really does throw that exception. This has the beneficial effect of being a placeholder for that exception, so you can actually start throwing the exception later without requiring changes to existing code. It’s also important for creating abstract base classes and interfaces whose derived classes or implementations may need to throw exceptions.

Catching any exception

It is possible to create a handler that catches any type of exception. You do this by catching the base-class exception type Exception (there are other types of base exceptions, but Exception is the base that’s pertinent to virtually all programming activities):

catch(Exception e) {
  System.out.println("caught an exception");
}

This will catch any exception, so if you use it you’ll want to put it at the end of your list of handlers to avoid preempting any exception handlers that might otherwise follow it.

Since the Exception class is the base of all the exception classes that are important to the programmer, you don’t get much specific information about the exception, but you can call the methods that come from its base type Throwable:

String getMessage( )
String getLocalizedMessage ( )
Gets the detail message, or a message adjusted for this particular locale.

String toString( )
Returns a short description of the Throwable, including the detail message if there is one.

void printStackTrace( )
void printStackTrace(PrintStream)
void printStackTrace ( PrintWriter )
Prints the Throwable and the Throwable’s call stack trace. The call stack shows the sequence of method calls that brought you to the point at which the exception was thrown. The first version prints to standard error, the second and third prints to a stream of your choice (in Chapter 11, you’ll understand why there are two types of streams).

Throwable fillInStackTrace ( )
Records within this Throwable object information about the current state of the stack frames. Useful when an application is rethrowing an error or exception (more about this shortly).

In addition, you get some other methods from Throwable’s base type Object (everybody’s base type). The one that might come in handy for exceptions is getClass( ), which returns an object representing the class of this object. You can in turn query this Class object for its name with getName( ) or toString( ). You can also do more sophisticated things with Class objects that aren’t necessary in exception handling. Class objects will be studied later in this book.

Here’s an example that shows the use of the basic Exception methods:

//: c10:ExceptionMethods.java
// Demonstrating the Exception Methods.

public class ExceptionMethods {
  public static void main(String[] args) {
    try {
      throw new Exception("Here's my Exception");
    } catch(Exception e) {
      System.out.println("Caught Exception");
      System.out.println(
        "e.getMessage(): " + e.getMessage());
      System.out.println(
        "e.getLocalizedMessage(): " +
         e.getLocalizedMessage());
      System.out.println("e.toString(): " + e);
      System.out.println("e.printStackTrace():");
      e.printStackTrace();
      e.printStackTrace(System.err);
    }
  }
} ///:~

The output for this program is:

Caught Exception
e.getMessage(): Here's my Exception
e.getLocalizedMessage(): Here's my Exception
e.toString(): java.lang.Exception:
   Here's my Exception
e.printStackTrace():
java.lang.Exception: Here's my Exception
 at ExceptionMethods.main(ExceptionMethods.java:7)
java.lang.Exception:
   Here's my Exception
 at ExceptionMethods.main(ExceptionMethods.java:7)

You can see that the methods provide successively more information—each is effectively a superset of the previous one.

Rethrowing an exception

Sometimes you’ll want to rethrow the exception that you just caught, particularly when you use Exception to catch any exception. Since you already have the reference to the current exception, you can simply rethrow that reference:

catch(Exception e) {
  System.out.println("An exception was thrown");
  throw e;
}

Rethrowing an exception causes the exception to go to the exception handlers in the next-higher context. Any further catch clauses for the same try block are still ignored. In addition, everything about the exception object is preserved, so the handler at the higher context that catches the specific exception type can extract all the information from that object.

If you simply rethrow the current exception, the information that you print about that exception in printStackTrace( ) will pertain to the exception’s origin, not the place where you rethrow it. If you want to install new stack trace information, you can do so by calling fillInStackTrace( ), which returns an exception object that it creates by stuffing the current stack information into the old exception object. Here’s what it looks like:

//: c10:Rethrowing.java
// Demonstrating fillInStackTrace()

public class Rethrowing {
  public static void f() throws Exception {
    System.out.println(
      "originating the exception in f()");
    throw new Exception("thrown from f()");
  }
  public static void g() throws Throwable {
    try {
      f();
    } catch(Exception e) {
      System.out.println(
        "Inside g(), e.printStackTrace()");
      e.printStackTrace();
      throw e; // 17
      // throw e.fillInStackTrace(); // 18
    }
  }
  public static void
  main(String[] args) throws Throwable {
    try {
      g();
    } catch(Exception e) {
      System.out.println(
        "Caught in main, e.printStackTrace()");
      e.printStackTrace();
    }
  }
} ///:~

The important line numbers are marked as comments. With line 17 uncommented (as shown), the output is:

originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
        at Rethrowing.f(Rethrowing.java:8)
        at Rethrowing.g(Rethrowing.java:12)
        at Rethrowing.main(Rethrowing.java:24)
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
        at Rethrowing.f(Rethrowing.java:8)
        at Rethrowing.g(Rethrowing.java:12)
        at Rethrowing.main(Rethrowing.java:24)

So the exception stack trace always remembers its true point of origin, no matter how many times it gets rethrown.

With line 17 commented and line 18 uncommented, fillInStackTrace( ) is used instead, and the result is:

originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
        at Rethrowing.f(Rethrowing.java:8)
        at Rethrowing.g(Rethrowing.java:12)
        at Rethrowing.main(Rethrowing.java:24)
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
        at Rethrowing.g(Rethrowing.java:18)
        at Rethrowing.main(Rethrowing.java:24)

Because of fillInStackTrace( ), line 18 becomes the new point of origin of the exception.

The class Throwable must appear in the exception specification for g( ) and main( ) because fillInStackTrace( ) produces a reference to a Throwable object. Since Throwable is a base class of Exception, it’s possible to get an object that’s a Throwable but not an Exception, so the handler for Exception in main( ) might miss it. To make sure everything is in order, the compiler forces an exception specification for Throwable. For example, the exception in the following program is not caught in main( ):

//: c10:ThrowOut.java
public class ThrowOut {
  public static void
  main(String[] args) throws Throwable {
    try {
      throw new Throwable();
    } catch(Exception e) {
      System.out.println("Caught in main()");
    }
  }
} ///:~

It’s also possible to rethrow a different exception from the one you caught. If you do this, you get a similar effect as when you use fillInStackTrace( ): the information about the original site of the exception is lost, and what you’re left with is the information pertaining to the new throw:

//: c10:RethrowNew.java
// Rethrow a different object
// from the one that was caught.

public class RethrowNew {
  public static void f() throws Exception {
    System.out.println(
      "originating the exception in f()");
    throw new Exception("thrown from f()");
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(Exception e) {
      System.out.println(
        "Caught in main, e.printStackTrace()");
      e.printStackTrace();
      throw new NullPointerException("from main");
    }
  }
} ///:~

The output is:

originating the exception in f()
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
        at RethrowNew.f(RethrowNew.java:9)
        at RethrowNew.main(RethrowNew.java:13)
Exception in thread "main"
java.lang.NullPointerException: from main
        at RethrowNew.main(RethrowNew.java:18)

The final exception knows only that it came from main( ), and not from f( ). Note that Throwable isn’t necessary in any of the exception specifications.

You never have to worry about cleaning up the previous exception, or any exceptions for that matter. They’re all heap-based objects created with new, so the garbage collector automatically cleans them all up.

Standard Java exceptions

The Java class Throwable describes anything that can be thrown as an exception. There are two general types of Throwable objects (“types of” = “inherited from”). Error represents compile-time and system errors that you don’t worry about catching (except in special cases). Exception is the basic type that can be thrown from any of the standard Java library class methods and from your methods and run-time accidents. So the Java programmer’s base type of interest is Exception.

The best way to get an overview of the exceptions is to browse online Java documentation from http://java.sun.com. (Of course, it’s easier to download it first.) It’s worth doing this once just to get a feel for the various exceptions, but you’ll soon see that there isn’t anything special between one exception and the next except for the name. Also, the number of exceptions in Java keeps expanding; basically it’s pointless to print them in a book. Any new library you get from a third-party vendor will probably have its own exceptions as well. The important thing to understand is the concept and what you should do with the exceptions.

java.lang.Exception

This is the basic exception class your program will catch. Other exceptions are derived from this. The basic idea is that the name of the exception represents the problem that occurred and the exception name is intended to be relatively self-explanatory. The exceptions are not all defined in java.lang; some are created to support other libraries such as util, net, and io, which you can see from their full class names or what they are inherited from. For example, all IO exceptions are inherited from java.io.IOException.

The special case of RuntimeException

The first example in this chapter was

if(t == null)
  throw new NullPointerException();

It can be a bit horrifying to think that you must check for null on every reference that is passed into a method (since you can’t know if the caller has passed you a valid reference). Fortunately, you don’t—this is part of the standard run-time checking that Java performs for you, and if any call is made to a null reference, Java will automatically throw a NullPointerException. So the above bit of code is always superfluous.

There’s a whole group of exception types that are in this category. They’re always thrown automatically by Java and you don’t need to include them in your exception specifications. Conveniently enough, they’re all grouped together by putting them under a single base class called RuntimeException, which is a perfect example of inheritance: it establishes a family of types that have some characteristics and behaviors in common. Also, you never need to write an exception specification saying that a method might throw a RuntimeException, since that’s just assumed. Because they indicate bugs, you virtually never catch a RuntimeException—it’s dealt with automatically. If you were forced to check for RuntimeExceptions your code could get messy. Even though you don’t typically catch RuntimeExceptions, in your own packages you might choose to throw some of the RuntimeExceptions.

What happens when you don’t catch such exceptions? Since the compiler doesn’t enforce exception specifications for these, it’s quite plausible that a RuntimeException could percolate all the way out to your main( ) method without being caught. To see what happens in this case, try the following example:

//: c10:NeverCaught.java
// Ignoring RuntimeExceptions.

public class NeverCaught {
  static void f() {
    throw new RuntimeException("From f()");
  }
  static void g() {
    f();
  }
  public static void main(String[] args) {
    g();
  }
} ///:~

You can already see that a RuntimeException (or anything inherited from it) is a special case, since the compiler doesn’t require an exception specification for these types.

The output is:

Exception in thread "main"
java.lang.RuntimeException: From f()
        at NeverCaught.f(NeverCaught.java:9)
        at NeverCaught.g(NeverCaught.java:12)
        at NeverCaught.main(NeverCaught.java:15)

So the answer is: If a RuntimeException gets all the way out to main( ) without being caught, printStackTrace( ) is called for that exception as the program exits.

Keep in mind that it’s possible to ignore only RuntimeExceptions in your coding, since all other handling is carefully enforced by the compiler. The reasoning is that a RuntimeException represents a programming error:

  1. An error you cannot catch (receiving a null reference handed to your method by a client programmer, for example)
  2. An error that you, as a programmer, should have checked for in your code (such as ArrayIndexOutOfBoundsException where you should have paid attention to the size of the array).

You can see what a tremendous benefit it is to have exceptions in this case, since they help in the debugging process.

It’s interesting to notice that you cannot classify Java exception handling as a single-purpose tool. Yes, it is designed to handle those pesky run-time errors that will occur because of forces outside your code’s control, but it’s also essential for certain types of programming bugs that the compiler cannot detect.

Creating your own exceptions

You’re not stuck using the Java exceptions. This is important because you’ll often need to create your own exceptions to denote a special error that your library is capable of creating, but which was not foreseen when the Java exception hierarchy was created.

To create your own exception class, you’re forced to inherit from an existing type of exception, preferably one that is close in meaning to your new exception (this is often not possible, however). Inheriting an exception is quite simple:

//: c10:Inheriting.java
// Inheriting your own exceptions.

class MyException extends Exception {
  public MyException() {}
  public MyException(String msg) {
    super(msg);
  }
}

public class Inheriting {
  public static void f() throws MyException {
    System.out.println(
      "Throwing MyException from f()");
    throw new MyException();
  }
  public static void g() throws MyException {
    System.out.println(
      "Throwing MyException from g()");
    throw new MyException("Originated in g()");
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(MyException e) {
      e.printStackTrace();
    }
    try {
      g();
    } catch(MyException e) {
      e.printStackTrace();
    }
  }
} ///:~

The inheritance occurs in the creation of the new class:

class MyException extends Exception {
  public MyException() {}
  public MyException(String msg) {
    super(msg);
  }
}

The key phrase here is extends Exception, which says “it’s everything an Exception is and more.” The added code is small—the addition of two constructors that define the way MyException is created. Remember that the compiler automatically calls the base-class default constructor if you don’t explicitly call a base-class constructor, as in the MyException( ) default constructor. In the second constructor, the base-class constructor with a String argument is explicitly invoked by using the super keyword.

The output of the program is:

Throwing MyException from f()
MyException
        at Inheriting.f(Inheriting.java:16)
        at Inheriting.main(Inheriting.java:24)
Throwing MyException from g()
MyException: Originated in g()
        at Inheriting.g(Inheriting.java:20)
        at Inheriting.main(Inheriting.java:29)

You can see the absence of the detail message in the MyException thrown from f( ).

The process of creating your own exceptions can be taken further. You can add extra constructors and members:

//: c10:Inheriting2.java
// Inheriting your own exceptions.

class MyException2 extends Exception {
  public MyException2() {}
  public MyException2(String msg) {
    super(msg);
  }
  public MyException2(String msg, int x) {
    super(msg);
    i = x;
  }
  public int val() { return i; }
  private int i;
}

public class Inheriting2 {
  public static void f() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from f()");
    throw new MyException2();
  }
  public static void g() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from g()");
    throw new MyException2("Originated in g()");
  }
  public static void h() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from h()");
    throw new MyException2(
      "Originated in h()", 47);
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(MyException2 e) {
      e.printStackTrace();
    }
    try {
      g();
    } catch(MyException2 e) {
      e.printStackTrace();
    }
    try {
      h();
    } catch(MyException2 e) {
      e.printStackTrace();
      System.out.println("e.val() = " + e.val());
    }
  }
} ///:~

A data member i has been added, along with a method that reads that value and an additional constructor that sets it. The output is:

Throwing MyException2 from f()
MyException2
        at Inheriting2.f(Inheriting2.java:22)
        at Inheriting2.main(Inheriting2.java:34)
Throwing MyException2 from g()
MyException2: Originated in g()
        at Inheriting2.g(Inheriting2.java:26)
        at Inheriting2.main(Inheriting2.java:39)
Throwing MyException2 from h()
MyException2: Originated in h()
        at Inheriting2.h(Inheriting2.java:30)
        at Inheriting2.main(Inheriting2.java:44)
e.val() = 47

Since an exception is just another kind of object, you can continue this process of embellishing the power of your exception classes. Keep in mind, however, that all this dressing up might be lost on the client programmers using your packages, since they might simply look for the exception to be thrown and nothing more. (That’s the way most of the Java library exceptions are used.) If this is the case, it’s possible to create a new exception type with almost no code at all:

//: c10:SimpleException.java
class SimpleException extends Exception {
} ///:~

This relies on the compiler to create the default constructor (which automatically calls the base-class default constructor). Of course, in this case you don’t get a SimpleException(String) constructor, but in practice that isn’t used much.

Exception restrictions

When you override a method, you can throw only the exceptions that have been specified in the base-class version of the method. This is a useful restriction, since it means that code that works with the base class will automatically work with any object derived from the base class (a fundamental OOP concept, of course), including exceptions.

This example demonstrates the kinds of restrictions imposed (at compile-time) for exceptions:

//: c10:StormyInning.java
// Overridden methods may throw only the
// exceptions specified in their base-class
// versions, or exceptions derived from the
// base-class exceptions.

class BaseballException extends Exception {}
class Foul extends BaseballException {}
class Strike extends BaseballException {}

abstract class Inning {
  Inning() throws BaseballException {}
  void event () throws BaseballException {
   // Doesn't actually have to throw anything
  }
  abstract void atBat() throws Strike, Foul;
  void walk() {} // Throws nothing
}

class StormException extends Exception {}
class RainedOut extends StormException {}
class PopFoul extends Foul {}

interface Storm {
  void event() throws RainedOut;
  void rainHard() throws RainedOut;
}

public class StormyInning extends Inning
    implements Storm {
  // OK to add new exceptions for
  // constructors, but you must deal
  // with the base constructor exceptions:
  StormyInning() throws RainedOut,
    BaseballException {}
  StormyInning(String s) throws Foul,
    BaseballException {}
  // Regular methods must conform to base class:
//! void walk() throws PopFoul {} //Compile error
  // Interface CANNOT add exceptions to existing
  // methods from the base class:
//! public void event() throws RainedOut {}
  // If the method doesn't already exist in the
  // base class, the exception is OK:
  public void rainHard() throws RainedOut {}
  // You can choose to not throw any exceptions,
  // even if base version does:
  public void event() {}
  // Overridden methods can throw
  // inherited exceptions:
  void atBat() throws PopFoul {}
  public static void main(String[] args) {
    try {
      StormyInning si = new StormyInning();
      si.atBat();
    } catch(PopFoul e) {
    } catch(RainedOut e) {
    } catch(BaseballException e) {}
    // Strike not thrown in derived version.
    try {
      // What happens if you upcast?
      Inning i = new StormyInning();
      i.atBat();
      // You must catch the exceptions from the
      // base-class version of the method:
    } catch(Strike e) {
    } catch(Foul e) {
    } catch(RainedOut e) {
    } catch(BaseballException e) {}
  }
} ///:~

In Inning, you can see that both the constructor and the event( ) method say they will throw an exception, but they never do. This is legal because it allows you to force the user to catch any exceptions that might be added in overridden versions of event( ). The same idea holds for abstract methods, as seen in atBat( ).

The interface Storm is interesting because it contains one method (event( )) that is defined in Inning, and one method that isn’t. Both methods throw a new type of exception, RainedOut. When StormyInning extends Inning and implements Storm, you’ll see that the event( ) method in Storm cannot change the exception interface of event( ) in Inning. Again, this makes sense because otherwise you’d never know if you were catching the correct thing when working with the base class. Of course, if a method described in an interface is not in the base class, such as rainHard( ), then there’s no problem if it throws exceptions.

The restriction on exceptions does not apply to constructors. You can see in StormyInning that a constructor can throw anything it wants, regardless of what the base-class constructor throws. However, since a base-class constructor must always be called one way or another (here, the default constructor is called automatically), the derived-class constructor must declare any base-class constructor exceptions in its exception specification. Note that a derived-class constructor cannot catch exceptions thrown by its base-class constructor.

The reason StormyInning.walk( ) will not compile is that it throws an exception, while Inning.walk( ) does not. If this was allowed, then you could write code that called Inning.walk( ) and that didn’t have to handle any exceptions, but then when you substituted an object of a class derived from Inning, exceptions would be thrown so your code would break. By forcing the derived-class methods to conform to the exception specifications of the base-class methods, substitutability of objects is maintained.

The overridden event( ) method shows that a derived-class version of a method may choose not to throw any exceptions, even if the base-class version does. Again, this is fine since it doesn’t break any code that is written assuming the base-class version throws exceptions. Similar logic applies to atBat( ), which throws PopFoul, an exception that is derived from Foul thrown by the base-class version of atBat( ). This way, if someone writes code that works with Inning and calls atBat( ), they must catch the Foul exception. Since PopFoul is derived from Foul, the exception handler will also catch PopFoul.

The last point of interest is in main( ). Here you can see that if you’re dealing with exactly a StormyInning object, the compiler forces you to catch only the exceptions that are specific to that class, but if you upcast to the base type then the compiler (correctly) forces you to catch the exceptions for the base type. All these constraints produce much more robust exception handling code [52] .

It’s useful to realize that although exception specifications are enforced by the compiler during inheritance, the exception specifications are not part of the type of a method, which is comprised of only the method name and argument types. Therefore, you cannot overload methods based on exception specifications. In addition, just because an exception specification exists in a base-class version of a method doesn’t mean that it must exist in the derived-class version of the method. This is quite different from inheritance rules, where a method in the base class must also exist in the derived class. Put another way, the “exception specification interface” for a particular method may narrow during inheritance and overriding, but it may not widen—this is precisely the opposite of the rule for the class interface during inheritance.

Performing cleanup
with finally

There’s often some piece of code that you want to execute whether or not an exception is thrown within a try block. This usually pertains to some operation other than memory recovery (since that’s taken care of by the garbage collector). To achieve this effect, you use a finally clause [53] at the end of all the exception handlers. The full picture of an exception handling section is thus:

try {
  // The guarded region: Dangerous activities
  // that might throw A, B, or C
} catch (A a1) {
  // Handler for situation A
} catch (B b1) {
  // Handler for situation B
} catch (C c1) {
  // Handler for situation C
} finally {
  // Activities that happen every time
}

To demonstrate that the finally clause always runs, try this program:

//: c10:FinallyWorks.java
// The finally clause is always executed.

public class FinallyWorks {
  static int count = 0;
  public static void main(String[] args) {
    while(true) {
      try {
        // Post-increment is zero first time:
        if(count++ == 0)
          throw new Exception();
        System.out.println("No exception");
      } catch(Exception e) {
        System.out.println("Exception thrown");
      } finally {
        System.out.println("In finally clause");
        if(count == 2) break; // out of "while"
      }
    }
  }
} ///:~

This program also gives a hint for how you can deal with the fact that exceptions in Java (like exceptions in C++) do not allow you to resume back to where the exception was thrown, as discussed earlier. If you place your try block in a loop, you can establish a condition that must be met before you continue the program. You can also add a static counter or some other device to allow the loop to try several different approaches before giving up. This way you can build a greater level of robustness into your programs.

The output is:

Exception thrown
In finally clause
No exception
In finally clause

Whether an exception is thrown or not, the finally clause is always executed.

What’s finally for?

In a language without garbage collection and without automatic destructor calls [54] , finally is important because it allows the programmer to guarantee the release of memory regardless of what happens in the try block. But Java has garbage collection, so releasing memory is virtually never a problem. Also, it has no destructors to call. So when do you need to use finally in Java?

finally is necessary when you need to set something other than memory back to its original state. This is usually something like an open file or network connection, something you’ve drawn on the screen or even a switch in the outside world, as modeled in the following example:

//: c10:Switch.java
public class Switch {
  boolean state = false;
  boolean read() { return state; }
  void on() { state = true; }
  void off() { state = false; }
} ///:~
//: c10:OnOffSwitch.java
// Why use finally?

public class OnOffSwitch {
  static Switch sw = new Switch();
  public static void main(String[] args) {
    try {
      sw.on();
      // Code that can throw exceptions...
      sw.off();
    } catch(NullPointerException e) {
      System.out.println("NullPointerException");
      sw.off();
    } catch(IllegalArgumentException e) {
      System.out.println("IOException");
      sw.off();
    }
  }
} ///:~

The goal here is to make sure that the switch is off when main( ) is completed, so sw.off( ) is placed at the end of the try block and at the end of each exception handler. But it’s possible that an exception could be thrown that isn’t caught here, so sw.off( ) would be missed. However, with finally you can place the cleanup code from a try block in just one place:

//: c10:WithFinally.java
// Finally Guarantees cleanup.

public class WithFinally {
  static Switch sw = new Switch();
  public static void main(String[] args) {
    try {
      sw.on();
      // Code that can throw exceptions...
    } catch(NullPointerException e) {
      System.out.println("NullPointerException");
    } catch(IllegalArgumentException e) {
      System.out.println("IOException");
    } finally {
      sw.off();
    }
  }
} ///:~

Here the sw.off( ) has been moved to just one place, where it’s guaranteed to run no matter what happens.

Even in cases in which the exception is not caught in the current set of catch clauses, finally will be executed before the exception handling mechanism continues its search for a handler at the next higher level:

//: c10:AlwaysFinally.java
// Finally is always executed.

class Ex extends Exception {}

public class AlwaysFinally {
  public static void main(String[] args) {
    System.out.println(
      "Entering first try block");
    try {
      System.out.println(
        "Entering second try block");
      try {
        throw new Ex();
      } finally {
        System.out.println(
          "finally in 2nd try block");
      }
    } catch(Ex e) {
      System.out.println(
        "Caught Ex in first try block");
    } finally {
      System.out.println(
        "finally in 1st try block");
    }
  }
} ///:~

The output for this program shows you what happens:

Entering first try block
Entering second try block
finally in 2nd try block
Caught Ex in first try block
finally in 1st try block

The finally statement will also be executed in situations in which break and continue statements are involved. Note that, along with the labeled break and labeled continue, finally eliminates the need for a goto statement in Java.

Pitfall: the lost exception

In general, Java’s exception implementation is quite outstanding, but unfortunately there’s a flaw. Although exceptions are an indication of a crisis in your program and should never be ignored, it’s possible for an exception to simply be lost. This happens with a particular configuration using a finally clause:

//: c10:LostMessage.java
// How an exception can be lost.

class VeryImportantException extends Exception {
  public String toString() {
    return "A very important exception!";
  }
}

class HoHumException extends Exception {
  public String toString() {
    return "A trivial exception";
  }
}

public class LostMessage {
  void f() throws VeryImportantException {
    throw new VeryImportantException();
  }
  void dispose() throws HoHumException {
    throw new HoHumException();
  }
  public static void main(String[] args)
      throws Exception {
    LostMessage lm = new LostMessage();
    try {
      lm.f();
    } finally {
      lm.dispose();
    }
  }
} ///:~

The output is:

Exception in thread "main" A trivial exception
    at LostMessage.dispose(LostMessage.java:21)
    at LostMessage.main(LostMessage.java:29)

You can see that there’s no evidence of the VeryImportantException, which is simply replaced by the HoHumException in the finally clause. This is a rather serious pitfall, since it means that an exception can be completely lost, and in a far more subtle and difficult-to-detect fashion than the example above. In contrast, C++ treats the situation in which a second exception is thrown before the first one is handled as a dire programming error. Perhaps a future version of Java will repair the problem.

Constructors

When writing code with exceptions, it’s particularly important that you always ask, “If an exception occurs, will this be properly cleaned up?” Most of the time you’re fairly safe, but in constructors there’s a problem. The constructor puts the object into a safe starting state, but it might perform some operation—such as opening a file—that doesn’t get cleaned up until the user is finished with the object and calls a special cleanup method. If you throw an exception from inside a constructor, these cleanup behaviors might not occur properly. This means that you must be especially diligent while you write your constructor.

Since you’ve just learned about finally, you might think that it is the correct solution. But it’s not quite that simple, because finally performs the cleanup code every time, even in the situations in which you don’t want the cleanup code executed until the cleanup method runs. Thus, if you do perform cleanup in finally, you must set some kind of flag when the constructor finishes normally so that you don’t do anything in the finally block if the flag is set. Because this isn’t particularly elegant (you are coupling your code from one place to another), it’s best if you try to avoid performing this kind of cleanup in finally unless you are forced to.

In the following example, a class called InputFile is created that opens a file and allows you to read it one line (converted into a String) at a time. It uses the classes FileReader and BufferedReader from the Java standard IO library that will be discussed in Chapter 11, but which are simple enough that you probably won’t have any trouble understanding their basic use:

//: c10:Cleanup.java
// Paying attention to exceptions
// in constructors.
import java.io.*;

class InputFile {
  private BufferedReader in;
  InputFile(String fname) throws Exception {
    try {
      in =
        new BufferedReader(
          new FileReader(fname));
      // Other code that might throw exceptions
    } catch(FileNotFoundException e) {
      System.out.println(
        "Could not open " + fname);
      // Wasn't open, so don't close it
      throw e;
    } catch(Exception e) {
      // All other exceptions must close it
      try {
        in.close();
      } catch(IOException e2) {
        System.out.println(
          "in.close() unsuccessful");
      }
      throw e;
    } finally {
      // Don't close it here!!!
    }
  }
  String getLine() {
    String s;
    try {
      s = in.readLine();
    } catch(IOException e) {
      System.out.println(
        "readLine() unsuccessful");
      s = "failed";
    }
    return s;
  }
  void cleanup() {
    try {
      in.close();
    } catch(IOException e2) {
      System.out.println(
        "in.close() unsuccessful");
    }
  }
}

public class Cleanup {
  public static void main(String[] args) {
    try {
      InputFile in =
        new InputFile("Cleanup.java");
      String s;
      int i = 1;
      while((s = in.getLine()) != null)
        System.out.println(""+ i++ + ": " + s);
      in.cleanup();
    } catch(Exception e) {
      System.out.println(
        "Caught in main, e.printStackTrace()");
      e.printStackTrace();
    }
  }
} ///:~

The constructor for InputFile takes a String argument, which is the name of the file you want to open. Inside a try block, it creates a FileReader using the file name. A FileReader isn’t particularly useful until you turn around and use it to create a BufferedReader that you can actually talk to—notice that one of the benefits of InputFile is that it combines these two actions.

If the FileReader constructor is unsuccessful, it throws a FileNotFoundException, which must be caught separately because that’s the one case in which you don’t want to close the file since it wasn’t successfully opened. Any other catch clauses must close the file because it was opened by the time those catch clauses are entered. (Of course, this is trickier if more than one method can throw a FileNotFoundException. In that case, you might want to break things into several try blocks.) The close( ) method might throw an exception so it is tried and caught even though it’s within the block of another catch clause—it’s just another pair of curly braces to the Java compiler. After performing local operations, the exception is rethrown, which is appropriate because this constructor failed, and you wouldn’t want the calling method to assume that the object had been properly created and was valid.

In this example, which doesn’t use the aforementioned flagging technique, the finally clause is definitely not the place to close( ) the file, since that would close it every time the constructor completed. Since we want the file to be open for the useful lifetime of the InputFile object this would not be appropriate.

The getLine( ) method returns a String containing the next line in the file. It calls readLine( ), which can throw an exception, but that exception is caught so getLine( ) doesn’t throw any exceptions. One of the design issues with exceptions is whether to handle an exception completely at this level, to handle it partially and pass the same exception (or a different one) on, or whether to simply pass it on. Passing it on, when appropriate, can certainly simplify coding. The getLine( ) method becomes:

String getLine() throws IOException {
  return in.readLine();
}

But of course, the caller is now responsible for handling any IOException that might arise.

The cleanup( ) method must be called by the user when finished using the InputFile object. This will release the system resources (such as file handles) that are used by the BufferedReader and/or FileReader objects [55] . You don’t want to do this until you’re finished with the InputFile object, at the point you’re going to let it go. You might think of putting such functionality into a finalize( ) method, but as mentioned in Chapter 4 you can’t always be sure that finalize( ) will be called (even if you can be sure that it will be called, you don’t know when). This is one of the downsides to Java—all cleanup other than memory cleanup doesn’t happen automatically, so you must inform the client programmer that they are responsible, and possibly guarantee that cleanup occurs using finalize( ).

In Cleanup.java an InputFile is created to open the same source file that creates the program, the file is read in a line at a time, and line numbers are added. All exceptions are caught generically in main( ), although you could choose greater granularity.

One of the benefits of this example is to show you why exceptions are introduced at this point in the book—you can’t do basic I/O without using exceptions. Exceptions are so integral to programming in Java, especially because the compiler enforces them, that you can accomplish only so much without knowing how to work with them.

Exception matching

When an exception is thrown, the exception handling system looks through the “nearest” handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs.

Matching an exception doesn’t require a perfect match between the exception and its handler. A derived-class object will match a handler for the base class, as shown in this example:

//: c10:Human.java
// Catching exception hierarchies.

class Annoyance extends Exception {}
class Sneeze extends Annoyance {}

public class Human {
  public static void main(String[] args) {
    try {
      throw new Sneeze();
    } catch(Sneeze s) {
      System.out.println("Caught Sneeze");
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    }
  }
} ///:~

The Sneeze exception will be caught by the first catch clause that it matches, which is the first one, of course. However, if you remove the first catch clause, leaving only:

    try {
      throw new Sneeze();
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    }

The code will still work because it’s catching the base class of Sneeze. Put another way, catch(Annoyance e) will catch a Annoyance or any class derived from it. This is useful because if you decide to add more exceptions to a method, if they’re all inherited from the same base class then the client programmer’s code will not need changing, assuming they catch the base class, at the very least.

If you try to “mask” the derived-class exceptions by putting the base-class catch clause first, like this:

    try {
      throw new Sneeze();
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    } catch(Sneeze s) {
      System.out.println("Caught Sneeze");
    }

the compiler will give you an error message, since it sees that the Sneeze catch-clause can never be reached.

Exception guidelines

Use exceptions to:

  1. Fix the problem and call the method that caused the exception again.
  2. Patch things up and continue without retrying the method.
  3. Calculate some alternative result instead of what the method was supposed to produce.
  4. Do whatever you can in the current context and rethrow the same exception to a higher context.
  5. Do whatever you can in the current context and throw a different exception to a higher context.
  6. Terminate the program.
  7. Simplify. If your exception scheme makes things more complicated, then it is painful and annoying to use.
  8. Make your library and program safer. This is a short-term investment (for debugging) and a long-term investment (for application robustness).

Summary

Improved error recovery is one of the most powerful ways that you can increase the robustness of your code. Error recovery is a fundamental concern for every program you write, and it’s especially important in Java, in which one of the primary goals is to create program components for others to use. To create a robust system, each component must be robust.

The goals for exception handling in Java are to simplify the creation of large, reliable programs using less code than currently possible, and with more confidence that your application doesn’t have an unhandled error.

Exceptions are not terribly difficult to learn, and are one of those features that provide immediate and significant benefits to your project. Fortunately, Java enforces all aspects of exceptions so it’s guaranteed that they will be used consistently by both library designer and client programmer."_Toc375545380">"_Toc477589610">

Exercises
  1. Create a class with a main( ) that throws an object of class Exception inside a try block. Give the constructor for Exception a String argument. Catch the exception inside a catch clause and print the String argument. Add a finally clause and print a message to prove you were there.
  2. Create your own exception class using the extends keyword. Write a constructor for this class that takes a String argument and stores it inside the object with a String reference. Write a method that prints out the stored String. Create a try-catch clause to exercise your new exception.
  3. Write a class with a method that throws an exception of the type created in Exercise 2. Try compiling it without an exception specification to see what the compiler says. Add the appropriate exception specification. Try out your class and its exception inside a try-catch clause.
  4. Define an object reference and initialize it to null. Try to call a method through this reference. Now wrap the code in a try-catch clause to catch the exception.
  5. Create a class with two methods, f( ) and g( ). In g( ), throw an exception of a new type that you define. In f( ), call g( ), catch its exception and, in the catch clause, throw a different exception (of a second type that you define). Test your code in main( ).
  6. Create three new types of exceptions. Write a class with a method that throws all three. In main( ), call the method but only use a single catch clause that will catch all three types of exceptions.
  7. Write code to generate and catch an ArrayIndexOutOfBoundsException.
  8. Create your own resumption-like behavior using a while loop that repeats until an exception is no longer thrown.
  9. Create a three-level hierarchy of exceptions. Now create a base-class A with a method that throws an exception at the base of your hierarchy. Inherit B from A and override the method so it throws an exception at level two of your hierarchy. Repeat by inheriting class C from B. In main( ), create a C and upcast it to A, then call the method.
  10. Demonstrate that a derived-class constructor cannot catch exceptions thrown by its base-class constructor.
  11. Show that OnOffSwitch.java can fail by throwing a RuntimeException inside the try block.
  12. Show that WithFinally.java doesn’t fail by throwing a RuntimeException inside the try block.
  13. Modify Exercise 6 by adding a finally clause. Verify your finally clause is executed, even if a NullPointerException is thrown.
  14. Create an example where you use a flag to control whether cleanup code is called, as described in the second paragraph after the heading “Constructors.”
  15. Modify StormyInning.java by adding an UmpireArgument exception type, and methods that throw this exception. Test the modified hierarchy.
  16. Remove the first catch clause in Human.java and verify that the code still compiles and runs properly.
  17. Add a second level of exception loss to LostMessage.java so that the HoHumException is itself replaced by a third exception.
  18. In chapter 5, find the two programs called Assert.java and modify these to throw their own type of exception instead of printing to System.err. This exception should be an inner class that extends RuntimeException.
  19. Add an appropriate set of exceptions to c08:GreenhouseControls.java.

[50] The C programmer can look up the return value of printf( ) for an example of this.

[51] This is a significant improvement over C++ exception handling, which doesn’t catch violations of exception specifications until run time, when it’s not very useful.

[52] ISO C++ added similar constraints that require derived-method exceptions to be the same as, or derived from, the exceptions thrown by the base-class method. This is one case in which C++ is actually able to check exception specifications at compile-time.

[53] C++ exception handling does not have the finally clause because it relies on destructors to accomplish this sort of cleanup.

[54] A destructor is a function that’s always called when an object becomes unused. You always know exactly where and when the destructor gets called. C++ has automatic destructor calls, but Delphi’s Object Pascal versions 1 and 2 do not (which changes the meaning and use of the concept of a destructor for that language).

[55] In C++, a destructor would handle this for you.


11: The Java IO System

11: The Java
IO System

Creating a good input/output (IO) system is one of the more difficult tasks for the language designer.

This is evidenced by the number of different approaches. The challenge seems to be in covering all eventualities. Not only are there different kinds of IO that you want to communicate with (files, the console, network connections), but you need to talk to them in a wide variety of ways (sequential, random-access, binary, character, by lines, by words, etc.).

The Java library designers attacked the problem by creating lots of classes. In fact, there are so many classes for Java’s IO system that it can be intimidating at first (ironically, the Java IO design actually prevents an explosion of classes). There was also a significant change in the IO library after Java 1.0. Instead of simply replacing the old library with a new one, the designers at Sun extended the old library and added the new one alongside it. As a result you can sometimes end up mixing the old and new libraries and creating even more intimidating code.

This chapter will help you understand the variety of IO classes in the standard Java library and how to use them. The first portion of the chapter will introduce the “old” Java 1.0 IO stream library, since there is a significant amount of existing code that uses that library. The remainder of the chapter will introduce the new features in the Java 1.1 IO library. Note that when you compile some of the code in the first part of the chapter with a Java 2 compiler you can get a “deprecated feature” warning message at compile-time. The code still works; the compiler is just suggesting that you use certain new features that are described in the latter part of this chapter. It is valuable, however, to see the difference between the old and new way of doing things and that’s why it was left in—to increase your understanding (and to allow you to read code written for Java 1.0).

Input and output

The Java library classes for IO are divided by input and output, as you can see by looking at the online Java class hierarchy with your Web browser. By inheritance, all classes derived from InputStream have basic methods called read( ) for reading a single byte or array of bytes. Likewise, all classes derived from OutputStream have basic methods called write( ) for writing a single byte or array of bytes. However, you won’t generally use these methods; they exist so more sophisticated classes can use them as they provide a more useful interface. Thus, you’ll rarely create your stream object by using a single class, but instead will layer multiple objects together to provide your desired functionality. The fact that you create more than one object to create a single resulting stream is the primary reason that Java’s stream library is confusing.

It’s helpful to categorize the classes by their functionality. The library designers started by deciding that all classes that had anything to do with input would be inherited from InputStream and all classes that were associated with output would be inherited from OutputStream.

Types of InputStream

InputStream’s job is to represent classes that produce input from different sources. These sources can be (and each has an associated subclass of InputStream):

  1. An array of bytes
  2. A String object
  3. A file
  4. A “pipe,” which works like a physical pipe: you put things in one end and they come out the other
  5. A sequence of other streams, so you can collect them together into a single stream
  6. Other sources, such as an Internet connection. (This will be discussed in a later chapter.)

In addition, the FilterInputStream is also a type of InputStream, to provide a base class for "decorator" classes that attach attributes or useful interfaces to input streams. This is discussed later.

Table 10-1. Types of InputStream

Class | Function | Constructor Arguments
How to use it
ByteArray-InputStream | Allows a buffer in memory to be used as an InputStream | The buffer from which to extract the bytes.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
StringBuffer-InputStream | Converts a String into an InputStream | A String. The underlying implementation actually uses a StringBuffer.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
File-InputStream | For reading information from a file | A String representing the file name, or a File or FileDescriptor object.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.

Piped-InputStream | Produces the data that’s being written to the associated PipedOutput-Stream. Implements the “piping” concept. | PipedOutputStream
As a source of data in multithreading. Connect it to a FilterInputStream object to provide a useful interface.
Sequence-InputStream | Converts two or more InputStream objects into a single InputStream. | Two InputStream objects or an Enumeration for a container of InputStream objects.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
Filter-InputStream | Abstract class which is an interface for decorators that provide useful functionality to the other InputStream classes. See Table 10-3. | See Table 10-3.
See Table 10-3.

Types of OutputStream

This category includes the classes that decide where your output will go: an array of bytes (no String, however; presumably you can create one using the array of bytes), a file, or a “pipe.”

In addition, the FilterOutputStream provides a base class for "decorator" classes that attach attributes or useful interfaces to output streams. This is discussed later.

Table 10-2. Types of OutputStream

Class | Function | Constructor Arguments
How to use it
ByteArray-OutputStream | Creates a buffer in memory. All the data that you send to the stream is placed in this buffer. | Optional initial size of the buffer.
To designate the destination of your data. Connect it to a FilterOutputStream object to provide a useful interface.
File-OutputStream | For sending information to a file. | A String representing the file name, or a File or FileDescriptor object.
To designate the destination of your data. Connect it to a FilterOutputStream object to provide a useful interface.
Piped-OutputStream | Any information you write to this automatically ends up as input for the associated PipedInput-Stream. Implements the “piping” concept. | PipedInputStream
To designate the destination of your data for multithreading. Connect it to a FilterOutputStream object to provide a useful interface.
Filter-OutputStream | Abstract class which is an interface for decorators that provide useful functionality to the other OutputStream classes. See Table 10-4. | See Table 10-4.
See Table 10-4.

Adding attributes
and useful interfaces

The use of layered objects to dynamically and transparently add responsibilities to individual objects is referred to as the decorator pattern. (Patterns [56] are the subject of Thinking in Patterns with Java, downloadable at www.BruceEckel.com.) The decorator pattern specifies that all objects that wrap around your initial object have the same interface, to make the use of the decorators transparent—you send the same message to an object whether it’s been decorated or not. This is the reason for the existence of the “filter” classes in the Java IO library: the abstract “filter” class is the base class for all the decorators. (A decorator must have the same interface as the object it decorates, but the decorator can also extend the interface, which occurs in several of the “filter” classes).

Decorators are often used when subclassing requires a large number of subclasses to support every possible combination needed—so many that subclassing becomes impractical. The Java IO library requires many different combinations of features which is why the decorator pattern is a good approach. There is a drawback to the decorator pattern, however. Decorators give you much more flexibility while you’re writing a program (since you can easily mix and match attributes), but they add complexity to your code. The reason that the Java IO library is awkward to use is that you must create many classes—the “core” IO type plus all the decorators—in order to get the single IO object that you want.

The classes that provide the decorator interface to control a particular InputStream or OutputStream are the FilterInputStream and FilterOutputStream—which don’t have very intuitive names. They are derived, respectively, from InputStream and OutputStream, and they are abstract classes, in theory to provide a common interface for all the different ways you want to talk to a stream. In fact, FilterInputStream and FilterOutputStream simply mimic their base classes, which is the key requirement of the decorator.

Reading from an InputStream
with FilterInputStream

The FilterInputStream classes accomplish two significantly different things. DataInputStream allows you to read different types of primitive data as well as String objects. (All the methods start with “read,” such as readByte( ), readFloat( ), etc.) This, along with its companion DataOutputStream, allows you to move primitive data from one place to another via a stream. These “places” are determined by the classes in Table 10-1. If you’re reading data in blocks and parsing it yourself, you won’t need DataInputStream, but in most other cases you will want to use it to automatically format the data you read.

The remaining classes modify the way an InputStream behaves internally: whether it’s buffered or unbuffered, if it keeps track of the lines it’s reading (allowing you to ask for line numbers or set the line number), and whether you can push back a single character. The last two classes look a lot like support for building a compiler (that is, they were added to support the construction of the Java compiler), so you probably won’t use them in general programming.

You’ll probably need to buffer your input almost every time, regardless of the IO device you’re connecting to, so it would have made more sense for the IO library to make a special case for unbuffered input rather than buffered input.

Table 10-3. Types of FilterInputStream

Class | Function | Constructor Arguments
How to use it
Data-InputStream | Used in concert with DataOutputStream, so you can read primitives (int, char, long, etc.) from a stream in a portable fashion. | InputStream
Contains a full interface to allow you to read primitive types.

Buffered-InputStream | Use this to prevent a physical read every time you want more data. You’re saying “Use a buffer.” | InputStream, with optional buffer size.
This doesn’t provide an interface per se, just a requirement that a buffer be used. Attach an interface object.
LineNumber-InputStream | Keeps track of line numbers in the input stream; you can call getLineNumber( ) and setLineNumber(int). | InputStream
This just adds line numbering, so you’ll probably attach an interface object.
Pushback-InputStream | Has a one byte push-back buffer so that you can push back the last character read. | InputStream
Generally used in the scanner for a compiler and probably included because the Java compiler needed it. You probably won’t use this.

Writing to an OutputStream
with FilterOutputStream

The complement to DataInputStream is DataOutputStream, which formats each of the primitive types and String objects onto a stream in such a way that any DataInputStream, on any machine, can read them. All the methods start with “write,” such as writeByte( ), writeFloat( ), etc.

If you want to do true formatted output, for example, to the console, use a PrintStream. This is the endpoint that allows you to print all of the primitive data types and String objects in a viewable format as opposed to DataOutputStream, whose goal is to put them on a stream in a way that DataInputStream can portably reconstruct them. The System.out static object is a PrintStream.

The two important methods in PrintStream are print( ) and println( ), which are overloaded to print all the various types. The difference between print( ) and println( ) is that the latter adds a newline when it’s done.

BufferedOutputStream is a modifier and tells the stream to use buffering so you don’t get a physical write every time you write to the stream. You’ll probably always want to use this with files, and possibly console IO.

Table 10-4. Types of FilterOutputStream

Class | Function | Constructor Arguments
How to use it
Data-OutputStream | Used in concert with DataInputStream so you can write primitives (int, char, long, etc.) to a stream in a portable fashion. | OutputStream
Contains full interface to allow you to write primitive types.
PrintStream | For producing formatted output. While DataOutputStream handles the storage of data, PrintStream handles display. | OutputStream, with optional boolean indicating that the buffer is flushed with every newline.
Should be the “final” wrapping for your OutputStream object. You’ll probably use this a lot.
Buffered-OutputStream | Use this to prevent a physical write every time you send a piece of data. You’re saying “Use a buffer.” You can call flush( ) to flush the buffer. | OutputStream, with optional buffer size.
This doesn’t provide an interface per se, just a requirement that a buffer is used. Attach an interface object.

Off by itself:
RandomAccessFile

RandomAccessFile is used for files containing records of known size so that you can move from one record to another using seek( ), then read or change the records. The records don’t have to be the same size; you just have to be able to determine how big they are and where they are placed in the file.

At first it’s a little bit hard to believe that RandomAccessFile is not part of the InputStream or OutputStream hierarchy. It has no association with those hierarchies other than that it happens to implement the DataInput and DataOutput interfaces (which are also implemented by DataInputStream and DataOutputStream). It doesn’t even use any of the functionality of the existing InputStream or OutputStream classes—it’s a completely separate class, written from scratch, with all of its own (mostly native) methods. The reason for this may be that RandomAccessFile has essentially different behavior than the other IO types, since you can move forward and backward within a file. In any event, it stands alone, as a direct descendant of Object.

Essentially, a RandomAccessFile works like a DataInputStream pasted together with a DataOutputStream and the methods getFilePointer( ) to find out where you are in the file, seek( ) to move to a new point in the file, and length( ) to determine the maximum size of the file. In addition, the constructors require a second argument (identical to fopen( ) in C) indicating whether you are just randomly reading (“r”) or reading and writing (“rw”). There’s no support for write-only files, which could suggest that RandomAccessFile might have worked well if it were inherited from DataInputStream.

What’s even more frustrating is that you could easily imagine wanting to seek within other types of streams, such as a ByteArrayInputStream, but the seeking methods are available only in RandomAccessFile, which works for files only. BufferedInputStream does allow you to mark( ) a position (whose value is held in a single internal variable) and reset( ) to that position, but this is limited and not too useful.

The File class

The File class has a deceiving name—you might think it refers to a file, but it doesn’t. It can represent either the name of a particular file or the names of a set of files in a directory. If it’s a set of files, you can ask for the set with the list( ) method, and this returns an array of String. It makes sense to return an array rather than one of the flexible container classes because the number of elements is fixed, and if you want a different directory listing you just create a different File object. In fact, “FilePath” would have been a better name. This section shows a complete example of the use of this class, including the associated FilenameFilter interface.

A directory lister

Suppose you’d like to see a directory listing. The File object can be listed in two ways. If you call list( ) with no arguments, you’ll get the full list that the File object contains. However, if you want a restricted list, for example, all of the files with an extension of .java, then you use a “directory filter,” which is a class that tells how to select the File objects for display.

Here’s the code for the example. Note that the result has been effortlessly sorted (alphabetically) using the java.utils.Array.sort( ) method and the AlphabeticComparator defined in Chapter 9:

//: c11:DirList.java
// Displays directory listing.
import java.io.*;
import java.util.*;
import com.bruceeckel.util.*;

public class DirList {
  public static void main(String[] args) {
    try {
      File path = new File(".");
      String[] list;
      if(args.length == 0)
        list = path.list();
      else
        list = path.list(new DirFilter(args[0]));
      Arrays.sort(list,
        new AlphabeticComparator());
      for(int i = 0; i < list.length; i++)
        System.out.println(list[i]);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
}

class DirFilter implements FilenameFilter {
  String afn;
  DirFilter(String afn) { this.afn = afn; }
  public boolean accept(File dir, String name) {
    // Strip path information:
    String f = new File(name).getName();
    return f.indexOf(afn) != -1;
  }
} ///:~

The DirFilter class “implements” the interface FilenameFilter. (Interfaces were covered in Chapter 8.) It’s useful to see how simple the FilenameFilter interface is:

public interface FilenameFilter {
  boolean accept(File dir, String name);
}

It says that all this type of object does is provide a method called accept( ). The whole reason behind the creation of this class is to provide the accept( ) method to the list( ) method so that list( ) can call back accept( ) to determine which file names should be included in the list. Thus, this technique is often referred to as a callback or sometimes a functor (that is, DirFilter is a functor because its only job is to hold a method). Because list( ) takes a FilenameFilter object as its argument, it means that you can pass an object of any class that implements FilenameFilter to choose (even at run-time) how the list( ) method will behave. The purpose of a callback is to provide flexibility in the behavior of code.

DirFilter shows that just because an interface contains only a set of methods, you’re not restricted to writing only those methods. (You must at least provide definitions for all the methods in an interface, however.) In this case, the DirFilter constructor is also created.

The accept( ) method must accept a File object representing the directory that a particular file is found in, and a String containing the name of that file. You might choose to use or ignore either of these arguments, but you will probably at least use the file name. Remember that the list( ) method is calling accept( ) for each of the file names in the directory object to see which one should be included—this is indicated by the boolean result returned by accept( ).

To make sure that what you’re working with is only the name and contains no path information, all you have to do is take the String object and create a File object out of it, then call getName( ) which strips away all the path information (in a platform-independent way). Then accept( ) uses the String class indexOf( ) method to see if the search string afn appears anywhere in the name of the file. If afn is found within the string, the return value is the starting index of afn, but if it’s not found the return value is -1. Keep in mind that this is a simple string search and does not have regular expression “wildcard” matching such as “fo?.b?r*” which is much more difficult to implement.

The list( ) method returns an array. You can query this array for its length and then move through it selecting the array elements. This ability to easily pass an array in and out of a method is a tremendous improvement over the behavior of C and C++.

Anonymous inner classes

This example is ideal for rewriting using an anonymous inner class (described in Chapter 8). As a first cut, a method filter( ) is created that returns a reference to a FilenameFilter:

//: c11:DirList2.java
// Uses anonymous inner classes.
import java.io.*;
import java.util.*;
import com.bruceeckel.util.*;

public class DirList2 {
  public static FilenameFilter
  filter(final String afn) {
    // Creation of anonymous inner class:
    return new FilenameFilter() {
      String fn = afn;
      public boolean accept(File dir, String n) {
        // Strip path information:
        String f = new File(n).getName();
        return f.indexOf(fn) != -1;
      }
    }; // End of anonymous inner class
  }
  public static void main(String[] args) {
    try {
      File path = new File(".");
      String[] list;
      if(args.length == 0)
        list = path.list();
      else
        list = path.list(filter(args[0]));
      Arrays.sort(list,
        new AlphabeticComparator());
      for(int i = 0; i < list.length; i++)
        System.out.println(list[i]);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

Note that the argument to filter( ) must be final. This is required by the anonymous inner class so that it can use an object from outside its scope.

This design is an improvement because the FilenameFilter class is now tightly bound to DirList2. However, you can take this approach one step further and define the anonymous inner class as an argument to list( ), in which case it’s even smaller:

//: c11:DirList3.java
// Building the anonymous inner class "in-place."
import java.io.*;
import java.util.*;
import com.bruceeckel.util.*;

public class DirList3 {
  public static void main(final String[] args) {
    try {
      File path = new File(".");
      String[] list;
      if(args.length == 0)
        list = path.list();
      else
        list = path.list(
          new FilenameFilter() {
            public boolean
            accept(File dir, String n) {
              String f = new File(n).getName();
              return f.indexOf(args[0]) != -1;
            }
          });
      Arrays.sort(list,
        new AlphabeticComparator());
      for(int i = 0; i < list.length; i++)
        System.out.println(list[i]);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

The argument to main( ) is now final, since the anonymous inner class uses args[0] directly.

This shows you how anonymous inner classes allow the creation of quick-and-dirty classes to solve problems. Since everything in Java revolves around classes, this can be a useful coding technique. One benefit is that it keeps the code that solves a particular problem isolated together in one spot. On the other hand, it is not always as easy to read, so you must use it judiciously.

Checking for and creating directories

The File class is more than just a representation for an existing directory path, file, or group of files. You can also use a File object to create a new directory or an entire directory path if it doesn’t exist. You can also look at the characteristics of files (size, last modification date, read/write), see whether a File object represents a file or a directory, and delete a file. This program shows the remaining methods available with the File class:

//: c11:MakeDirectories.java
// Demonstrates the use of the File class to
// create directories and manipulate files.
import java.io.*;

public class MakeDirectories {
  private final static String usage =
    "Usage:MakeDirectories path1 ...\n" +
    "Creates each path\n" +
    "Usage:MakeDirectories -d path1 ...\n" +
    "Deletes each path\n" +
    "Usage:MakeDirectories -r path1 path2\n" +
    "Renames from path1 to path2\n";
  private static void usage() {
    System.err.println(usage);
    System.exit(1);
  }
  private static void fileData(File f) {
    System.out.println(
      "Absolute path: " + f.getAbsolutePath() +
      "\n Can read: " + f.canRead() +
      "\n Can write: " + f.canWrite() +
      "\n getName: " + f.getName() +
      "\n getParent: " + f.getParent() +
      "\n getPath: " + f.getPath() +
      "\n length: " + f.length() +
      "\n lastModified: " + f.lastModified());
    if(f.isFile())
      System.out.println("it's a file");
    else if(f.isDirectory())
      System.out.println("it's a directory");
  }
  public static void main(String[] args) {
    if(args.length < 1) usage();
    if(args[0].equals("-r")) {
      if(args.length != 3) usage();
      File
        old = new File(args[1]),
        rname = new File(args[2]);
      old.renameTo(rname);
      fileData(old);
      fileData(rname);
      return; // Exit main
    }
    int count = 0;
    boolean del = false;
    if(args[0].equals("-d")) {
      count++;
      del = true;
    }
    for( ; count < args.length; count++) {
      File f = new File(args[count]);
      if(f.exists()) {
        System.out.println(f + " exists");
        if(del) {
          System.out.println("deleting..." + f);
          f.delete();
        }
      }
      else { // Doesn't exist
        if(!del) {
          f.mkdirs();
          System.out.println("created " + f);
        }
      }
      fileData(f);
    }
  }
} ///:~

In fileData( ) you can see the various file investigation methods put to use to display information about the file or directory path.

The first method that’s exercised by main( ) is renameTo( ), which allows you to rename (or move) a file to an entirely new path represented by the argument, which is another File object. This also works with directories of any length.

If you experiment with the above program, you’ll find that you can make a directory path of any complexity because mkdirs( ) will do all the work for you. In Java 1.0, the -d flag reports that the directory is deleted but it’s still there; in Java 1.1 the directory is actually deleted.

Typical uses of IO streams

Although there are a lot of IO stream classes in the library that can be combined in many different ways, there are just a few ways that you’ll probably end up using them. However, they require attention to get the correct combinations. The following rather long example shows the creation and use of typical IO configurations so you can use it as a reference when writing your own code. Note that each configuration begins with a commented number and title that corresponds to the heading for the appropriate explanation that follows in the text.

//: c11:IOStreamDemo.java
// Typical IO stream configurations.
import java.io.*;
import com.bruceeckel.tools.*;

public class IOStreamDemo {
  public static void main(String[] args) {
    try {
      // 1. Buffered input file
      DataInputStream in =
        new DataInputStream(
          new BufferedInputStream(
            new FileInputStream(args[0])));
      String s, s2 = new String();
      while((s = in.readLine())!= null)
        s2 += s + "\n";
      in.close();

      // 2. Input from memory
      StringBufferInputStream in2 =
          new StringBufferInputStream(s2);
      int c;
      while((c = in2.read()) != -1)
        System.out.print((char)c);

      // 3. Formatted memory input
      try {
        DataInputStream in3 =
          new DataInputStream(
            new StringBufferInputStream(s2));
        while(true)
          System.out.print((char)in3.readByte());
      } catch(EOFException e) {
        System.out.println(
          "End of stream encountered");
      }

      // 4. Line numbering & file output
      try {
        LineNumberInputStream li =
          new LineNumberInputStream(
            new StringBufferInputStream(s2));
        DataInputStream in4 =
          new DataInputStream(li);
        PrintStream out1 =
          new PrintStream(
            new BufferedOutputStream(
              new FileOutputStream(
                "IODemo.out")));
        while((s = in4.readLine()) != null )
          out1.println(
            "Line " + li.getLineNumber() + s);
        out1.close(); // finalize() not reliable!
      } catch(EOFException e) {
        System.out.println(
          "End of stream encountered");
      }

      // 5. Storing & recovering data
      try {
        DataOutputStream out2 =
          new DataOutputStream(
            new BufferedOutputStream(
              new FileOutputStream("Data.txt")));
        out2.writeBytes(
          "Here's the value of pi: \n");
        out2.writeDouble(3.14159);
        out2.close();
        DataInputStream in5 =
          new DataInputStream(
            new BufferedInputStream(
              new FileInputStream("Data.txt")));
        System.out.println(in5.readLine());
        System.out.println(in5.readDouble());
      } catch(EOFException e) {
        System.out.println(
          "End of stream encountered");
      }

      // 6. Reading/writing random access files
      RandomAccessFile rf =
        new RandomAccessFile("rtest.dat", "rw");
      for(int i = 0; i < 10; i++)
        rf.writeDouble(i*1.414);
      rf.close();

      rf =
        new RandomAccessFile("rtest.dat", "rw");
      rf.seek(5*8);
      rf.writeDouble(47.0001);
      rf.close();

      rf =
        new RandomAccessFile("rtest.dat", "r");
      for(int i = 0; i < 10; i++)
        System.out.println(
          "Value " + i + ": " +
          rf.readDouble());
      rf.close();

      // 7. File input shorthand
      InFile in6 = new InFile(args[0]);
      System.out.println(
        "First line in file: " +
        in6.readLine());
        in6.close();

      // 8. Formatted file output shorthand
      PrintFile out3 = new PrintFile("Data2.txt");
      out3.print("Test of PrintFile");
      out3.close();

      // 9. Data file output shorthand
      OutFile out4 = new OutFile("Data3.txt");
      out4.writeBytes("Test of outDataFile\n\r");
      out4.writeChars("Test of outDataFile\n\r");
      out4.close();

    } catch(FileNotFoundException e) {
      System.out.println(
        "File Not Found:" + args[0]);
    } catch(IOException e) {
      System.out.println("IO Exception");
    }
  }
} ///:~

Input streams

Of course, one common thing you’ll want to do is print formatted output to the console, but that’s already been simplified in the package com.bruceeckel.tools created in Chapter 5.

Parts 1 through 4 demonstrate the creation and use of input streams (although part 4 also shows the simple use of an output stream as a testing tool).

1. Buffered input file

To open a file for input, you use a FileInputStream with a String or a File object as the file name. For speed, you’ll want that file to be buffered so you give the resulting reference to the constructor for a BufferedInputStream. To read input in a formatted fashion, you give that resulting reference to the constructor for a DataInputStream, which is your final object and the interface you read from.

In this example, only the readLine( ) method is used, but of course any of the DataInputStream methods are available. When you reach the end of the file, readLine( ) returns null so that is used to break out of the while loop.

The String s2 is used to accumulate the entire contents of the file (including newlines that must be added since readLine( ) strips them off). s2 is then used in the later portions of this program. Finally, close( ) is called to close the file. Technically, close( ) will be called when finalize( ) is run, and this is supposed to happen (whether or not garbage collection occurs) as the program exits. However, Java 1.0 has a rather important bug, so this doesn’t happen. In Java 1.1 you must explicitly call System.runFinalizersOnExit(true) to guarantee that finalize( ) will be called for every object in the system. The safest approach is to explicitly call close( ) for files.

2. Input from memory

This piece takes the String s2 that now contains the entire contents of the file and uses it to create a StringBufferInputStream. (A String, not a StringBuffer, is required as the constructor argument.) Then read( ) is used to read each character one at a time and send it out to the console. Note that read( ) returns the next byte as an int and thus it must be cast to a char to print properly.

3. Formatted memory input

The interface for StringBufferInputStream is limited, so you usually enhance it by wrapping it inside a DataInputStream. However, if you choose to read the characters out a byte at a time using readByte( ), any value is valid so the return value cannot be used to detect the end of input. Instead, you can use the available( ) method to find out how many more characters are available. Here’s an example that shows how to read a file one byte at a time:

//: c11:TestEOF.java
// Testing for the end of file
// while reading a byte at a time.
import java.io.*;

public class TestEOF {
  public static void main(String[] args) {
    try {
      DataInputStream in =
        new DataInputStream(
         new BufferedInputStream(
          new FileInputStream("TestEof.java")));
      while(in.available() != 0)
        System.out.print((char)in.readByte());
    } catch (IOException e) {
      System.err.println("IOException");
    }
  }
} ///:~

Note that available( ) works differently depending on what sort of medium you’re reading from—it’s literally “the number of bytes that can be read without blocking.” With a file this means the whole file, but with a different kind of stream this might not be true, so use it thoughtfully.

You could also detect the end of input in cases like these by catching an exception. However, the use of exceptions for control flow is considered a misuse of that feature.

4. Line numbering and file output

This example shows the use of the LineNumberInputStream to keep track of the input line numbers. Here, you cannot simply gang all the constructors together, since you have to keep a reference to the LineNumberInputStream. (Note that this is not an inheritance situation, so you cannot simply cast in4 to a LineNumberInputStream.) Thus, li holds the reference to the LineNumberInputStream, which is then used to create a DataInputStream for easy reading.

This example also shows how to write formatted data to a file. First, a FileOutputStream is created to connect to the file. For efficiency, this is made a BufferedOutputStream, which is what you’ll virtually always want to do, but you’re forced to do it explicitly. Then for the formatting it’s turned into a PrintStream. The data file created this way is readable as an ordinary text file.

One of the methods that indicates when a DataInputStream is exhausted is readLine( ), which returns null when there are no more strings to read. Each line is printed to the file along with its line number, which is acquired through li.

You’ll see an explicit close( ) for out1, which would make sense if the program were to turn around and read the same file again. However, this program ends without ever looking at the file IODemo.out. As mentioned before, if you don’t call close( ) for all your output files, you might discover that the buffers don’t get flushed so they’re incomplete.

Output streams

The two primary kinds of output streams are separated by the way they write data: one writes it for human consumption, and the other writes it to be re-acquired by a DataInputStream. The RandomAccessFile stands alone, although its data format is compatible with the DataInputStream and DataOutputStream.

5. Storing and recovering data

A PrintStream formats data so it’s readable by a human. To output data so that it can be recovered by another stream, you use a DataOutputStream to write the data and a DataInputStream to recover the data. Of course, these streams could be anything, but here a file is used, buffered for both reading and writing.

Note that the character string is written using writeBytes( ) and not writeChars( ). If you use the latter, you’ll be writing the 16-bit Unicode characters. Since there is no complementary “readChars” method in DataInputStream, you’re stuck pulling these characters off one at a time with readChar( ). So for ASCII, it’s easier to write the characters as bytes followed by a newline; then use readLine( ) to read back the bytes as a regular ASCII line.

The writeDouble( ) stores the double number to the stream and the complementary readDouble( ) recovers it. But for any of the reading methods to work correctly, you must know the exact placement of the data item in the stream, since it would be equally possible to read the stored double as a simple sequence of bytes, or as a char, etc. So you must either have a fixed format for the data in the file or extra information must be stored in the file that you parse to determine where the data is located.

6. Reading and writing random access files

As previously noted, the RandomAccessFile is almost totally isolated from the rest of the IO hierarchy, save for the fact that it implements the DataInput and DataOutput interfaces. So you cannot combine it with any of the aspects of the InputStream and OutputStream subclasses. Even though it might make sense to treat a ByteArrayInputStream as a random access element, you can use RandomAccessFile to only open a file. You must assume a RandomAccessFile is properly buffered since you cannot add that.

The one option you have is in the second constructor argument: you can open a RandomAccessFile to read (“r”) or read and write (“rw”).

Using a RandomAccessFile is like using a combined DataInputStream and DataOutputStream (because it implements the equivalent interfaces). In addition, you can see that seek( ) is used to move about in the file and change one of the values.

Shorthand for file manipulation

Since there are certain canonical forms that you’ll be using regularly with files, you may wonder why you have to do all of that typing—this is one of the drawbacks of the decorator pattern. This portion shows the creation and use of shorthand versions of typical file reading and writing configurations. These shorthands are placed in the package com.bruceeckel.tools that was begun in Chapter 5 (See page 249). To add each class to the library, simply place it in the appropriate directory and add the package statement.

7. File input shorthand

The creation of an object that reads a file from a buffered DataInputStream can be encapsulated into a class called InFile:

//: com:bruceeckel:tools:InFile.java
// Shorthand class for opening an input file.
package com.bruceeckel.tools;
import java.io.*;

public class InFile extends DataInputStream {
  public InFile(String filename)
    throws FileNotFoundException {
    super(
      new BufferedInputStream(
        new FileInputStream(filename)));
  }
  public InFile(File file)
    throws FileNotFoundException {
    this(file.getPath());
  }
} ///:~

Both the String versions of the constructor and the File versions are included, to parallel the creation of a FileInputStream.

Now you can reduce your chances of repetitive stress syndrome while creating files, as seen in the example.

8. Formatted file output shorthand

The same kind of approach can be taken to create a PrintStream that writes to a buffered file. Here’s the extension to com.bruceeckel.tools:

//: com:bruceeckel:tools:PrintFile.java
// Shorthand class for opening an output file
// for human-readable output.
package com.bruceeckel.tools;
import java.io.*;

public class PrintFile extends PrintStream {
  public PrintFile(String filename)
    throws IOException {
    super(
      new BufferedOutputStream(
        new FileOutputStream(filename)));
  }
  public PrintFile(File file)
    throws IOException {
    this(file.getPath());
  }
} ///:~

Note that it is not possible for a constructor to catch an exception that’s thrown by a base-class constructor.

9. Data file output shorthand

Finally, the same kind of shorthand can create a buffered output file for data storage (as opposed to human-readable storage):

//: com:bruceeckel:tools:OutFile.java
// Shorthand class for opening an output file
// for data storage.
package com.bruceeckel.tools;
import java.io.*;

public class OutFile extends DataOutputStream {
  public OutFile(String filename)
    throws IOException {
    super(
      new BufferedOutputStream(
        new FileOutputStream(filename)));
  }
  public OutFile(File file)
    throws IOException {
    this(file.getPath());
  }
} ///:~

It is curious (and unfortunate) that the Java library designers didn’t think to provide these conveniences as part of their standard.

Reading from standard input

Following the approach pioneered in Unix of “standard input,” “standard output,” and “standard error output,” Java has System.in, System.out, and System.err. Throughout this book you’ve seen how to write to standard output using System.out, which is already prewrapped as a PrintStream object. System.err is likewise a PrintStream, but System.in is a raw InputStream, with no wrapping. This means that while you can use System.out and System.err right away, System.in must be wrapped before you can read from it.

Typically, you’ll want to read input a line at a time using readLine( ), so you’ll want to wrap System.in in a DataInputStream. This is the “old” Java 1.0 way to do line input. A bit later in the chapter you’ll see the Java 1.1 solution. Here’s an example that simply echoes each line that you type in:

//: c11:Echo.java
// How to read from standard input.
import java.io.*;

public class Echo {
  public static void main(String[] args) {
    DataInputStream in =
      new DataInputStream(
        new BufferedInputStream(System.in));
    String s;
    try {
      while((s = in.readLine()).length() != 0)
        System.out.println(s);
      // An empty line terminates the program
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
} ///:~

The reason for the try block is that readLine( ) can throw an IOException. Note that System.in should also be buffered, as with most streams.

It’s a bit inconvenient that you’re forced to wrap System.in in a DataInputStream in each program, but perhaps it was designed this way to allow maximum flexibility.

Piped streams

The PipedInputStream and PipedOutputStream have been mentioned only briefly in this chapter. This is not to suggest that they aren’t useful, but their value is not apparent until you begin to understand multithreading, since the piped streams are used to communicate between threads. This is covered along with an example in Chapter 14.

StreamTokenizer

Although StreamTokenizer is not derived from InputStream or OutputStream, it works only with InputStream objects, so it rightfully belongs in the IO portion of the library.

The StreamTokenizer class is used to break any InputStream into a sequence of “tokens,” which are bits of text delimited by whatever you choose. For example, your tokens could be words, and then they would be delimited by white space and punctuation.

Consider a program to count the occurrence of words in a text file:

//: c11:SortedWordCount.java
// Counts words from a file, outputs
// results in sorted form.
import java.io.*;
import java.util.*;

class Counter {
  private int i = 1;
  int read() { return i; }
  void increment() { i++; }
}

public class SortedWordCount {
  private FileReader file;
  private StreamTokenizer st;
  // A TreeMap keeps keys in sorted order:
  private TreeMap counts = new TreeMap();
  SortedWordCount(String filename)
    throws FileNotFoundException {
    try {
      file = new FileReader(filename);
      st = new StreamTokenizer(
        new BufferedReader(file));
      st.ordinaryChar('.');
      st.ordinaryChar('-');
    } catch(FileNotFoundException e) {
      System.out.println(
        "Could not open " + filename);
      throw e;
    }
  }
  void cleanup() {
    try {
      file.close();
    } catch(IOException e) {
      System.out.println(
        "file.close() unsuccessful");
    }
  }
  void countWords() {
    try {
      while(st.nextToken() !=
        StreamTokenizer.TT_EOF) {
        String s;
        switch(st.ttype) {
          case StreamTokenizer.TT_EOL:
            s = new String("EOL");
            break;
          case StreamTokenizer.TT_NUMBER:
            s = Double.toString(st.nval);
            break;
          case StreamTokenizer.TT_WORD:
            s = st.sval; // Already a String
            break;
          default: // single character in ttype
            s = String.valueOf((char)st.ttype);
        }
        if(counts.containsKey(s))
          ((Counter)counts.get(s)).increment();
        else
          counts.put(s, new Counter());
      }
    } catch(IOException e) {
      System.out.println(
        "st.nextToken() unsuccessful");
    }
  }
  Collection values() {
    return counts.values();
  }
  Set keySet() { return counts.keySet(); }
  Counter getCounter(String s) {
    return (Counter)counts.get(s);
  }
  public static void main(String[] args) {
    try {
      SortedWordCount wc =
        new SortedWordCount(args[0]);
      wc.countWords();
      Iterator keys = wc.keySet().iterator();
      while(keys.hasNext()) {
        String key = (String)keys.next();
        System.out.println(key + ": "
                 + wc.getCounter(key).read());
      }
      wc.cleanup();
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

It makes sense to present these in a sorted form, which is easy enough to do with by storing the data in a TreeMap, which automatically organizes its keys in sorted order (see Chapter 9). When you get a set of keys using keySet( ), they will also be in sorted order.

To open the file, a FileInputStream is used, and to turn the file into words a StreamTokenizer is created from the FileInputStream. In StreamTokenizer, there is a default list of separators, and you can add more with a set of methods. Here, ordinaryChar( ) is used to say “This character has no significance that I’m interested in,” so the parser doesn’t include it as part of any of the words that it creates. For example, saying st.ordinaryChar('.') means that periods will not be included as parts of the words that are parsed. You can find more information in the online documentation that comes with Java.

In countWords( ), the tokens are pulled one at a time from the stream, and the ttype information is used to determine what to do with each token, since a token can be an end-of-line, a number, a string, or a single character.

Once a token is found, the TreeMap counts is queried to see if it already contains the token as a key. If it does, the corresponding Counter object is incremented to indicate that another instance of this word has been found. If not, a new Counter is created—since the Counter constructor initializes its value to one, this also acts to count the word.

SortedWordCount is not a type of TreeMap, so it wasn’t inherited. It performs a specific type of functionality, so even though the keys( ) and values( ) methods must be re-exposed, that still doesn’t mean that inheritance should be used since a number of TreeMap methods are inappropriate here. In addition, other methods like getCounter( ), which get the Counter for a particular String, and sortedKeys( ), which produces an Iterator, finish the change in the shape of SortedWordCount’s interface.

In main( ) you can see the use of a SortedWordCount to open and count the words in a file—it just takes two lines of code. Then an Iterator to a sorted list of keys (words) is extracted, and this is used to pull out each key and associated Count. Note that the call to cleanup( ) is necessary to ensure that the file is closed.

StringTokenizer

Although it isn’t part of the IO library, the StringTokenizer has sufficiently similar functionality to StreamTokenizer that it will be described here.

The StringTokenizer returns the tokens within a string one at a time. These tokens are consecutive characters delimited by tabs, spaces, and newlines. Thus, the tokens of the string “Where is my cat?” are “Where”, “is”, “my”, and “cat?” Like the StreamTokenizer, you can tell the StringTokenizer to break up the input in any way that you want, but with StringTokenizer you do this by passing a second argument to the constructor, which is a String of the delimiters you wish to use. In general, if you need more sophistication, use a StreamTokenizer.

You ask a StringTokenizer object for the next token in the string using the nextToken( ) method, which either returns the token or an empty string to indicate that no tokens remain.

As an example, the following program performs a limited analysis of a sentence, looking for key phrase sequences to indicate whether happiness or sadness is implied.

//: c11:AnalyzeSentence.java
// Look for particular sequences
// within sentences.
import java.util.*;

public class AnalyzeSentence {
  public static void main(String[] args) {
    analyze("I am happy about this");
    analyze("I am not happy about this");
    analyze("I am not! I am happy");
    analyze("I am sad about this");
    analyze("I am not sad about this");
    analyze("I am not! I am sad");
    analyze("Are you happy about this?");
    analyze("Are you sad about this?");
    analyze("It's you! I am happy");
    analyze("It's you! I am sad");
  }
  static StringTokenizer st;
  static void analyze(String s) {
    prt("\nnew sentence >> " + s);
    boolean sad = false;
    st = new StringTokenizer(s);
    while (st.hasMoreTokens()) {
      String token = next();
      // Look until you find one of the
      // two starting tokens:
      if(!token.equals("I") &&
         !token.equals("Are"))
        continue; // Top of while loop
      if(token.equals("I")) {
        String tk2 = next();
        if(!tk2.equals("am")) // Must be after I
          break; // Out of while loop
        else {
          String tk3 = next();
          if(tk3.equals("sad")) {
            sad = true;
            break; // Out of while loop
          }
          if (tk3.equals("not")) {
            String tk4 = next();
            if(tk4.equals("sad"))
              break; // Leave sad false
            if(tk4.equals("happy")) {
              sad = true;
              break;
            }
          }
        }
      }
      if(token.equals("Are")) {
        String tk2 = next();
        if(!tk2.equals("you"))
          break; // Must be after Are
        String tk3 = next();
        if(tk3.equals("sad"))
          sad = true;
        break; // Out of while loop
      }
    }
    if(sad) prt("Sad detected");
  }
  static String next() {
    if(st.hasMoreTokens()) {
      String s = st.nextToken();
      prt(s);
      return s;
    }
    else
      return "";
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~

For each string being analyzed, a while loop is entered and tokens are pulled off the string. Notice the first if statement, which says to continue (go back to the beginning of the loop and start again) if the token is neither an “I” nor an “Are.” This means that it will get tokens until an “I” or an “Are” is found. You might think to use the == instead of the equals( ) method, but that won’t work correctly, since == compares reference values while equals( ) compares contents.

The logic of the rest of the analyze( ) method is that the pattern that’s being searched for is “I am sad,” “I am not happy,” or “Are you sad?” Without the break statement, the code for this would be even messier than it is. You should be aware that a typical parser (this is a primitive example of one) normally has a table of these tokens and a piece of code that moves through the states in the table as new tokens are read.

You should think of the StringTokenizer only as shorthand for a simple and specific kind of StreamTokenizer. However, if you have a String that you want to tokenize and StringTokenizer is too limited, all you have to do is turn it into a stream with StringBufferInputStream and then use that to create a much more powerful StreamTokenizer.

Java 1.1 IO streams

At this point you might be scratching your head, wondering if there is another design for IO streams that could require more typing. Could someone have come up with an odder design?” Prepare yourself: Java 1.1 makes some significant modifications to the IO stream library. When you see the Reader and Writer classes your first thought (like mine) might be that these were meant to replace the InputStream and OutputStream classes. But that’s not the case. Although some aspects of the original streams library are deprecated (if you use them you will receive a warning from the compiler), the old streams have been left in for backward compatibility and:

  1. New classes have been put into the old hierarchy, so it’s obvious that Sun is not abandoning the old streams.
  2. There are times when you’re supposed to use classes in the old hierarchy in combination with classes in the new hierarchy and to accomplish this there are “bridge” classes: InputStreamReader converts an InputStream to a Reader and OutputStreamWriter converts an OutputStream to a Writer.

As a result there are situations in which you have more layers of wrapping with the new IO stream library than with the old. Again, this is a drawback of the decorator pattern—the price you pay for added flexibility.

The most important reason for adding the Reader and Writer hierarchies in Java 1.1 is for internationalization. The old IO stream hierarchy supports only 8-bit byte streams and doesn’t handle the 16-bit Unicode characters well. Since Unicode is used for internationalization (and Java’s native char is 16-bit Unicode), the Reader and Writer hierarchies were added to support Unicode in all IO operations. In addition, the new libraries are designed for faster operations than the old.

As is the practice in this book, I will attempt to provide an overview of the classes but assume that you will use online documentation to determine all the details, such as the exhaustive list of methods.

Sources and sinks of data

Almost all of the Java 1.0 IO stream classes have corresponding Java 1.1 classes to provide native Unicode manipulation. It would be easiest to say “Always use the new classes, never use the old ones,” but things are not that simple. Sometimes you are forced into using the Java 1.0 IO stream classes because of the library design; in particular, the java.util.zip libraries are new additions to the old stream library and they rely on old stream components. So the most sensible approach to take is to try to use the Reader and Writer classes whenever you can, and you’ll discover the situations when you have to drop back into the old libraries because your code won’t compile.

Here is a table that shows the correspondence between the sources and sinks of information (that is, where the data physically comes from or goes to) in the old and new libraries.

Sources & Sinks: Java 1.0 class | Corresponding Java 1.1 class
InputStream | Reader converter: InputStreamReader
OutputStream | Writer converter: OutputStreamWriter
FileInputStream | FileReader
FileOutputStream | FileWriter
StringBufferInputStream | StringReader
(no corresponding class) | StringWriter
ByteArrayInputStream | CharArrayReader
ByteArrayOutputStream | CharArrayWriter
PipedInputStream | PipedReader
PipedOutputStream | PipedWriter

In general, you’ll find that the interfaces in the old library components and the new ones are similar if not identical.

Modifying stream behavior

In Java 1.0, streams were adapted for particular needs using “decorator” subclasses of FilterInputStream and FilterOutputStream. Java 1.1 IO streams continues the use of this idea, but the model of deriving all of the decorators from the same “filter” base class is not followed. This can make it a bit confusing if you’re trying to understand it by looking at the class hierarchy.

In the following table, the correspondence is a rougher approximation than in the previous table. The difference is because of the class organization: while BufferedOutputStream is a subclass of FilterOutputStream, BufferedWriter is not a subclass of FilterWriter (which, even though it is abstract, has no subclasses and so appears to have been put in either as a placeholder or simply so you wouldn’t wonder where it was). However, the interfaces to the classes are quite a close match and it’s apparent that you’re supposed to use the new versions instead of the old whenever possible (that is, except in cases where you’re forced to produce a Stream instead of a Reader or Writer).

Filters: Java 1.0 class | Corresponding Java 1.1 class
FilterInputStream | FilterReader
FilterOutputStream | FilterWriter (abstract class with no subclasses)
BufferedInputStream | BufferedReader (also has readLine( ))
BufferedOutputStream | BufferedWriter
DataInputStream | use DataInputStream (Except when you need to use readLine( ), when you should use a BufferedReader)
PrintStream | PrintWriter
LineNumberInputStream | LineNumberReader
StreamTokenizer | StreamTokenizer (use constructor that takes a Reader instead)
PushBackInputStream | PushBackReader

There’s one direction that’s quite clear: Whenever you want to use readLine( ), you shouldn’t do it with a DataInputStream any more (this is met with a deprecation message at compile-time), but instead use a BufferedReader. Other than this, DataInputStream is still a “preferred” member of the Java 1.1 IO library.

To make the transition to using a PrintWriter easier, it has constructors that take any OutputStream object. However, PrintWriter has no more support for formatting than PrintStream does; the interfaces are virtually the same.

Unchanged Classes

Apparently, the Java library designers felt that they got some of the classes right the first time so there were no changes to these and you can go on using them as they are:

Java 1.0 classes without corresponding Java 1.1 classes

DataOutputStream

File

RandomAccessFile

SequenceInputStream

The DataOutputStream, in particular, is used without change, so for storing and retrieving data in a transportable format you’re forced to stay in the InputStream and OutputStream hierarchies.

An example

To see the effect of the new classes, let’s look at the appropriate portion of the IOStreamDemo.java example modified to use the Reader and Writer classes:

//: c11:NewIODemo.java
// Java 1.1 IO typical usage.
import java.io.*;

public class NewIODemo {
  public static void main(String[] args) {
    try {
      // 1. Reading input by lines:
      BufferedReader in =
        new BufferedReader(
          new FileReader(args[0]));
      String s, s2 = new String();
      while((s = in.readLine())!= null)
        s2 += s + "\n";
      in.close();

      // 1b. Reading standard input:
      BufferedReader stdin =
        new BufferedReader(
          new InputStreamReader(System.in));
      System.out.print("Enter a line:");
      System.out.println(stdin.readLine());

      // 2. Input from memory
      StringReader in2 = new StringReader(s2);
      int c;
      while((c = in2.read()) != -1)
        System.out.print((char)c);

      // 3. Formatted memory input
      try {
        DataInputStream in3 =
          new DataInputStream(
            // Oops: must use deprecated class:
            new StringBufferInputStream(s2));
        while(true)
          System.out.print((char)in3.readByte());
      } catch(EOFException e) {
        System.out.println("End of stream");
      }

      // 4. Line numbering & file output
      try {
        LineNumberReader li =
          new LineNumberReader(
            new StringReader(s2));
        BufferedReader in4 =
          new BufferedReader(li);
        PrintWriter out1 =
          new PrintWriter(
            new BufferedWriter(
              new FileWriter("IODemo.out")));
        while((s = in4.readLine()) != null )
          out1.println(
            "Line " + li.getLineNumber() + s);
        out1.close();
      } catch(EOFException e) {
        System.out.println("End of stream");
      }

      // 5. Storing & recovering data
      try {
        DataOutputStream out2 =
          new DataOutputStream(
            new BufferedOutputStream(
              new FileOutputStream("Data.txt")));
        out2.writeDouble(3.14159);
        out2.writeBytes("That was pi");
        out2.close();
        DataInputStream in5 =
          new DataInputStream(
            new BufferedInputStream(
              new FileInputStream("Data.txt")));
        BufferedReader in5br =
          new BufferedReader(
            new InputStreamReader(in5));
        // Must use DataInputStream for data:
        System.out.println(in5.readDouble());
        // Can now use the "proper" readLine():
        System.out.println(in5br.readLine());
      } catch(EOFException e) {
        System.out.println("End of stream");
      }

      // 6. Reading and writing random access
      // files is the same as before.
      // (not repeated here)

    } catch(FileNotFoundException e) {
      System.out.println(
        "File Not Found:" + args[1]);
    } catch(IOException e) {
      System.out.println("IO Exception");
    }
  }
} ///:~

In general, you’ll see that the conversion is fairly straightforward and the code looks quite similar. There are some important differences, though. First of all, since random access files have not changed, section 6 is not repeated.

Section 1 shrinks a bit because if all you’re doing is reading line input you need only to wrap a BufferedReader around a FileReader. Section 1b shows the new way to wrap System.in for reading console input, and this expands because System.in is a DataInputStream and BufferedReader needs a Reader argument, so InputStreamReader is brought in to perform the translation.

In section 2 you can see that if you have a String and want to read from it you just use a StringReader instead of a StringBufferInputStream and the rest of the code is identical.

Section 3 shows a bug in the design of the new IO stream library. If you have a String and you want to read from it, you’re not supposed to use a StringBufferInputStream any more. When you compile code involving a StringBufferInputStream constructor, you get a deprecation message telling you to not use it. Instead, you’re supposed to use a StringReader. However, if you want to do formatted memory input as in section 3, you’re forced to use a DataInputStream—there is no “DataReader” to replace it—and a DataInputStream constructor requires an InputStream argument. So you have no choice but to use the deprecated StringBufferInputStream class. The compiler will give you a deprecation message but there’s nothing you can do about it. [57]

Section 4 is a reasonably straightforward translation from the old streams to the new, with no surprises. In section 5, you’re forced to use all the old streams classes because DataOutputStream and DataInputStream require them and there are no alternatives. However, you don’t get any deprecation messages at compile-time. If a stream is deprecated, typically its constructor produces a deprecation message to prevent you from using the entire class, but in the case of DataInputStream only the readLine( ) method is deprecated since you’re supposed to use a BufferedReader for readLine( ) (but a DataInputStream for all other formatted input).

If you compare section 5 with that section in IOStreamDemo.java, you’ll notice that in this version, the data is written before the text. That’s because a bug was introduced in Java 1.1, which is shown in the following code:

//: c11:IOBug.java
// Java 1.1 (and higher?) IO Bug.
import java.io.*;

public class IOBug {
  public static void main(String[] args)
  throws Exception {
    DataOutputStream out =
      new DataOutputStream(
        new BufferedOutputStream(
          new FileOutputStream("Data.txt")));
    out.writeDouble(3.14159);
    out.writeBytes("That was the value of pi\n");
    out.writeBytes("This is pi/2:\n");
    out.writeDouble(3.14159/2);
    out.close();

    DataInputStream in =
      new DataInputStream(
        new BufferedInputStream(
          new FileInputStream("Data.txt")));
    BufferedReader inbr =
      new BufferedReader(
        new InputStreamReader(in));
    // The doubles written BEFORE the line of text
    // read back correctly:
    System.out.println(in.readDouble());
    // Read the lines of text:
    System.out.println(inbr.readLine());
    System.out.println(inbr.readLine());
    // Trying to read the doubles after the line
    // produces an end-of-file exception:
    System.out.println(in.readDouble());
  }
} ///:~

It appears that anything you write after a call to writeBytes( ) is not recoverable. This is a rather limiting bug, and we can hope that it will be fixed by the time you read this. You should run the above program to test it; if you don’t get an exception and the values print correctly then you’re out of the woods.

Redirecting standard IO

Java 1.1 has added methods in class System that allow you to redirect the standard input, output, and error IO streams using simple static method calls:

setIn(InputStream)
setOut(PrintStream)
setErr(PrintStream)

Redirecting output is especially useful if you suddenly start creating a large amount of output on your screen and it’s scrolling past faster than you can read it. Redirecting input is valuable for a command-line program in which you want to test a particular user-input sequence repeatedly. Here’s a simple example that shows the use of these methods:

//: c11:Redirecting.java
// Demonstrates the use of redirection
// for standard IO in Java 1.1
import java.io.*;

class Redirecting {
  public static void main(String[] args) {
    try {
      BufferedInputStream in =
        new BufferedInputStream(
          new FileInputStream(
            "Redirecting.java"));
      // Produces deprecation message:
      PrintStream out =
        new PrintStream(
          new BufferedOutputStream(
            new FileOutputStream("test.out")));
      System.setIn(in);
      System.setOut(out);
      System.setErr(out);

      BufferedReader br =
        new BufferedReader(
          new InputStreamReader(System.in));
      String s;
      while((s = br.readLine()) != null)
        System.out.println(s);
      out.close(); // Remember this!
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
} ///:~

This program attaches standard input to a file, and redirects standard output and standard error to another file.

This is another example in which a deprecation message is inevitable. The message you can get when compiling with the -deprecation flag is:

Note: The constructor java.io.PrintStream(java.io.OutputStream)
has been deprecated.

However, both System.setOut( ) and System.setErr( ) require a PrintStream object as an argument, so you are forced to call the PrintStream constructor. You might wonder, if Java 1.1 deprecates the entire PrintStream class by deprecating the constructor, why the library designers, at the same time as they added this deprecation, also add new methods to System that required a PrintStream rather than a PrintWriter, which is the new and preferred replacement. It’s a mystery.

Compression

Java 1.1 has also added some classes to support reading and writing streams in a compressed format. These are wrapped around existing IO classes to provide compression functionality.

One aspect of these Java 1.1 classes stands out: They are not derived from the new Reader and Writer classes, but instead are part of the InputStream and OutputStream hierarchies. So you might be forced to mix the two types of streams. (Remember that you can use InputStreamReader and OutputStreamWriter to provide easy conversion between one type and another.)

Java 1.1 Compression class | Function
CheckedInputStream | GetCheckSum( ) produces checksum for any InputStream (not just decompression)
CheckedOutputStream | GetCheckSum( ) produces checksum for any OutputStream (not just compression)
DeflaterOutputStream | Base class for compression classes
ZipOutputStream | A DeflaterOutputStream that compresses data into the Zip file format
GZIPOutputStream | A DeflaterOutputStream that compresses data into the GZIP file format
InflaterInputStream | Base class for decompression classes
ZipInputStream | A DeflaterInputStream that Decompresses data that has been stored in the Zip file format
GZIPInputStream | A DeflaterInputStream that decompresses data that has been stored in the GZIP file format

Although there are many compression algorithms, Zip and GZIP are possibly the most commonly used. Thus you can easily manipulate your compressed data with the many tools available for reading and writing these formats.

Simple compression with GZIP

The GZIP interface is simple and thus is probably more appropriate when you have a single stream of data that you want to compress (rather than a container of dissimilar pieces of data). Here’s an example that compresses a single file:

//: c11:ZipCompress.java
// Uses Java 1.1 Zip compression to compress
// any number of files whose names are passed
// on the command line.
import java.io.*;
import java.util.*;
import java.util.zip.*;

public class ZipCompress {
  public static void main(String[] args) {
    try {
      FileOutputStream f =
        new FileOutputStream("test.zip");
      CheckedOutputStream csum =
        new CheckedOutputStream(
          f, new Adler32());
      ZipOutputStream out =
        new ZipOutputStream(
          new BufferedOutputStream(csum));
      out.setComment("A test of Java Zipping");
      // Can't read the above comment, though
      for(int i = 0; i < args.length; i++) {
        System.out.println(
          "Writing file " + args[i]);
        BufferedReader in =
          new BufferedReader(
            new FileReader(args[i]));
        out.putNextEntry(new ZipEntry(args[i]));
        int c;
        while((c = in.read()) != -1)
          out.write(c);
        in.close();
      }
      out.close();
      // Checksum valid only after the file
      // has been closed!
      System.out.println("Checksum: " +
        csum.getChecksum().getValue());
      // Now extract the files:
      System.out.println("Reading file");
      FileInputStream fi =
         new FileInputStream("test.zip");
      CheckedInputStream csumi =
        new CheckedInputStream(
          fi, new Adler32());
      ZipInputStream in2 =
        new ZipInputStream(
          new BufferedInputStream(csumi));
      ZipEntry ze;
      System.out.println("Checksum: " +
        csumi.getChecksum().getValue());
      while((ze = in2.getNextEntry()) != null) {
        System.out.println("Reading file " + ze);
        int x;
        while((x = in2.read()) != -1)
          System.out.write(x);
      }
      in2.close();
      // Alternative way to open and read
      // zip files:
      ZipFile zf = new ZipFile("test.zip");
      Enumeration e = zf.entries();
      while(e.hasMoreElements()) {
        ZipEntry ze2 = (ZipEntry)e.nextElement();
        System.out.println("File: " + ze2);
        // ... and extract the data as before
      }
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

The use of the compression classes is straightforward—you simply wrap your output stream in a GZIPOutputStream or ZipOutputStream and your input stream in a GZIPInputStream or ZipInputStream. All else is ordinary IO reading and writing. This is, however, a good example of when you’re forced to mix the old IO streams with the new: in uses the Reader classes, whereas GZIPOutputStream’s constructor can accept only an OutputStream object, not a Writer object.

Multi-file storage with Zip

The Java 1.1 library that supports the Zip format is much more extensive. With it you can easily store multiple files, and there’s even a separate class to make the process of reading a Zip file easy. The library uses the standard Zip format so that it works seamlessly with all the tools currently downloadable on the Internet. The following example has the same form as the previous example, but it handles as many command-line arguments as you want. In addition, it shows the use of the Checksum classes to calculate and verify the checksum for the file. There are two Checksum types: Adler32 (which is faster) and CRC32 (which is slower but slightly more accurate).

//: c11:ZipCompress.java
// Uses Java 1.1 Zip compression to compress
// any number of files whose names are passed
// on the command line.
import java.io.*;
import java.util.*;
import java.util.zip.*;

public class ZipCompress {
  public static void main(String[] args) {
    try {
      FileOutputStream f =
        new FileOutputStream("test.zip");
      CheckedOutputStream csum =
        new CheckedOutputStream(
          f, new Adler32());
      ZipOutputStream out =
        new ZipOutputStream(
          new BufferedOutputStream(csum));
      out.setComment("A test of Java Zipping");
      // Can't read the above comment, though
      for(int i = 0; i < args.length; i++) {
        System.out.println(
          "Writing file " + args[i]);
        BufferedReader in =
          new BufferedReader(
            new FileReader(args[i]));
        out.putNextEntry(new ZipEntry(args[i]));
        int c;
        while((c = in.read()) != -1)
          out.write(c);
        in.close();
      }
      out.close();
      // Checksum valid only after the file
      // has been closed!
      System.out.println("Checksum: " +
        csum.getChecksum().getValue());
      // Now extract the files:
      System.out.println("Reading file");
      FileInputStream fi =
         new FileInputStream("test.zip");
      CheckedInputStream csumi =
        new CheckedInputStream(
          fi, new Adler32());
      ZipInputStream in2 =
        new ZipInputStream(
          new BufferedInputStream(csumi));
      ZipEntry ze;
      System.out.println("Checksum: " +
        csumi.getChecksum().getValue());
      while((ze = in2.getNextEntry()) != null) {
        System.out.println("Reading file " + ze);
        int x;
        while((x = in2.read()) != -1)
          System.out.write(x);
      }
      in2.close();
      // Alternative way to open and read
      // zip files:
      ZipFile zf = new ZipFile("test.zip");
      Enumeration e = zf.entries();
      while(e.hasMoreElements()) {
        ZipEntry ze2 = (ZipEntry)e.nextElement();
        System.out.println("File: " + ze2);
        // ... and extract the data as before
      }
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

For each file to add to the archive, you must call putNextEntry( ) and pass it a ZipEntry object. The ZipEntry object contains an extensive interface that allows you to get and set all the data available on that particular entry in your Zip file: name, compressed and uncompressed sizes, date, CRC checksum, extra field data, comment, compression method, and whether it’s a directory entry. However, even though the Zip format has a way to set a password, this is not supported in Java’s Zip library. And although CheckedInputStream and CheckedOutputStream support both Adler32 and CRC32 checksums, the ZipEntry class supports only an interface for CRC. This is a restriction of the underlying Zip format, but it might limit you from using the faster Adler32.

To extract files, ZipInputStream has a getNextEntry( ) method that returns the next ZipEntry if there is one. As a more succinct alternative, you can read the file using a ZipFile object, which has a method entries( ) to return an Enumeration to the ZipEntries.

In order to read the checksum you must somehow have access to the associated Checksum object. Here, a reference to the CheckedOutputStream and CheckedInputStream objects is retained, but you could also just hold onto a reference to the Checksum object.

A baffling method in Zip streams is setComment( ). As shown above, you can set a comment when you’re writing a file, but there’s no way to recover the comment in the ZipInputStream. Comments appear to be supported fully on an entry-by-entry basis only via ZipEntry.

Of course, you are not limited to files when using the GZIP or Zip libraries—you can compress anything, including data to be sent through a network connection.

The Java archive (jar) utility

The Zip format is also used in the Java 1.1 JAR (Java ARchive) file format, which is a way to collect a group of files into a single compressed file, just like Zip. However, like everything else in Java, JAR files are cross-platform so you don’t need to worry about platform issues. You can also include audio and image files as well as class files.

JAR files are particularly helpful when you deal with the Internet. Before JAR files, your Web browser would have to make repeated requests of a Web server in order to download all of the files that make up an applet. In addition, each of these files was uncompressed. By combining all of the files for a particular applet into a single JAR file, only one server request is necessary and the transfer is faster because of compression. And each entry in a JAR file can be digitally signed for security (refer to the Java documentation for details).

A JAR file consists of a single file containing a collection of zipped files along with a “manifest” that describes them. (You can create your own manifest file; otherwise the jar program will do it for you.) You can find out more about JAR manifests in the online documentation.

The jar utility that comes with Sun’s JDK automatically compresses the files of your choice. You invoke it on the command line:

jar [options] destination [manifest] inputfile(s)
The options are simply a collection of letters (no hyphen or any other indicator is necessary). These are:

c | Creates a new or empty archive.
t | Lists the table of contents.
x | Extracts all files
x file | Extracts the named file
f | Says: “I’m going to give you the name of the file.” If you don’t use this, jar assumes that its input will come from standard input, or, if it is creating a file, its output will go to standard output.
m | Says that the first argument will be the name of the user-created manifest file
v | Generates verbose output describing what jar is doing
0 | Only store the files; doesn’t compress the files (use to create a JAR file that you can put in your classpath)
M | Don’t automatically create a manifest file

If a subdirectory is included in the files to be put into the JAR file, that subdirectory is automatically added, including all of its subdirectories, etc. Path information is also preserved.

Here are some typical ways to invoke jar:

jar cf myJarFile.jar *.class

This creates a JAR file called myJarFile.jar that contains all of the class files in the current directory, along with an automatically-generated manifest file.

jar cmf myJarFile.jar myManifestFile.mf *.class

Like the previous example, but adding a user-created manifest file called myManifestFile.mf.

jar tf myJarFile.jar

Produces a table of contents of the files in myJarFile.jar.

jar tvf myJarFile.jar

Adds the “verbose” flag to give more detailed information about the files in myJarFile.jar.

jar cvf myApp.jar audio classes image

Assuming audio, classes, and image are subdirectories, this combines all of the subdirectories into the file myApp.jar. The “verbose” flag is also included to give extra feedback while the jar program is working.

If you create a JAR file using the 0 option, that file can be placed in your CLASSPATH:

CLASSPATH="lib1.jar;lib2.jar;"

Then Java can search lib1.jar and lib2.jar for class files.

The jar tool isn’t as useful as a zip utility. For example, you can’t add or update files to an existing JAR file; you can create JAR files only from scratch. Also, you can’t move files into a JAR file, erasing them as they are moved. However, a JAR file created on one platform will be transparently readable by the jar tool on any other platform (a problem that sometimes plagues zip utilities).

As you will see in Chapter 13, JAR files are also used to package Java Beans.

Object serialization

Java 1.1 has added an interesting feature called object serialization that allows you to take any object that implements the Serializable interface and turn it into a sequence of bytes that can later be restored fully into the original object. This is even true across a network, which means that the serialization mechanism automatically compensates for differences in operating systems. That is, you can create an object on a Windows machine, serialize it, and send it across the network to a Unix machine where it will be correctly reconstructed. You don’t have to worry about the data representations on the different machines, the byte ordering, or any other details.

By itself, object serialization is interesting because it allows you to implement lightweight persistence. Remember that persistence means an object’s lifetime is not determined by whether a program is executing—the object lives in between invocations of the program. By taking a serializable object and writing it to disk, then restoring that object when the program is reinvoked, you’re able to produce the effect of persistence. The reason it’s called “lightweight” is that you can’t simply define an object using some kind of “persistent” keyword and let the system take care of the details (although this might happen in the future). Instead, you must explicitly serialize and de-serialize the objects in your program.

Object serialization was added to the language to support two major features. Java 1.1’s remote method invocation (RMI) allows objects that live on other machines to behave as if they live on your machine. When sending messages to remote objects, object serialization is necessary to transport the arguments and return values. RMI is discussed in Chapter 15.

Object serialization is also necessary for Java Beans, introduced in Java 1.1. When a Bean is used, its state information is generally configured at design time. This state information must be stored and later recovered when the program is started; object serialization performs this task.

Serializing an object is quite simple, as long as the object implements the Serializable interface (this interface is just a flag and has no methods). In Java 1.1, many standard library classes have been changed so they’re serializable, including all of the wrappers for the primitive types, all of the container classes, and many others. Even Class objects can be serialized. (See Chapter 12 for the implications of this.)

To serialize an object, you create some sort of OutputStream object and then wrap it inside an ObjectOutputStream object. At this point you need only call writeObject( ) and your object is serialized and sent to the OutputStream. To reverse the process, you wrap an InputStream inside an ObjectInputStream and call readObject( ). What comes back is, as usual, a reference to an upcast Object, so you must downcast to set things straight.

A particularly clever aspect of object serialization is that it not only saves an image of your object but it also follows all the references contained in your object and saves those objects, and follows all the references in each of those objects, etc. This is sometimes referred to as the “web of objects” that a single object can be connected to, and it includes arrays of references to objects as well as member objects. If you had to maintain your own object serialization scheme, maintaining the code to follow all these links would be a bit mind–boggling. However, Java object serialization seems to pull it off flawlessly, no doubt using an optimized algorithm that traverses the web of objects. The following example tests the serialization mechanism by making a “worm” of linked objects, each of which has a link to the next segment in the worm as well as an array of references to objects of a different class, Data:

//: c11:Worm.java
// Demonstrates object serialization.
import java.io.*;

class Data implements Serializable {
  private int i;
  Data(int x) { i = x; }
  public String toString() {
    return Integer.toString(i);
  }
}

public class Worm implements Serializable {
  // Generate a random int value:
  private static int r() {
    return (int)(Math.random() * 10);
  }
  private Data[] d = {
    new Data(r()), new Data(r()), new Data(r())
  };
  private Worm next;
  private char c;
  // Value of i == number of segments
  Worm(int i, char x) {
    System.out.println(" Worm constructor: " + i);
    c = x;
    if(--i > 0)
      next = new Worm(i, (char)(x + 1));
  }
  Worm() {
    System.out.println("Default constructor");
  }
  public String toString() {
    String s = ":" + c + "(";
    for(int i = 0; i < d.length; i++)
      s += d[i].toString();
    s += ")";
    if(next != null)
      s += next.toString();
    return s;
  }
  public static void main(String[] args) {
    Worm w = new Worm(6, 'a');
    System.out.println("w = " + w);
    try {
      ObjectOutputStream out =
        new ObjectOutputStream(
          new FileOutputStream("worm.out"));
      out.writeObject("Worm storage");
      out.writeObject(w);
      out.close(); // Also flushes output
      ObjectInputStream in =
        new ObjectInputStream(
          new FileInputStream("worm.out"));
      String s = (String)in.readObject();
      Worm w2 = (Worm)in.readObject();
      System.out.println(s + ", w2 = " + w2);
    } catch(Exception e) {
      e.printStackTrace();
    }
    try {
      ByteArrayOutputStream bout =
        new ByteArrayOutputStream();
      ObjectOutputStream out =
        new ObjectOutputStream(bout);
      out.writeObject("Worm storage");
      out.writeObject(w);
      out.flush();
      ObjectInputStream in =
        new ObjectInputStream(
          new ByteArrayInputStream(
            bout.toByteArray()));
      String s = (String)in.readObject();
      Worm w3 = (Worm)in.readObject();
      System.out.println(s + ", w3 = " + w3);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

To make things interesting, the array of Data objects inside Worm are initialized with random numbers. (This way you don’t suspect the compiler of keeping some kind of meta-information.) Each Worm segment is labeled with a char that’s automatically generated in the process of recursively generating the linked list of Worms. When you create a Worm, you tell the constructor how long you want it to be. To make the next reference it calls the Worm constructor with a length of one less, etc. The final next reference is left as null, indicating the end of the Worm.

The point of all this was to make something reasonably complex that couldn’t easily be serialized. The act of serializing, however, is quite simple. Once the ObjectOutputStream is created from some other stream, writeObject( ) serializes the object. Notice the call to writeObject( ) for a String, as well. You can also write all the primitive data types using the same methods as DataOutputStream (they share the same interface).

There are two separate try blocks that look similar. The first writes and reads a file and the second, for variety, writes and reads a ByteArray. You can read and write an object using serialization to any DataInputStream or DataOutputStream including, as you will see in the networking chapter, a network. The output from one run was:

Worm constructor: 6
Worm constructor: 5
Worm constructor: 4
Worm constructor: 3
Worm constructor: 2
Worm constructor: 1
w = :a(262):b(100):c(396):d(480):e(316):f(398)
Worm storage, w2 = :a(262):b(100):c(396):d(480):e(316):f(398)
Worm storage, w3 = :a(262):b(100):c(396):d(480):e(316):f(398)

You can see that the deserialized object really does contain all of the links that were in the original object.

Note that no constructor, not even the default constructor, is called in the process of deserializing a Serializable object. The entire object is restored by recovering data from the InputStream.

Object serialization is another Java 1.1 feature that is not part of the new Reader and Writer hierarchies, but instead uses the old InputStream and OutputStream hierarchies. Thus you might encounter situations in which you’re forced to mix the two hierarchies.

Finding the class

You might wonder what’s necessary for an object to be recovered from its serialized state. For example, suppose you serialize an object and send it as a file or through a network to another machine. Could a program on the other machine reconstruct the object using only the contents of the file?

The best way to answer this question is (as usual) by performing an experiment. The following file goes in the subdirectory for this chapter:

//: c11:Alien.java
// A serializable class.
import java.io.*;

public class Alien implements Serializable {
} ///:~

The file that creates and serializes an Alien object goes in the same directory:

//: c11:FreezeAlien.java
// Create a serialized output file.
import java.io.*;

public class FreezeAlien {
  public static void main(String[] args)
      throws Exception {
    ObjectOutput out =
      new ObjectOutputStream(
        new FileOutputStream("file.x"));
    Alien zorcon = new Alien();
    out.writeObject(zorcon);
  }
} ///:~

Rather than catching and handling exceptions, this program takes the quick and dirty approach of passing the exceptions out of main( ), so they’ll be reported on the command line.

Once the program is compiled and run, copy the resulting file.x to a subdirectory called xfiles, where the following code goes:

//: c11:xfiles:ThawAlien.java
// Try to recover a serialized file without the
// class of object that's stored in that file.
package c11.xfiles;
import java.io.*;

public class ThawAlien {
  public static void main(String[] args)
      throws Exception {
    ObjectInputStream in =
      new ObjectInputStream(
        new FileInputStream("file.x"));
    Object mystery = in.readObject();
    System.out.println(mystery.getClass());
  }
} ///:~

This program opens the file and reads in the object mystery successfully. However, as soon as you try to find out anything about the object—which requires the Class object for Alien—the Java Virtual Machine (JVM) cannot find Alien.class (unless it happens to be in the Classpath, which it shouldn’t be in this example). You’ll get a ClassNotFoundException. (Once again, all evidence of alien life vanishes before proof of its existence can be verified!)

If you expect to do much after you’ve recovered an object that has been serialized, you must make sure that the JVM can find the associated .class file either in the local class path or somewhere on the Internet.

Controlling serialization

As you can see, the default serialization mechanism is trivial to use. But what if you have special needs? Perhaps you have special security issues and you don’t want to serialize portions of your object, or perhaps it just doesn’t make sense for one sub-object to be serialized if that part needs to be created anew when the object is recovered.

You can control the process of serialization by implementing the Externalizable interface instead of the Serializable interface. The Externalizable interface extends the Serializable interface and adds two methods, writeExternal( ) and readExternal( ), that are automatically called for your object during serialization and deserialization so that you can perform your special operations.

The following example shows simple implementations of the Externalizable interface methods. Note that Blip1 and Blip2 are nearly identical except for a subtle difference (see if you can discover it by looking at the code):

//: c11:Blips.java
// Simple use of Externalizable & a pitfall.
import java.io.*;
import java.util.*;

class Blip1 implements Externalizable {
  public Blip1() {
    System.out.println("Blip1 Constructor");
  }
  public void writeExternal(ObjectOutput out)
      throws IOException {
    System.out.println("Blip1.writeExternal");
  }
  public void readExternal(ObjectInput in)
     throws IOException, ClassNotFoundException {
    System.out.println("Blip1.readExternal");
  }
}

class Blip2 implements Externalizable {
  Blip2() {
    System.out.println("Blip2 Constructor");
  }
  public void writeExternal(ObjectOutput out)
      throws IOException {
    System.out.println("Blip2.writeExternal");
  }
  public void readExternal(ObjectInput in)
     throws IOException, ClassNotFoundException {
    System.out.println("Blip2.readExternal");
  }
}

public class Blips {
  public static void main(String[] args) {
    System.out.println("Constructing objects:");
    Blip1 b1 = new Blip1();
    Blip2 b2 = new Blip2();
    try {
      ObjectOutputStream o =
        new ObjectOutputStream(
          new FileOutputStream("Blips.out"));
      System.out.println("Saving objects:");
      o.writeObject(b1);
      o.writeObject(b2);
      o.close();
      // Now get them back:
      ObjectInputStream in =
        new ObjectInputStream(
          new FileInputStream("Blips.out"));
      System.out.println("Recovering b1:");
      b1 = (Blip1)in.readObject();
      // OOPS! Throws an exception:
//!   System.out.println("Recovering b2:");
//!   b2 = (Blip2)in.readObject();
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

The output for this program is:

Constructing objects:
Blip1 Constructor
Blip2 Constructor
Saving objects:
Blip1.writeExternal
Blip2.writeExternal
Recovering b1:
Blip1 Constructor
Blip1.readExternal

The reason that the Blip2 object is not recovered is that trying to do so causes an exception. Can you see the difference between Blip1 and Blip2? The constructor for Blip1 is public, while the constructor for Blip2 is not, and that causes the exception upon recovery. Try making Blip2’s constructor public and removing the //! comments to see the correct results.

When b1 is recovered, the Blip1 default constructor is called. This is different from recovering a Serializable object, in which the object is constructed entirely from its stored bits, with no constructor calls. With an Externalizable object, all the normal default construction behavior occurs (including the initializations at the point of field definition), and then readExternal( ) is called. You need to be aware of this—in particular the fact that all the default construction always takes place—to produce the correct behavior in your Externalizable objects.

Here’s an example that shows what you must do to fully store and retrieve an Externalizable object:

//: c11:Blip3.java
// Reconstructing an externalizable object.
import java.io.*;
import java.util.*;

class Blip3 implements Externalizable {
  int i;
  String s; // No initialization
  public Blip3() {
    System.out.println("Blip3 Constructor");
    // s, i not initialized
  }
  public Blip3(String x, int a) {
    System.out.println("Blip3(String x, int a)");
    s = x;
    i = a;
    // s & i initialized only in non-default
    // constructor.
  }
  public String toString() { return s + i; }
  public void writeExternal(ObjectOutput out)
      throws IOException {
    System.out.println("Blip3.writeExternal");
    // You must do this:
    out.writeObject(s); out.writeInt(i);
  }
  public void readExternal(ObjectInput in)
     throws IOException, ClassNotFoundException {
    System.out.println("Blip3.readExternal");
    // You must do this:
    s = (String)in.readObject();
    i =in.readInt();
  }
  public static void main(String[] args) {
    System.out.println("Constructing objects:");
    Blip3 b3 = new Blip3("A String ", 47);
    System.out.println(b3);
    try {
      ObjectOutputStream o =
        new ObjectOutputStream(
          new FileOutputStream("Blip3.out"));
      System.out.println("Saving object:");
      o.writeObject(b3);
      o.close();
      // Now get it back:
      ObjectInputStream in =
        new ObjectInputStream(
          new FileInputStream("Blip3.out"));
      System.out.println("Recovering b3:");
      b3 = (Blip3)in.readObject();
      System.out.println(b3);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

The fields s and i are initialized only in the second constructor, but not in the default constructor. This means that if you don’t initialize s and i in readExternal, it will be null (since the storage for the object gets wiped to zero in the first step of object creation). If you comment out the two lines of code following the phrases “You must do this” and run the program, you’ll see that when the object is recovered, s is null and i is zero.

If you are inheriting from an Externalizable object, you’ll typically call the base-class versions of writeExternal( ) and readExternal( ) to provide proper storage and retrieval of the base-class components.

So to make things work correctly you must not only write the important data from the object during the writeExternal( ) method (there is no default behavior that writes any of the member objects for an Externalizable object), but you must also recover that data in the readExternal( ) method. This can be a bit confusing at first because the default construction behavior for an Externalizable object can make it seem like some kind of storage and retrieval takes place automatically. It does not.

The transient keyword

When you’re controlling serialization, there might be a particular subobject that you don’t want Java’s serialization mechanism to automatically save and restore. This is commonly the case if that subobject represents sensitive information that you don’t want to serialize, such as a password. Even if that information is private in the object, once it’s serialized it’s possible for someone to access it by reading a file or intercepting a network transmission.

One way to prevent sensitive parts of your object from being serialized is to implement your class as Externalizable, as shown previously. Then nothing is automatically serialized and you can explicitly serialize only the necessary parts inside writeExternal( ).

If you’re working with a Serializable object, however, all serialization happens automatically. To control this, you can turn off serialization on a field-by-field basis using the transient keyword, which says “Don’t bother saving or restoring this—I’ll take care of it.”

For example, consider a Login object that keeps information about a particular login session. Suppose that, once you verify the login, you want to store the data, but without the password. The easiest way to do this is by implementing Serializable and marking the password field as transient. Here’s what it looks like:

//: c11:Logon.java
// Demonstrates the "transient" keyword.
import java.io.*;
import java.util.*;

class Logon implements Serializable {
  private Date date = new Date();
  private String username;
  private transient String password;
  Logon(String name, String pwd) {
    username = name;
    password = pwd;
  }
  public String toString() {
    String pwd =
      (password == null) ? "(n/a)" : password;
    return "logon info: \n   " +
      "username: " + username +
      "\n   date: " + date +
      "\n   password: " + pwd;
  }
  public static void main(String[] args) {
    Logon a = new Logon("Hulk", "myLittlePony");
    System.out.println( "logon a = " + a);
    try {
      ObjectOutputStream o =
        new ObjectOutputStream(
          new FileOutputStream("Logon.out"));
      o.writeObject(a);
      o.close();
      // Delay:
      int seconds = 5;
      long t = System.currentTimeMillis()
             + seconds * 1000;
      while(System.currentTimeMillis() < t)
        ;
      // Now get them back:
      ObjectInputStream in =
        new ObjectInputStream(
          new FileInputStream("Logon.out"));
      System.out.println(
        "Recovering object at " + new Date());
      a = (Logon)in.readObject();
      System.out.println( "logon a = " + a);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

You can see that the date and username fields are ordinary (not transient), and thus are automatically serialized. However, the password is transient, and so is not stored to disk; also the serialization mechanism makes no attempt to recover it. The output is:

logon a = logon info:
   username: Hulk
   date: Sun Mar 23 18:25:53 PST 1997
   password: myLittlePony
Recovering object at Sun Mar 23 18:25:59 PST 1997
logon a = logon info:
   username: Hulk
   date: Sun Mar 23 18:25:53 PST 1997
   password: (n/a)

When the object is recovered, the password field is null. Note that toString( ) must check for a null value of password because if you try to assemble a String object using the overloaded ‘+’ operator, and that operator encounters a null reference, you’ll get a NullPointerException. (Newer versions of Java might contain code to avoid this problem.)

You can also see that the date field is stored to and recovered from disk and not generated anew.

Since Externalizable objects do not store any of their fields by default, the transient keyword is for use with Serializable objects only.

An alternative to Externalizable

If you’re not keen on implementing the Externalizable interface, there’s another approach. You can implement the Serializable interface and add (notice I say “add” and not “override” or “implement”) methods called writeObject( ) and readObject( ) that will automatically be called when the object is serialized and deserialized, respectively. That is, if you provide these two methods they will be used instead of the default serialization.

The methods must have these exact signatures:

private void
  writeObject(ObjectOutputStream stream)
    throws IOException;

private void
  readObject(ObjectInputStream stream)
    throws IOException, ClassNotFoundException

From a design standpoint, things get really weird here. First of all, you might think that because these methods are not part of a base class or the Serializable interface, they ought to be defined in their own interface(s). But notice that they are defined as private, which means they are to be called only by other members of this class. However, you don’t actually call them from other members of this class, but instead the writeObject( ) and readObject( ) methods of the ObjectOutputStream and ObjectInputStream objects call your object’s writeObject( ) and readObject( ) methods. (Notice my tremendous restraint in not launching into a long diatribe about using the same method names here. In a word: confusing.) You might wonder how the ObjectOutputStream and ObjectInputStream objects have access to private methods of your class. We can only assume that this is part of the serialization magic.

In any event, anything defined in an interface is automatically public so if writeObject( ) and readObject( ) must be private, then they can’t be part of an interface. Since you must follow the signatures exactly, the effect is the same as if you’re implementing an interface.

It would appear that when you call ObjectOutputStream.writeObject( ), the Serializable object that you pass it to is interrogated (using reflection, no doubt) to see if it implements its own writeObject( ). If so, the normal serialization process is skipped and the writeObject( ) is called. The same sort of situation exists for readObject( ).

There’s one other twist. Inside your writeObject( ), you can choose to perform the default writeObject( ) action by calling defaultWriteObject( ). Likewise, inside readObject( ) you can call defaultReadObject( ). Here is a simple example that demonstrates how you can control the storage and retrieval of a Serializable object:

//: c11:SerialCtl.java
// Controlling serialization by adding your own
// writeObject() and readObject() methods.
import java.io.*;

public class SerialCtl implements Serializable {
  String a;
  transient String b;
  public SerialCtl(String aa, String bb) {
    a = "Not Transient: " + aa;
    b = "Transient: " + bb;
  }
  public String toString() {
    return a + "\n" + b;
  }
  private void
    writeObject(ObjectOutputStream stream)
      throws IOException {
    stream.defaultWriteObject();
    stream.writeObject(b);
  }
  private void
    readObject(ObjectInputStream stream)
      throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    b = (String)stream.readObject();
  }
  public static void main(String[] args) {
    SerialCtl sc =
      new SerialCtl("Test1", "Test2");
    System.out.println("Before:\n" + sc);
    ByteArrayOutputStream buf =
      new ByteArrayOutputStream();
    try {
      ObjectOutputStream o =
        new ObjectOutputStream(buf);
      o.writeObject(sc);
      // Now get it back:
      ObjectInputStream in =
        new ObjectInputStream(
          new ByteArrayInputStream(
            buf.toByteArray()));
      SerialCtl sc2 = (SerialCtl)in.readObject();
      System.out.println("After:\n" + sc2);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

In this example, one String field is ordinary and the other is transient, to prove that the non-transient field is saved by the defaultWriteObject( ) method and the transient field is saved and restored explicitly. The fields are initialized inside the constructor rather than at the point of definition to prove that they are not being initialized by some automatic mechanism during deserialization.

If you are going to use the default mechanism to write the non-transient parts of your object, you must call defaultWriteObject( ) as the first operation in writeObject( ) and defaultReadObject( ) as the first operation in readObject( ). These are strange method calls. It would appear, for example, that you are calling defaultWriteObject( ) for an ObjectOutputStream and passing it no arguments, and yet it somehow turns around and knows the reference to your object and how to write all the non-transient parts. Spooky.

The storage and retrieval of the transient objects uses more familiar code. And yet, think about what happens here. In main( ), a SerialCtl object is created, and then it’s serialized to an ObjectOutputStream. (Notice in this case that a buffer is used instead of a file—it’s all the same to the ObjectOutputStream.) The serialization occurs in the line:

o.writeObject(sc);

The writeObject( ) method must be examining sc to see if it has its own writeObject( ) method. (Not by checking the interface—there isn’t one—or the class type, but by actually hunting for the method using reflection.) If it does, it uses that. A similar approach holds true for readObject( ). Perhaps this was the only practical way that they could solve the problem, but it’s certainly strange.

Versioning

It’s possible that you might want to change the version of a serializable class (objects of the original class might be stored in a database, for example). This is supported but you’ll probably do it only in special cases, and it requires an extra depth of understanding that we will not attempt to achieve here. The JDK1.1 HTML documents downloadable from Sun (which might be part of your Java package’s online documents) cover this topic quite thoroughly.

Using persistence

It’s quite appealing to use serialization technology to store some of the state of your program so that you can easily restore the program to the current state later. But before you can do this, some questions must be answered. What happens if you serialize two objects that both have a reference to a third object? When you restore those two objects from their serialized state, do you get only one occurrence of the third object? What if you serialize your two objects to separate files and deserialize them in different parts of your code?

Here’s an example that shows the problem:

//: c11:MyWorld.java
import java.io.*;
import java.util.*;

class House implements Serializable {}

class Animal implements Serializable {
  String name;
  House preferredHouse;
  Animal(String nm, House h) {
    name = nm;
    preferredHouse = h;
  }
  public String toString() {
    return name + "[" + super.toString() +
      "], " + preferredHouse + "\n";
  }
}

public class MyWorld {
  public static void main(String[] args) {
    House house = new House();
    ArrayList  animals = new ArrayList();
    animals.add(
      new Animal("Bosco the dog", house));
    animals.add(
      new Animal("Ralph the hamster", house));
    animals.add(
      new Animal("Fronk the cat", house));
    System.out.println("animals: " + animals);

    try {
      ByteArrayOutputStream buf1 =
        new ByteArrayOutputStream();
      ObjectOutputStream o1 =
        new ObjectOutputStream(buf1);
      o1.writeObject(animals);
      o1.writeObject(animals); // Write a 2nd set
      // Write to a different stream:
      ByteArrayOutputStream buf2 =
        new ByteArrayOutputStream();
      ObjectOutputStream o2 =
        new ObjectOutputStream(buf2);
      o2.writeObject(animals);
      // Now get them back:
      ObjectInputStream in1 =
        new ObjectInputStream(
          new ByteArrayInputStream(
            buf1.toByteArray()));
      ObjectInputStream in2 =
        new ObjectInputStream(
          new ByteArrayInputStream(
            buf2.toByteArray()));
      ArrayList animals1 = (ArrayList)in1.readObject();
      ArrayList animals2 = (ArrayList)in1.readObject();
      ArrayList animals3 = (ArrayList)in2.readObject();
      System.out.println("animals1: " + animals1);
      System.out.println("animals2: " + animals2);
      System.out.println("animals3: " + animals3);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

One thing that’s interesting here is that it’s possible to use object serialization to and from a byte array as a way of doing a “deep copy” of any object that’s Serializable. (A deep copy means that you’re duplicating the entire web of objects, rather than just the basic object and its references.) Copying is covered in depth in Appendix A.

Animal objects contain fields of type House. In main( ), an ArrayList of these Animals is created and it is serialized twice to one stream and then again to a separate stream. When these are deserialized and printed, you see the following results for one run (the objects will be in different memory locations each run):

animals: [Bosco the dog[Animal@1cc76c], House@1cc769
, Ralph the hamster[Animal@1cc76d], House@1cc769
, Fronk the cat[Animal@1cc76e], House@1cc769
]
animals1: [Bosco the dog[Animal@1cca0c], House@1cca16
, Ralph the hamster[Animal@1cca17], House@1cca16
, Fronk the cat[Animal@1cca1b], House@1cca16
]
animals2: [Bosco the dog[Animal@1cca0c], House@1cca16
, Ralph the hamster[Animal@1cca17], House@1cca16
, Fronk the cat[Animal@1cca1b], House@1cca16
]
animals3: [Bosco the dog[Animal@1cca52], House@1cca5c
, Ralph the hamster[Animal@1cca5d], House@1cca5c
, Fronk the cat[Animal@1cca61], House@1cca5c
]

Of course you expect that the deserialized objects have different addresses from their originals. But notice that in animals1 and animals2 the same addresses appear, including the references to the House object that both share. On the other hand, when animals3 is recovered the system has no way of knowing that the objects in this other stream are aliases of the objects in the first stream, so it makes a completely different web of objects.

As long as you’re serializing everything to a single stream, you’ll be able to recover the same web of objects that you wrote, with no accidental duplication of objects. Of course, you can change the state of your objects in between the time you write the first and the last, but that’s your responsibility—the objects will be written in whatever state they are in (and with whatever connections they have to other objects) at the time you serialize them.

The safest thing to do if you want to save the state of a system is to serialize as an “atomic” operation. If you serialize some things, do some other work, and serialize some more, etc., then you will not be storing the system safely. Instead, put all the objects that comprise the state of your system in a single container and simply write that container out in one operation. Then you can restore it with a single method call as well.

The following example is an imaginary computer-aided design (CAD) system that demonstrates the approach. In addition, it throws in the issue of static fields—if you look at the documentation you’ll see that Class is Serializable, so it should be easy to store the static fields by simply serializing the Class object. That seems like a sensible approach, anyway.

//: c11:CADState.java
// Saving and restoring the state of a
// pretend CAD system.
import java.io.*;
import java.util.*;

abstract class Shape implements Serializable {
  public static final int
    RED = 1, BLUE = 2, GREEN = 3;
  private int xPos, yPos, dimension;
  private static Random r = new Random();
  private static int counter = 0;
  abstract public void setColor(int newColor);
  abstract public int getColor();
  public Shape(int xVal, int yVal, int dim) {
    xPos = xVal;
    yPos = yVal;
    dimension = dim;
  }
  public String toString() {
    return getClass() +
      " color[" + getColor() +
      "] xPos[" + xPos +
      "] yPos[" + yPos +
      "] dim[" + dimension + "]\n";
  }
  public static Shape randomFactory() {
    int xVal = r.nextInt() % 100;
    int yVal = r.nextInt() % 100;
    int dim = r.nextInt() % 100;
    switch(counter++ % 3) {
      default:
      case 0: return new Circle(xVal, yVal, dim);
      case 1: return new Square(xVal, yVal, dim);
      case 2: return new Line(xVal, yVal, dim);
    }
  }
}

class Circle extends Shape {
  private static int color = RED;
  public Circle(int xVal, int yVal, int dim) {
    super(xVal, yVal, dim);
  }
  public void setColor(int newColor) {
    color = newColor;
  }
  public int getColor() {
    return color;
  }
}

class Square extends Shape {
  private static int color;
  public Square(int xVal, int yVal, int dim) {
    super(xVal, yVal, dim);
    color = RED;
  }
  public void setColor(int newColor) {
    color = newColor;
  }
  public int getColor() {
    return color;
  }
}

class Line extends Shape {
  private static int color = RED;
  public static void
  serializeStaticState(ObjectOutputStream os)
      throws IOException {
    os.writeInt(color);
  }
  public static void
  deserializeStaticState(ObjectInputStream os)
      throws IOException {
    color = os.readInt();
  }
  public Line(int xVal, int yVal, int dim) {
    super(xVal, yVal, dim);
  }
  public void setColor(int newColor) {
    color = newColor;
  }
  public int getColor() {
    return color;
  }
}

public class CADState {
  public static void main(String[] args)
      throws Exception {
    ArrayList shapeTypes, shapes;
    if(args.length == 0) {
      shapeTypes = new ArrayList();
      shapes = new ArrayList();
      // Add references to the class objects:
      shapeTypes.add(Circle.class);
      shapeTypes.add(Square.class);
      shapeTypes.add(Line.class);
      // Make some shapes:
      for(int i = 0; i < 10; i++)
        shapes.add(Shape.randomFactory());
      // Set all the static colors to GREEN:
      for(int i = 0; i < 10; i++)
        ((Shape)shapes.get(i))
          .setColor(Shape.GREEN);
      // Save the state vector:
      ObjectOutputStream out =
        new ObjectOutputStream(
          new FileOutputStream("CADState.out"));
      out.writeObject(shapeTypes);
      Line.serializeStaticState(out);
      out.writeObject(shapes);
    } else { // There's a command-line argument
      ObjectInputStream in =
        new ObjectInputStream(
          new FileInputStream(args[0]));
      // Read in the same order they were written:
      shapeTypes = (ArrayList)in.readObject();
      Line.deserializeStaticState(in);
      shapes = (ArrayList)in.readObject();
    }
    // Display the shapes:
    System.out.println(shapes);
  }
} ///:~

The Shape class implements Serializable, so anything that is inherited from Shape is automatically Serializable as well. Each Shape contains data, and each derived Shape class contains a static field that determines the color of all of those types of Shapes. (Placing a static field in the base class would result in only one field, since static fields are not duplicated in derived classes.) Methods in the base class can be overridden to set the color for the various types (static methods are not dynamically bound, so these are normal methods). The randomFactory( ) method creates a different Shape each time you call it, using random values for the Shape data.

Circle and Square are straightforward extensions of Shape; the only difference is that Circle initializes color at the point of definition and Square initializes it in the constructor. We’ll leave the discussion of Line for later.

In main( ), one ArrayList is used to hold the Class objects and the other to hold the shapes. If you don’t provide a command line argument the shapeTypes ArrayList is created and the Class objects are added, and then the shapes ArrayList is created and Shape objects are added. Next, all the static color values are set to GREEN, and everything is serialized to the file CADState.out.

If you provide a command line argument (presumably CADState.out), that file is opened and used to restore the state of the program. In both situations, the resulting ArrayList of Shapes is printed. The results from one run are:

>java CADState
[class Circle color[3] xPos[-51] yPos[-99] dim[38]
, class Square color[3] xPos[2] yPos[61] dim[-46]
, class Line color[3] xPos[51] yPos[73] dim[64]
, class Circle color[3] xPos[-70] yPos[1] dim[16]
, class Square color[3] xPos[3] yPos[94] dim[-36]
, class Line color[3] xPos[-84] yPos[-21] dim[-35]
, class Circle color[3] xPos[-75] yPos[-43] dim[22]
, class Square color[3] xPos[81] yPos[30] dim[-45]
, class Line color[3] xPos[-29] yPos[92] dim[17]
, class Circle color[3] xPos[17] yPos[90] dim[-76]
]

>java CADState CADState.out
[class Circle color[1] xPos[-51] yPos[-99] dim[38]
, class Square color[0] xPos[2] yPos[61] dim[-46]
, class Line color[3] xPos[51] yPos[73] dim[64]
, class Circle color[1] xPos[-70] yPos[1] dim[16]
, class Square color[0] xPos[3] yPos[94] dim[-36]
, class Line color[3] xPos[-84] yPos[-21] dim[-35]
, class Circle color[1] xPos[-75] yPos[-43] dim[22]
, class Square color[0] xPos[81] yPos[30] dim[-45]
, class Line color[3] xPos[-29] yPos[92] dim[17]
, class Circle color[1] xPos[17] yPos[90] dim[-76]
]

You can see that the values of xPos, yPos, and dim were all stored and recovered successfully, but there’s something wrong with the retrieval of the static information. It’s all ‘3’ going in, but it doesn’t come out that way. Circles have a value of 1 (RED, which is the definition), and Squares have a value of 0 (remember, they are initialized in the constructor). It’s as if the statics didn’t get serialized at all! That’s right—even though class Class is Serializable, it doesn’t do what you expect. So if you want to serialize statics, you must do it yourself.

This is what the serializeStaticState( ) and deserializeStaticState( ) static methods in Line are for. You can see that they are explicitly called as part of the storage and retrieval process. (Note that the order of writing to the serialize file and reading back from it must be maintained.) Thus to make CADState.java run correctly you must (1) Add a serializeStaticState( ) and deserializeStaticState( ) to the shapes, (2) Remove the ArrayList shapeTypes and all code related to it, and (3) Add calls to the new serialize and deserialize static methods in the shapes.

Another issue you might have to think about is security, since serialization also saves private data. If you have a security issue, those fields should be marked as transient. But then you have to design a secure way to store that information so that when you do a restore you can reset those private variables.

Checking capitalization style

In this section we’ll look at a more complete example of the use of Java IO. This project is directly useful because it performs a style check to make sure that your capitalization conforms to the Java style as found at http://java.sun.com/docs/codeconv/index.html. It opens each .java file in the current directory and extracts all the class names and identifiers, then shows you if any of them don’t meet the Java style.

For the program to operate correctly, you must first build a class name repository to hold all the class names in the standard Java library. You do this by moving into all the source code subdirectories for the standard Java library and running ClassScanner in each subdirectory. Provide as arguments the name of the repository file (using the same path and name each time) and the -a command-line option to indicate that the class names should be added to the repository.

To use the program to check your code, run it and hand it the path and name of the repository to use. It will check all the classes and identifiers in the current directory and tell you which ones don’t follow the typical Java capitalization style.

You should be aware that the program isn’t perfect; there are a few times when it will point out what it thinks is a problem but on looking at the code you’ll see that nothing needs to be changed. This is a little annoying, but it’s still much easier than trying to find all these cases by staring at your code.

The explanation immediately follows the listing:

//: c11:ClassScanner.java
// Scans all files in directory for classes
// and identifiers, to check capitalization.
// Assumes properly compiling code listings.
// Doesn't do everything right, but is a very
// useful aid.
import java.io.*;
import java.util.*;

class MultiStringMap extends HashMap {
  public void add(String key, String value) {
    if(!containsKey(key))
      put(key, new ArrayList());
    ((ArrayList)get(key)).add(value);
  }
  public ArrayList getArrayList(String key) {
    if(!containsKey(key)) {
      System.err.println(
        "ERROR: can't find key: " + key);
      System.exit(1);
    }
    return (ArrayList)get(key);
  }
  public void printValues(PrintStream p) {
    Iterator k = keySet().iterator();
    while(k.hasNext()) {
      String oneKey = (String)k.next();
      ArrayList val = getArrayList(oneKey);
      for(int i = 0; i < val.size(); i++)
        p.println((String)val.get(i));
    }
  }
}

public class ClassScanner {
  private File path;
  private String[] fileList;
  private Properties classes = new Properties();
  private MultiStringMap
    classMap = new MultiStringMap(),
    identMap = new MultiStringMap();
  private StreamTokenizer in;
  public ClassScanner() {
    path = new File(".");
    fileList = path.list(new JavaFilter());
    for(int i = 0; i < fileList.length; i++) {
      System.out.println(fileList[i]);
      scanListing(fileList[i]);
    }
  }
  void scanListing(String fname) {
    try {
      in = new StreamTokenizer(
          new BufferedReader(
            new FileReader(fname)));
      // Doesn't seem to work:
      // in.slashStarComments(true);
      // in.slashSlashComments(true);
      in.ordinaryChar('/');
      in.ordinaryChar('.');
      in.wordChars('_', '_');
      in.eolIsSignificant(true);
      while(in.nextToken() !=
            StreamTokenizer.TT_EOF) {
        if(in.ttype == '/')
          eatComments();
        else if(in.ttype ==
                StreamTokenizer.TT_WORD) {
          if(in.sval.equals("class") ||
             in.sval.equals("interface")) {
            // Get class name:
               while(in.nextToken() !=
                     StreamTokenizer.TT_EOF
                     && in.ttype !=
                     StreamTokenizer.TT_WORD)
                 ;
               classes.put(in.sval, in.sval);
               classMap.add(fname, in.sval);
          }
          if(in.sval.equals("import") ||
             in.sval.equals("package"))
            discardLine();
          else // It's an identifier or keyword
            identMap.add(fname, in.sval);
        }
      }
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
  void discardLine() {
    try {
      while(in.nextToken() !=
            StreamTokenizer.TT_EOF
            && in.ttype !=
            StreamTokenizer.TT_EOL)
        ; // Throw away tokens to end of line
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
  // StreamTokenizer's comment removal seemed
  // to be broken. This extracts them:
  void eatComments() {
    try {
      if(in.nextToken() !=
         StreamTokenizer.TT_EOF) {
        if(in.ttype == '/')
          discardLine();
        else if(in.ttype != '*')
          in.pushBack();
        else
          while(true) {
            if(in.nextToken() ==
              StreamTokenizer.TT_EOF)
              break;
            if(in.ttype == '*')
              if(in.nextToken() !=
                StreamTokenizer.TT_EOF
                && in.ttype == '/')
                break;
          }
      }
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
  public String[] classNames() {
    String[] result = new String[classes.size()];
    Iterator e = classes.keySet().iterator();
    int i = 0;
    while(e.hasNext())
      result[i++] = (String)e.next();
    return result;
  }
  public void checkClassNames() {
    Iterator files = classMap.keySet().iterator();
    while(files.hasNext()) {
      String file = (String)files.next();
      ArrayList cls = classMap.getArrayList(file);
      for(int i = 0; i < cls.size(); i++) {
        String className = (String)cls.get(i);
        if(Character.isLowerCase(
             className.charAt(0)))
          System.out.println(
            "class capitalization error, file: "
            + file + ", class: "
            + className);
      }
    }
  }
  public void checkIdentNames() {
    Iterator files = identMap.keySet().iterator();
    ArrayList reportSet = new ArrayList();
    while(files.hasNext()) {
      String file = (String)files.next();
      ArrayList ids = identMap.getArrayList(file);
      for(int i = 0; i < ids.size(); i++) {
        String id = (String)ids.get(i);
        if(!classes.contains(id)) {
          // Ignore identifiers of length 3 or
          // longer that are all uppercase
          // (probably static final values):
          if(id.length() >= 3 &&
             id.equals(
               id.toUpperCase()))
            continue;
          // Check to see if first char is upper:
          if(Character.isUpperCase(id.charAt(0))){
            if(reportSet.indexOf(file + id)
                == -1){ // Not reported yet
              reportSet.add(file + id);
              System.out.println(
                "Ident capitalization error in:"
                + file + ", ident: " + id);
            }
          }
        }
      }
    }
  }
  static final String usage =
    "Usage: \n" +
    "ClassScanner classnames -a\n" +
    "\tAdds all the class names in this \n" +
    "\tdirectory to the repository file \n" +
    "\tcalled 'classnames'\n" +
    "ClassScanner classnames\n" +
    "\tChecks all the java files in this \n" +
    "\tdirectory for capitalization errors, \n" +
    "\tusing the repository file 'classnames'";
  private static void usage() {
    System.err.println(usage);
    System.exit(1);
  }
  public static void main(String[] args) {
    if(args.length < 1 || args.length > 2)
      usage();
    ClassScanner c = new ClassScanner();
    File old = new File(args[0]);
    if(old.exists()) {
      try {
        // Try to open an existing
        // properties file:
        InputStream oldlist =
          new BufferedInputStream(
            new FileInputStream(old));
        c.classes.load(oldlist);
        oldlist.close();
      } catch(IOException e) {
        System.err.println("Could not open "
          + old + " for reading");
        System.exit(1);
      }
    }
    if(args.length == 1) {
      c.checkClassNames();
      c.checkIdentNames();
    }
    // Write the class names to a repository:
    if(args.length == 2) {
      if(!args[1].equals("-a"))
        usage();
      try {
        BufferedOutputStream out =
          new BufferedOutputStream(
            new FileOutputStream(args[0]));
        c.classes.save(out,
          "Classes found by ClassScanner.java");
        out.close();
      } catch(IOException e) {
        System.err.println(
          "Could not write " + args[0]);
        System.exit(1);
      }
    }
  }
}

class JavaFilter implements FilenameFilter {
  public boolean accept(File dir, String name) {
    // Strip path information:
    String f = new File(name).getName();
    return f.trim().endsWith(".java");
  }
} ///:~

The class MultiStringMap is a tool that allows you to map a group of strings onto each key entry. As in the previous example, it uses a HashMap (this time with inheritance) with the key as the single string that’s mapped onto the ArrayList value. The add( ) method simply checks to see if there’s a key already in the HashMap, and if not it puts one there. The getArrayList( ) method produces an ArrayList for a particular key, and printValues( ), which is primarily useful for debugging, prints out all the values ArrayList by ArrayList.

To keep life simple, the class names from the standard Java libraries are all put into a Properties object (from the standard Java library). Remember that a Properties object is a HashMap that holds only String objects for both the key and value entries. However, it can be saved to disk and restored from disk in one method call, so it’s ideal for the repository of names. Actually, we need only a list of names, and a HashMap can’t accept null for either its key or its value entry. So the same object will be used for both the key and the value.

For the classes and identifiers that are discovered for the files in a particular directory, two MultiStringMaps are used: classMap and identMap. Also, when the program starts up it loads the standard class name repository into the Properties object called classes, and when a new class name is found in the local directory that is also added to classes as well as to classMap. This way, classMap can be used to step through all the classes in the local directory, and classes can be used to see if the current token is a class name (which indicates a definition of an object or method is beginning, so grab the next tokens—until a semicolon—and put them into identMap).

The default constructor for ClassScanner creates a list of file names (using the JavaFilter implementation of FilenameFilter, as described in Chapter 11). Then it calls scanListing( ) for each file name.

Inside scanListing( ) the source code file is opened and turned into a StreamTokenizer. In the documentation, passing true to slashStarComments( ) and slashSlashComments( ) is supposed to strip those comments out, but this seems to be a bit flawed (it doesn’t quite work in Java 1.0). Instead, those lines are commented out and the comments are extracted by another method. To do this, the ‘/’ must be captured as an ordinary character rather than letting the StreamTokenizer absorb it as part of a comment, and the ordinaryChar( ) method tells the StreamTokenizer to do this. This is also true for dots (‘.’), since we want to have the method calls pulled apart into individual identifiers. However, the underscore, which is ordinarily treated by StreamTokenizer as an individual character, should be left as part of identifiers since it appears in such static final values as TT_EOF etc., used in this very program. The wordChars( ) method takes a range of characters you want to add to those that are left inside a token that is being parsed as a word. Finally, when parsing for one-line comments or discarding a line we need to know when an end-of-line occurs, so by calling eolIsSignificant(true) the eol will show up rather than being absorbed by the StreamTokenizer.

The rest of scanListing( ) reads and reacts to tokens until the end of the file, signified when nextToken( ) returns the final static value StreamTokenizer.TT_EOF.

If the token is a ‘/’ it is potentially a comment, so eatComments( ) is called to deal with it. The only other situation we’re interested in here is if it’s a word, of which there are some special cases.

If the word is class or interface then the next token represents a class or interface name, and it is put into classes and classMap. If the word is import or package, then we don’t want the rest of the line. Anything else must be an identifier (which we’re interested in) or a keyword (which we’re not, but they’re all lowercase anyway so it won’t spoil things to put those in). These are added to identMap.

The discardLine( ) method is a simple tool that looks for the end of a line. Note that any time you get a new token, you must check for the end of the file.

The eatComments( ) method is called whenever a forward slash is encountered in the main parsing loop. However, that doesn’t necessarily mean a comment has been found, so the next token must be extracted to see if it’s another forward slash (in which case the line is discarded) or an asterisk. But if it’s neither of those, it means the token you’ve just pulled out is needed back in the main parsing loop! Fortunately, the pushBack( ) method allows you to “push back” the current token onto the input stream so that when the main parsing loop calls nextToken( ) it will get the one you just pushed back.

For convenience, the classNames( ) method produces an array of all the names in the classes container. This method is not used in the program but is helpful for debugging.

The next two methods are the ones in which the actual checking takes place. In checkClassNames( ), the class names are extracted from the classMap (which, remember, contains only the names in this directory, organized by file name so the file name can be printed along with the errant class name). This is accomplished by pulling each associated ArrayList and stepping through that, looking to see if the first character is lowercase. If so, the appropriate error message is printed.

In checkIdentNames( ), a similar approach is taken: each identifier name is extracted from identMap. If the name is not in the classes list, it’s assumed to be an identifier or keyword. A special case is checked: if the identifier length is 3 or more and all the characters are uppercase, this identifier is ignored because it’s probably a static final value such as TT_EOF. Of course, this is not a perfect algorithm, but it assumes that you’ll eventually notice any all-uppercase identifiers that are out of place.

Instead of reporting every identifier that starts with an uppercase character, this method keeps track of which ones have already been reported in an ArrayList called reportSet( ). This treats the ArrayList as a “set” that tells you whether an item is already in the set. The item is produced by concatenating the file name and identifier. If the element isn’t in the set, it’s added and then the report is made.

The rest of the listing is comprised of main( ), which busies itself by handling the command line arguments and figuring out whether you’re building a repository of class names from the standard Java library or checking the validity of code you’ve written. In both cases it makes a ClassScanner object.

Whether you’re building a repository or using one, you must try to open the existing repository. By making a File object and testing for existence, you can decide whether to open the file and load( ) the Properties list classes inside ClassScanner. (The classes from the repository add to, rather than overwrite, the classes found by the ClassScanner constructor.) If you provide only one command-line argument it means that you want to perform a check of the class names and identifier names, but if you provide two arguments (the second being “-a”) you’re building a class name repository. In this case, an output file is opened and the method Properties.save( ) is used to write the list into a file, along with a string that provides header file information.

Summary

The Java IO stream library does seem to satisfy the basic requirements: you can perform reading and writing with the console, a file, a block of memory, or even across the Internet (as you will see in Chapter 15). It’s possible (by inheriting from InputStream and OutputStream) to create new types of input and output objects. And you can even add a simple extensibility to the kinds of objects a stream will accept by redefining the toString( ) method that’s automatically called when you pass an object to a method that’s expecting a String (Java’s limited “automatic type conversion”).

There are questions left unanswered by the documentation and design of the IO stream library. For example, it would have been nice if you could say that you want an exception thrown if you try to overwrite a file when opening it for output—some programming systems allow you to specify that you want to open an output file, but only if it doesn’t already exist. In Java, it appears that you are supposed to use a File object to determine whether a file exists, because if you open it as an FileOutputStream or FileWriter it will always get overwritten. By representing both files and directory paths, the File class also suggests poor design by violating the maxim “Don’t try to do too much in a single class.”

The IO stream library brings up mixed feelings. It does much of the job and it’s portable. But if you don’t already understand the decorator pattern, the design is non-intuitive, so there’s extra overhead in learning and teaching it. It’s also incomplete: there’s no support for the kind of output formatting that almost every other language’s IO package supports. (This was not remedied in Java 1.1, which missed the opportunity to change the library design completely, and instead added even more special cases and complexity.) The Java 1.1 changes to the IO library haven’t been replacements, but rather additions, and it seems that the library designers couldn’t quite get straight which features are deprecated and which are preferred, resulting in annoying deprecation messages that show up the contradictions in the library design.

However, once you do understand the decorator pattern and begin using the library in situations that require its flexibility, you can begin to benefit from this design, at which point its cost in extra lines of code may not bother you as much.

Exercises
  1. Open a text file so that you can read the file one line at a time. Read each line as a String and place that String object into a LinkedList. Print all of the lines in the LinkedList in reverse order.
  2. Modify Exercise 1 so that the name of the file you read is provided as a command-line argument.
  3. Modify Exercise 2 to also open a text file so you can write text into it. Write the lines in the ArrayList, along with line numbers, out to the file.
  4. Modify Exercise 2 to force all the lines in the ArrayList to upper case and send the results to System.out.
  5. Modify Exercise 2 to take additional arguments of words to find in the file. Print any lines in which the words match.
  6. Create a class called SortedDirList with a constructor that takes file path information and builds a sorted directory list from the files at that path. Create two overloaded list( ) methods that will either produce the whole list or a subset of the list based on an argument. Add a size( ) method that takes a file name and produces the size of that file.
  7. In Blips.java, copy the file and rename it to BlipCheck.java and rename the class Blip2 to BlipCheck (making it public and removing the public scope from the class Blips in the process). Remove the //! marks in the file and execute the program including the offending lines. Next, comment out the default constructor for BlipCheck. Run it and explain why it works. Note that after compiling, you must execute the program with ‘java Blips’ because the main( ) method is still in class Blips.
  8. In Blip3.java, comment out the two lines after the phrases “You must do this:” and run the program. Explain the result and why it differs from when the two lines are in the program.
  9. Modify the SortedWordCount.java program so that it produces an alphabetic sort instead, using the tool from Chapter 9.
  10. Modify SortedWordCount.java so that it uses a class containing a String and a count value to store each different word, and a Set of these objects to maintain the list of words.
  11. Convert the SortedWordCount.java program to use the Java 1.1 IO Streams.
  12. Repair the program CADState.java as described in the text.
  13. (Intermediate) In Chapter 8, locate the GreenhouseControls.java example, which consists of three files. In GreenhouseControls.java, the Restart( ) inner class has a hard-coded set of events. Change the program so that it reads the events and their relative times from a text file. (Challenging: Use a factory method from to build the events—see Thinking in Patterns with Java, downloadable at www.BruceEckel.com.)

[56] Design Patterns, Erich Gamma et al., Addison-Wesley 1995.

[57] Perhaps by the time you read this, the bug will be fixed.


12: Run-time Type Identification

The idea of run-time type identification (RTTI) seems fairly simple at first: it lets you find the exact type of an object when you only have a reference to the base type.

However, the need for RTTI uncovers a whole plethora of interesting (and often perplexing) OO design issues and raises fundamental questions of how you should structure your programs.

This chapter looks at the ways that Java allows you to discover information about objects and classes at run-time. This takes two forms: “traditional” RTTI, which assumes that you have all the types available at compile-time and run-time, and the “reflection” mechanism, which allows you to discover class information solely at run-time. The “traditional” RTTI will be covered first, followed by a discussion of reflection.

The need for RTTI

Consider the now familiar example of a class hierarchy that uses polymorphism. The generic type is the base class Shape, and the specific derived types are Circle, Square, and Triangle:

This is a typical class hierarchy diagram, with the base class at the top and the derived classes growing downward. The normal goal in object-oriented programming is for the bulk of your code to manipulate references to the base type (Shape, in this case), so if you decide to extend the program by adding a new class (Rhomboid, derived from Shape, for example), the bulk of the code is not affected. In this example, the dynamically bound method in the Shape interface is draw( ), so the intent is for the client programmer to call draw( ) through a generic Shape reference. draw( ) is overridden in all of the derived classes, and because it is a dynamically bound method, the proper behavior will occur even though it is called through a generic Shape reference. That’s polymorphism.

Thus, you generally create a specific object (Circle, Square, or Triangle), upcast it to a Shape (forgetting the specific type of the object), and use that anonymous Shape reference in the rest of the program.

As a brief review of polymorphism and upcasting, you might code the above example as follows:

//: c12:Shapes.java
import java.util.*;

class Shape {
  void draw() {
    System.out.println(this + ".draw()");
  }
}

class Circle extends Shape {
  public String toString() { return "Circle"; }
}

class Square extends Shape {
  public String toString() { return "Square"; }
}

class Triangle extends Shape {
  public String toString() { return "Triangle"; }
}

public class Shapes {
  public static void main(String[] args) {
    ArrayList s = new ArrayList();
    s.add(new Circle());
    s.add(new Square());
    s.add(new Triangle());
    Iterator e = s.iterator();
    while(e.hasNext())
      ((Shape)e.next()).draw();
  }
} ///:~

The base class contains a draw( ) method that indirectly uses toString( ) to print an identifier for the class by passing this to System.out.println( ). If that function sees an object, it automatically calls the toString( ) method to produce a String representation.

Each of the derived classes overrides the toString( ) method (from Object) so that draw( ) ends up printing something different in each case. In main( ), specific types of Shape are created and then added to an ArrayList. This is the point at which the upcast occurs because the ArrayList holds only Objects. Since everything in Java (with the exception of primitives) is an Object, an ArrayList can also hold Shape objects. But during an upcast to Object, it also loses any specific information, including the fact that the objects are Shapes. To the ArrayList, they are just Objects.

At the point you fetch an element out of the ArrayList with next( ), things get a little busy. Since ArrayList holds only Objects, next( ) naturally produces an Object reference. But we know it’s really a Shape reference, and we want to send Shape messages to that object. So a cast to Shape is necessary using the traditional “(Shape)” cast. This is the most basic form of RTTI, since in Java all casts are checked at run-time for correctness. That’s exactly what RTTI means: at run-time, the type of an object is identified.

In this case, the RTTI cast is only partial: the Object is cast to a Shape, and not all the way to a Circle, Square, or Triangle. That’s because the only thing we know at this point is that the ArrayList is full of Shapes. At compile-time, this is enforced only by your own self-imposed rules, but at run-time the cast ensures it.

Now polymorphism takes over and the exact method that’s called for the Shape is determined by whether the reference is for a Circle, Square, or Triangle. And in general, this is how it should be; you want the bulk of your code to know as little as possible about specific types of objects, and to just deal with the general representation of a family of objects (in this case, Shape). As a result, your code will be easier to write, read, and maintain, and your designs will be easier to implement, understand, and change. So polymorphism is the general goal in object-oriented programming.

But what if you have a special programming problem that’s easiest to solve if you know the exact type of a generic reference? For example, suppose you want to allow your users to highlight all the shapes of any particular type by turning them purple. This way, they can find all the triangles on the screen by highlighting them. This is what RTTI accomplishes: you can ask a reference to a Shape exactly what type it’s referring to.

The Class object

To understand how RTTI works in Java, you must first know how type information is represented at run-time. This is accomplished through a special kind of object called the Class object, which contains information about the class. (This is sometimes called a meta-class.) In fact, the Class object is used to create all of the “regular” objects of your class.

There’s a Class object for each class that is part of your program. That is, each time you write and compile a new class, a single Class object is also created (and stored, appropriately enough, in an identically named .class file). At run-time, when you want to make an object of that class, the Java Virtual Machine (JVM) that’s executing your program first checks to see if the Class object for that type is loaded. If not, the JVM loads it by finding the .class file with that name. Thus, a Java program isn’t completely loaded before it begins, which is different from many traditional languages.

Once the Class object for that type is in memory, it is used to create all objects of that type.

If this seems shadowy or if you don’t really believe it, here’s a demonstration program to prove it:

//: c12:SweetShop.java
// Examination of the way the class loader works.

class Candy {
  static {
    System.out.println("Loading Candy");
  }
}

class Gum {
  static {
    System.out.println("Loading Gum");
  }
}

class Cookie {
  static {
    System.out.println("Loading Cookie");
  }
}

public class SweetShop {
  public static void main(String[] args) {
    System.out.println("inside main");
    new Candy();
    System.out.println("After creating Candy");
    try {
      Class.forName("Gum");
    } catch(ClassNotFoundException e) {
      e.printStackTrace();
    }
    System.out.println(
      "After Class.forName(\"Gum\")");
    new Cookie();
    System.out.println("After creating Cookie");
  }
} ///:~

Each of the classes Candy, Gum, and Cookie has a static clause that is executed as the class is loaded for the first time. Information will be printed to tell you when loading occurs for that class. In main( ), the object creations are spread out between print statements to help detect the time of loading.

A particularly interesting line is:

Class.forName("Gum");

This method is a static member of Class (to which all Class objects belong). A Class object is like any other object and so you can get and manipulate a reference to it. (That’s what the loader does.) One of the ways to get a reference to the Class object is forName( ), which takes a String containing the textual name (watch the spelling and capitalization!) of the particular class you want a reference for. It returns a Class reference.

The output of this program for one JVM is:

inside main
Loading Candy
After creating Candy
Loading Gum
After Class.forName("Gum")
Loading Cookie
After creating Cookie

You can see that each Class object is loaded only when it’s needed, and the static initialization is performed upon class loading.

Class literals

Java provides a second way to produce the reference to the Class object, using a class literal. In the above program this would look like:

Gum.class;

which is not only simpler, but also safer since it’s checked at compile-time. Because it eliminates the method call, it’s also more efficient.

Class literals work with regular classes as well as interfaces, arrays, and primitive types. In addition, there’s a standard field called TYPE that exists for each of the primitive wrapper classes. The TYPE field produces a reference to the Class object for the associated primitive type, such that:

... is equivalent to ...
boolean.class | Boolean.TYPE
char.class | Character.TYPE
byte.class | Byte.TYPE
short.class | Short.TYPE
int.class | Integer.TYPE
long.class | Long.TYPE
float.class | Float.TYPE
double.class | Double.TYPE
void.class | Void.TYPE

My preference is to use the “.class” versions if you can, since they’re more consistent with regular classes.

Checking before a cast

So far, you’ve seen RTTI forms including:

  1. The classic cast, e.g., “(Shape),” which uses RTTI to make sure the cast is correct and throws a ClassCastException if you’ve performed a bad cast.
  2. The Class object representing the type of your object. The Class object can be queried for useful run-time information.

In C++, the classic cast “(Shape)” does not perform RTTI. It simply tells the compiler to treat the object as the new type. In Java, which does perform the type check, this cast is often called a “type safe downcast.” The reason for the term “downcast” is the historical arrangement of the class hierarchy diagram. If casting a Circle to a Shape is an upcast, then casting a Shape to a Circle is a downcast. However, you know a Circle is also a Shape, and the compiler freely allows an upcast assignment, but you don’t know that a Shape is necessarily a Circle, so the compiler doesn’t allow you to perform a downcast assignment without using an explicit cast.

There’s a third form of RTTI in Java. This is the keyword instanceof that tells you if an object is an instance of a particular type. It returns a boolean so you use it in the form of a question, like this:

if(x instanceof Dog)
  ((Dog)x).bark();

The above if statement checks to see if the object x belongs to the class Dog before casting x to a Dog. It’s important to use instanceof before a downcast when you don’t have other information that tells you the type of the object; otherwise you’ll end up with a ClassCastException.

Ordinarily, you might be hunting for one type (triangles to turn purple, for example), but you can easily tally all of the objects using instanceof. Suppose you have a family of Pet classes:

//: c12:Pets.java
class Pet {}
class Dog extends Pet {}
class Pug extends Dog {}
class Cat extends Pet {}
class Rodent extends Pet {}
class Gerbil extends Rodent {}
class Hamster extends Rodent {}

class Counter { int i; } ///:~

The Counter class is used to keep track of the number of any particular type of Pet. You could think of it as an Integer that can be modified.

Using instanceof, all the pets can be counted:

//: c12:PetCount.java
// Using instanceof.
import java.util.*;

public class PetCount {
  static String[] typenames = {
    "Pet", "Dog", "Pug", "Cat",
    "Rodent", "Gerbil", "Hamster",
  };
  public static void main(String[] args) {
    ArrayList pets = new ArrayList();
    try {
      Class[] petTypes = {
        Class.forName("Dog"),
        Class.forName("Pug"),
        Class.forName("Cat"),
        Class.forName("Rodent"),
        Class.forName("Gerbil"),
        Class.forName("Hamster"),
      };
      for(int i = 0; i < 15; i++)
        pets.add(
          petTypes[
            (int)(Math.random()*petTypes.length)]
            .newInstance());
    } catch(InstantiationException e) {}
      catch(IllegalAccessException e) {}
      catch(ClassNotFoundException e) {}
    HashMap h = new HashMap();
    for(int i = 0; i < typenames.length; i++)
      h.put(typenames[i], new Counter());
    for(int i = 0; i < pets.size(); i++) {
      Object o = pets.get(i);
      if(o instanceof Pet)
        ((Counter)h.get("Pet")).i++;
      if(o instanceof Dog)
        ((Counter)h.get("Dog")).i++;
      if(o instanceof Pug)
        ((Counter)h.get("Pug")).i++;
      if(o instanceof Cat)
        ((Counter)h.get("Cat")).i++;
      if(o instanceof Rodent)
        ((Counter)h.get("Rodent")).i++;
      if(o instanceof Gerbil)
        ((Counter)h.get("Gerbil")).i++;
      if(o instanceof Hamster)
        ((Counter)h.get("Hamster")).i++;
    }
    for(int i = 0; i < pets.size(); i++)
      System.out.println(pets.get(i).getClass());
    for(int i = 0; i < typenames.length; i++)
      System.out.println(
        typenames[i] + " quantity: " +
        ((Counter)h.get(typenames[i])).i);
  }
} ///:~

There’s a rather narrow restriction on instanceof: You can compare it to a named type only, and not to a Class object. In the example above you might feel that it’s tedious to write out all of those instanceof expressions, and you’re right. But there is no way to cleverly automate instanceof by creating an ArrayList of Class objects and comparing it to those instead (stay tuned—you’ll see an alternative). This isn’t as great a restriction as you might think, because you’ll eventually understand that your design is probably flawed if you end up writing a lot of instanceof expressions.

Of course this example is contrived—you’d probably put a static data member in each type and increment it in the constructor to keep track of the counts. You would do something like that if you had control of the source code for the class and could change it. Since this is not always the case, RTTI can come in handy.

Using class literals

It’s interesting to see how the PetCount.java example can be rewritten using class literals. The result is cleaner in many ways:

//: c12:PetCount2.java
// Using class literals.
import java.util.*;

public class PetCount2 {
  public static void main(String[] args) {
    ArrayList pets = new ArrayList();
    Class[] petTypes = {
      // Class literals:
      Pet.class,
      Dog.class,
      Pug.class,
      Cat.class,
      Rodent.class,
      Gerbil.class,
      Hamster.class,
    };
    try {
      for(int i = 0; i < 15; i++) {
        // Offset by one to eliminate Pet.class:
        int rnd = 1 + (int)(
          Math.random() * (petTypes.length - 1));
        pets.add(
          petTypes[rnd].newInstance());
      }
    } catch(InstantiationException e) {}
      catch(IllegalAccessException e) {}
    HashMap h = new HashMap();
    for(int i = 0; i < petTypes.length; i++)
      h.put(petTypes[i].toString(),
        new Counter());
    for(int i = 0; i < pets.size(); i++) {
      Object o = pets.get(i);
      if(o instanceof Pet)
        ((Counter)h.get("class Pet")).i++;
      if(o instanceof Dog)
        ((Counter)h.get("class Dog")).i++;
      if(o instanceof Pug)
        ((Counter)h.get("class Pug")).i++;
      if(o instanceof Cat)
        ((Counter)h.get("class Cat")).i++;
      if(o instanceof Rodent)
        ((Counter)h.get("class Rodent")).i++;
      if(o instanceof Gerbil)
        ((Counter)h.get("class Gerbil")).i++;
      if(o instanceof Hamster)
        ((Counter)h.get("class Hamster")).i++;
    }
    for(int i = 0; i < pets.size(); i++)
      System.out.println(pets.get(i).getClass());
    Iterator keys = h.keySet().iterator();
    while(keys.hasNext()) {
      String nm = (String)keys.next();
      Counter cnt = (Counter)h.get(nm);
      System.out.println(
        nm.substring(nm.lastIndexOf('.') + 1) +
        " quantity: " + cnt.i);
    }
  }
} ///:~

Here, the typenames array has been removed in favor of getting the type name strings from the Class object. Notice that the system can distinguish between classes and interfaces.

You can also see that the creation of petTypes does not need to be surrounded by a try block since it’s evaluated at compile-time and thus won’t throw any exceptions, unlike Class.forName( ).

When the Pet objects are dynamically created, you can see that the random number is restricted so it is between 1 and petTypes.length and does not include zero. That’s because zero refers to Pet.class, and presumably a generic Pet object is not interesting. However, since Pet.class is part of petTypes the result is that all of the pets get counted.

A dynamic instanceof

The Class isInstance method provides a way to dynamically call the instanceof operator. Thus, all those tedious instanceof statements can be removed in the PetCount example:

//: c12:PetCount3.java
// Using isInstance().
import java.util.*;

public class PetCount3 {
  public static void main(String[] args) {
    ArrayList pets = new ArrayList();
    Class[] petTypes = {
      Pet.class,
      Dog.class,
      Pug.class,
      Cat.class,
      Rodent.class,
      Gerbil.class,
      Hamster.class,
    };
    try {
      for(int i = 0; i < 15; i++) {
        // Offset by one to eliminate Pet.class:
        int rnd = 1 + (int)(
          Math.random() * (petTypes.length - 1));
        pets.add(
          petTypes[rnd].newInstance());
      }
    } catch(InstantiationException e) {}
      catch(IllegalAccessException e) {}
    HashMap h = new HashMap();
    for(int i = 0; i < petTypes.length; i++)
      h.put(petTypes[i].toString(),
        new Counter());
    for(int i = 0; i < pets.size(); i++) {
      Object o = pets.get(i);
      // Using isInstance to eliminate individual
      // instanceof expressions:
      for (int j = 0; j < petTypes.length; ++j)
        if (petTypes[j].isInstance(o)) {
          String key = petTypes[j].toString();
          ((Counter)h.get(key)).i++;
        }
    }
    for(int i = 0; i < pets.size(); i++)
      System.out.println(pets.get(i).getClass());
    Iterator keys = h.keySet().iterator();
    while(keys.hasNext()) {
      String nm = (String)keys.next();
      Counter cnt = (Counter)h.get(nm);
      System.out.println(
        nm.substring(nm.lastIndexOf('.') + 1) +
        " quantity: " + cnt.i);
    }
  }
} ///:~

You can see that the isInstance( ) method has eliminated the need for the instanceof expressions. In addition, this means that you can add new types of pets simply by changing the petTypes array; the rest of the program does not need modification (as it did when using the instanceof expressions).

instanceof vs. Class equivalence

When querying for type information, there’s an important difference between either form of instanceof (that is, instanceof or isInstance( ), which produce equivalent results) and the direct comparison of the Class objects. Here’s an example that demonstrates the difference:

//: c12:FamilyVsExactType.java
// The difference between instanceof and class

class Base {}
class Derived extends Base {}

public class FamilyVsExactType {
  static void test(Object x) {
    System.out.println("Testing x of type " +
      x.getClass());
    System.out.println("x instanceof Base " +
      (x instanceof Base));
    System.out.println("x instanceof Derived " +
      (x instanceof Derived));
    System.out.println("Base.isInstance(x) " +
      Base.class.isInstance(x));
    System.out.println("Derived.isInstance(x) " +
      Derived.class.isInstance(x));
    System.out.println(
      "x.getClass() == Base.class " +
      (x.getClass() == Base.class));
    System.out.println(
      "x.getClass() == Derived.class " +
      (x.getClass() == Derived.class));
    System.out.println(
      "x.getClass().equals(Base.class)) " +
      (x.getClass().equals(Base.class)));
    System.out.println(
      "x.getClass().equals(Derived.class)) " +
      (x.getClass().equals(Derived.class)));
  }
  public static void main(String[] args) {
    test(new Base());
    test(new Derived());
  }
} ///:~

The test( ) method performs type checking with its argument using both forms of instanceof. It then gets the Class reference and uses == and equals( ) to test for equality of the Class objects. Here is the output:

Testing x of type class Base
x instanceof Base true
x instanceof Derived false
Base.isInstance(x) true
Derived.isInstance(x) false
x.getClass() == Base.class true
x.getClass() == Derived.class false
x.getClass().equals(Base.class)) true
x.getClass().equals(Derived.class)) false
Testing x of type class Derived
x instanceof Base true
x instanceof Derived true
Base.isInstance(x) true
Derived.isInstance(x) true
x.getClass() == Base.class false
x.getClass() == Derived.class true
x.getClass().equals(Base.class)) false
x.getClass().equals(Derived.class)) true

Reasuringly, instanceof and isInstance( ) produce exactly the same results, as do equals( ) and ==. But the tests themselves draw different conclusions. In keeping with the concept of type, instanceof says “are you this class, or a class derived from this class?” On the other hand, if you compare the actual Class objects using ==, there is no concern with inheritance—it’s either the exact type or it isn’t.

RTTI syntax

Java performs its RTTI using the Class object, even if you’re doing something like a cast. The class Class also has a number of other ways you can use RTTI.

First, you must get a reference to the appropriate Class object. One way to do this, as shown in the previous example, is to use a string and the Class.forName( ) method. This is convenient because you don’t need an object of that type in order to get the Class reference. However, if you do already have an object of the type you’re interested in, you can fetch the Class reference by calling a method that’s part of the Object root class: getClass( ). This returns the Class reference representing the actual type of the object. Class has many interesting methods, demonstrated in the following example:

//: c12:ToyTest.java
// Testing class Class.

interface HasBatteries {}
interface Waterproof {}
interface ShootsThings {}
class Toy {
  // Comment out the following default
  // constructor to see
  // NoSuchMethodError from (*1*)
  Toy() {}
  Toy(int i) {}
}

class FancyToy extends Toy
    implements HasBatteries,
      Waterproof, ShootsThings {
  FancyToy() { super(1); }
}

public class ToyTest {
  public static void main(String[] args) {
    Class c = null;
    try {
      c = Class.forName("FancyToy");
    } catch(ClassNotFoundException e) {}
    printInfo(c);
    Class[] faces = c.getInterfaces();
    for(int i = 0; i < faces.length; i++)
      printInfo(faces[i]);
    Class cy = c.getSuperclass();
    Object o = null;
    try {
      // Requires default constructor:
      o = cy.newInstance(); // (*1*)
    } catch(InstantiationException e) {}
      catch(IllegalAccessException e) {}
    printInfo(o.getClass());
  }
  static void printInfo(Class cc) {
    System.out.println(
      "Class name: " + cc.getName() +
      " is interface? [" +
      cc.isInterface() + "]");
  }
} ///:~

You can see that class FancyToy is quite complicated, since it inherits from Toy and implements the interfaces of HasBatteries, Waterproof, and ShootsThings. In main( ), a Class reference is created and initialized to the FancyToy Class using forName( ) inside an appropriate try block.

The Class.getInterfaces( ) method returns an array of Class objects representing the interfaces that are contained in the Class object of interest.

If you have a Class object you can also ask it for its direct base class using getSuperclass( ). This, of course, returns a Class reference that you can further query. This means that, at run-time, you can discover an object’s entire class hierarchy.

The newInstance( ) method of Class can, at first, seem like just another way to clone( ) an object. However, you can create a new object with newInstance( ) without an existing object, as seen here, because there is no Toy object, only cy, which is a reference to y’s Class object. This is a way to implement a “virtual constructor,” which allows you to say “I don’t know exactly what type you are, but create yourself properly anyway.” In the example above, cy is just a Class reference with no further type information known at compile-time. And when you create a new instance, you get back an Object reference. But that reference is pointing to a Toy object. Of course, before you can send any messages other than those accepted by Object, you have to investigate it a bit and do some casting. In addition, the class that’s being created with newInstance( ) must have a default constructor. In the next section, you’ll see how to dynamically create objects of classes using any constructor, with the Java reflection API.

The final method in the listing is printInfo( ), which takes a Class reference and gets its name with getName( ), and finds out whether it’s an interface with isInterface( ).

The output from this program is:

Class name: FancyToy is interface? [false]
Class name: HasBatteries is interface? [true]
Class name: Waterproof is interface? [true]
Class name: ShootsThings is interface? [true]
Class name: Toy is interface? [false]

Thus, with the Class object you can find out just about everything you want to know about an object.

Reflection: run-time
class information

If you don’t know the precise type of an object, RTTI will tell you. However, there’s a limitation: the type must be known at compile-time in order for you to be able to detect it using RTTI and do something useful with the information. Put another way, the compiler must know about all the classes you’re working with for RTTI.

This doesn’t seem like that much of a limitation at first, but suppose you’re given a reference to an object that’s not in your program space. In fact, the class of the object isn’t even available to your program at compile-time. For example, suppose you get a bunch of bytes from a disk file or from a network connection and you’re told that those bytes represent a class. Since the compiler can’t know about the class while it’s compiling the code, how can you possibly use such a class?

In a traditional programming environment this seems like a far-fetched scenario. But as we move into a larger programming world there are important cases in which this happens. The first is component-based programming in which you build projects using Rapid Application Development (RAD) in an application builder tool. This is a visual approach to creating a program (which you see on the screen as a “form”) by moving icons that represent components onto the form. These components are then configured by setting some of their values at program time. This design-time configuration requires that any component be instantiable, that it exposes parts of itself, and that it allows its values to be read and set. In addition, components that handle GUI events must expose information about appropriate methods so that the RAD environment can assist the programmer in overriding these event-handling methods. Reflection provides the mechanism to detect the available methods and produce the method names. Java provides a structure for component-based programming through Java Beans (described in Chapter 13).

Another compelling motivation for discovering class information at run-time is to provide the ability to create and execute objects on remote platforms across a network. This is called Remote Method Invocation (RMI) and it allows a Java program to have objects distributed across many machines. This distribution can happen for a number of reasons: for example, perhaps you’re doing a computation-intensive task and you want to break it up and put pieces on machines that are idle in order to speed things up. In some situations you might want to place code that handles particular types of tasks (e.g., “Business Rules” in a multi-tier client/server architecture) on a particular machine so that machine becomes a common repository describing those actions and it can be easily changed to affect everyone in the system. (This is an interesting development since the machine exists solely to make software changes easy!) Along these lines, distributed computing also supports specialized hardware that might be good at a particular task—matrix inversions, for example—but inappropriate or too expensive for general purpose programming.

The class Class (described previously in this chapter) supports the concept of reflection, and there’s an additional library, java.lang.reflect, with classes Field, Method, and Constructor (each of which implement the Member interface). Objects of these types are created by the JVM at run-time to represent the corresponding member in the unknown class. You can then use the Constructors to create new objects, the get( ) and set( ) methods to read and modify the fields associated with Field objects, and the invoke( ) method to call a method associated with a Method object. In addition, you can call the convenience methods getFields( ), getMethods( ), getConstructors( ), etc., to return arrays of the objects representing the fields, methods, and constructors. (You can find out more by looking up the class Class in your online documentation.) Thus, the class information for anonymous objects can be completely determined at run-time, and nothing need be known at compile-time.

It’s important to realize that there’s nothing magic about reflection. When you’re using reflection to interact with an object of an unknown type, the JVM will simply look at the object and see that it belongs to a particular class (just like ordinary RTTI) but then, before it can do anything else, the Class object must be loaded. Thus, the .class file for that particular type must still be available to the JVM, either on the local machine or across the network. So the true difference between RTTI and reflection is that with RTTI, the compiler opens and examines the .class file at compile-time. Put another way, you can call all the methods of an object in the “normal” way. With reflection, the .class file is unavailable at compile-time; it is opened and examined by the run-time environment.

A class method extractor

You’ll rarely need to use the reflection tools directly; they’re in the language to support the other Java features such as object serialization (Chapter 11), Java Beans (Chapter 13), and RMI (Chapter 15). However, there are times when it’s quite useful to be able to dynamically extract information about a class. One extremely useful tool is a class method extractor. As mentioned before, looking at a class definition source code or online documentation shows only the methods that are defined or overridden within that class definition. But there could be dozens more available to you that have come from base classes. To locate these is both tedious and time consuming [58] . Fortunately, reflection provides a way to write a simple tool that will automatically show you the entire interface. Here’s the way it works:

//: c12:ShowMethods.java
// Using reflection to show all the methods of
// a class, even if the methods are defined in
// the base class.
import java.lang.reflect.*;

public class ShowMethods {
  static final String usage =
    "usage: \n" +
    "ShowMethods qualified.class.name\n" +
    "To show all methods in class or: \n" +
    "ShowMethods qualified.class.name word\n" +
    "To search for methods involving 'word'";
  public static void main(String[] args) {
    if(args.length < 1) {
      System.out.println(usage);
      System.exit(0);
    }
    try {
      Class c = Class.forName(args[0]);
      Method[] m = c.getMethods();
      Constructor[] ctor = c.getConstructors();
      if(args.length == 1) {
        for (int i = 0; i < m.length; i++)
          System.out.println(m[i]);
        for (int i = 0; i < ctor.length; i++)
          System.out.println(ctor[i]);
      } else {
        for (int i = 0; i < m.length; i++)
          if(m[i].toString()
             .indexOf(args[1])!= -1)
            System.out.println(m[i]);
        for (int i = 0; i < ctor.length; i++)
          if(ctor[i].toString()
             .indexOf(args[1])!= -1)
          System.out.println(ctor[i]);
      }
    } catch (ClassNotFoundException e) {
      System.out.println("No such class: " + e);
    }
  }
} ///:~

The Class methods getMethods( ) and getConstructors( ) return an array of Method and Constructor, respectively. Each of these classes has further methods to dissect the names, arguments, and return values of the methods they represent. But you can also just use toString( ), as is done here, to produce a String with the entire method signature. The rest of the code is just for extracting command line information, determining if a particular signature matches with your target string (using indexOf( )), and printing the results.

This shows reflection in action, since the result produced by Class.forName( ) cannot be known at compile-time, and therefore all the method signature information is being extracted at run-time. If you investigate your online documentation on reflection, you’ll see that there is enough support to actually set up and make a method call on an object that’s totally unknown at compile-time (there will be examples of this later in this book). Again, this is something you may never need to do yourself—the support is there for RMI and so a programming environment can manipulate JavaBeans—but it’s interesting.

An interesting experiment is to run java ShowMethods ShowMethods. This produces a listing that includes a public default constructor, even though you can see from the code that no constructor was defined. The constructor you see is the one that’s automatically synthesized by the compiler. If you then make ShowMethods a non-public class (that is, friendly), the synthesized default constructor no longer shows up in the output. The synthesized default constructor is automatically given the same access as the class.

The output for ShowMethods is still a little tedious. For example, here’s a portion of the output produced by invoking java ShowMethods java.lang.String:

public boolean
  java.lang.String.startsWith(java.lang.String,int)
public boolean
  java.lang.String.startsWith(java.lang.String)
public boolean
  java.lang.String.endsWith(java.lang.String)

It would be even nicer if the qualifiers like java.lang could be stripped off. The StreamTokenizer class introduced in the previous chapter can help solve this problem:

//: c12:ShowMethodsClean.java
// ShowMethods with the qualifiers stripped
// to make the results easier to read.
import java.lang.reflect.*;
import java.io.*;

public class ShowMethodsClean {
  static final String usage =
    "usage: \n" +
    "ShowMethodsClean qualified.class.name\n" +
    "To show all methods in class or: \n" +
    "ShowMethodsClean qualif.class.name word\n" +
    "To search for methods involving 'word'";
  public static void main(String[] args) {
    if(args.length < 1) {
      System.out.println(usage);
      System.exit(0);
    }
    try {
      Class c = Class.forName(args[0]);
      Method[] m = c.getMethods();
      Constructor[] ctor = c.getConstructors();
      // Convert to an array of cleaned Strings:
      String[] n =
        new String[m.length + ctor.length];
      for(int i = 0; i < m.length; i++) {
        String s = m[i].toString();
        n[i] = StripQualifiers.strip(s);
      }
      for(int i = 0; i < ctor.length; i++) {
        String s = ctor[i].toString();
        n[i + m.length] =
          StripQualifiers.strip(s);
      }
      if(args.length == 1)
        for (int i = 0; i < n.length; i++)
          System.out.println(n[i]);
      else
        for (int i = 0; i < n.length; i++)
          if(n[i].indexOf(args[1])!= -1)
            System.out.println(n[i]);
    } catch (ClassNotFoundException e) {
      System.out.println("No such class: " + e);
    }
  }
}

class StripQualifiers {
  private StreamTokenizer st;
  public StripQualifiers(String qualified) {
      st = new StreamTokenizer(
        new StringReader(qualified));
      st.ordinaryChar(' '); // Keep the spaces
  }
  public String getNext() {
    String s = null;
    try {
      if(st.nextToken() !=
            StreamTokenizer.TT_EOF) {
        switch(st.ttype) {
          case StreamTokenizer.TT_EOL:
            s = null;
            break;
          case StreamTokenizer.TT_NUMBER:
            s = Double.toString(st.nval);
            break;
          case StreamTokenizer.TT_WORD:
            s = new String(st.sval);
            break;
          default: // single character in ttype
            s = String.valueOf((char)st.ttype);
        }
      }
    } catch(IOException e) {
      System.out.println(e);
    }
    return s;
  }
  public static String strip(String qualified) {
    StripQualifiers sq =
      new StripQualifiers(qualified);
    String s = "", si;
    while((si = sq.getNext()) != null) {
      int lastDot = si.lastIndexOf('.');
      if(lastDot != -1)
        si = si.substring(lastDot + 1);
      s += si;
    }
    return s;
  }
} ///:~

The class ShowMethodsClean is quite similar to the previous ShowMethods, except that it takes the arrays of Method and Constructor and converts them into a single array of String. Each of these String objects is then passed through StripQualifiers.Strip( ) to remove all the method qualification. As you can see, this uses the StreamTokenizer and String manipulation to do its work.

This tool can be a real time-saver while you’re programming, when you can’t remember if a class has a particular method and you don’t want to go walking through the class hierarchy in the online documentation, or if you don’t know whether that class can do anything with, for example, Color objects.

Chapter 13 contains a GUI version of this program so you can leave it running while you’re writing code, to allow quick lookups.

Summary

RTTI allows you to discover type information from an anonymous base-class reference. Thus, it’s ripe for misuse by the novice since it might make sense before polymorphic method calls do. For many people coming from a procedural background, it’s difficult not to organize their programs into sets of switch statements. They could accomplish this with RTTI and thus lose the important value of polymorphism in code development and maintenance. The intent of Java is that you use polymorphic method calls throughout your code, and you use RTTI only when you must.

However, using polymorphic method calls as they are intended requires that you have control of the base-class definition because at some point in the extension of your program you might discover that the base class doesn’t include the method you need. If the base class comes from a library or is otherwise controlled by someone else, a solution to the problem is RTTI: You can inherit a new type and add your extra method. Elsewhere in the code you can detect your particular type and call that special method. This doesn’t destroy the polymorphism and extensibility of the program because adding a new type will not require you to hunt for switch statements in your program. However, when you add new code in your main body that requires your new feature, you must use RTTI to detect your particular type.

Putting a feature in a base class might mean that, for the benefit of one particular class, all of the other classes derived from that base require some meaningless stub of a method. This makes the interface less clear and annoys those who must override abstract methods when they derive from that base class. For example, consider a class hierarchy representing musical instruments. Suppose you wanted to clear the spit valves of all the appropriate instruments in your orchestra. One option is to put a ClearSpitValve( ) method in the base class Instrument, but this is confusing because it implies that Percussion and Electronic instruments also have spit valves. RTTI provides a much more reasonable solution in this case because you can place the method in the specific class (Wind in this case), where it’s appropriate. However, a more appropriate solution is to put a prepareInstrument( ) method in the base class, but you might not see this when you’re first solving the problem and could mistakenly assume that you must use RTTI.

Finally, RTTI will sometimes solve efficiency problems. If your code nicely uses polymorphism, but it turns out that one of your objects reacts to this general purpose code in a horribly inefficient way, you can pick out that type using RTTI and write case-specific code to improve the efficiency. Be wary, however, of programming for efficiency too soon. It’s a seductive trap. It’s best to get the program working first, then decide if it’s running fast enough, and only then should you attack efficiency issues—with a profiler.

Exercises
  1. Add Rhomboid to Shapes.java. Create a Rhomboid, upcast it to a Shape, then downcast it back to a Rhomboid. Try downcasting to a Circle and see what happens.
  2. Modify Exercise 1 so that it uses instanceof to check the type before performing the downcast.
  3. Modify Shapes.java so that it can “highlight” (set a flag) in all shapes of a particular type. The toString( ) method for each derived Shape should indicate whether that Shape is “highlighted.”
  4. Modify SweetShop.java so that each type of object creation is controlled by a command-line argument. That is, if your command line is “java SweetShop Candy,” then only the Candy object is created. Notice how you can control which Class objects are loaded via the command-line argument.
  5. Add a new type of Pet to PetCount3.java. Verify that it is created and counted correctly in main( ).
  6. Write a method that takes an object and recursively prints all the classes in that object’s hierarchy.
  7. Modify Exercise 6 so that it uses Class. getDeclaredFields ( ) to also display information about the fields in a class.
  8. In ToyTest.java, comment out Toy’s default constructor and explain what happens.
  9. Incorporate a new kind of interface into ToyTest.java and verify that it is detected and displayed properly.
  10. Create a new type of container that uses a private ArrayList to hold the objects. Capture the type of the first object you put in it, and then allow the user to insert objects of only that type from then on.
  11. Write a program to determine whether an array of char is a primitive type or a true object.
  12. Implement clearSpitValve( ) as described in the summary.
  13. Implement the rotate(Shape) method described in this chapter, such that it checks to see if it is rotating a Circle (and, if so, doesn’t perform the operation).
  14. Modify Exercise 6 so that it uses reflection instead of RTTI.
  15. Modify Exercise 7 so that it uses reflection instead of RTTI.
  16. In ToyTest.java, use reflection to create a Toy object using the non-default constructor.
  17. Look up the interface for java.lang.Class in the HTML Java documentation from http://java.sun.com. Write a program that takes the name of a class as a command-line argument, then uses the Class methods to dump all the information available for that class. Test your program with a standard library class and a class you

    [58] Especially in the past. However, Sun has greatly improved its HTML Java documentation so that it’s easier to see base-class methods.

    create.


13: Creating Windows & Applets

13: Creating Windows
& Applets

[[[ Note: please notice that only the code files have been changed to be Swing-compliant, and the prose is essentially the way it was before, and refers to the old programs—that is to say, more work is going to be done here. So please concentrate on the code listings, and if you see changes that should be made, please include the entire file after you’ve made the improvements, using the correction page attached to this HTML file or at www.BruceEckel.com ]]]

A fundamental design guideline is “make simple things easy, and difficult things possible [59] .”

The original design goal of the graphical user interface (GUI) library in Java 1.0 was to allow the programmer to build a GUI that looks good on all platforms. That goal was not achieved. Instead, the Java 1.0 Abstract Window Toolkit (AWT) produces a GUI that looks equally mediocre on all systems. In addition it’s restrictive: you can use only four fonts and you cannot access any of the more sophisticated GUI elements that exist in your operating system. The Java 1.0 AWT programming model is also awkward and non-object-oriented. A student in one of my seminars (who had been at Sun during the creation of Java) explained why: the original AWT had been conceptualized, designed and implemented in a month. Certainly a marvel of productivity, and also an object lesson in why design is important.

Much of the situation improved with the Java 1.1 AWT event model, which takes a much clearer, object-oriented approach, along with the addition of Java Beans, a component programming model that is oriented toward the easy creation of visual programming environments. Java 2 finishes the transformation away from the old Java 1.0 AWT by adding the Java Foundation Classes (JFC), the GUI portion of which is called “Swing.” These are a rich set of easy-to-use, easy-to-understand Java Beans that can be dragged and dropped (as well as hand programmed) to create a GUI that you can (finally) be satisfied with. The “revision 3” rule of the software industry (a product isn’t good until revision 3) seems to hold true with programming languages as well.

This chapter does not cover anything but the modern, Java 2 Swing library, and makes the reasonable assumption that Swing is the final destination GUI library for Java. If for some reason you need to use the original “old” AWT (because you’re supporting old code or old browsers), you can find that introduction in the online first edition of this book at www.BruceEckel.com.

Early in this chapter, you’ll see how things are different when you want to create an applet vs. a regular application using Swing, and how to create programs that are both applets and applications so they can be run either inside a browser or from the command line. Almost all the GUI examples in this book will be runnable as either applets or applications.

Please be aware that this is not a comprehensive glossary of all the methods for the described classes. The Swing library is vast, and the goal of this chapter is only to get you started with the essentials and comfortable with the concepts. When you’re looking for more sophistication, make sure you go to the HTML JDK library documents from http://java.sun.com to look for the javax.swing classes and methods—because of the simplicity of Swing, this will often be enough information to solve your problem. There are numerous (rather thick) books dedicated solely to Swing and you’ll want to go to those if you need to modify the default Swing behavior.

As you learn about Swing you’ll discover:

  1. Swing is a much better programming model than you’ve probably seen in other languages and development environments. Java Beans is the framework for that library.
  2. “GUI builders” (visual programming environments) are de rigeur for all development systems. Java Beans and the Swing allow the GUI builder to write code for you as you place components onto forms using graphical tools. This not only rapidly speeds development during GUI building, but it allows for greater experimentation and thus the ability to try out more designs and presumably come up with a better one.
  3. The simplicity and well-designed nature of Swing means that even if you do use a GUI builder, the resulting code will still be comprehensible—this has been a big problem with GUI builders in the past.

The basic applet

One of Java’s primary design goals is to create applets, which are little programs that run inside a Web browser. Because they must be safe, applets are limited in what they can accomplish. However, applets are a powerful tool that supports client-side programming, a major issue for the Web.

Programming within an applet is so restrictive that it’s often referred to as being “inside the sandbox,” since you always have someone—the Java run-time security system—watching over you. Java offers digital signing for applets so you can choose to allow trusted applets to have access to your machine. However, you can also step outside the sandbox and write regular applications, in which case you can access the other features of your OS. We’ve been writing regular applications all along in this book, but they’ve been console applications without any graphical components. Swing can also be used to build GUI interfaces for regular applications.

Libraries are often grouped according to their functionality. Some libraries, for example, are used as is, off the shelf. The standard Java library String and ArrayList classes are examples of these. Other libraries are designed specifically as building blocks to build other classes. A certain class of library is the application framework, whose goal is to help you build applications by providing a class or set of classes that produces the basic behavior that you need in every application of a particular type. Then, to customize the behavior to your own needs you inherit from the application class and override the methods of interest. The application framework’s default control mechanism will call your overridden methods at the appropriate time. An application framework is a good example of “separating the things that change from the things that stay the same,” since it attempts to localize all the unique parts of a program in the overridden methods [60] .

Applets are built using an application framework. You inherit from class JApplet and override the appropriate methods. There are a few methods that control the creation and execution of an applet on a Web page:

Method | Operation
init( ) | Called when the applet is first created to perform first-time initialization of the applet, including component layout. You’ll always override this method.
start( ) | Called every time the applet moves into sight on the Web browser to allow the applet to start up its normal operations (especially those that are shut off by stop( )). Also called after init( ).
stop( ) | Called every time the applet moves out of sight on the Web browser to allow the applet to shut off expensive operations. Also called right before destroy( ).
destroy( ) | Called when the applet is being unloaded from the page to perform final release of resources when the applet is no longer used

With this information you can create a simple applet:

//: c13:Applet1.java
// Very simple applet.
import javax.swing.*;
import java.awt.*;

public class Applet1 extends JApplet {
  public void init() {
    getContentPane().add(new JLabel("Applet!"));
  }
} ///:~

Note that applets are not required to have a main( ). That’s all wired in to the application framework; you put any startup code in init( ).

Running applets inside a Web browser

To run this program you must place it inside a Web page and view that page inside your Java-enabled Web browser. To place an applet inside a Web page you put a special tag inside the HTML source for that Web page [61] to tell the page how to load and run the applet.

This process used to be very simple, when Java was simple and everyone was on the same bandwagon and incorporated the same Java support inside their Web browsers. Then you might have been able to get away with a very simple bit of HTML inside your Web page, like this:

<applet code=Applet1 width=100 height=50>
</applet>

Then along came the browser & language wars, and we (programmers and end users alike) lost. After awhile, JavaSoft realized that we could no longer expect browsers to support the correct flavor of Java, and the only solution was to provide some kind of add-on that would conform to a browser’s extension mechanism. By using the extension mechanism (which a browser vendor cannot disable—in an attempt to gain competitive advantage—without breaking all the 3rd party extensions) JavaSoft guarantees that Java cannot be shut out of the Web browser by an antagonistic vendor.

With Internet Explorer, the extension mechanism is the ActiveX control, and with Netscape the extension mechanism is the plug-in. In your html code, you must provide tags to support both. Here’s what the simplest resulting HTML page looks like for Applet1:

//:! c13:Applet1.html
<html><head><title>Applet1</title></head><hr>
<OBJECT
  classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"
  width="100" height="50" align="baseline"  codebase="http://java.sun.com/products/plugin/1.2.2/jinstall-1_2_2-win.cab#Version=1,2,2,0">
<PARAM NAME="code" VALUE="Applet1.class">
<PARAM NAME="codebase" VALUE=".">
<PARAM NAME="type" VALUE="application/x-java-applet;version=1.2.2">
<COMMENT>
  <EMBED type=
    "application/x-java-applet;version=1.2.2"
    width="200" height="200" align="baseline"
    code="Applet1.class" codebase="."
pluginspage="http://java.sun.com/products/plugin/1.2/plugin-install.html">
  <NOEMBED>
</COMMENT>
   No Java 2 support for APPLET!!
  </NOEMBED>
</EMBED>
</OBJECT>
<hr></body></html>
///:~

Some of these lines were too long and had to be wrapped to fit on the page. The code in this book’s source code (on the book’s CD ROM, and downloadable from www.BruceEckel.com) will work without having to worry about correcting line wraps.

The code value gives the name of the .class file where the applet resides. The width and height specify the initial size of the applet (in pixels, as before). There are other items you can place within the applet tag: a place to find other .class files on the Internet (codebase), alignment information (align), a special identifier that makes it possible for applets to communicate with each other (name), and applet parameters to provide information that the applet can retrieve. Parameters are in the form

<param name="identifier" value = "information">

and there can be as many as you want.

Using Appletviewer

Sun’s JDK (freely downloadable from http://java.sun.com) contains a tool called the Appletviewer that picks the <applet> tags out of the HTML file and runs the applets without displaying the surrounding HTML text. Because the appletviewer ignores everything but APPLET tags, you can put those tags in the Java source file as comments:

// <applet code=MyApplet width=200 height=100>
// </applet>

This way, you can run "Appletviewer MyApplet.java" and you don’t need to create tiny HTML files to run tests. Also, you can see from looking at the example the HTML tag that must be inserted into a Web page.

For example, you can add the commented HTML tags to Applet1.java:

//: c13:Applet1b.java
// Embedding the applet tag for Appletviewer.
// <applet code=Applet1b width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;

public class Applet1b extends JApplet {
  public void init() {
    getContentPane().add(new JLabel("Applet!"));
  }
} ///:~

Now you can invoke the applet with the command

Appletviewer Applet1b.java

For “plain” applets in this book, this form will be used for easy testing (later, you’ll see another coding approach which will allow you to execute applets from the command line without the Appletviewer).

Testing applets

You can perform a simple test without any network connection by starting up your Web browser and opening the HTML file containing the applet tag. As the HTML file is loaded, the browser will discover the applet tag and go hunt for the .class file specified by the code value. Of course, it looks at the CLASSPATH to find out where to hunt, and if your .class file isn’t in the CLASSPATH then it will give an error message on the status line of the browser to the effect that it couldn’t find that .class file.

When you want to try this out on your Web site things are a little more complicated. First of all, you must have a Web site, which for most people means a third-party Internet Service Provider (ISP) at a remote location (since the applet is just a file or set of files, the ISP does not have to provide any special support for Java). Then you must have a way to move the HTML files and the .class files from your site to the correct directory (your WWW directory) on the ISP machine. This is typically done with a File Transfer Protocol (FTP) program, of which there are many different types available for free or as shareware. So it would seem that all you need to do is move the files to the ISP machine with FTP, then connect to the site and HTML file using your browser; if the applet comes up and works, then everything checks out, right?

Here’s where you can get fooled. If the browser on the client machine cannot locate the .class file on the server, it will hunt through the CLASSPATH on your local machine. Thus, the applet might not be loading properly from the server, but to you it looks fine because the browser finds it on your machine. When someone else logs in, however, his or her browser can’t find it. So when you’re testing, make sure you erase the relevant .class files on your machine to be safe.

One of the most insidious places where this happened to me is when I innocently placed an applet inside a package. After uploading the HTML file and applet, it turned out that the server path to the applet was confused because of the package name. However, my browser found it in the local CLASSPATH. So I was the only one who could properly load the applet. It took some time to discover that the package statement was the culprit. In general, you’ll want to leave the package statement out of an applet.

Making windows and applets

It’s possible to see that for safety’s sake you can have only limited behavior within an applet. In a real sense, the applet is a temporary extension to the Web browser so its functionality must be limited along with its knowledge and control. There are times, however, when you’d like to make a windowed program do something else than sit on a Web page, and perhaps you’d like it to do some of the things a “regular” application can do and yet have the vaunted instant portability provided by Java. In previous chapters in this book we’ve made command-line applications, but in some operating environments (the Macintosh, for example) there isn’t a command line. So for any number of reasons you’d like to build a windowed, non-applet program using Java. This is certainly a reasonable desire.

A Java windowed application can have menus and dialog boxes (impossible or difficult with an applet), and yet if you’re using an older version of Java you sacrifice the native operating environment’s look and feel. The JFC/Swing library allows you to make an application that preserves the look and feel of the underlying operating environment. If you want to build windowed applications, it makes sense to do so only if you can use the latest version of Java and associated tools so you can deliver applications that won’t confound your users. If for some reason you’re forced to use an older version of Java, think hard before committing to building a significant windowed application.

Often you’ll want to be able to create a class that can be invoked as either a window or an applet. This is especially convenient when you’re testing the applets, since it’s typically much faster and easier to run the resulting applet-application from the command line than it is to start up a Web browser or the Appletviewer.

To create an applet that can be run from the console command line, you simply add a main( ) to your applet that builds an instance of the applet inside a Frame. As a simple example, let’s look at Applet1b.java modified to work as both an application and an applet:

//: c13:Applet1c.java
// An application and an applet.
// <applet code=Applet1c width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Applet1c extends JApplet {
  public void init() {
    getContentPane().add(new JLabel("Applet!"));
  }
  // A main() for the application:
  public static void main(String[] args) {
    JApplet applet = new Applet1b();
    JFrame frame = new JFrame("Applet1b");
    // To close the application:
    Console.setupClosing(frame);
    frame.getContentPane().add(applet);
    frame.setSize(100,50);
    applet.init();
    applet.start();
    frame.setVisible(true);
  }
} ///:~

main( ) is the only element added to the applet, and the rest of the applet is untouched. In fact, you can usually copy and paste main( ) into your own applets with little modification. You can see that in main( ), the applet is explicitly initialized and started since in this case the browser isn’t available to do it for you. Of course, this doesn’t provide the full behavior of the browser, which also calls stop( ) and destroy( ), but for most situations it’s acceptable. If it’s a problem, you can:

  1. Make the reference applet a static member of the class (instead of a local variable of main( )), and then:
  2. 	Call
applet.stop( ) and applet.destroy( ) inside
WindowAdapter.windowClosing( ) before you call
System.exit( ).

Notice the last line:

frame.setVisible(true);

Without this, you won’t see anything on the screen.

A display framework

Although the programs that are both applets and applications can be valuable, if used everywhere they become distracting and waste paper. Instead, a display framework will be used for the examples in the rest of this chapter:

//: com:bruceeckel:swing:Console.java
// Tool for running Swing demos from the
// console, both applets and JFrames.
package com.bruceeckel.swing;
import javax.swing.*;
import java.awt.event.*;

public class Console {
  // Create a title string from the class name:
  public static String title(Object o) {
    String t = o.getClass().toString();
    // Remove the word "class":
    if(t.indexOf("class") != -1)
      t = t.substring(6);
    return t;
  }
  public static void setupClosing(JFrame frame) {
    // The JDK 1.2 Solution as an
    // anonymous inner class:
    frame.addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        System.exit(0);
      }
    });
    // The improved solution in JDK 1.3:
    // frame.setDefaultCloseOperation(
    //     EXIT_ON_CLOSE);
  }
  public static void
  run(JFrame frame, int width, int height) {
    setupClosing(frame);
    frame.setSize(width, height);
    frame.setVisible(true);
  }
  public static void
  run(JApplet applet, int width, int height) {
    JFrame frame = new JFrame(title(applet));
    setupClosing(frame);
    frame.getContentPane().add(applet);
    frame.setSize(width, height);
    applet.init();
    applet.start();
    frame.setVisible(true);
  }
  public static void
  run(JPanel panel, int width, int height) {
    JFrame frame = new JFrame(title(panel));
    setupClosing(frame);
    frame.getContentPane().add(panel);
    frame.setSize(width, height);
    frame.setVisible(true);
  }
} ///:~

The common base class between JApplet and JPanel is Container. Only if it’s a JApplet is init( ) and start( ) called.

Note that the title for the JFrame is produced using RTTI.

// The BorderLayout argument will become clear later in this chapter.

Closing the window

The method setupClosing( ) adds a window listener which is a static object that is responsible for closing the window.

You’ll see on the lines below this a more succinct form to close the window:

//#frame.setDefaultCloseOperation(EXIT_ON_CLOSE);

This works with JDK 1.3, but is commented out here for backward compatibility with JDK 1.2.

Now any applet can be run from the command line by creating a main( ) containing the a line like this:

Console.run(new MyClass(), 500, 300);

in which the last two arguments are the display width and height. Here’s Applet1c.java modified to use Console:

//: c13:Applet1d.java
// Console runs applets from the command line.
// <applet code=Applet1d width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Applet1d extends JApplet {
  public void init() {
    getContentPane().add(new JLabel("Applet!"));
  }
  public static void main(String[] args) {
    Console.run(new Applet1c(), 100, 50);
  }
} ///:~

This allows the elimination of repeated code while providing the greatest flexibility in running the examples.

Using the Windows Explorer

If you’re using Windows, you can simplify the process of running a command-line Java program by configuring the Windows Explorer (this is the file browser in Windows, not the Internet Explorer) so that you can simply double-click on a .class file to execute it. There are several steps in this process.

First, download and install the Perl programming language from www.Perl.org. You’ll find the instructions on that site.

Next, create the following script (it is part of this book’s source-code package):

//:! c13:RunJava.bat
@rem = '--*-Perl-*--
@echo off
perl -x -S "%0" %1 %2 %3 %4 %5 %6 %7 %8 %9
goto endofperl
@rem ';
#!perl
$file = $ARGV[0];
$file =~ s/(.*)\..*/\1/;
$file =~ s/(.*\\)*(.*)/$+/;
´java $file´;
__END__
:endofperl
///:~

Now, open the Windows Explorer, select “View,” “Folder Options,” then click on the “File Types” tab. Press the “New Type” button. For “Description of Type” enter “Java class file.” Under “Actions” press the “New” button. For “Action” enter “Open,” and for “Application used to perform action” enter a line like this:

"c:\aaa\Perl\RunJava.bat" "%L"

Where the path to “RunJava.bat” must be customized to wherever you place the batch file.

Making a button

Making a button is quite simple: you just call the JButton constructor with the label you want on the button. (You can also use the default constructor if you want a button with no label, but this is not very useful.) Usually you’ll want to create a reference for the button so you can refer to it later.

The JButton is a component, like its own little window, that will automatically get repainted as part of an update. This means that you don’t explicitly paint a button or any other kind of control; you simply place them on the form and let them automatically take care of painting themselves. So to place a button on a form you override init( ) instead of overriding paint( ):

//: c13:Button1.java
// Putting buttons on an applet.
// <applet code=Button1 width=200 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Button1 extends JApplet {
  JButton
    b1 = new JButton("Button 1"),
    b2 = new JButton("Button 2");
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
  }
  public static void main(String[] args) {
    Console.run(new Button1(), 200, 50);
  }
} ///:~

It’s not enough to create the Button (or any other control). You must also call the Applet add( ) method to cause the button to be placed on the applet’s form. This seems a lot simpler than it is, because the call to add( ) actually decides, implicitly, where to place the control on the form. Controlling the layout of a form is examined shortly.

Capturing an event

You’ll notice that if you compile and run the applet above, nothing happens when you press the buttons. This is where you must step in and write some code to determine what will happen. The basis of event-driven programming, which comprises a lot of what a GUI is about, is tying events to code that responds to those events.

After working your way this far through this book and grasping some of the fundamentals of object-oriented programming, you might think that of course there will be some sort of object-oriented approach to handling events. For example, you might have to inherit each button and override some “button pressed” method (this, it turns out, is too tedious and restrictive). You might also think there’s some master “event” class that contains a method for each event you want to respond to.

Before objects, the typical approach to handling events was the “giant switch statement.” Each event would have a unique integer value and inside the master event handling method you’d write a switch on that value.

Swing in Java 1.0 doesn’t use any object-oriented approach. Neither does it use a giant switch statement that relies on the assignment of numbers to events. Instead, you must create a cascaded set of if statements. What you’re trying to do with the if statements is detect the object that was the target of the event. That is, if you click on a button, then that particular button is the target. Normally, that’s all you care about—if a button is the target of an event, then it was most certainly a mouse click and you can continue based on that assumption. However, events can contain other information as well. For example, if you want to find out the pixel location where a mouse click occurred so you can draw a line to that location, the Event object will contain the location. (You should also be aware that Java 1.0 components can be limited in the kinds of events they generate, while Java 1.1 and Swing/JFC components produce a full set of events.)

The Java 1.0 AWT method where your cascaded if statement resides is called action( ). Although the whole Java 1.0 Event model has been deprecated in Java 1.1, it is still widely used for simple applets and in systems that do not yet support Java 1.1, so I recommend you become comfortable with it, including the use of the following action( ) method approach.

action( ) has two arguments: the first is of type Event and contains all the information about the event that triggered this call to action( ). For example, it could be a mouse click, a normal keyboard press or release, a special key press or release, the fact that the component got or lost the focus, mouse movements, or drags, etc. The second argument is usually the target of the event, which you’ll often ignore. The second argument is also encapsulated in the Event object so it is redundant as an argument.

The situations in which action( ) gets called are extremely limited: When you place controls on a form, some types of controls (buttons, check boxes, drop-down lists, menus) have a “standard action” that occurs, which causes the call to action( ) with the appropriate Event object. For example, with a button the action( ) method is called when the button is pressed and at no other time. Usually this is just fine, since that’s what you ordinarily look for with a button. However, it’s possible to deal with many other types of events via the handleEvent( ) method as we will see later in this chapter.

The previous example can be extended to handle button clicks as follows:

//: c13:Button2.java
// Capturing button presses.
// <applet code=Button2 width=200 height=50>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Button2 extends JApplet {
  JButton
    b1 = new JButton("Button 1"),
    b2 = new JButton("Button 2");
  class BL implements ActionListener {
    public void actionPerformed(ActionEvent e){
      String name =
        ((JButton)e.getSource()).getText();
      getAppletContext().showStatus(name);
    }
  }
  BL al = new BL();
  public void init() {
    b1.addActionListener(al);
    b2.addActionListener(al);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
  }
  public static void main(String[] args) {
    Console.run(new Button2(), 200, 50);
  }
} ///:~

To see what the target is, ask the Event object what its target member is and then use the equals( ) method to see if it matches the target object reference you’re interested in. When you’ve written handlers for all the objects you’re interested in you must call super.action(evt, arg) in the else statement at the end, as shown above. Remember from Chapter 7 (polymorphism) that your overridden method is called instead of the base class version. However, the base-class version contains code to handle all of the cases that you’re not interested in, and it won’t get called unless you call it explicitly. The return value indicates whether you’ve handled it or not, so if you do match an event you should return true, otherwise return whatever the base-class event( ) returns.

For this example, the simplest action is to print what button is pressed. Some systems allow you to pop up a little window with a message in it, but applets discourage this. However, you can put a message at the bottom of the Web browser window on its status line by calling the Applet method getAppletContext( ) to get access to the browser and then showStatus( ) to put a string on the status line. [62] You can print a complete description of an event the same way, with getAppletContext( ).showStatus(evt + "" ). (The empty String forces the compiler to convert evt to a String.) Both of these reports are really useful only for testing and debugging since the browser might overwrite your message.

It is often more convenient to code the ActionListener as an anonymous inner class, especially since you tend to only use a single instance of each listener class. Button2.java can be modified to use an anonymous inner class as follows:

//: c13:Button2b.java
// Using anonymous inner classes.
// <applet code=Button2b width=200 height=50>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Button2b extends JApplet {
  JButton
    b1 = new JButton("Button 1"),
    b2 = new JButton("Button 2");
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      String name =
        ((JButton)e.getSource()).getText();
      getAppletContext().showStatus(name);
    }
  };
  public void init() {
    b1.addActionListener(al);
    b2.addActionListener(al);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
  }
  public static void main(String[] args) {
    Console.run(new Button2b(), 200, 50);
  }
} ///:~

The approach of using an anonymous inner class will be preferred for the examples in this book.

Text fields

A TextField is a one line area that allows the user to enter and edit text. TextField is inherited from TextComponent, which lets you select text, get the selected text as a String, get or set the text, and set whether the TextField is editable, along with other associated methods that you can find in your online reference. The following example demonstrates some of the functionality of a TextField; you can see that the method names are fairly obvious:

//: c13:TextField1.java
// Using the JTextField control.
// <applet code=TextField1 width=350 height=75>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class TextField1 extends JApplet {
  JButton
    b1 = new JButton("Get Text"),
    b2 = new JButton("Set Text");
  JTextField
    t = new JTextField("Starting text: ", 30);
  String s = new String();
  ActionListener a1 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      getAppletContext().showStatus(t.getText());
      s = t.getSelectedText();
      if(s == null)
        s = t.getText();
      t.setEditable(true);
    }
  };
  ActionListener a2 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      t.setText("Inserted by Button 2: " + s);
      t.setEditable(false);
    }
  };
  public void init() {
    b1.addActionListener(a1);
    b2.addActionListener(a2);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
    cp.add(t);
  }
  public static void main(String[] args) {
    Console.run(new TextField1(), 350, 75);
  }
} ///:~

There are several ways to construct a TextField; the one shown here provides an initial string and sets the size of the field in characters.

Pressing button 1 either gets the text you’ve selected with the mouse or it gets all the text in the field and places the result in String s. It also allows the field to be edited. Pressing button 2 puts a message and s into the text field and prevents the field from being edited (although you can still select the text). The editability of the text is controlled by passing setEditable( ) a true or false.

Text areas

A TextArea is like a TextField except that it can have multiple lines and has significantly more functionality. In addition to what you can do with a TextField, you can append text and insert or replace text at a given location. It seems like this functionality could be useful for TextField as well, so it’s a little confusing to try to detect how the distinction is made. You might think that if you want TextArea functionality everywhere you can simply use a one line TextArea in places where you would otherwise use a TextField. In Java 1.0, you also got scroll bars with a TextArea even when they weren’t appropriate; that is, you got both vertical and horizontal scroll bars for a one line TextArea. In Java 1.1 this was remedied with an extra constructor that allows you to select which scroll bars (if any) are present. The following example shows only the Java 1.0 behavior, in which the scrollbars are always on. Later in the chapter you’ll see an example that demonstrates Java 1.1 TextAreas.

//: c13:TextArea1.java
// Using the JTextArea control.
// <applet code=TextArea1 width=350 height=200>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class TextArea1 extends JApplet {
  JButton
    b1 = new JButton("Text Area 1"),
    b2 = new JButton("Text Area 2"),
    b3 = new JButton("Replace Text"),
    b4 = new JButton("Insert Text");
  JTextArea
    t1 = new JTextArea("t1", 1, 30),
    t2 = new JTextArea("t2", 4, 30);
  ActionListener a1 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      getAppletContext().showStatus(t1.getText());
    }
  };
  ActionListener a2 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      t2.setText("Inserted by Button 2");
      t2.append(": " + t1.getText());
      getAppletContext().showStatus(t2.getText());
    }
  };
  ActionListener a3 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      String s = " Replacement ";
      t2.replaceRange(s, 3, 3 + s.length());
    }
  };
  ActionListener a4 = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      t2.insert(" Inserted ", 10);
    }
  };
  public void init() {
    b1.addActionListener(a1);
    b2.addActionListener(a2);
    b3.addActionListener(a3);
    b4.addActionListener(a4);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(t1);
    cp.add(b2);
    cp.add(t2);
    cp.add(b3);
    cp.add(b4);
  }
  public static void main(String[] args) {
    Console.run(new TextArea1(), 350, 200);
  }
} ///:~

There are several different TextArea constructors, but the one shown here gives a starting string and the number of rows and columns. The different buttons show getting, appending, replacing, and inserting text.

Labels

A JLabel does exactly what it sounds like it should: places a label on the form. This is particularly important for text fields and text areas that don’t have labels of their own, and can also be useful if you simply want to place textual information on a form. You can, as shown in the first example in this chapter, use drawString( ) inside paint( ) to place text in an exact location. When you use a JLabel it allows you to (approximately) associate the text with some other component via the layout manager (which will be discussed later in this chapter).

With the constructor you can create a blank label or a label with initial text in it (which is what you’ll typically do). Less frequently used options include changing the alignment of labels. You can also change the label with setText( ) and and read the value with getText( ). This example shows things you might commonly do with labels:

//: c13:Label1.java
// Using JLabels.
// <applet code=Label1 width=200 height=100>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Label1 extends JApplet {
  JTextField t1 = new JTextField("t1", 10);
  JLabel
    labl1 = new JLabel("TextField t1"),
    labl2 = new JLabel("                   ");
  JButton
    b1 = new JButton("Test 1"),
    b2 = new JButton("Test 2");
  ActionListener a1 = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      t1.setText("Button 1");
      labl2.setText("Text set into Label");
    }
  };
  ActionListener a2 = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      t1.setText("Button 2");
      labl1.setText("Hello");
    }
  };
  public void init() {
    b1.addActionListener(a1);
    b2.addActionListener(a2);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(labl1);
    cp.add(t1);
    cp.add(b1);
    cp.add(labl2);
    cp.add(b2);
  }
  public static void main(String[] args) {
    Console.run(new Label1(), 200, 100);
  }
} ///:~

The first use of the label is the most typical: labeling a TextField or TextArea. In the second part of the example, a bunch of empty spaces are reserved and when you press the “Test 1” button setText( ) is used to insert text into the field.

HTML text on Swing components

Any component that can take text can also take HTML text, which it will reformat according to HTML rules. This means you can very easily add fancy text to a Swing component. For example,

JButton bigButton =
  new JButton( "<html><b><font size=+2>" +
    "Hello!<br><i>Press me now!" ));

This produces [[ etc. ]]

[[rewrite]] The same trick works for JLabel, JTabbedPane, JMenuItem, and JToolTip. Future releases of Swing may also add HTML support to JRadioButton and JCheckBox

Check boxes

A check box provides a way to make a single on-off choice; it consists of a tiny box and a label. The box typically holds a little ‘x’ (or some other indication that it is set) or is empty depending on whether that item was selected.

You’ll normally create a Checkbox using a constructor that takes the label as an argument. You can get and set the state, and also get and set the label if you want to read or change it after the Checkbox has been created. Note that the capitalization of Checkbox is inconsistent with the other controls, which could catch you by surprise since you might expect it to be “CheckBox.”

Whenever a Checkbox is set or cleared an event occurs, which you can capture the same way you do a button. The following example uses a TextArea to enumerate all the check boxes that have been checked:

//: c13:CheckBox1.java
// Using JCheckBoxes.
// <applet code=CheckBox1 width=200 height=200>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class CheckBox1 extends JApplet {
  JTextArea t = new JTextArea(6, 15);
  JCheckBox
    cb1 = new JCheckBox("Check Box 1"),
    cb2 = new JCheckBox("Check Box 2"),
    cb3 = new JCheckBox("Check Box 3");
  public void init() {
    cb1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        trace("1", cb1);
      }
    });
    cb2.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        trace("2", cb2);
      }
    });
    cb3.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        trace("3", cb3);
      }
    });
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(new JScrollPane(t));
    cp.add(cb1);
    cp.add(cb2);
    cp.add(cb3);
  }
  void trace(String b, JCheckBox cb) {
    if(cb.isSelected())
      t.append("Box " + b + " Set\n");
    else
      t.append("Box " + b + " Cleared\n");
  }
  public static void main(String[] args) {
    Console.run(new CheckBox1(), 200, 200);
  }
} ///:~

Notice that in this example, the anonymous inner class that implements ActionListener is created inline, so no intermediate variable is used.

The trace( ) method sends the name of the selected Checkbox and its current state to the TextArea using appendText( ) so you’ll see a cumulative list of the checkboxes that were selected and what their state is.

Radio buttons

The concept of a radio button in GUI programming comes from pre-electronic car radios with mechanical buttons: when you push one in, any other button that was pressed pops out. Thus it allows you to force a single choice among many.

Swing does not have a separate class to represent the radio button; instead it reuses the Checkbox. However, to put the Checkbox in a radio button group (and to change its shape so it’s visually different from an ordinary Checkbox) you must use a special constructor that takes a CheckboxGroup object as an argument. (You can also call setCheckboxGroup( ) after the Checkbox has been created.)

A CheckboxGroup has no constructor argument; its sole reason for existence is to collect some Checkboxes into a group of radio buttons. One of the Checkbox objects must have its state set to true before you try to display the group of radio buttons; otherwise you’ll get an exception at run-time. If you try to set more than one radio button to true then only the final one set will be true.

Here’s a simple example of the use of radio buttons. Note that you capture radio button events like all others:

//: c13:RadioButton1.java
// Using JRadioButtons.
// <applet code=RadioButton1
// width=200 height=75> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class RadioButton1 extends JApplet {
  JTextField t =
    new JTextField("Radio button 2", 15);
  ButtonGroup g = new ButtonGroup();
  JRadioButton
    rb1 = new JRadioButton("one", false),
    rb2 = new JRadioButton("two", true),
    rb3 = new JRadioButton("three",false);
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      t.setText("Radio button " +
        ((JRadioButton)e.getSource()).getText());
    }
  };
  public void init() {
    rb1.addActionListener(al);
    rb2.addActionListener(al);
    rb3.addActionListener(al);
    g.add(rb1); g.add(rb2); g.add(rb3);
    t.setEditable(false);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    cp.add(rb1);
    cp.add(rb2);
    cp.add(rb3);
  }
  public static void main(String[] args) {
    Console.run(new RadioButton1(), 200, 75);
  }
} ///:~

To display the state, a text field is used. This field is set to non-editable because it’s used only to display data, not to collect it. This is shown as an alternative to using a Label. Notice the text in the field is initialized to “Radio button two” since that’s the initial selected radio button.

You can have any number of CheckboxGroups on a form.

Drop-down lists

Like a group of radio buttons, a drop-down list is a way to force the user to select only one element from a group of possibilities. However, it’s a much more compact way to accomplish this, and it’s easier to change the elements of the list without surprising the user. (You can change radio buttons dynamically, but that tends to be visibly jarring).

Java’s Choice box is not like the combo box in Windows, which lets you select from a list or type in your own selection. With a Choice box you choose one and only one element from the list. In the following example, the Choice box starts with a certain number of entries and then new entries are added to the box when a button is pressed. This allows you to see some interesting behaviors in Choice boxes:

//: c13:Choice1.java
// Using drop-down lists.
// <applet code=Choice1
// width=200 height=100> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Choice1 extends JApplet {
  String[] description = { "Ebullient", "Obtuse",
    "Recalcitrant", "Brilliant", "Somnescent",
    "Timorous", "Florid", "Putrescent" };
  JTextField t = new JTextField(15);
  JComboBox c = new JComboBox();
  JButton b = new JButton("Add items");
  int count = 0;
  public void init() {
    for(int i = 0; i < 4; i++)
      c.addItem(description[count++]);
    t.setEditable(false);
    b.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        if(count < description.length)
          c.addItem(description[count++]);
      }
    });
    c.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        t.setText("index: "+ c.getSelectedIndex()
          + "   " + ((JComboBox)e.getSource())
          .getSelectedItem());
      }
    });
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    cp.add(c);
    cp.add(b);
  }
  public static void main(String[] args) {
    Console.run(new Choice1(), 200, 100);
  }
} ///:~

The TextField displays the “selected index,” which is the sequence number of the currently selected element, as well as the String representation of the second argument of action( ), which is in this case the string that was selected.

When you run this applet, pay attention to the determination of the size of the Choice box: in Windows, the size is fixed from the first time you drop down the list. This means that if you drop down the list, then add more elements to the list, the elements will be there but the drop-down list won’t get any longer [63] (you can scroll through the elements). However, if you add all the elements before the first time the list is dropped down, then it will be sized correctly. Of course, the user will expect to see the whole list when it’s dropped down, so this behavior puts some significant limitations on adding elements to Choice boxes.

List boxes

List boxes are significantly different from Choice boxes, and not just in appearance. While a Choice box drops down when you activate it, a List occupies some fixed number of lines on a screen all the time and doesn’t change. In addition, a List allows multiple selection: if you click on more than one item the original item stays highlighted and you can select as many as you want. If you want to see the items in a list, you simply call getSelectedItems( ), which produces an array of String of the items that have been selected. To remove an item from a group you have to click it again.

A problem with a List is that the default action is double clicking, not single clicking. A single click adds or removes elements from the selected group and a double click calls action( ). One way around this is to re-educate your user, which is the assumption made in the following program:

//: c13:List1.java
// Using JLists.
// <applet code=List1
// width=200 height=350> </applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class List1 extends JApplet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  JList list = new JList(flavors);
  JTextArea t =
    new JTextArea(flavors.length + 1, 15);
  public void init() {
    t.setEditable(false);
    list.addListSelectionListener(
      new ListSelectionListener() {
      public void
      valueChanged(ListSelectionEvent e) {
        t.setText(""); // Erase the text area
        Object[] items= list.getSelectedValues();
        for(int i = 0; i < items.length; i++)
          t.append(items[i] + "\n");
      }
    });
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    cp.add(list);
  }
  public static void main(String[] args) {
    Console.run(new List1(), 200, 350);
  }
} ///:~

When you press the button it adds items to the top of the list (because of the second argument 0 to addItem( )). Adding elements to a List is more reasonable than the Choice box because users expect to scroll a list box (for one thing, it has a built-in scroll bar) but they don’t expect to have to figure out how to get a drop-down list to scroll, as in the previous example.

However, the only way for action( ) to be called is through a double-click. If you need to monitor other activities that the user is doing on your List (in particular, single clicks) you must take an alternative approach.

Tabbed panes

The JTabbedPane allows you to create a “tabbed dialog,” which has file-folder tabs running across one edge, and all you have to do is press a tab to bring forward a different dialog.

//: c13:TabbedPane1.java
// Demonstrates the Tabbed Pane.
// <applet code=TabbedPane1
// width=350 height=200> </applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class TabbedPane1 extends JApplet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  JTabbedPane tabs = new JTabbedPane();
  public void init() {
    for(int i = 0; i < flavors.length; i++)
      tabs.addTab(flavors[i],
        new JButton("Tabbed pane " + i));
    tabs.addChangeListener(new ChangeListener(){
      public void stateChanged(ChangeEvent e) {
        getAppletContext().showStatus(
          "Tab selected: " +
            tabs.getSelectedIndex());
      }
    });
    getContentPane().add(tabs);
  }
  public static void main(String[] args) {
    Console.run(new TabbedPane1(), 350, 200);
  }
} ///:~

In Java, the use of some sort of “tabbed panel” mechanism is quite important because (as you’ll see later) in applet programming the use of pop-up dialogs is heavily discouraged.

When you run the program you’ll see that the JTabbedPane automatically stacks the tabs if there are too many of them to fit on one row. You can see this by resizing the window when you run the program from the console command line.

Message boxes

Windowing environments commonly contain a standard set of message boxes that allow you to quickly post information to the user or to capture information from the user. In Swing, these message boxes are contained in JOptionPane. You have many different possibilities (some quite sophisticated), but the ones you’ll most commonly use are probably the message dialog and confirmation dialog, invoked using the static JOptionPane.showMessageDialog( ) and JOptionPane. showConfirmDialog( ).

//: c13:MessageBoxes.java
// Demonstrates JoptionPane.
// <applet code=MessageBoxes
// width=200 height=100> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class MessageBoxes extends JApplet {
  JButton[] b = { new JButton("alert"),
    new JButton("yes/no"), new JButton("Color"),
    new JButton("input"), new JButton("3 vals")
  };
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      String id =
        ((JButton)e.getSource()).getText();
      if(id.equals("alert"))
        JOptionPane.showMessageDialog(null,
          "There's a bug on you!", "Hey!",
          JOptionPane.ERROR_MESSAGE);
      else if(id.equals("yes/no"))
        JOptionPane.showConfirmDialog(null,
          "or no", "choose yes",
          JOptionPane.YES_NO_OPTION);
      else if(id.equals("Color")) {
        Object[] options = { "Red", "Green" };
        int sel = JOptionPane.showOptionDialog(
          null, "Choose a Color!", "Warning",
          JOptionPane.DEFAULT_OPTION,
          JOptionPane.WARNING_MESSAGE, null,
          options, options[0]);
          if(sel != JOptionPane.CLOSED_OPTION)
            getAppletContext().showStatus(
              "Color Selected: " + options[sel]);
      } else if(id.equals("input")) {
        String val = JOptionPane.showInputDialog(
            "How many fingers do you see?");
        getAppletContext().showStatus(val);
      } else if(id.equals("3 vals")) {
        Object[] selections = {
          "First", "Second", "Third" };
        Object val = JOptionPane.showInputDialog(
          null, "Choose one", "Input",
          JOptionPane.INFORMATION_MESSAGE,
          null, selections, selections[0]);
        if(val != null)
          getAppletContext().showStatus(
            val.toString());
      }
    }
  };
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < b.length; i++) {
      b[i].addActionListener(al);
      cp.add(b[i]);
    }
  }
  public static void main(String[] args) {
    Console.run(new MessageBoxes(), 200, 100);
  }
} ///:~

Menus

//: c13:SimpleMenus.java
// <applet code=SimpleMenus
// width=200 height=75> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class SimpleMenus extends JApplet {
  JTextField t =
    new JTextField(15);
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      t.setText(
        ((JMenuItem)e.getSource()).getText());
    }
  };
  JMenu[] menus = { new JMenu("Winken"),
    new JMenu("Blinken"), new JMenu("Nod") };
  JMenuItem[] items = {
    new JMenuItem("Fee"), new JMenuItem("Fi"),
    new JMenuItem("Fo"),  new JMenuItem("Zip"),
    new JMenuItem("Zap"), new JMenuItem("Zot"),
    new JMenuItem("Olly"), new JMenuItem("Oxen"),
    new JMenuItem("Free") };
  public void init() {
    JMenuBar mb = new JMenuBar();
    setJMenuBar(mb);
    for(int i = 0; i < items.length; i++) {
      items[i].addActionListener(al);
      menus[i%3].add(items[i]);
    }
    for(int i = 0; i < menus.length; i++)
      mb.add(menus[i]);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
  }
  public static void main(String[] args) {
    Console.run(new SimpleMenus(), 200, 75);
  }
} ///:~

Dialog Boxes

//: c13:DialogDemo.java
// Creating and using Dialog Boxes.
// <applet code=DialogDemo width=125 height=50>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

class MyDialog extends JDialog {
  public MyDialog(JFrame parent) {
    super(parent, "My dialog", true);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(new JLabel("Here is my dialog"));
    JButton ok = new JButton("OK");
    ok.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        setVisible(false); // Closes the dialog
      }
    });
    cp.add(ok);
    setSize(150,125);
  }
}

public class DialogDemo extends JApplet {
  JButton b1 = new JButton("Dialog Box");
  MyDialog dlg = new MyDialog(null);
  public void init() {
    b1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        dlg.show();
      }
    });
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
  }
  public static void main(String[] args) {
    Console.run(new DialogDemo(), 125, 50);
  }
} ///:~

If you’ve been running these programs, you’ll note that anything that pops up out of an applet, including dialog boxes, is “untrusted.” That is, you get a warning in the window that’s been popped up. This is because, in theory, it would be possible to fool the user into thinking that they’re dealing with a regular native application and to get them to type in their credit card number, which then goes across the Web. An applet is always attached to a Web page and visible within your Web browser, while a dialog box is detached so in theory it could be possible. As a result it is not so common to see an applet that uses a dialog box.

Controlling layout

The way that you place components on a form in Java is probably different from any other GUI system you’ve used. First, it’s all code; there are no “resources” that control placement of components. Second, the way components are placed on a form is controlled by a “layout manager” that decides how the components lie based on the order that you add( ) them. The size, shape, and placement of components will be remarkably different from one layout manager to another. In addition, the layout managers adapt to the dimensions of your applet or application window, so if that window dimension is changed (for example, in the HTML page’s applet specification) the size, shape, and placement of the components could change.

Both the Applet and Frame classes are derived from Container, whose job it is to contain and display Components. (The Container is a Component so it can also react to events.) In Container, there’s a method called setLayout( ) that allows you to choose a different layout manager.

In this section we’ll explore the various layout managers by placing buttons in them (since that’s the simplest thing to do). There won’t be any capturing of button events since this is just intended to show how the buttons are laid out.

FlowLayout

So far, all the applets that have been created seem to have laid out their components using some mysterious internal logic. That’s because the applet uses a default layout scheme: the FlowLayout. This simply “flows” the components onto the form, from left to right until the top space is full, then moves down a row and continues flowing the components.

Here’s an example that explicitly (redundantly) sets the layout manager in an applet to FlowLayout and then places buttons on the form. You’ll notice that with FlowLayout the components take on their “natural” size. A Button, for example, will be the size of its string.

//: c13:FlowLayout1.java
// Demonstrates FlowLayout.
// <applet code=FlowLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class FlowLayout1 extends JApplet {
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < 20; i++)
      cp.add(new JButton("Button " + i));
  }
  public static void main(String[] args) {
    Console.run(new FlowLayout1(), 300, 250);
  }
} ///:~

All components will be compacted to their smallest size in a FlowLayout, so you might get a little bit of surprising behavior. For example, a label will be the size of its string, so right-justifying it yields an unchanged display.

BorderLayout

This layout manager has the concept of four border regions and a center area. When you add something to a panel that’s using a BorderLayout you must use an add( ) method that takes a String object as its first argument, and that string must specify (with proper capitalization) “North” (top), “South” (bottom), “East” (right), “West” (left), or “Center.” If you misspell or mis-capitalize, you won’t get a compile-time error, but the applet simply won’t do what you expect. Fortunately, as you will see shortly, there’s a much-improved approach in Java 1.1.

Here’s a simple example:

//: c13:BorderLayout1.java
// Demonstrates BorderLayout.
// <applet code=BorderLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class BorderLayout1 extends JApplet {
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new BorderLayout());
    cp.add(BorderLayout.NORTH,
      new JButton("North"));
    cp.add(BorderLayout.SOUTH,
      new JButton("South"));
    cp.add(BorderLayout.EAST,
      new JButton("East"));
    cp.add(BorderLayout.WEST,
      new JButton("West"));
    cp.add(BorderLayout.CENTER,
      new JButton("Center"));
  }
  public static void main(String[] args) {
    Console.run(new BorderLayout1(), 300, 250);
  }
} ///:~

For every placement but “Center,” the element that you add is compressed to fit in the smallest amount of space along one dimension while it is stretched to the maximum along the other dimension. “Center,” however, spreads out along both dimensions to occupy the middle.

The BorderLayout is the default layout manager for applications and dialogs.

GridLayout

A GridLayout allows you to build a table of components, and as you add them they are placed left-to-right and top-to-bottom in the grid. In the constructor you specify the number of rows and columns that you need and these are laid out in equal proportions.

//: c13:GridLayout1.java
// Demonstrates GridLayout.
// <applet code=GridLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class GridLayout1 extends JApplet {
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(7,3));
    for(int i = 0; i < 20; i++)
      cp.add(new JButton("Button " + i));
  }
  public static void main(String[] args) {
    Console.run(new GridLayout1(), 300, 250);
  }
} ///:~

In this case there are 21 slots but only 20 buttons. The last slot is left empty; no “balancing” goes on with a GridLayout.

GridBagLayout

Some time ago, it was believed that all the stars, planets, the sun, and the moon revolved around the earth. It seemed intuitive from observation. But then astronomers became more sophisticated and started tracking the motion of individual objects, some of which seemed at times to go backward in their paths. Since it was known that everything revolved around the earth, those astronomers spent large amounts of time coming up with equations and theories to explain the motion of the stellar objects.

When trying to work with GridBagLayout, you can consider yourself the analog of one of those early astronomers. The basic precept (decreed, interestingly enough, by the designers at “Sun”) is that everything should be done in code. The Copernican revolution (again dripping with irony, the discovery that the planets in the solar system revolve around the sun) is the use of resources to determine the layout and make the programmer’s job easy. Until these are added to Java, you’re stuck (to continue the metaphor) in the Spanish Inquisition of GridBagLayout and GridBagConstraints.

My recommendation is to avoid GridBagLayout. Instead, use the other layout managers and especially the technique of combining several panels using different layout managers within a single program. Your applets won’t look that different; at least not enough to justify the trouble that GridBagLayout entails. For my part, it’s just too painful to come up with an example for this (and I wouldn’t want to encourage this kind of library design). Instead, I’ll refer you to Core Java 2 by Horstmann & Cornell (Prentice-Hall, 1999) to get you started.

BoxLayout

Because so many people had so much trouble understanding and working with GridBagLayout, Javasoft introduced BoxLayout in Java 2, which gives you many of the benefits of GridBagLayout without the grief. BoxLayout allows you to control the placement of components either vertically or horizontally, and control the space between the components using something call “Struts and Glue.” First, let’s look at using the BoxLayout directly, as all the other layout managers are used:

//: c13:BoxLayout1.java
// Vertical and horizontal BoxLayouts.
// <applet code=BoxLayout1
// width=450 height=200> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class BoxLayout1 extends JApplet {
  public void init() {
    JPanel jpv = new JPanel();
    jpv.setLayout(
      new BoxLayout(jpv, BoxLayout.Y_AXIS));
    for(int i = 0; i < 5; i++)
      jpv.add(new JButton("" + i));
    JPanel jph = new JPanel();
    jph.setLayout(
      new BoxLayout(jph, BoxLayout.X_AXIS));
    for(int i = 0; i < 5; i++)
      jph.add(new JButton("" + i));
    Container cp = getContentPane();
    cp.add(BorderLayout.EAST, jpv);
    cp.add(BorderLayout.SOUTH, jph);
  }
  public static void main(String[] args) {
    Console.run(new BoxLayout1(), 450, 200);
  }
} ///:~

The constructor for BoxLayout is a bit different than the other layout managers—you provide the Container that is to be controlled by the BoxLayout as the first argument, and the direction of the layout as the second argument.

To simplify matters, there’s a special container called Box that uses BoxLayout as its native manager. The following example lays out components horizontally and vertically using Box, which has two static methods to create boxes with vertical and horizontal alignment:

//: c13:Box1.java
// Vertical and horizontal BoxLayouts.
// <applet code=Box1
// width=450 height=200> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Box1 extends JApplet {
  public void init() {
    Box bv = Box.createVerticalBox();
    for(int i = 0; i < 5; i++)
      bv.add(new JButton("" + i));
    Box bh = Box.createHorizontalBox();
    for(int i = 0; i < 5; i++)
      bh.add(new JButton("" + i));
    Container cp = getContentPane();
    cp.add(BorderLayout.EAST, bv);
    cp.add(BorderLayout.SOUTH, bh);
  }
  public static void main(String[] args) {
    Console.run(new Box1(), 450, 200);
  }
} ///:~
//: c13:Box2.java
// Adding struts.
// <applet code=Box2
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Box2 extends JApplet {
  public void init() {
    Box bv = Box.createVerticalBox();
    for(int i = 0; i < 5; i++) {
      bv.add(new JButton("" + i));
      bv.add(Box.createVerticalStrut(i*10));
    }
    Box bh = Box.createHorizontalBox();
    for(int i = 0; i < 5; i++) {
      bh.add(new JButton("" + i));
      bh.add(Box.createHorizontalStrut(i*10));
    }
    Container cp = getContentPane();
    cp.add(BorderLayout.EAST, bv);
    cp.add(BorderLayout.SOUTH, bh);
  }
  public static void main(String[] args) {
    Console.run(new Box2(), 450, 300);
  }
} ///:~
//: c13:Box3.java
// Using Glue.
// <applet code=Box3
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Box3 extends JApplet {
  public void init() {
    Box bv = Box.createVerticalBox();
    bv.add(new JLabel("Hello"));
    bv.add(Box.createVerticalGlue());
    bv.add(new JLabel("Applet"));
    bv.add(Box.createVerticalGlue());
    bv.add(new JLabel("World"));
    Box bh = Box.createHorizontalBox();
    bh.add(new JLabel("Hello"));
    bh.add(Box.createHorizontalGlue());
    bh.add(new JLabel("Applet"));
    bh.add(Box.createHorizontalGlue());
    bh.add(new JLabel("World"));
    bv.add(Box.createVerticalGlue());
    bv.add(bh);
    bv.add(Box.createVerticalGlue());
    getContentPane().add(bv);
  }
  public static void main(String[] args) {
    Console.run(new Box3(), 450, 300);
  }
} ///:~
//: c13:Box4.java
// Rigid Areas are like pairs of struts.
// <applet code=Box4
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Box4 extends JApplet {
  public void init() {
    Box bv = Box.createVerticalBox();
    bv.add(new JButton("Top"));
    bv.add(Box.createRigidArea(
      new Dimension(120, 90)));
    bv.add(new JButton("Bottom"));
    Box bh = Box.createHorizontalBox();
    bh.add(new JButton("Left"));
    bh.add(Box.createRigidArea(
      new Dimension(160, 80)));
    bh.add(new JButton("Right"));
    bv.add(bh);
    getContentPane().add(bv);
  }
  public static void main(String[] args) {
    Console.run(new Box4(), 450, 300);
  }
} ///:~

You should be aware that rigid areas are a bit controversial. Since they use absolute values, some people feel that they cause more trouble than they are worth.

Absolute positioning

It is possible to set the absolute position of the graphical components in your Frame or Applet in this way:

  1. Set a null layout manager for your Container: setLayout(null).
  2. Call setBounds( ) or reshape (depending on the language version) for each component, passing a bounding rectangle in pixel coordinates. You can do it in the constructor, or in paint( ), depending on what you want to achieve.

Alternatives to action

As noted previously, action( ) isn’t the only method that’s automatically called by handleEvent( ) once it sorts everything out for you. There are three other sets of methods that are called, and if you want to capture certain types of events (keyboard, mouse, and focus events) all you have to do is override the provided method. These methods are defined in the base class Component, so they’re available in virtually all the controls that you might place on a form. However, you should be aware that this approach is deprecated in Java 1.1, so although you might see legacy code using this technique you should use the Java 1.1 approaches (described later in this chapter) instead.

Component method | When it’s called
action (Event evt, Object what) | When the “typical” event occurs for this component (for example, when a button is pushed or a drop-down list item is selected)
keyDown (Event evt, int key) | A key is pressed when this component has the focus. The second argument is the key that was pressed and is redundantly copied from evt.key.
keyUp(Event evt, int key) | A key is released when this component has the focus.
lostFocus(Event evt, Object what) | The focus has moved away from the target. Normally, what is redundantly copied from evt.arg.
gotFocus(Event evt, Object what) | The focus has moved into the target.
mouseDown(Event evt, int x, int y) | A mouse down has occurred over the component, at the coordinates x, y.
mouseUp(Event evt, int x, int y) | A mouse up has occurred over the component.
mouseMove(Event evt, int x, int y) | The mouse has moved while it’s over the component.
mouseDrag(Event evt, int x, int y) | The mouse is being dragged after a mouseDown occurred over the component. All drag events are reported to the component in which the mouseDown occurred until there is a mouseUp.
mouseEnter(Event evt, int x, int y) | The mouse wasn’t over the component before, but now it is.
mouseExit(Event evt, int x, int y) | The mouse used to be over the component, but now it isn’t.

You can see that each method receives an Event object along with some information that you’ll typically need when you’re handling that particular situation—with a mouse event, for example, it’s likely that you’ll want to know the coordinates where the mouse event occurred. It’s interesting to note that when Component’s handleEvent( ) calls any of these methods (the typical case), the extra arguments are always redundant as they are contained within the Event object. In fact, if you look at the source code for Component.handleEvent( ) you can see that it explicitly plucks the additional arguments out of the Event object. (This might be considered inefficient coding in some languages, but remember that Java’s focus is on safety, not necessarily speed.)

To prove to yourself that these events are in fact being called and as an interesting experiment, it’s worth creating an applet that overrides each of the methods above (except for action( ), which is overridden in many other places in this chapter) and displays data about each of the events as they happen.

This example also shows you how to make your own button object because that’s what is used as the target of all the events of interest. You might first (naturally) assume that to make a new button, you’d inherit from Button. But this doesn’t work. Instead, you inherit from Canvas (a much more generic component) and paint your button on that canvas by overriding the paint( ) method. As you’ll see, it’s really too bad that overriding Button doesn’t work, since there’s a bit of code involved to paint the button. (If you don’t believe me, try exchanging Button for Canvas in this example, and remember to call the base-class constructor super(label). You’ll see that the button doesn’t get painted and the events don’t get handled.)

The myButton class is specific: it works only with a TrackEvent “parent window” (not a base class, but the window in which this button is created and lives). With this knowledge, myButton can reach into the parent window and manipulate its text fields, which is what’s necessary to be able to write the status information into the fields of the parent. Of course this is a much more limited solution, since myButton can be used only in conjunction with TrackEvent. This kind of code is sometimes called “highly coupled.” However, to make myButton more generic requires a lot more effort that isn’t warranted for this example (and possibly for many of the applets that you will write). Again, keep in mind that the following code uses APIs that are deprecated in Java 1.1.

//: c13:TrackEvent.java
// Show events as they happen.
// <applet code=TrackEvent
//  width=700 height=500></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

class MyButton extends JButton {
  HashMap h;
  public MyButton(TrackEvent parent,
    Color color, String label) {
    super(label);
    h = parent.h;
    setBackground(color);
    addFocusListener(fl);
    addKeyListener(kl);
    addMouseListener(ml);
    addMouseMotionListener(mml);
  }
  void report(String field, String msg) {
    ((JTextField)h.get(field)).setText(msg);
  }
  FocusListener fl = new FocusListener() {
    public void focusGained(FocusEvent e) {
      report("focusGained", e.paramString());
    }
    public void focusLost(FocusEvent e) {
      report("focusLost", e.paramString());
    }
  };
  KeyListener kl = new KeyListener() {
    public void keyPressed(KeyEvent e) {
      report("keyPressed", e.paramString());
    }
    public void keyReleased(KeyEvent e) {
      report("keyReleased", e.paramString());
    }
    public void keyTyped(KeyEvent e) {
      report("keyTyped", e.paramString());
    }
  };
  MouseListener ml = new MouseListener() {
    public void mouseClicked(MouseEvent e) {
      report("mouseClicked", e.paramString());
    }
    public void mouseEntered(MouseEvent e) {
      report("mouseEntered", e.paramString());
    }
    public void mouseExited(MouseEvent e) {
      report("mouseExited", e.paramString());
    }
    public void mousePressed(MouseEvent e) {
      report("mousePressed", e.paramString());
    }
    public void mouseReleased(MouseEvent e) {
      report("mouseReleased", e.paramString());
    }
  };
  MouseMotionListener mml =
    new MouseMotionListener() {
    public void mouseDragged(MouseEvent e) {
      report("mouseDragged", e.paramString());
    }
    public void mouseMoved(MouseEvent e) {
      report("mouseMoved", e.paramString());
    }
  };
}

public class TrackEvent extends JApplet {
  HashMap h = new HashMap();
  String[] event = {
    "focusGained", "focusLost", "keyPressed",
    "keyReleased", "keyTyped", "mouseClicked",
    "mouseEntered", "mouseExited","mousePressed",
    "mouseReleased", "mouseDragged", "mouseMoved"
  };
  MyButton
    b1 = new MyButton(this, Color.blue, "test1"),
    b2 = new MyButton(this, Color.red, "test2");
  public void init() {
    Container c = getContentPane();
    c.setLayout(new GridLayout(event.length+1,2));
    for(int i = 0; i < event.length; i++) {
      JTextField t = new JTextField();
      t.setEditable(false);
      c.add(new JLabel(event[i], JLabel.RIGHT));
      c.add(t);
      h.put(event[i], t);
    }
    c.add(b1);
    c.add(b2);
  }
  public static void main(String[] args) {
    Console.run(new TrackEvent(), 700, 500);
  }
} ///:~

You can see the constructor uses the technique of using the same name for the argument as what it’s assigned to, and differentiating between the two using this:

this.label = label;

The paint( ) method starts out simple: it fills a “round rectangle” with the button’s color, and then draws a black line around it. Notice the use of size( ) to determine the width and height of the component (in pixels, of course). After this, paint( ) seems quite complicated because there’s a lot of calculation going on to figure out how to center the button’s label inside the button using the “font metrics.” You can get a pretty good idea of what’s going on by looking at the method call, and it turns out that this is pretty stock code, so you can just cut and paste it when you want to center a label inside any component.

You can’t understand exactly how the keyDown( ), keyUp( ), etc. methods work until you look down at the TrackEvent class. This contains a HashMap to hold the strings representing the type of event and the TextField where information about that event is held. Of course, these could have been created statically rather than putting them in a HashMap, but I think you’ll agree that it’s a lot easier to use and change. In particular, if you need to add or remove a new type of event in TrackEvent, you simply add or remove a string in the event array—everything else happens automatically.

The place where you look up the strings is in the keyDown( ), keyUp( ), etc. methods back in MyButton. Each of these methods uses the parent reference to reach back to the parent window. Since that parent is an TrackEvent it contains the HashMap h, and the get( ) method, when provided with the appropriate String, will produce a reference to an object that we happen to know is a TextField—so it is cast to that. Then the Event object is converted to its String representation, which is displayed in the TextField.

It turns out this example is rather fun to play with since you can really see what’s going on with the events in your program.

Applet restrictions

For safety’s sake, applets are quite restricted and there are many things you can’t do. You can generally answer the question of what an applet is able to do by looking at what it is supposed to do: extend the functionality of a Web page in a browser. Since, as a net surfer, you never really know if a Web page is from a friendly place or not, you want any code that it runs to be safe. So the biggest restrictions you’ll notice are probably:

1) An applet can’t touch the local disk. This means writing or reading, since you wouldn’t want an applet to read and transmit important information about you across the Web. Writing is prevented, of course, since that would be an open invitation to a virus. These restrictions can be relaxed when digital signing is fully implemented.

Many applet restrictions are relaxed for trusted applets (those signed by a trusted source) in newer browsers.

There are other issues when thinking about applet development:

  • Applets take longer to download since you must download the whole thing every time, including a separate server hit for each different class. Your browser can cache the applet, but there are no guarantees. One improvement in Java 1.1 is the JAR (Java ARchive) file that allows packaging of all the applet components (including other .class files as well as images and sounds) together into a single compressed file that can be downloaded in a single server transaction. “Digital signing” (the ability to verify the creator of a class) is available for each individual entry in the JAR file.
  • Because of security issues you must work harder to do certain things such as accessing databases and sending email. In addition, the security restrictions make accessing multiple hosts difficult, since everything has to be routed through the Web server, which then becomes a performance bottleneck and a single failure point that can stop the entire process.
  • An applet within the browser doesn’t have the same kind of control that a native application does. For example, you can’t have a modal dialog box within an applet, since the user can always switch the page. When the user does change from a Web page or even exit the browser, the results can be catastrophic for your applet—there’s no way to save the state so if you’re in the middle of a transaction or other operation the information can be lost. In addition, different browsers do different things to your applet when you leave a Web page so the results are essentially undefined.

Applet advantages

If you can live within the restrictions, applets have definite advantages, especially when building client/server or other networked applications:

  • There is no installation issue. An applet has true platform independence (including the ability to easily play audio files, etc.) so you don’t need to make any changes in your code for different platforms nor does anyone have to perform any “tweaking” upon installation. In fact, installation is automatic every time the user loads the Web page along with the applets, so updates happen silently and automatically. In traditional client/server systems, building and installing a new version of the client software is often a nightmare.
  • Because of the security built into the core Java language and the applet structure, you don’t have to worry about bad code causing damage to someone’s system. This, along with the previous point, makes Java (as well as alternative client-side Web programming tools like JavaScript and VBScript) popular for so-called intranet client/server applications that live only within the company and don’t move out onto the Internet.
  • Because applets are automatically integrated with HTML, you have a built-in platform-independent documentation system to support the applet. It’s an interesting twist, since we’re used to having the documentation part of the program rather than vice versa.

Windowed applications

[Probably remove this, or move the example]

Here’s an example of a windowed application:

//: c13:ButtonApp.java
// Creating an application.
// <applet code=ButtonApp
//  width=200 height=200></applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class ButtonApp extends JApplet {
  JButton
    b1 = new JButton("Hello"),
    b2 = new JButton("Howdy");
  JTextField t = new JTextField(15);
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e){
      String name =
        ((JButton)e.getSource()).getText();
      t.setText(name);
    }
  };
  public void init() {
    b1.addActionListener(al);
    b2.addActionListener(al);
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
    cp.add(t);
  }
  public static void main(String[] args) {
    Console.run(new ButtonApp(), 200, 200);
  }
} ///:~

Menus

There are four different types inherited from the abstract class MenuComponent: MenuBar (you can have one MenuBar only on a particular Frame), Menu to hold one individual drop-down menu or submenu, MenuItem to represent one single element on a menu, and CheckboxMenuItem, which is derived from MenuItem and produces a checkmark to indicate whether that menu item is selected.

Unlike a system that uses resources, with Java and Swing you must hand assemble all the menus in source code. Here are the ice cream flavors again, used to create menus:

//: c13:Menu1.java
// Shows submenus, checkbox menu
// items, and swapping menus
// <applet code=Menu1
//  width=300 height=100> </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Menu1 extends JApplet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie"
  };
  JTextField t = new JTextField("No flavor", 30);
  JMenuBar mb1 = new JMenuBar();
  JMenu
    f = new JMenu("File"),
    m = new JMenu("Flavors"),
    s = new JMenu("Safety");
  // Alternative approach:
  JCheckBoxMenuItem[] safety = {
    new JCheckBoxMenuItem("Guard"),
    new JCheckBoxMenuItem("Hide")
  };
  JMenuItem[] file = {
    new JMenuItem("Open"),
    new JMenuItem("Exit")
  };
  // A second menu bar to swap to:
  JMenuBar mb2 = new JMenuBar();
  JMenu fooBar = new JMenu("fooBar");
  JMenuItem[] other = {
    new JMenuItem("Foo"),
    new JMenuItem("Bar"),
    new JMenuItem("Baz"),
  };
  JButton b = new JButton("Swap Menus");
  public void init() {
    for(int i = 0; i < flavors.length; i++) {
      JMenuItem mi = new JMenuItem(flavors[i]);
      mi.addActionListener(al);
      m.add(mi);
      // Add separators at intervals:
      if((i+1) % 3 == 0)
        m.addSeparator();
    }
    for(int i = 0; i < safety.length; i++) {
      safety[i].addActionListener(al);
      s.add(safety[i]);
    }
    f.add(s);
    for(int i = 0; i < file.length; i++) {
      file[i].addActionListener(al);
      f.add(file[i]);
    }
    mb1.add(f);
    mb1.add(m);
    t.setEditable(false);
    Container cp = getContentPane();
    cp.add(t, BorderLayout.CENTER);
    // Set up the system for swapping menus:
    b.addActionListener(al);
    cp.add(b, BorderLayout.NORTH);
    for(int i = 0; i < other.length; i++) {
      other[i].addActionListener(al);
      fooBar.add(other[i]);
    }
    mb2.add(fooBar);
    setJMenuBar(mb1);
  }
  ActionListener al = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      String arg = e.getActionCommand();
      Object source = e.getSource();
      if(source.equals(b)) {
        JMenuBar m = getJMenuBar();
        setJMenuBar(m == mb1 ? mb2 : mb1);
        validate(); // Refresh the frame
      } else if(source instanceof JMenuItem) {
        if(arg.equals("Open")) {
          String s = t.getText();
          boolean chosen = false;
          for(int i = 0; i < flavors.length; i++)
            if(s.equals(flavors[i]))
              chosen = true;
          if(!chosen)
            t.setText("Choose a flavor first!");
          else
            t.setText("Opening "+s+". Mmm, mm!");
        } else if(source.equals(file[1]))
          System.exit(0);
        // CheckboxMenuItems cannot use String
        // matching; you must match getSource():
        else if(source.equals(safety[0]))
          t.setText("Guard the Ice Cream! " +
            "Guarding is "+safety[0].getState());
        else if(source.equals(safety[1]))
          t.setText("Hide the Ice Cream! " +
            "Is it cold? "+safety[1].getState());
        else
          t.setText(arg);
      }
    }
  };
  public static void main(String[] args) {
    Console.run(new Menu1(), 300, 100);
  }
} ///:~

In this program I avoided the typical long lists of add( ) calls for each menu because that seemed like a lot of unnecessary typing. Instead, I placed the menu items into arrays and then simply stepped through each array calling add( ) in a for loop. This makes adding or subtracting a menu item less tedious.

As an alternative approach (which I find less desirable since it requires more typing), the CheckboxMenuItems are created in an array of references called safety; this is true for the arrays file and other as well.

This program creates not one but two MenuBars to demonstrate that menu bars can be actively swapped while the program is running. You can see how a MenuBar is made up of Menus, and each Menu is made up of MenuItems, CheckboxMenuItems, or even other Menus (which produce submenus). When a MenuBar is assembled it can be installed into the current program with the setMenuBar( ) method. Note that when the button is pressed, it checks to see which menu is currently installed using getMenuBar( ), then puts the other menu bar in its place.

When testing for “Open,” notice that spelling and capitalization are critical, but Java signals no error if there is no match with “Open.” This kind of string comparison is a clear source of programming errors.

The checking and un-checking of the menu items is taken care of automatically, but dealing with CheckboxMenuItems can be a bit surprising since for some reason they don’t allow string matching. (Although string matching isn’t a good approach, this seems inconsistent.) So you can match only the target object and not its label. As shown, the getState( ) method can be used to reveal the state. You can also change the state of a CheckboxMenuItem with setState( ).

You might think that one menu could reasonably reside on more than one menu bar. This does seem to make sense because all you’re passing to the MenuBar add( ) method is a reference. However, if you try this, the behavior will be strange and not what you expect. (It’s difficult to know if this is a bug or if they intended it to work this way.)

This example also shows what you need to do to create an application instead of an applet. (Again, because an application can support menus and an applet cannot directly have a menu.) Instead of inheriting from Applet, you inherit from Frame. Instead of init( ) to set things up, you make a constructor for your class. Finally, you create a main( ) and in that you build an object of your new type, resize it, and then call show( ). It’s different from an applet in only a few small places, but it’s now a standalone windowed application and you’ve got menus.

Dialog boxes

A dialog box is a window that pops up out of another window. Its purpose is to deal with some specific issue without cluttering the original window with those details. Dialog boxes are heavily used in windowed programming environments, but as mentioned previously, rarely used in applets.

To create a dialog box, you inherit from Dialog, which is just another kind of Window, like a Frame. Unlike a Frame, a Dialog cannot have a menu bar or change the cursor, but other than that they’re quite similar. A dialog has a layout manager (which defaults to BorderLayout) and you override action( ) etc., or handleEvent( ) to deal with events. One significant difference you’ll want to note in handleEvent( ): when the WINDOW_DESTROY event occurs, you don’t want to shut down the application! Instead, you release the resources used by the dialog’s window by calling dispose( ).

In the following example, the dialog box is made up of a grid (using GridLayout) of a special kind of button that is defined here as class ToeButton. This button draws a frame around itself and, depending on its state, a blank, an “x,” or an “o” in the middle. It starts out blank, and then depending on whose turn it is, changes to an “x” or an “o.” However, it will also flip back and forth between “x” and “o” when you click on the button. (This makes the tic-tac-toe concept only slightly more annoying than it already is.) In addition, the dialog box can be set up for any number of rows and columns by changing numbers in the main application window.

//: c13:TicTacToe.java
// Demonstration of dialog boxes
// and creating your own components.
// <applet code=TicTacToe
//  width=200 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

class ToeButton extends JPanel {
  int state = ToeDialog.BLANK;
  ToeDialog p;
  public ToeButton(ToeDialog parent) {
    p = parent;
    addMouseListener(ml);
  }
  MouseListener ml = new MouseAdapter() {
    public void mousePressed(MouseEvent e) {
      if(state == ToeDialog.BLANK) {
        state = p.turn;
        p.turn = (p.turn == ToeDialog.XX ?
          ToeDialog.OO : ToeDialog.XX);
      } else
        state = (state == ToeDialog.XX ?
          ToeDialog.OO : ToeDialog.XX);
      repaint();
    }
  };
  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    int x1 = 0;
    int y1 = 0;
    int x2 = getWidth() - 1;
    int y2 = getHeight() - 1;
    g.drawRect(x1, y1, x2, y2);
    x1 = x2/4;
    y1 = y2/4;
    int wide = x2/2;
    int high = y2/2;
    if(state == ToeDialog.XX) {
      g.drawLine(x1, y1, x1 + wide, y1 + high);
      g.drawLine(x1, y1 + high, x1 + wide, y1);
    }
    if(state == ToeDialog.OO) {
      g.drawOval(x1, y1, x1+wide/2, y1+high/2);
    }
  }
}

class ToeDialog extends JDialog {
  static final int BLANK = 0, XX = 1, OO = 2;
  int turn = XX; // Start with x's turn
  // w = number of cells wide
  // h = number of cells high
  public ToeDialog(int w, int h) {
    setTitle("The game itself");
   // JDK 1.3 close dialog:
   //#setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    // JDK 1.2 close dialog:
    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e){
        dispose();
      }
    });
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(w, h));
    for(int i = 0; i < w * h; i++)
      cp.add(new ToeButton(this));
    setSize(w * 50, h * 50);
  }
}

public class TicTacToe extends JApplet {
  JTextField
    rows = new JTextField("3"),
    cols = new JTextField("3");
  JButton go = new JButton("go");
  public void init() {
    JPanel p = new JPanel();
    p.setLayout(new GridLayout(2,2));
    p.add(new JLabel("Rows", JLabel.CENTER));
    p.add(rows);
    p.add(new JLabel("Columns", JLabel.CENTER));
    p.add(cols);
    Container cp = getContentPane();
    cp.add(p, BorderLayout.NORTH);
    cp.add(go, BorderLayout.SOUTH);
    go.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        JDialog d = new ToeDialog(
          Integer.parseInt(rows.getText()),
          Integer.parseInt(cols.getText()));
        d.setVisible(true);
      }
    });
  }
  public static void main(String[] args) {
    Console.run(new TicTacToe(), 200, 100);
  }
} ///:~

The ToeButton class keeps a reference to its parent, which must be of type ToeDialog. As before, this introduces high coupling because a ToeButton can be used only with a ToeDialog, but it solves a number of problems, and in truth it doesn’t seem like such a bad solution because there’s no other kind of dialog that’s keeping track of whose turn it is. Of course, you can take another approach, which is to make ToeDialog.turn a static member of ToeButton. This eliminates the coupling, but prevents you from having more than one ToeDialog at a time. (More than one that works properly, anyway.)

The paint( ) method is concerned with the graphics: drawing the square around the button and drawing the “x” or the “o.” This is full of tedious calculations, but it’s straightforward.

A mouse click is captured by the overridden mouseDown( ) method, which first checks to see if the button has anything written on it. If not, the parent window is queried to find out whose turn it is and that is used to establish the state of the button. Note that the button then reaches back into the parent and changes the turn. If the button is already displaying an “x” or an “o” then that is flopped. You can see in these calculations the convenient use of the ternary if-else described in Chapter 3. After a button state change, the button is repainted.

The constructor for ToeDialog is quite simple: it adds into a GridLayout as many buttons as you request, then resizes it for 50 pixels on a side for each button. (If you don’t resize a Window, it won’t show up!) Note that handleEvent( ) just calls dispose( ) for a WINDOW_DESTROY so the whole application doesn’t go away.

TicTacToe sets up the whole application by creating the TextFields (for inputting the rows and columns of the button grid) and the “go” button. You’ll see in action( ) that this program uses the less-desirable “string match” technique for detecting the button press (make sure you get spelling and capitalization right!). When the button is pressed, the data in the TextFields must be fetched, and, since they are in String form, turned into ints using the static Integer.parseInt( ) method. Once the Dialog is created, the show( ) method must be called to display and activate it.

You’ll notice that the ToeDialog object is assigned to a Dialog reference d. This is an example of upcasting, although it really doesn’t make much difference here since all that’s happening is the show( ) method is called. However, if you wanted to call some method that existed only in ToeDialog you would want to assign to a ToeDialog reference and not lose the information in an upcast.

File dialogs

Some operating systems have a number of special built-in dialog boxes to handle the selection of things such as fonts, colors, printers, and the like. Virtually all graphical operating systems support the opening and saving of files, however, and so Java’s JFileChooser encapsulates these for easy use.

The following application exercises two forms of JFileChooser dialogs, one for opening and one for saving. Most of the code should by now be familiar, and all the interesting activities happen in the actionlisteners for the two different button clicks:

//: c13:FileChooserTest.java
// Demonstration of File dialog boxes.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class FileChooserTest extends JFrame {
  JTextField
    filename = new JTextField(),
    dir = new JTextField();
  JButton
    open = new JButton("Open"),
    save = new JButton("Save");
  public FileChooserTest() {
    setTitle("File Dialog Test");
    JPanel p = new JPanel();
    open.addActionListener(new OpenL());
    p.add(open);
    save.addActionListener(new SaveL());
    p.add(save);
    Container cp = getContentPane();
    cp.add(p, BorderLayout.SOUTH);
    dir.setEditable(false);
    filename.setEditable(false);
    p = new JPanel();
    p.setLayout(new GridLayout(2,1));
    p.add(filename);
    p.add(dir);
    cp.add(p, BorderLayout.NORTH);
  }
  class OpenL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JFileChooser c = new JFileChooser();
      // Demonstrate "Open" dialog:
      int rVal =
        c.showOpenDialog(FileChooserTest.this);
      if(rVal == JFileChooser.APPROVE_OPTION) {
        filename.setText(
          c.getSelectedFile().getName());
          dir.setText(
            c.getCurrentDirectory().toString());
      }
      if(rVal == JFileChooser.CANCEL_OPTION) {
        filename.setText("You pressed cancel");
        dir.setText("");
      }
    }
  }
  class SaveL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JFileChooser c = new JFileChooser();
      // Demonstrate "Save" dialog:
      int rVal =
        c.showSaveDialog(FileChooserTest.this);
      if(rVal == JFileChooser.APPROVE_OPTION) {
        filename.setText(
          c.getSelectedFile().getName());
          dir.setText(
            c.getCurrentDirectory().toString());
      }
      if(rVal == JFileChooser.CANCEL_OPTION) {
        filename.setText("You pressed cancel");
        dir.setText("");
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new FileChooserTest(), 250, 110);
  }
} ///:~

Note that there are many variations you can apply to JFileChooser, including filters to narrow the file names that you will allow.

For an “open file” dialog, you use the constructor that takes two arguments; the first is the parent window reference and the second is the title for the title bar of the FileDialog. The method setFile( ) provides an initial file name—presumably the native OS supports wildcards, so in this example all the .java files will initially be displayed. The setDirectory( ) method chooses the directory where the file selection will begin. (In general, the OS allows the user to change directories.)

The show( ) command doesn’t return until the dialog is closed. The FileDialog object still exists, so you can read data from it. If you call getFile( ) and it returns null it means the user canceled out of the dialog. Both the file name and the results of getDirectory( ) are displayed in the TextFields.

The button for saving works the same way, except that it uses a different constructor for the FileDialog. This constructor takes three arguments and the third argument must be either FileDialog.SAVE or FileDialog.OPEN.

The event model

In the new event model a component can initiate (“fire”) an event. Each type of event is represented by a distinct class. When an event is fired, it is received by one or more “listeners,” which act on that event. Thus, the source of an event and the place where the event is handled can be separate.

Each event listener is an object of a class that implements a particular type of listener interface. So as a programmer, all you do is create a listener object and register it with the component that’s firing the event. This registration is performed by calling a addXXXListener( ) method in the event-firing component, in which XXX represents the type of event listened for. You can easily know what types of events can be handled by noticing the names of the addListener methods, and if you try to listen for the wrong events you’ll find out your mistake at compile-time. Java Beans also uses the names of the addListener methods to determine what a Bean can do.

All of your event logic, then, will go inside a listener class. When you create a listener class, the sole restriction is that it must implement the appropriate interface. You can create a global listener class, but this is a situation in which inner classes tend to be quite useful, not only because they provide a logical grouping of your listener classes inside the UI or business logic classes they are serving, but because (as you shall see later) the fact that an inner class object keeps a reference to its parent object provides a nice way to call across class and subsystem boundaries.

A simple example will make this clear. Consider the Button2.java example from earlier in this chapter.

//: c13:Button2New.java
// Capturing button presses.
// <applet code=Button2New
// width=200 height=50> </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*; // Must add this
import com.bruceeckel.swing.*;

public class Button2New extends JApplet {
  JButton
    b1 = new JButton("Button 1"),
    b2 = new JButton("Button 2");
  public void init() {
    b1.addActionListener(new B1());
    b2.addActionListener(new B2());
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
  }
  class B1 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      getAppletContext().showStatus("Button 1");
    }
  }
  class B2 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      getAppletContext().showStatus("Button 2");
    }
  }
  public static void main(String[] args) {
    Console.run(new Button2New(), 200, 50);
  }
} ///:~

So you can compare the two approaches, the old code is left in as a comment. In init( ), the only change is the addition of the two lines:

b1.addActionListener(new B1());
b2.addActionListener(new B2());

addActionListener( ) tells a button which object to activate when the button is pressed. The classes B1 and B2 are inner classes that implement the interface ActionListener. This interface contains a single method actionPerformed( ) (meaning “This is the action that will be performed when the event is fired”). Note that actionPerformed( ) does not take a generic event, but rather a specific type of event, ActionEvent. So you don’t need to bother testing and downcasting the argument if you want to extract specific ActionEvent information.

One of the nicest things about actionPerformed( ) is how simple it is. It’s just a method that gets called. Compare it to the old action( ) method, in which you must figure out what happened and act appropriately, and also worry about calling the base class version of action( ) and return a value to indicate whether it’s been handled. With the new event model you know that all the event-detection logic is taken care of so you don’t have to figure that out; you just say what happens and you’re done. If you don’t already prefer this approach over the old one, you will soon.

Event and listener types

All Swing components have been changed to include addXXXListener( ) and removeXXXListener( ) methods so that the appropriate types of listeners can be added and removed from each component. You’ll notice that the “XXX” in each case also represents the argument for the method, for example, addFooListener(FooListener fl). The following table includes the associated events, listeners, methods, and the components that support those particular events by providing the addXXXListener( ) and removeXXXListener( ) methods.

Event, listener interface and add- and remove-methods | Components supporting this event
ActionEvent ActionListener addActionListener( ) removeActionListener( ) | Button, List, TextField, MenuItem, and its derivatives including CheckboxMenuItem, Menu, and PopupMenu
AdjustmentEvent AdjustmentListener addAdjustmentListener( ) removeAdjustmentListener( ) | Scrollbar Anything you create that implements the Adjustable interface
ComponentEvent ComponentListener addComponentListener( ) removeComponentListener( ) | Component and its derivatives, including Button, Canvas, Checkbox, Choice, Container, Panel, Applet, ScrollPane, Window, Dialog, FileDialog, Frame, Label, List, Scrollbar, TextArea, and TextField
ContainerEvent ContainerListener addContainerListener( ) removeContainerListener( ) | Container and its derivatives, including Panel, Applet, ScrollPane, Window, Dialog, FileDialog, and Frame
FocusEvent FocusListener addFocusListener( ) removeFocusListener( ) | Component and its derivatives, including Button, Canvas, Checkbox, Choice, Container, Panel, Applet, ScrollPane, Window, Dialog, FileDialog, Frame, Label, List, Scrollbar, TextArea, and TextField
KeyEvent KeyListener addKeyListener( ) removeKeyListener( ) | Component and its derivatives, including Button, Canvas, Checkbox, Choice, Container, Panel, Applet, ScrollPane, Window, Dialog, FileDialog, Frame, Label, List, Scrollbar, TextArea, and TextField
MouseEvent (for both clicks and motion) MouseListener addMouseListener( ) removeMouseListener( ) | Component and its derivatives, including Button, Canvas, Checkbox, Choice, Container, Panel, Applet, ScrollPane, Window, Dialog, FileDialog, Frame, Label, List, Scrollbar, TextArea, and TextField
MouseEvent [64] (for both clicks and motion) MouseMotionListener addMouseMotionListener( ) removeMouseMotionListener( ) | Component and its derivatives, including Button, Canvas, Checkbox, Choice, Container, Panel, Applet, ScrollPane, Window, Dialog, FileDialog, Frame, Label, List, Scrollbar, TextArea, and TextField
WindowEvent WindowListener addWindowListener( ) removeWindowListener( ) | Window and its derivatives, including Dialog, FileDialog, and Frame
ItemEvent ItemListener addItemListener( ) removeItemListener( ) | Checkbox, CheckboxMenuItem, Choice, List, and anything that implements the ItemSelectable interface
TextEvent TextListener addTextListener( ) removeTextListener( ) | Anything derived from TextComponent, including TextArea and TextField

You can see that each type of component supports only certain types of events. It’s helpful to see the events supported by each component, as shown in the following table:

Component type | Events supported by this component
Adjustable | AdjustmentEvent
Applet | ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Button | ActionEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Canvas | FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Checkbox | ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
CheckboxMenuItem | ActionEvent, ItemEvent
Choice | ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Component | FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Container | ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Dialog | ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
FileDialog | ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Frame | ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Label | FocusEvent, KeyEvent, MouseEvent, ComponentEvent
List | ActionEvent, FocusEvent, KeyEvent, MouseEvent, ItemEvent, ComponentEvent
Menu | ActionEvent
MenuItem | ActionEvent
Panel | ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
PopupMenu | ActionEvent
Scrollbar | AdjustmentEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
ScrollPane | ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextArea | TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextComponent | TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextField | ActionEvent, TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Window | ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent

Once you know which events a particular component supports, you don’t need to look anything up to react to that event. You simply:

  1. Take the name of the event class and remove the word “Event.” Add the word “Listener” to what remains. This is the listener interface you need to implement in your inner class.
  2. Implement the interface above and write out the methods for the events you want to capture. For example, you might be looking for mouse movements, so you write code for the mouseMoved( ) method of the MouseMotionListener interface. (You must implement the other methods, of course, but there’s a shortcut for that which you’ll see soon.)
  3. Create an object of the listener class in step 2. Register it with your component with the method produced by prefixing “add” to your listener name. For example, addMouseMotionListener( ).

To finish what you need to know, here are the listener interfaces:

Listener interface w/ adapter | Methods in interface
ActionListener | actionPerformed(ActionEvent)
AdjustmentListener | adjustmentValueChanged( AdjustmentEvent)
ComponentListener ComponentAdapter | componentHidden(ComponentEvent) componentShown(ComponentEvent) componentMoved(ComponentEvent) componentResized(ComponentEvent)
ContainerListener ContainerAdapter | componentAdded(ContainerEvent) componentRemoved(ContainerEvent)
FocusListener FocusAdapter | focusGained(FocusEvent) focusLost(FocusEvent)
KeyListener KeyAdapter | keyPressed(KeyEvent) keyReleased(KeyEvent) keyTyped(KeyEvent)
MouseListener MouseAdapter | mouseClicked(MouseEvent) mouseEntered(MouseEvent) mouseExited(MouseEvent) mousePressed(MouseEvent) mouseReleased(MouseEvent)
MouseMotionListener MouseMotionAdapter | mouseDragged(MouseEvent) mouseMoved(MouseEvent)
WindowListener WindowAdapter | windowOpened(WindowEvent) windowClosing(WindowEvent) windowClosed(WindowEvent) windowActivated(WindowEvent) windowDeactivated(WindowEvent) windowIconified(WindowEvent) windowDeiconified(WindowEvent)
ItemListener | itemStateChanged(ItemEvent)
TextListener | textValueChanged(TextEvent)

Using listener adapters for simplicity

In the table above, you can see that some listener interfaces have only one method. These are trivial to implement since you’ll implement them only when you want to write that particular method. However, the listener interfaces that have multiple methods could be less pleasant to use. For example, something you must always do when creating an application is provide a WindowListener to the Frame so that when you get the windowClosing( ) event you can call System.exit(0) to exit the application. But since WindowListener is an interface, you must implement all of the other methods even if they don’t do anything. This can be annoying.

To solve the problem, each of the listener interfaces that have more than one method are provided with adapters, the names of which you can see in the table above. Each adapter provides default methods for each of the interface methods. (Alas, WindowAdapter does not have a default windowClosing( ) that calls System.exit(0).) Then all you need to do is inherit from the adapter and override only the methods you need to change. For example, the typical WindowListener you’ll use looks like this:

class MyWindowListener extends WindowAdapter {
  public void windowClosing(WindowEvent e) {
    System.exit(0);
  }
}

The whole point of the adapters is to make the creation of listener classes easy.

There is a downside to adapters, however, in the form of a pitfall. Suppose you write a WindowAdapter like the one above:

class MyWindowListener extends WindowAdapter {
  public void WindowClosing(WindowEvent e) {
    System.exit(0);
  }
}

This doesn’t work, but it will drive you crazy trying to figure out why, since everything will compile and run fine—except that closing the window won’t exit the program. Can you see the problem? It’s in the name of the method: WindowClosing( ) instead of windowClosing( ). A simple slip in capitalization results in the addition of a completely new method. However, this is not the method that’s called when the window is closing, so you don’t get the desired results.

Packaging the applet into a JAR file

An important JAR use is to optimize applet loading. In Java 1.0, people tended to try to cram all their code into a single Applet class so the client would need only a single server hit to download the applet code. Not only did this result in messy, hard to read (and maintain) programs, but the .class file was still uncompressed so downloading wasn’t as fast as it could have been.

JAR files change all of that by compressing all of your .class files into a single file that is downloaded by the browser. Now you don’t need to create an ugly design to minimize the number of classes you create, and the user will get a much faster download time.

Consider the example above. It looks like Button2NewB is a single class, but in fact it contains three inner classes, so that’s four in all. Once you’ve compiled the program, you package it into a JAR file with the line:

jar cf Button2NewB.jar *.class

This assumes that the only .class files in the current directory are the ones from Button2NewB.java (otherwise you’ll get extra baggage).

Now you can create an HTML page with the new archive tag to indicate the name of the JAR file, like this:

<head><title>Button2NewB Example Applet
</title></head>
<body>
<applet code=Button2NewB.class
        archive=Button2NewB.jar
        width=200 height=150>
</applet>
</body>

Everything else about applet tags in HTML files remains the same.

Revisiting the earlier examples

To see a number of examples using the new event model and to study the way a program can be converted from the old to the new event model, the following examples revisit many of the issues demonstrated in the first part of this chapter using the old event model. In addition, each program is now both an applet and an application so you can run it with or without a browser.

Demonstrating
the framework methods

It’s interesting to see some of the framework methods in action. (This example will look only at init( ), start( ), and stop( ) because paint( ) and destroy( ) are self-evident and not so easily traceable.) The following applet keeps track of the number of times these methods are called and displays them using paint( ):

//: c13:Applet3.java
// Shows init(), start() and stop() activities.
// <applet code=Applet3 width=250 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Applet3 extends JApplet {
  String s;
  int inits = 0;
  int starts = 0;
  int stops = 0;
  public void init() { inits++; }
  public void start() { starts++; }
  public void stop() { stops++; }
  public void paint(Graphics g) {
    s = "inits: " + inits +
      ", starts: " + starts +
      ", stops: " + stops;
    g.drawString(s, 10, 10);
  }
  public static void main(String[] args) {
    Console.run(new Applet3 (), 250, 50);
  }
} ///:~

Normally when you override a method you’ll want to look to see whether you need to call the base-class version of that method, in case it does something important. For example, with init( ) you might need to call super.init( ). However, the Applet documentation specifically states that the init( ), start( ), and stop( ) methods in Applet do nothing, so it’s not necessary to call them here.

When you experiment with this applet you’ll discover that if you minimize the Web browser or cover it up with another window you might not get calls to stop( ) and start( ). (This behavior seems to vary among implementations; you might wish to contrast the behavior of Web browsers with that of applet viewers.) The only time the calls will occur is when you move to a different Web page and then come back to the one containing the applet.

Text fields

This is similar to TextField1.java, but it adds significant extra behavior:

//: c13:TextNew.java
// Text fields and Java events.
// <applet code=TextNew width=375
// height=125></applet>
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class TextNew extends JApplet {
  JButton
    b1 = new JButton("Get Text"),
    b2 = new JButton("Set Text");
  JTextField
    t1 = new JTextField(30),
    t2 = new JTextField(30),
    t3 = new JTextField(30);
  String s = new String();
  UpperCaseDocument
    ucd = new UpperCaseDocument();
  public void init() {
    t1.setDocument(ucd);
    ucd.addDocumentListener(new T1());
    b1.addActionListener(new B1());
    b2.addActionListener(new B2());
    DocumentListener dl = new T1();
    t1.addActionListener(new T1A());
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(b1);
    cp.add(b2);
    cp.add(t1);
    cp.add(t2);
    cp.add(t3);
  }
  class T1 implements DocumentListener {
    public void changedUpdate(DocumentEvent e){}
    public void insertUpdate(DocumentEvent e){
      t2.setText(t1.getText());
      t3.setText("Text: "+ t1.getText());
    }
    public void removeUpdate(DocumentEvent e){
      t2.setText(t1.getText());
    }
  }
  class T1A implements ActionListener {
    private int count = 0;
    public void actionPerformed(ActionEvent e) {
      t3.setText("t1 Action Event " + count++);
    }
  }
  class B1 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(t1.getSelectedText() == null)
        s = t1.getText();
      else
        s = t1.getSelectedText();
      t1.setEditable(true);
    }
  }
  class B2 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      ucd.setUpperCase(false);
      t1.setText("Inserted by Button 2: " + s);
      ucd.setUpperCase(true);
      t1.setEditable(false);
    }
  }
  public static void main(String[] args) {
    Console.run(new TextNew(), 375, 125);
  }
}

class UpperCaseDocument extends PlainDocument {
  boolean upperCase = true;
  public void setUpperCase(boolean flag) {
    upperCase = flag;
  }
  public void insertString(int offset,
    String string, AttributeSet attributeSet)
    throws BadLocationException {
      if(upperCase)
        string = string.toUpperCase();
      super.insertString(offset,
        string, attributeSet);
  }
} ///:~

The TextField t3 is included as a place to report when the action listener for the TextField t1 is fired. You’ll see that the action listener for a TextField is fired only when you press the “enter” key.

The TextField t1 has several listeners attached to it. The T1 listener copies all text from t1 into t2 and the T1K listener forces all characters to upper case. You’ll notice that the two work together, and if you add the T1K listener after you add the T1 listener, it doesn’t matter: all characters will still be forced to upper case in both text fields. It would seem that keyboard events are always fired before TextComponent events, and if you want the characters in t2 to retain the original case that was typed in, you must do some extra work.

T1K has some other activities of interest. You must detect a backspace (since you’re controlling everything now) and perform the deletion. The caret must be explicitly set to the end of the field; otherwise it won’t behave as you expect. Finally, to prevent the original character from being handled by the default mechanism, the event must be “consumed” using the consume( ) method that exists for event objects. This tells the system to stop firing the rest of the event handlers for this particular event.

This example also quietly demonstrates one of the benefits of the design of inner classes. Note that in the inner class:

  class T1 implements TextListener {
    public void textValueChanged(TextEvent e) {
      t2.setText(t1.getText());
    }
  }

t1 and t2 are not members of T1, and yet they’re accessible without any special qualification. This is because an object of an inner class automatically captures a reference to the outer object that created it, so you can treat members and methods of the enclosing class object as if they’re yours. As you can see, this is quite convenient. [65]

Text areas

The most significant change to text areas in Java 1.1 concerns scroll bars. With the TextArea constructor, you can now control whether a TextArea will have scroll bars: vertical, horizontal, both, or neither. This example modifies the earlier Java 1.0 TextArea1.java to show the Java 1.1 scrollbar constructors:

//: c13:TextAreaNew.java
// Controlling scrollbars with JtextArea.
// <applet code=TextAreaNew width=300 height=725>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;

public class TextAreaNew extends JApplet {
  JButton
    b1 = new JButton("Text Area 1"),
    b2 = new JButton("Text Area 2"),
    b3 = new JButton("Replace Text"),
    b4 = new JButton("Insert Text");
  JTextArea
    t1 = new JTextArea("t1", 1, 20),
    t2 = new JTextArea("t2", 4, 20),
    t3 = new JTextArea("t3", 1, 20),
    t4 = new JTextArea("t4", 10, 10),
    t5 = new JTextArea("t5", 4, 20),
    t6 = new JTextArea("t6", 10, 10);
  JScrollPane
    sp3 = new JScrollPane(t3,
      JScrollPane.VERTICAL_SCROLLBAR_NEVER,
      JScrollPane.HORIZONTAL_SCROLLBAR_NEVER),
    sp4 = new JScrollPane(t4,
      JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
      JScrollPane.HORIZONTAL_SCROLLBAR_NEVER),
    sp5 = new JScrollPane(t5,
      JScrollPane.VERTICAL_SCROLLBAR_NEVER,
      JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS),
    sp6 = new JScrollPane(t6,
      JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
      JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
  class B1L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t5.append(t1.getText() + "\n");
    }
  }
  class B2L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t2.setText("Inserted by Button 2");
      t2.append(": " + t1.getText());
      t5.append(t2.getText() + "\n");
    }
  }
  class B3L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String s = " Replacement ";
      t2.replaceRange(s, 3, 3 + s.length());
    }
  }
  class B4L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t2.insert(" Inserted ", 10);
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    // Create Borders for components:
    Border brd = BorderFactory.createMatteBorder(
      1, 1, 1, 1, Color.black);
    t1.setBorder(brd);
    t2.setBorder(brd);
    sp3.setBorder(brd);
    sp4.setBorder(brd);
    sp5.setBorder(brd);
    sp6.setBorder(brd);
    // Initialize listeners and add components:
    b1.addActionListener(new B1L());
    cp.add(b1);
    cp.add(t1);
    b2.addActionListener(new B2L());
    cp.add(b2);
    cp.add(t2);
    b3.addActionListener(new B3L());
    cp.add(b3);
    b4.addActionListener(new B4L());
    cp.add(b4);
    cp.add(sp3);
    cp.add(sp4);
    cp.add(sp5);
    cp.add(sp6);
  }
  public static void main(String[] args) {
    Console.run(new TextAreaNew(), 300, 725);
  }
} ///:~

You’ll notice that you can control the scrollbars only at the time of construction of the TextArea. Also, even if a TextArea doesn’t have a scrollbar, you can move the cursor such that scrolling will be forced. (You can see this behavior by playing with the example.)

Check boxes and radio buttons

As noted previously, check boxes and radio buttons are both created with the same class, Checkbox, but radio buttons are Checkboxes placed into a CheckboxGroup. In either case, the interesting event is ItemEvent, for which you create an ItemListener.

When dealing with a group of check boxes or radio buttons, you have a choice. You can either create a new inner class to handle the event for each different Checkbox or you can create one inner class that determines which Checkbox was clicked and register a single object of that inner class with each Checkbox object. The following example shows both approaches:

//: c13:RadioCheckNew.java
// Radio buttons and Check Boxes.
// <applet code=RadioCheckNew
// width=325 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class RadioCheckNew extends JApplet {
  JTextField t = new JTextField(20);
  JCheckBox[] cb = {
    new JCheckBox("Check Box 1"),
    new JCheckBox("Check Box 2"),
    new JCheckBox("Check Box 3") };
  ButtonGroup group = new ButtonGroup();
  JRadioButton
    cb4 = new JRadioButton("four"),
    cb5 = new JRadioButton("five"),
    cb6 = new JRadioButton("six");
  // Checking the source:
  class ILCheck implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      for(int i = 0; i < cb.length; i++) {
        if(e.getSource().equals(cb[i])) {
          t.setText("Check box " + (i + 1));
          return;
        }
      }
    }
  }
  // vs. an individual class for each item:
  class IL4 implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      t.setText("Radio button four");
    }
  }
  class IL5 implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      t.setText("Radio button five");
    }
  }
  class IL6 implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      t.setText("Radio button six");
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    t.setEditable(false);
    cp.add(t);
    ILCheck il = new ILCheck();
    for(int i = 0; i < cb.length; i++) {
      cb[i].addItemListener(il);
      cp.add(cb[i]);
    }
    group.add(cb4);
    group.add(cb5);
    group.add(cb6);
    cb4.addItemListener(new IL4());
    cb5.addItemListener(new IL5());
    cb6.addItemListener(new IL6());
    cp.add(cb4); cp.add(cb5); cp.add(cb6);
  }
  public static void main(String[] args) {
    Console.run(new RadioCheckNew(), 325, 100);
  }
} ///:~

ILCheck has the advantage that it automatically adapts when you add or subtract Checkboxes. Of course, you can use this with radio buttons as well. It should be used, however, only when your logic is general enough to support this approach. Otherwise you’ll end up with a cascaded if statement, a sure sign that you should revert to using independent listener classes.

Drop-down lists

Drop-down lists (Choice) in Java 1.1 also use ItemListeners to notify you when a choice has changed:

//: c13:ChoiceNew.java
// Drop-down lists (combo boxes).
// <applet code=ChoiceNew
// width=450 height=175></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class ChoiceNew extends JApplet {
  String[] descriptions1 = { "Ebullient",
    "Obtuse", "Recalcitrant", "Brilliant" };
  String[] descriptions2 = { "Somnescent",
    "Timorous", "Florid", "Putrescent" };
  JTextArea t = new JTextArea(7, 40);
  JComboBox c = new JComboBox(descriptions1);
  JButton b = new JButton("Add items");
  int count = 0;
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    t.setLineWrap(true);
    t.setEditable(false);
    cp.add(t);
    cp.add(c);
    cp.add(b);
    c.addItemListener(new CL());
    b.addActionListener(new BL());
  }
  class CL implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      t.setText("index: " +  c.getSelectedIndex()
        + "   " + e);
    }
  }
  class BL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(count < descriptions2.length)
        c.addItem(descriptions2[count++]);
      if(count >=descriptions2.length)
        b.setEnabled(false);
    }
  }
  public static void main(String[] args) {
    Console.run(new ChoiceNew(), 450, 175);
  }
} ///:~

Nothing else here is particularly new (except that Java 1.1 has significantly fewer bugs in the UI classes).

Lists

You’ll recall that one of the problems with the Java 1.0 List design is that it took extra work to make it do what you’d expect: react to a single click on one of the list elements. Java 1.1 has solved this problem:

//: c13:ListNew.java
// <applet code=ListNew width=250
// height=325> </applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;

public class ListNew extends JApplet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  DefaultListModel lItems=new DefaultListModel();
  JList lst = new JList(lItems);
  JTextArea t = new JTextArea(flavors.length,20);
  JButton b = new JButton("Add Item");
  ActionListener bl = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      if(count < flavors.length) {
        lItems.add(0, flavors[count++]);
      } else {
        // Disable, since there are no more
        // flavors left to be added to the List
        b.setEnabled(false);
      }
    }
  };
  ListSelectionListener ll =
    new ListSelectionListener() {
      public void valueChanged(
        ListSelectionEvent e) {
          t.setText("");
          Object[] items=lst.getSelectedValues();
          for(int i = 0; i < items.length; i++)
            t.append(items[i] + "\n");
        }
    };
  int count = 0;
  public void init() {
    Container cp = getContentPane();
    t.setEditable(false);
    cp.setLayout(new FlowLayout());
    // Create Borders for components:
    Border brd = BorderFactory.createMatteBorder(
      1, 1, 2, 2, Color.black);
    lst.setBorder(brd);
    t.setBorder(brd);
    // Add the first four items to the List
    for(int i = 0; i < 4; i++)
      lItems.addElement(flavors[count++]);
    // Add items to the Content Pane for Display
    cp.add(t);
    cp.add(lst);
    cp.add(b);
    // Register event listeners
    lst.addListSelectionListener(ll);
    b.addActionListener(bl);
  }
  public static void main(String[] args) {
    Console.run(new ListNew(), 250, 325);
  }
} ///:~

You can see that no extra logic is required to support a single click on a list item. You just attach a listener like you do everywhere else.

Menus

The event handling for menus does seem to benefit from the Java 1.1 event model, but Java’s approach to menus is still messy and requires a lot of hand coding. The right medium for a menu seems to be a resource rather than a lot of code. Keep in mind that program-building tools will generally handle the creation of menus for you, so that will reduce the pain somewhat (as long as they will also handle the maintenance!).

In addition, you’ll find the events for menus are inconsistent and can lead to confusion: MenuItems use ActionListeners, but CheckboxMenuItems use ItemListeners. The Menu objects can also support ActionListeners, but that’s not usually helpful. In general, you’ll attach listeners to each MenuItem or CheckboxMenuItem, but the following example (revised from the earlier version) also shows ways to combine the capture of multiple menu components into a single listener class. As you’ll see, it’s probably not worth the hassle to do this.

//: c13:MenuNew.java
// Menu shortcuts and action commands.
// <applet code=MenuNew width=300
// height=100> </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class MenuNew extends JApplet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  JTextField t = new JTextField("No flavor", 30);
  JMenuBar mb1 = new JMenuBar();
  JMenu
    f = new JMenu("File"),
    m = new JMenu("Flavors"),
    s = new JMenu("Safety");
  // Alternative approach:
  JCheckBoxMenuItem[] safety = {
    new JCheckBoxMenuItem("Guard"),
    new JCheckBoxMenuItem("Hide")
  };
  JMenuItem[] file = {
    // No menu shortcut:
    new JMenuItem("Open"),
    // Adding a menu shortcut is very simple:
    new JMenuItem("Exit", KeyEvent.VK_E)
  };
  // A second menu bar to swap to:
  JMenuBar mb2 = new JMenuBar();
  JMenu fooBar = new JMenu("fooBar");
  JMenuItem[] other = {
    new JMenuItem("Foo"),
    new JMenuItem("Bar"),
    new JMenuItem("Baz"),
  };
  JButton b = new JButton("Swap Menus");
  class BL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JMenuBar m = getJMenuBar();
      setJMenuBar(m == mb1 ? mb2 : mb1);
      validate(); // Refresh the frame ///////// Necessary???
    }
  }
  class ML implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JMenuItem target = (JMenuItem)e.getSource();
      String actionCommand =
        target.getActionCommand();
      if(actionCommand.equals("Open")) {
        String s = t.getText();
        boolean chosen = false;
        for(int i = 0; i < flavors.length; i++)
          if(s.equals(flavors[i])) chosen = true;
        if(!chosen)
          t.setText("Choose a flavor first!");
        else
          t.setText("Opening "+ s +". Mmm, mm!");
      } else if(actionCommand.equals("Exit")) {
        // This trick won't work with applets
        // because MenuNew.this doesn't produce
        // a Window if this is a JApplet:
        //dispatchEvent(
        //  new WindowEvent(MenuNew.this,
        //    WindowEvent.WINDOW_CLOSING));
      }
    }
  }
  class FL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JMenuItem target = (JMenuItem)e.getSource();
      t.setText(target.getText());
    }
  }
  // Alternatively, you can create a different
  // class for each different MenuItem. Then you
  // Don't have to figure out which one it is:
  class FooL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t.setText("Foo selected");
    }
  }
  class BarL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t.setText("Bar selected");
    }
  }
  class BazL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t.setText("Baz selected");
    }
  }
  class CMIL implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      JCheckBoxMenuItem target =
        (JCheckBoxMenuItem)e.getSource();
      String actionCommand =
        target.getActionCommand();
      if(actionCommand.equals("Guard"))
        t.setText("Guard the Ice Cream! " +
          "Guarding is " + target.getState());
      else if(actionCommand.equals("Hide"))
        t.setText("Hide the Ice Cream! " +
          "Is it cold? " + target.getState());
    }
  }
  public void init() {
    ML ml = new ML();
    CMIL cmil = new CMIL();
    safety[0].setActionCommand("Guard");
    safety[0].addItemListener(cmil);
    safety[1].setActionCommand("Hide");
    safety[1].addItemListener(cmil);
    file[0].setActionCommand("Open");
    file[0].addActionListener(ml);
    file[1].setActionCommand("Exit");
    file[1].addActionListener(ml);
    other[0].addActionListener(new FooL());
    other[1].addActionListener(new BarL());
    other[2].addActionListener(new BazL());
    FL fl = new FL();
    for(int i = 0; i < flavors.length; i++) {
      JMenuItem mi = new JMenuItem(flavors[i]);
      mi.addActionListener(fl);
      m.add(mi);
      // Add separators at intervals:
      if((i+1) % 3 == 0)
        m.addSeparator();
    }
    for(int i = 0; i < safety.length; i++)
      s.add(safety[i]);
    f.add(s);
    for(int i = 0; i < file.length; i++)
      f.add(file[i]);
    mb1.add(f);
    mb1.add(m);
    setJMenuBar(mb1);
    t.setEditable(false);
    Container cp = getContentPane();
    cp.add(t, BorderLayout.CENTER);
    // Set up the system for swapping menus:
    b.addActionListener(new BL());
    cp.add(b, BorderLayout.NORTH);
    for(int i = 0; i < other.length; i++)
      fooBar.add(other[i]);
    mb2.add(fooBar);
  }
  public static void main(String[] args) {
    Console.run(new MenuNew(), 300, 100);
  }
} ///:~

This code is similar to the previous (Java 1.0) version, until you get to the init( ) method. Here you can see the ItemListeners and ActionListeners attached to the various menu components.

Java 1.1 supports “menu shortcuts,” so you can select a menu item using the keyboard instead of the mouse. These are quite simple; you just use the overloaded MenuItem constructor that takes as a second argument a MenuShortcut object. The constructor for MenuShortcut takes the key of interest, which magically appears on the menu item when it drops down. The example above adds Control-E to the “Exit” menu item.

You can also see the use of setActionCommand( ). This seems a bit strange because in each case the “action command” is exactly the same as the label on the menu component. Why not just use the label instead of this alternative string? The problem is internationalization. If you retarget this program to another language, you want to change only the label in the menu, and not go through the code changing all the logic that will no doubt introduce new errors. So to make this easy for code that checks the text string associated with a menu component, the “action command” can be immutable while the menu label can change. All the code works with the “action command,” so it’s unaffected by changes to the menu labels. Note that in this program, not all the menu components are examined for their action commands, so those that aren’t don’t have their action command set.

Much of the constructor is the same as before, with the exception of a couple of calls to add listeners. The bulk of the work happens in the listeners. In BL, the MenuBar swapping happens as in the previous example. In ML, the “figure out who rang” approach is taken by getting the source of the ActionEvent and casting it to a MenuItem, then getting the action command string to pass it through a cascaded if statement. Much of this is the same as before, but notice that if “Exit” is chosen, a new WindowEvent is created, passing in the reference of the enclosing class object (MenuNew.this) and creating a WINDOW_CLOSING event. This is handed to the dispatchEvent( ) method of the enclosing class object, which then ends up calling windowClosing( ) inside the window listener for the Frame (this listener is created as an anonymous inner class, inside main( )), just as if the message had been generated the “normal” way. Through this mechanism, you can dispatch any message you want in any circumstances, so it’s quite powerful.

The FL listener is simple even though it’s handling all the different flavors in the flavor menu. This approach is useful if you have enough simplicity in your logic, but in general, you’ll want to take the approach used with FooL, BarL, and BazL, in which they are each attached to only a single menu component so no extra detection logic is necessary and you know exactly who called the listener. Even with the profusion of classes generated this way, the code inside tends to be smaller and the process is more foolproof.

Dialog boxes

This is a direct rewrite of the earlier TicTacToe.java. In this version, however, everything is placed inside an inner class. Although this completely eliminates the need to keep track of the object that spawned any class, as was the case in TicTacToe.java, it could be taking the concept of inner classes a bit too far. At one point, the inner classes are nested four deep! This is the kind of design in which you need to decide whether the benefit of inner classes is worth the increased complexity. In addition, when you create a non-static inner class you’re tying that class to its surrounding class. Sometimes a standalone class can more easily be reused.

//: c13:TicTacToeInner.java
// TicTacToe.java with heavy use of inner classes.
// <applet code=TicTacToeInner
//  width=200 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class TicTacToeInner extends JApplet {
  JTextField
    rows = new JTextField("3"),
    cols = new JTextField("3");
  static final int BLANK = 0, XX = 1, OO = 2;
  class ToeDialog extends JDialog {
    int turn = XX; // Start with x's turn
    // w = number of cells wide
    // h = number of cells high
    public ToeDialog(int w, int h) {
      setTitle("The game itself");
      Container cp = getContentPane();
      cp.setLayout(new GridLayout(w, h));
      for(int i = 0; i < w * h; i++)
        cp.add(new ToeButton());
      setSize(w * 50, h * 50);
      addWindowListener(new WindowAdapter() {
        public void windowClosing(WindowEvent e){
          dispose();
        }
      });
    }
    class ToeButton extends JPanel {
      int state = BLANK;
      public ToeButton() {
        addMouseListener(new ML());
      }
      public void paintComponent(Graphics g) {
        super.paintComponent(g);
        int x1 = 0;
        int y1 = 0;
        int x2 = getSize().width - 1;
        int y2 = getSize().height - 1;
        g.drawRect(x1, y1, x2, y2);
        x1 = x2/4;
        y1 = y2/4;
        int wide = x2/2;
        int high = y2/2;
        if(state == XX) {
          g.drawLine(x1, y1,
            x1 + wide, y1 + high);
          g.drawLine(x1, y1 + high,
            x1 + wide, y1);
        }
        if(state == OO) {
          g.drawOval(x1, y1,
            x1 + wide/2, y1 + high/2);
        }
      }
      class ML extends MouseAdapter {
        public void mousePressed(MouseEvent e) {
          if(state == BLANK) {
            state = turn;
            turn = (turn == XX ? OO : XX);
          }
          else
            state = (state == XX ? OO : XX);
          repaint();
        }
      }
    }
  }
  class BL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      JDialog d = new ToeDialog(
        Integer.parseInt(rows.getText()),
        Integer.parseInt(cols.getText()));
      d.setVisible(true);
    }
  }
  public void init() {
    JPanel p = new JPanel();
    p.setLayout(new GridLayout(2,2));
    p.add(new JLabel("Rows", JLabel.CENTER));
    p.add(rows);
    p.add(new JLabel("Columns", JLabel.CENTER));
    p.add(cols);
    Container cp = getContentPane();
    cp.add(p, BorderLayout.NORTH);
    JButton b = new JButton("go");
    b.addActionListener(new BL());
    cp.add(b, BorderLayout.SOUTH);
  }
  public static void main(String[] args) {
    Console.run(new TicTacToeInner(), 200, 100);
  }
} ///:~

Because statics can be at only the outer level of the class, inner classes cannot have static data or static inner classes.

Selecting Look & Feel

One of the very interesting aspects of Swing is the “Pluggable Look & Feel.” This allows your program to emulate the look and feel of various operating environments. You can even do all sorts of fancy things like dynamically changing the look and feel while the program is executing. However, you generally just want to do one of two things, either select the “cross platform” look and feel (which is Swing’s “metal”) or select the look and feel for the system you are currently on, so your Java program looks like it was created specifically for that system. The code to select either of these behaviors is quite simple, but you must execute it before you create any visual components because the components will be made based on the current look and feel and will not be changed just because you happen to change the look and feel midway during the program (that process is more complicated and uncommon, and is relegated to Swing-specific books).

Actually, if you want to use the cross-platform (“metal”) look and feel that is characteristic of Swing programs, you don’t have to do anything—it’s the default. But if you want instead to use the current operating environment’s look and feel, you just insert the following code, typically at the beginning of your main( ) but somehow before any components are added:

try {
  UIManager.setLookAndFeel(UIManager.
    getSystemLookAndFeelClassName());
} catch (Exception ex) { }

You don’t need anything in the catch clause because the UIManager will default to the cross-platform look and feel if your attempts to set up any of the alternatives fail. However, during debugging the exception can be quite useful.

Here is a program that takes a command-line argument to select a look and feel, and shows how a selection of components look:

//: c13:LookAndFeel.java
// Selecting different looks & feels.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

public class LookAndFeel extends JFrame {
  String[] choices = {
    "eeny", "meeny", "minie", "moe", "toe", "you"
  };
  Component[] samples = {
    new JButton("JButton"),
    new JTextField("JTextField"),
    new JLabel("JLabel"),
    new JCheckBox("JCheckBox"),
    new JRadioButton("Radio"),
    new JComboBox(choices),
    new JList(choices),
  };
  public LookAndFeel() {
    super("Look And Feel");
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < samples.length; i++)
      cp.add(samples[i]);
  }
  private static void usageError() {
    System.out.println(
      "Usage:LookAndFeel [cross|system|motif]");
    System.exit(1);
  }
  public static void main(String[] args) {
    if(args.length == 0) usageError();
    if(args[0].equals("cross")) {
      try {
        UIManager.setLookAndFeel(UIManager.
          getCrossPlatformLookAndFeelClassName());
      } catch (Exception ex) {
          ex.printStackTrace();
      }
    } else if(args[0].equals("system")) {
      try {
        UIManager.setLookAndFeel(UIManager.
          getSystemLookAndFeelClassName());
      } catch (Exception ex) {
          ex.printStackTrace();
      }
    } else if(args[0].equals("motif")) {
      try {
        UIManager.setLookAndFeel("com.sun.java."+
          "swing.plaf.motif.MotifLookAndFeel");
      } catch (Exception ex) {
          ex.printStackTrace();
      }
    } else usageError();
    // Note the look & feel must be set before
    // any components are created.
    Console.run(new LookAndFeel(), 300, 200);
  }
} ///:~

You can see that one option is to explicitly specify a string for a look and feel, as seen with MotifLookAndFeel. However, that one and the default “metal” look and feel are the only ones that can legally be used on any platform; even though there are strings for Windows and Macintosh look and feels, those can only be used on their respective platforms (these are produced when you call getSystemLookAndFeelClassName( ) and you’re on that particular platform).

It is also possible to create a custom look and feel package, for example if you are building a framework for a company that wants a distinctive appearance. This is a big job and is far beyond the scope of this book (in fact, you’ll discover it is beyond the scope of many dedicated Swing books!).

Binding events dynamically

One of the benefits of the new AWT event model is flexibility. In the old model you were forced to hard code the behavior of your program, but with the new model you can add and remove event behavior with single method calls. The following example demonstrates this:

//: c13:DynamicEvents.java
// You can change event behavior dynamically.
// Also shows multiple actions for an event.
// <applet code=DynamicEvents
//  width=200 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

public class DynamicEvents extends JApplet {
  ArrayList v = new ArrayList();
  int i = 0;
  JButton
    b1 = new JButton("Button1"),
    b2 = new JButton("Button2");
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    b1.addActionListener(new B());
    b1.addActionListener(new B1());
    b2.addActionListener(new B());
    b2.addActionListener(new B2());
    cp.add(b1);
    cp.add(b2);
  }
  class B implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("A button was pressed");
    }
  }
  class CountListener implements ActionListener {
    int index;
    public CountListener(int i) { index = i; }
    public void actionPerformed(ActionEvent e) {
      System.out.println(
        "Counted Listener " + index);
    }
  }
  class B1 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("Button 1 pressed");
      ActionListener a = new CountListener(i++);
      v.add(a);
      b2.addActionListener(a);
    }
  }
  class B2 implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("Button2 pressed");
      int end = v.size() - 1;
      if(end >= 0) {
        b2.removeActionListener(
          (ActionListener)v.get(end));
        v.remove(end);
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new DynamicEvents(), 200, 100);
  }
} ///:~

The new twists in this example are:

  1. There is more than one listener attached to each Button. Usually, components handle events as multicast, meaning that you can register many listeners for a single event. In the special components in which an event is handled as unicast, you’ll get a TooManyListenersException.
  2. During the execution of the program, listeners are dynamically added and removed from the Button b2. Adding is accomplished in the way you’ve seen before, but each component also has a removeXXXListener( ) method to remove each type of listener.

This kind of flexibility provides much greater power in your programming.

You should notice that event listeners are not guaranteed to be called in the order they are added (although most implementations do in fact work that way).

Separating business logic
from UI logic

In general you’ll want to design your classes so that each one does “only one thing.” This is particularly important when user-interface code is concerned, since it’s easy to wrap up “what you’re doing” with “how you’re displaying it.” This kind of coupling prevents code reuse. It’s much more desirable to separate your “business logic” from the GUI. This way, you can not only reuse the business logic more easily, it’s also easier to reuse the GUI.

Another issue is multi-tiered systems, where the “business objects” reside on a completely separate machine. This central location of the business rules allows changes to be instantly effective for all new transactions, and is thus a compelling way to set up a system. However, these business objects can be used in many different applications and so should not be tied to any particular mode of display. They should just perform the business operations and nothing more.

The following example shows how easy it is to separate the business logic from the GUI code:

//: c13:Separation.java
// Separating GUI logic and business objects.
// <applet code=Separation
// width=250 height=100> </applet>
import javax.swing.*;
import java.awt.*;
import javax.swing.event.*;
import java.awt.event.*;
import java.applet.*;
import com.bruceeckel.swing.*;

class BusinessLogic {
  private int modifier;
  public BusinessLogic(int mod) {
    modifier = mod;
  }
  public void setModifier(int mod) {
    modifier = mod;
  }
  public int getModifier() {
    return modifier;
  }
  // Some business operations:
  public int calculation1(int arg) {
    return arg * modifier;
  }
  public int calculation2(int arg) {
    return arg + modifier;
  }
}

public class Separation extends JApplet {
  JTextField
    t = new JTextField(15),
    mod = new JTextField(15);
  BusinessLogic bl = new BusinessLogic(2);
  JButton
    calc1 = new JButton("Calculation 1"),
    calc2 = new JButton("Calculation 2");
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    calc1.addActionListener(new Calc1L());
    calc2.addActionListener(new Calc2L());
    JPanel p1 = new JPanel();
    p1.add(calc1);
    p1.add(calc2);
    cp.add(p1);
    mod.getDocument().
      addDocumentListener(new ModL());
    JPanel p2 = new JPanel();
    p2.add(new JLabel("Modifier:"));
    p2.add(mod);
    cp.add(p2);
  }
  static int getValue(JTextField tf) {
    try {
      return Integer.parseInt(tf.getText());
    } catch(NumberFormatException e) {
      return 0;
    }
  }
  class Calc1L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t.setText(Integer.toString(
        bl.calculation1(getValue(t))));
    }
  }
  class Calc2L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      t.setText(Integer.toString(
        bl.calculation2(getValue(t))));
    }
  }
  // If you want something to happen whenever
  // a JTextField changes, add this listener:
  class ModL implements DocumentListener {
    public void changedUpdate(DocumentEvent e) {}
    public void insertUpdate(DocumentEvent e) {
      bl.setModifier(getValue(mod));
    }
    public void removeUpdate(DocumentEvent e) {
      bl.setModifier(getValue(mod));
    }
  }
  public static void main(String[] args) {
    Console.run(new Separation(), 250, 100);
  }
} ///:~

You can see that BusinessLogic is a straightforward class that performs its operations without even a hint that it might be used in a GUI environment. It just does its job.

Separation keeps track of all the UI details, and it talks to BusinessLogic only through its public interface. All the operations are centered around getting information back and forth through the UI and the BusinessLogic object. So Separation, in turn, just does its job. Since Separation knows only that it’s talking to a BusinessLogic object (that is, it isn’t highly coupled), it could be massaged into talking to other types of objects without much trouble.

Thinking in terms of separating UI from business logic also makes life easier when you’re adapting legacy code to work with Java.

Recommended coding approaches

Inner classes, the new event model, and the fact that the old event model is still supported along with new library features that rely on old-style programming has added a new element of confusion. Now there are even more different ways for people to write unpleasant code. Unfortunately, this kind of code is showing up in books and article examples, and even in documentation and examples distributed from Sun! In this section we’ll look at some misunderstandings about what you should and shouldn’t do with the new AWT, and end by showing that except in extenuating circumstances you can always use listener classes (written as inner classes) to solve your event-handling needs. Since this is also the simplest and clearest approach, it should be a relief for you to learn this.

Before looking at anything else, you should know that although Java 1.1 is backward-compatible with Java 1.0 (that is, you can compile and run 1.0 programs with 1.1), you cannot mix the event models within the same program. That is, you cannot use the old-style action( ) method in the same program in which you employ listeners. This can be a problem in a larger program when you’re trying to integrate old code with a new program, since you must decide whether to use the old, hard-to-maintain approach with the new program or to update the old code. This shouldn’t be too much of a battle since the new approach is so superior to the old.

Baseline: the good way to do it

To give you something to compare with, here’s an example showing the recommended approach. By now it should be reasonably familiar and comfortable:

//: c13:GoodIdea.java
// The best way to design classes using the
// Java event model: use an inner class for
// each different event. This maximizes
// flexibility and modularity.
// <applet code=GoodIdea
//  width=200 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

public class GoodIdea extends JApplet {
  JButton
    b1 = new JButton("Button 1"),
    b2 = new JButton("Button 2");
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    b1.addActionListener(new B1L());
    b2.addActionListener(new B2L());
    cp.add(b1);
    cp.add(b2);
  }
  public class B1L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("Button 1 pressed");
    }
  }
  public class B2L implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("Button 2 pressed");
    }
  }
  public static void main(String[] args) {
    Console.run(new GoodIdea(), 200, 100);
  }
} ///:~

This is fairly trivial: each button has its own listener that prints something out to the console. But notice that there isn’t an if statement in the entire program, or any statement that says, “I wonder what caused this event.” Each piece of code is concerned with doing, not type-checking. This is the best way to write your code; not only is it easier to conceptualize, but much easier to read and maintain. Cutting and pasting to create new programs is also much easier.

JFC APIs

JFC includes important functionality, including focus traversal, desktop color access, printing “inside the sandbox,” the beginnings of clipboard support, and drag-and-drop.

Focus traversal is quite easy, since it’s transparently present in Swing library components and you don’t have to do anything to make it work. If you make your own components and want them to handle focus traversal, you override isFocusTraversable( ) to return true. If you want to capture the keyboard focus on a mouse click, you catch the mouse down event and call requestFocus( ).

Desktop colors

The desktop colors provide a way for you to know what the various color choices are on the current user’s desktop. This way, you can use those colors in your program if you desire. The colors are automatically initialized and placed in static members of class SystemColor, so all you need to do is read the member you’re interested in. The names are intentionally self-explanatory: desktop, activeCaption, activeCaptionText, activeCaptionBorder, inactiveCaption, inactiveCaptionText, inactiveCaptionBorder, window, windowBorder, windowText, menu, menuText, text, textText, textHighlight, textHighlightText, textInactiveText, control, controlText, controlHighlight, controlLtHighlight, controlShadow, controlDkShadow, scrollbar, info (for help), and infoText (for help text).

The clipboard

The JFC supports limited operations with the system clipboard (in the java.awt.datatransfer package). You can copy String objects to the clipboard as text, and you can paste text from the clipboard into String objects. Of course, the clipboard is designed to hold any type of data, but how this data is represented on the clipboard is up to the program doing the cutting and pasting. The Java clipboard API provides for extensibility through the concept of a “flavor.” When data comes off the clipboard, it has an associated set of flavors that it can be converted to (for example, a graph might be represented as a string of numbers or as an image) and you can see if that particular clipboard data supports the flavor you’re interested in.

The following program is a simple demonstration of cut, copy, and paste with String data in a TextArea. One thing you’ll notice is that the keyboard sequences you normally use for cutting, copying, and pasting also work. But if you look at any TextField or TextArea in any other program you’ll find that they also automatically support the clipboard key sequences. This example simply adds programmatic control of the clipboard, and you could use these techniques if you want to capture clipboard text into some non-TextComponent.

//: c13:CutAndPaste.java
// Using the clipboard.
// <applet code=CutAndPaste
//  width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.datatransfer.*;
import com.bruceeckel.swing.*;

public class CutAndPaste extends JApplet  {
  JMenuBar mb = new JMenuBar();
  JMenu edit = new JMenu("Edit");
  JMenuItem
    cut = new JMenuItem("Cut"),
    copy = new JMenuItem("Copy"),
    paste = new JMenuItem("Paste");
  JTextArea text = new JTextArea(20, 20);
  Clipboard clipbd =
    getToolkit().getSystemClipboard();
  public void init()  {
    cut.addActionListener(new CutL());
    copy.addActionListener(new CopyL());
    paste.addActionListener(new PasteL());
    edit.add(cut);
    edit.add(copy);
    edit.add(paste);
    mb.add(edit);
    setJMenuBar(mb);
    getContentPane().add(text);
  }
  class CopyL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String selection = text.getSelectedText();
      if (selection == null)
        return;
      StringSelection clipString =
        new StringSelection(selection);
      clipbd.setContents(clipString,clipString);
    }
  }
  class CutL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String selection = text.getSelectedText();
      if (selection == null)
        return;
      StringSelection clipString =
        new StringSelection(selection);
      clipbd.setContents(clipString, clipString);
      text.replaceRange("",
        text.getSelectionStart(),
        text.getSelectionEnd());
    }
  }
  class PasteL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      Transferable clipData =
        clipbd.getContents(CutAndPaste.this);
      try {
        String clipString =
          (String)clipData.
            getTransferData(
              DataFlavor.stringFlavor);
        text.replaceRange(clipString,
          text.getSelectionStart(),
          text.getSelectionEnd());
      } catch(Exception evt) {
        System.out.println("not String flavor");
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new CutAndPaste(), 300, 200);
  }
} ///:~

The creation and addition of the menu and TextArea should by now seem a pedestrian activity. What’s different is the creation of the Clipboard field clipbd, which is done through the Toolkit.

All the action takes place in the listeners. The CopyL and CutL listeners are the same except for the last line of CutL, which erases the line that’s been copied. The special two lines are the creation of a StringSelection object from the String and the call to setContents( ) with this StringSelection. That’s all there is to putting a String on the clipboard.

In PasteL, data is pulled off the clipboard using getContents( ). What comes back is a fairly anonymous Transferable object, and you don’t really know what it contains. One way to find out is to call getTransferDataFlavors( ), which returns an array of DataFlavor objects indicating which flavors are supported by this particular object. You can also ask it directly with isDataFlavorSupported( ), passing in the flavor you’re interested in. Here, however, the bold approach is taken: getTransferData( ) is called assuming that the contents supports the String flavor, and if it doesn’t the problem is sorted out in the exception handler.

In the future you can expect more data flavors to be supported.

Visual programming
and Beans

So far in this book you’ve seen how valuable Java is for creating reusable pieces of code. The “most reusable” unit of code has been the class, since it comprises a cohesive unit of characteristics (fields) and behaviors (methods) that can be reused either directly via composition or through inheritance.

Inheritance and polymorphism are essential parts of object-oriented programming, but in the majority of cases when you’re putting together an application, what you really want is components that do exactly what you need. You’d like to drop these parts into your design like the electronic engineer puts together chips on a circuit board (or even, in the case of Java, onto a Web page). It seems, too, that there should be some way to accelerate this “modular assembly” style of programming.

“Visual programming” first became successful—very successful—with Microsoft’s Visual Basic (VB), followed by a second-generation design in Borland’s Delphi (the primary inspiration for the Java Beans design). With these programming tools the components are represented visually, which makes sense since they usually display some kind of visual component such as a button or a text field. The visual representation, in fact, is often exactly the way the component will look in the running program. So part of the process of visual programming involves dragging a component from a palette and dropping it onto your form. The application builder tool writes code as you do this, and that code will cause the component to be created in the running program.

Simply dropping the component onto a form is usually not enough to complete the program. Often, you must change the characteristics of a component, such as what color it is, what text is on it, what database it’s connected to, etc. Characteristics that can be modified at design time are referred to as properties. You can manipulate the properties of your component inside the application builder tool, and when you create the program this configuration data is saved so that it can be rejuvenated when the program is started.

By now you’re probably used to the idea that an object is more than characteristics; it’s also a set of behaviors. At design-time, the behaviors of a visual component are partially represented by events, meaning “Here’s something that can happen to the component.” Ordinarily, you decide what you want to happen when an event occurs by tying code to that event.

Here’s the critical part: the application builder tool is able to dynamically interrogate (using reflection) the component to find out which properties and events the component supports. Once it knows what they are, it can display the properties and allow you to change those (saving the state when you build the program), and also display the events. In general, you do something like double clicking on an event and the application builder tool creates a code body and ties it to that particular event. All you have to do at that point is write the code that executes when the event occurs.

All this adds up to a lot of work that’s done for you by the application builder tool. As a result you can focus on what the program looks like and what it is supposed to do, and rely on the application builder tool to manage the connection details for you. The reason that visual programming tools have been so successful is that they dramatically speed up the process of building an application—certainly the user interface, but often other portions of the application as well.

What is a Bean?

After the dust settles, then, a component is really just a block of code, typically embodied in a class. The key issue is the ability for the application builder tool to discover the properties and events for that component. To create a VB component, the programmer had to write a fairly complicated piece of code following certain conventions to expose the properties and events. Delphi was a second-generation visual programming tool and the language was actively designed around visual programming so it is much easier to create a visual component. However, Java has brought the creation of visual components to its most advanced state with Java Beans, because a Bean is just a class. You don’t have to write any extra code or use special language extensions in order to make something a Bean. The only thing you need to do, in fact, is slightly modify the way that you name your methods. It is the method name that tells the application builder tool whether this is a property, an event, or just an ordinary method.

In the Java documentation, this naming convention is mistakenly termed a “design pattern.” This is unfortunate since design patterns (see Thinking in Patterns with Java, downloadable at www.BruceEckel.com) are challenging enough without this sort of confusion. It’s not a design pattern, it’s just a naming convention and it’s fairly simple:

  1. For a property named xxx, you typically create two methods: getXxx( ) and setXxx( ). Note that the first letter after get or set is automatically lowercased to produce the property name. The type produced by the “get” method is the same as the type of the argument to the “set” method. The name of the property and the type for the “get” and “set” are not related.
  2. For a boolean property, you can use the “get” and “set” approach above, but you can also use “is” instead of “get.”
  3. Ordinary methods of the Bean don’t conform to the above naming convention, but they’re public.
  4. For events, you use the “listener” approach. It’s exactly the same as you’ve been seeing: addFooBarListener(FooBarListener) and removeFooBarListener(FooBarListener) to handle a FooBarEvent. Most of the time the built-in events and listeners will satisfy your needs, but you can also create your own events and listener interfaces.

Point 1 above answers a question about something you might have noticed in the change from Java 1.0 to Java 1.1: a number of method names have had small, apparently meaningless name changes. Now you can see that most of those changes had to do with adapting to the “get” and “set” naming conventions in order to make that particular component into a Bean.

We can use these guidelines to create a simple Bean:

//: frogbean:Frog.java
// A trivial Java Bean.
package frogbean;
import java.awt.*;
import java.awt.event.*;

class Spots {}

public class Frog {
  private int jumps;
  private Color color;
  private Spots spots;
  private boolean jmpr;
  public int getJumps() { return jumps; }
  public void setJumps(int newJumps) {
    jumps = newJumps;
  }
  public Color getColor() { return color; }
  public void setColor(Color newColor) {
    color = newColor;
  }
  public Spots getSpots() { return spots; }
  public void setSpots(Spots newSpots) {
    spots = newSpots;
  }
  public boolean isJumper() { return jmpr; }
  public void setJumper(boolean j) { jmpr = j; }
  public void addActionListener(
      ActionListener l) {
    //...
  }
  public void removeActionListener(
      ActionListener l) {
    // ...
  }
  public void addKeyListener(KeyListener l) {
    // ...
  }
  public void removeKeyListener(KeyListener l) {
    // ...
  }
  // An "ordinary" public method:
  public void croak() {
    System.out.println("Ribbet!");
  }
} ///:~

First, you can see that it’s just a class. Usually, all your fields will be private, and accessible only through methods. Following the naming convention, the properties are jumps, color, spots, and jumper (notice the change in case of the first letter in the property name). Although the name of the internal identifier is the same as the name of the property in the first three cases, in jumper you can see that the property name does not force you to use any particular name for internal variables (or, indeed, to even have any internal variable for that property).

The events this Bean handles are ActionEvent and KeyEvent, based on the naming of the “add” and “remove” methods for the associated listener. Finally, you can see that the ordinary method croak( ) is still part of the Bean simply because it’s a public method, not because it conforms to any naming scheme.

Extracting BeanInfo
with the Introspector

One of the most critical parts of the Bean scheme occurs when you drag a Bean off a palette and plop it down on a form. The application builder tool must be able to create the Bean (which it can do if there’s a default constructor) and then, without access to the Bean’s source code, extract all the necessary information to create the property sheet and event handlers.

Part of the solution is already evident from the end of Chapter 12: Java 1.1 reflection allows all the methods of an anonymous class to be discovered. This is perfect for solving the Bean problem without requiring you to use any extra language keywords like those required in other visual programming languages. In fact, one of the prime reasons that reflection was added to Java 1.1 was to support Beans (although reflection also supports object serialization and remote method invocation). So you might expect that the creator of the application builder tool would have to reflect each Bean and hunt through its methods to find the properties and events for that Bean.

This is certainly possible, but the Java designers wanted to provide a standard interface for everyone to use, not only to make Beans simpler to use but also to provide a standard gateway to the creation of more complex Beans. This interface is the Introspector class, and the most important method in this class is the static getBeanInfo( ). You pass a Class reference to this method and it fully interrogates that class and returns a BeanInfo object that you can then dissect to find properties, methods, and events.

Usually you won’t care about any of this—you’ll probably get most of your Beans off the shelf from vendors, and you don’t need to know all the magic that’s going on underneath. You’ll simply drag your Beans onto your form, then configure their properties and write handlers for the events you’re interested in. However, it’s an interesting and educational exercise to use the Introspector to display information about a Bean, so here’s a tool that does it (you’ll find it in the frogbean subdirectory):

//: c13:BeanDumper.java
// A method to introspect a Bean.
import java.beans.*;
import java.lang.reflect.*;

public class BeanDumper {
  public static void dump(Class bean){
    BeanInfo bi = null;
    try {
      bi = Introspector.getBeanInfo(
        bean, java.lang.Object.class);
    } catch(IntrospectionException ex) {
      System.out.println("Couldn't introspect " +
        bean.getName());
      System.exit(1);
    }
    PropertyDescriptor[] properties =
      bi.getPropertyDescriptors();
    for(int i = 0; i < properties.length; i++) {
      Class p = properties[i].getPropertyType();
      System.out.println(
        "Property type:\n  " + p.getName());
      System.out.println(
        "Property name:\n  " +
        properties[i].getName());
      Method readMethod =
        properties[i].getReadMethod();
      if(readMethod != null)
        System.out.println(
          "Read method:\n  " + readMethod);
      Method writeMethod =
        properties[i].getWriteMethod();
      if(writeMethod != null)
        System.out.println(
          "Write method:\n  " + writeMethod);
      System.out.println("====================");
    }
    System.out.println("Public methods:");
    MethodDescriptor[] methods =
      bi.getMethodDescriptors();
    for(int i = 0; i < methods.length; i++)
      System.out.println(methods[i].getMethod());
    System.out.println("======================");
    System.out.println("Event support:");
    EventSetDescriptor[] events =
      bi.getEventSetDescriptors();
    for(int i = 0; i < events.length; i++) {
      System.out.println("Listener type:\n  " +
        events[i].getListenerType().getName());
      Method[] lm =
        events[i].getListenerMethods();
      for(int j = 0; j < lm.length; j++)
        System.out.println(
          "Listener method:\n  " +
          lm[j].getName());
      MethodDescriptor[] lmd =
        events[i].getListenerMethodDescriptors();
      for(int j = 0; j < lmd.length; j++)
        System.out.println(
          "Method descriptor:\n  " +
          lmd[j].getMethod());
      Method addListener =
        events[i].getAddListenerMethod();
      System.out.println(
          "Add Listener Method:\n  " +
          addListener);
      Method removeListener =
        events[i].getRemoveListenerMethod();
      System.out.println(
        "Remove Listener Method:\n  " +
        removeListener);
      System.out.println("====================");
    }
  }
  // Dump the class of your choice:
  public static void main(String[] args) {
    if(args.length < 1) {
      System.err.println("usage: \n" +
        "BeanDumper fully.qualified.class");
      System.exit(0);
    }
    Class c = null;
    try {
      c = Class.forName(args[0]);
    } catch(ClassNotFoundException ex) {
      System.err.println(
        "Couldn't find " + args[0]);
      System.exit(0);
    }
    dump(c);
  }
} ///:~

BeanDumper.dump( ) is the method that does all the work. First it tries to create a BeanInfo object, and if successful calls the methods of BeanInfo that produce information about properties, methods, and events. In Introspector.getBeanInfo( ), you’ll see there is a second argument. This tells the Introspector where to stop in the inheritance hierarchy. Here, it stops before it parses all the methods from Object, since we’re not interested in seeing those.

For properties, getPropertyDescriptors( ) returns an array of PropertyDescriptors. For each PropertyDescriptor you can call getPropertyType( ) to find the class of object that is passed in and out via the property methods. Then, for each property you can get its pseudonym (extracted from the method names) with getName( ), the method for reading with getReadMethod( ), and the method for writing with getWriteMethod( ). These last two methods return a Method object that can actually be used to invoke the corresponding method on the object (this is part of reflection).

For the public methods (including the property methods), getMethodDescriptors( ) returns an array of MethodDescriptors. For each one you can get the associated Method object and print its name.

For the events, getEventSetDescriptors( ) returns an array of (what else?) EventSetDescriptors. Each of these can be queried to find out the class of the listener, the methods of that listener class, and the add- and remove-listener methods. The BeanDumper program prints out all of this information.

If you invoke BeanDumper on the Frog class like this:

java BeanDumper frogbean.Frog

the output, after removing extra details that are unnecessary here, is:

class name: Frog
Property type:
  Color
Property name:
  color
Read method:
  public Color getColor()
Write method:
  public void setColor(Color)
====================
Property type:
  Spots
Property name:
  spots
Read method:
  public Spots getSpots()
Write method:
  public void setSpots(Spots)
====================
Property type:
  boolean
Property name:
  jumper
Read method:
  public boolean isJumper()
Write method:
  public void setJumper(boolean)
====================
Property type:
  int
Property name:
  jumps
Read method:
  public int getJumps()
Write method:
  public void setJumps(int)
====================
Public methods:
public void setJumps(int)
public void croak()
public void removeActionListener(ActionListener)
public void addActionListener(ActionListener)
public int getJumps()
public void setColor(Color)
public void setSpots(Spots)
public void setJumper(boolean)
public boolean isJumper()
public void addKeyListener(KeyListener)
public Color getColor()
public void removeKeyListener(KeyListener)
public Spots getSpots()
======================
Event support:
Listener type:
  KeyListener
Listener method:
  keyTyped
Listener method:
  keyPressed
Listener method:
  keyReleased
Method descriptor:
  public void keyTyped(KeyEvent)
Method descriptor:
  public void keyPressed(KeyEvent)
Method descriptor:
  public void keyReleased(KeyEvent)
Add Listener Method:
  public void addKeyListener(KeyListener)
Remove Listener Method:
  public void removeKeyListener(KeyListener)
====================
Listener type:
  ActionListener
Listener method:
  actionPerformed
Method descriptor:
  public void actionPerformed(ActionEvent)
Add Listener Method:
  public void addActionListener(ActionListener)
Remove Listener Method:
  public void removeActionListener(ActionListener)
====================

This reveals most of what the Introspector sees as it produces a BeanInfo object from your Bean. You can see that the type of the property and its name are independent. Notice the lowercasing of the property name. (The only time this doesn’t occur is when the property name begins with more than one capital letter in a row.) And remember that the method names you’re seeing here (such as the read and write methods) are actually produced from a Method object that can be used to invoke the associated method on the object.

The public method list includes the methods that are not associated with a property or event, such as croak( ), as well as those that are. These are all the methods that you can call programmatically for a Bean, and the application builder tool can choose to list all of these while you’re making method calls, to ease your task.

Finally, you can see that the events are fully parsed out into the listener, its methods, and the add- and remove-listener methods. Basically, once you have the BeanInfo, you can find out everything of importance for the Bean. You can also call the methods for that Bean, even though you don’t have any other information except the object (again, a feature of reflection).

A more sophisticated Bean

This next example is slightly more sophisticated, albeit frivolous. It’s a canvas that draws a little circle around the mouse whenever the mouse is moved. When you press the mouse, the word “Bang!” appears in the middle of the screen, and an action listener is fired.

The properties you can change are the size of the circle as well as the color, size, and text of the word that is displayed when you press the mouse. A BangBean also has its own addActionListener( ) and removeActionListener( ) so you can attach your own listener that will be fired when the user clicks on the BangBean. You should be able to recognize the property and event support:

//: bangbean:BangBean.java
// A graphical Bean.
package bangbean;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
import com.bruceeckel.swing.*;

public class BangBean extends JPanel
     implements Serializable {
  protected int xm, ym;
  protected int cSize = 20; // Circle size
  protected String text = "Bang!";
  protected int fontSize = 48;
  protected Color tColor = Color.red;
  protected ActionListener actionListener;
  public BangBean() {
    addMouseListener(new ML());
    addMouseMotionListener(new MML());
  }
  public int getCircleSize() { return cSize; }
  public void setCircleSize(int newSize) {
    cSize = newSize;
  }
  public String getBangText() { return text; }
  public void setBangText(String newText) {
    text = newText;
  }
  public int getFontSize() { return fontSize; }
  public void setFontSize(int newSize) {
    fontSize = newSize;
  }
  public Color getTextColor() { return tColor; }
  public void setTextColor(Color newColor) {
    tColor = newColor;
  }
  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.black);
    g.drawOval(xm - cSize/2, ym - cSize/2,
      cSize, cSize);
  }
  // This is a unicast listener, which is
  // the simplest form of listener management:
  public void addActionListener (
      ActionListener l)
        throws TooManyListenersException {
    if(actionListener != null)
      throw new TooManyListenersException();
    actionListener = l;
  }
  public void removeActionListener(
      ActionListener l) {
    actionListener = null;
  }
  class ML extends MouseAdapter {
    public void mousePressed(MouseEvent e) {
      Graphics g = getGraphics();
      g.setColor(tColor);
      g.setFont(
        new Font(
          "TimesRoman", Font.BOLD, fontSize));
      int width =
        g.getFontMetrics().stringWidth(text);
      g.drawString(text,
        (getSize().width - width) /2,
        getSize().height/2);
      g.dispose();
      // Call the listener's method:
      if(actionListener != null)
        actionListener.actionPerformed(
          new ActionEvent(BangBean.this,
            ActionEvent.ACTION_PERFORMED, null));
    }
  }
  class MML extends MouseMotionAdapter {
    public void mouseMoved(MouseEvent e) {
      xm = e.getX();
      ym = e.getY();
      repaint();
    }
  }
  public Dimension getPreferredSize() {
    return new Dimension(200, 200);
  }
  // During testing, send action
  // information to the console:
  static class BBL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("BangBean action");
    }
  }
  // Testing the BangBean:
  public static void main(String[] args) {
    BangBean bb = new BangBean();
    try {
      bb.addActionListener(new BBL());
    } catch(TooManyListenersException e) {}
    Console.run(bb, 300, 300);
  }
} ///:~

The first thing you’ll notice is that BangBean implements the Serializable interface. This means that the application builder tool can “pickle” all the information for the BangBean using serialization after the program designer has adjusted the values of the properties. When the Bean is created as part of the running application, these “pickled” properties are restored so that you get exactly what you designed.

You can see that all the fields are private, which is what you’ll usually do with a Bean—allow access only through methods, usually using the “property” scheme.

When you look at the signature for addActionListener( ), you’ll see that it can throw a TooManyListenersException. This indicates that it is unicast, which means it notifies only one listener when the event occurs. Ordinarily, you’ll use multicast events so that many listeners can be notified of an event. However, that runs into issues that you won’t be ready for until the next chapter, so it will be revisited there (under the heading “Java Beans revisited”). A unicast event sidesteps the problem.

When you press the mouse, the text is put in the middle of the BangBean, and if the actionListener field is not null, its actionPerformed( ) is called, creating a new ActionEvent object in the process. Whenever the mouse is moved, its new coordinates are captured and the canvas is repainted (erasing any text that’s on the canvas, as you’ll see).

The main( ) is added to allow you to test the program from the command line. When a Bean is in a development environment, main( ) will not be used, but it’s helpful to have a main( ) in each of your Beans because it provides for rapid testing. main( ) creates a Frame and places a BangBean within it, attaching a simple ActionListener to the BangBean to print to the console whenever an ActionEvent occurs. Usually, of course, the application builder tool would create most of the code that uses the Bean.

When you run the BangBean through BeanDumper or put the BangBean inside a Bean-enabled development environment, you’ll notice that there are many more properties and actions than are evident from the above code. That’s because BangBean is inherited from Canvas, and Canvas is a Bean, so you’re seeing its properties and events as well.

Packaging a Bean

Before you can bring a Bean into a Bean-enabled visual builder tool, it must be put into the standard Bean container, which is a JAR (Java ARchive) file that includes all the Bean classes as well as a “manifest” file that says “This is a Bean.” A manifest file is simply a text file that follows a particular form. For the BangBean, the manifest file looks like this:

Manifest-Version: 1.0

Name: bangbean/BangBean.class
Java-Bean: True

The first line indicates the version of the manifest scheme, which until further notice from Sun is 1.0. The second line (empty lines are ignored) names the BangBean.class file, and the third says, “It’s a Bean.” Without the third line, the program builder tool will not recognize the class as a Bean.

The only tricky part is that you must make sure that you get the proper path in the “Name:” field. If you look back at BangBean.java, you’ll see it’s in package bangbean (and thus in a subdirectory called “bangbean” that’s off of the classpath), and the name in the manifest file must include this package information. In addition, you must place the manifest file in the directory above the root of your package path, which in this case means placing the file in the directory above the “bangbean” subdirectory. Then you must invoke jar from the same directory as the manifest file, as follows:

jar cfm BangBean.jar BangBean.mf bangbean

This assumes that you want the resulting JAR file to be named BangBean.jar and that you’ve put the manifest in a file called BangBean.mf.

You might wonder “What about all the other classes that were generated when I compiled BangBean.java?” Well, they all ended up inside the bangbean subdirectory, and you’ll see that the last argument for the above jar command line is the bangbean subdirectory. When you give jar the name of a subdirectory, it packages that entire subdirectory into the jar file (including, in this case, the original BangBean.java source-code file—you might not choose to include the source with your own Beans). In addition, if you turn around and unpack the JAR file you’ve just created, you’ll discover that your manifest file isn’t inside, but that jar has created its own manifest file (based partly on yours) called MANIFEST.MF and placed it inside the subdirectory META-INF (for “meta-information”). If you open this manifest file you’ll also notice that digital signature information has been added by jar for each file, of the form:

Digest-Algorithms: SHA MD5
SHA-Digest: pDpEAG9NaeCx8aFtqPI4udSX/O0=
MD5-Digest: O4NcS1hE3Smnzlp2hj6qeg==

In general, you don’t need to worry about any of this, and if you make changes you can just modify your original manifest file and reinvoke jar to create a new JAR file for your Bean. You can also add other Beans to the JAR file simply by adding their information to your manifest.

One thing to notice is that you’ll probably want to put each Bean in its own subdirectory, since when you create a JAR file you hand the jar utility the name of a subdirectory and it puts everything in that subdirectory into the JAR file. You can see that both Frog and BangBean are in their own subdirectories.

Once you have your Bean properly inside a JAR file you can bring it into a Beans-enabled program-builder environment. The way you do this varies from one tool to the next, but Sun provides a freely-available test bed for Java Beans in their “Beans Development Kit” (BDK) called the “beanbox.” (Download the BDK from www.javasoft.com.) To place your Bean in the beanbox, copy the JAR file into the BDK’s “jars” subdirectory before you start up the beanbox.

More complex Bean support

You can see how remarkably simple it is to make a Bean. But you aren’t limited to what you’ve seen here. The Java Bean design provides a simple point of entry but can also scale to more complex situations. These situations are beyond the scope of this book but they will be briefly introduced here. You can find more details at http://java.sun.com/beans.

One place where you can add sophistication is with properties. The examples above have shown only single properties, but it’s also possible to represent multiple properties in an array. This is called an indexed property. You simply provide the appropriate methods (again following a naming convention for the method names) and the Introspector recognizes an indexed property so your application builder tool can respond appropriately.

Properties can be bound, which means that they will notify other objects via a PropertyChangeEvent. The other objects can then choose to change themselves based on the change to the Bean.

Properties can be constrained, which means that other objects can veto a change to that property if it is unacceptable. The other objects are notified using a PropertyChangeEvent, and they can throw a PropertyVetoException to prevent the change from happening and to restore the old values.

You can also change the way your Bean is represented at design time:

  1. You can provide a custom property sheet for your particular Bean. The ordinary property sheet will be used for all other Beans, but yours is automatically invoked when your Bean is selected.
  2. You can create a custom editor for a particular property, so the ordinary property sheet is used, but when your special property is being edited, your editor will automatically be invoked.
  3. You can provide a custom BeanInfo class for your Bean that produces information that’s different from the default created by the Introspector.
  4. It’s also possible to turn “expert” mode on and off in all FeatureDescriptors to distinguish between basic features and more complicated ones.

More to Beans

There’s another issue that couldn’t be addressed here. Whenever you create a Bean, you should expect that it will be run in a multithreaded environment. This means that you must understand the issues of threading, which will be introduced in the next chapter. You’ll find a section there called “Java Beans revisited” that will look at the problem and its solution.

Introduction to Swing[66]

After working your way through this chapter and seeing the huge changes that have occurred within Swing (although, if you can remember back that far, Sun claimed Java was a “stable” language when it first appeared), you might still have the feeling that it’s not quite done. Sure, there’s now a good event model, and JavaBeans is an excellent component-reuse design. But the GUI components still seem rather minimal, primitive, and awkward.

That’s where Swing comes in. The Swing library appeared after Java 1.1 so you might naturally assume that it’s part of Java 2. However, it is designed to work with Java 1.1 as an add-on. This way, you don’t have to wait for your platform to support Java 2 in order to enjoy a good UI component library. Your users might actually need to download the Swing library if it isn’t part of their Java 1.1 support, and this could cause a few snags. But it works.

Swing contains all the components that you’ve been missing throughout the rest of this chapter: those you expect to see in a modern UI, everything from buttons that contain pictures to trees and grids. It’s a big library, but it’s designed to have appropriate complexity for the task at hand—if something is simple, you don’t have to write much code but as you try to do more your code becomes increasingly complex. This means an easy entry point, but you’ve got the power if you need it.

Swing has great depth. This section does not attempt to be comprehensive, but instead introduces the power and simplicity of Swing to get you started using the library. Please be aware that what you see here is intended to be simple. If you need to do more, then Swing can probably give you what you want if you’re willing to do the research by hunting through the online documentation from Sun.

Benefits of Swing

When you begin to use the Swing library, you’ll see that it’s a huge step forward. Swing components are Beans (and thus use the Java 1.1 event model), so they can be used in any development environment that supports Beans. Swing provides a full set of UI components. For speed, all the components are lightweight (no “peer” components are used), and Swing is written entirely in Java for portability.

Much of what you’ll like about Swing could be called “orthogonality of use;” that is, once you pick up the general ideas about the library you can apply them everywhere. Primarily because of the Beans naming conventions, much of the time I was writing these examples I could guess at the method names and get it right the first time, without looking anything up. This is certainly the hallmark of a good library design. In addition, you can generally plug components into other components and things will work correctly.

Keyboard navigation is automatic—you can use a Swing application without the mouse, but you don’t have to do any extra programming (the old AWT required some ugly code to achieve keyboard navigation). Scrolling support is effortless—you simply wrap your component in a JScrollPane as you add it to your form. Other features such as tool tips typically require a single line of code to implement.

Swing also supports something called “pluggable look and feel,” which means that the appearance of the UI can be dynamically changed to suit the expectations of users working under different platforms and operating systems. It’s even possible to invent your own look and feel.

Easy conversion

If you’ve struggled long and hard to build your UI using Java 1.1, you don’t want to throw it away to convert to Swing. Fortunately, the library is designed to allow easy conversion—in many cases you can simply put a ‘J’ in front of the class names of each of your old AWT components. Here’s an example that should have a familiar flavor to it:

//: c13:ButtonDemo.java
// Looks like Java 1.1 but with J's added.
// <applet code=ButtonDemo width=250 height=75>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import com.bruceeckel.swing.*;

public class ButtonDemo extends JApplet {
  JButton
    b1 = new JButton("JButton 1"),
    b2 = new JButton("JButton 2");
  JTextField t = new JTextField(20);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    ActionListener al = new ActionListener() {
      public void actionPerformed(ActionEvent e){
        String name =
          ((JButton)e.getSource()).getText();
        t.setText(name + " Pressed");
      }
    };
    b1.addActionListener(al);
    cp.add(b1);
    b2.addActionListener(al);
    cp.add(b2);
    cp.add(t);
  }
  public static void main(String[] args) {
    Console.run(new ButtonDemo(), 250, 75);
  }
} ///:~

There’s a new import statement, but everything else looks like the Java 1.1 AWT with the addition of some J’s. Also, you don’t just add( ) something to a Swing JFrame, but you must get the “content pane” first, as seen above. But you can easily get many of the benefits of Swing with a simple conversion.

Because of the package statement, you’ll have to invoke this program by saying:

java c13.swing.JbuttonDemo

All of the programs in this section will require a similar form to run them.

Tool tips

Almost all of the classes that you’ll be using to create your user interfaces are derived from JComponent, which contains a method called setToolTipText(String). So, for virtually anything you place on your form, all you need to do is say (for an object jc of any JComponent-derived class):

jc.setToolTipText("My tip");

and when the mouse stays over that JComponent for a predetermined period of time, a tiny box containing your text will pop up next to the mouse.

Borders

JComponent also contains a method called setBorder( ), which allows you to place various interesting borders on any visible component. The following example demonstrates a number of the different borders that are available, using a method called showBorder( ) that creates a JPanel and puts on the border in each case. Also, it uses RTTI to find the name of the border that you’re using (stripping off all the path information), then puts that name in a JLabel in the middle of the panel:

//: c13:Borders.java
// Different Swing borders.
// <applet code=Borders
//  width=500 height=300></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;

public class Borders extends JApplet {
  static JPanel showBorder(Border b) {
    JPanel jp = new JPanel();
    jp.setLayout(new BorderLayout());
    String nm = b.getClass().toString();
    nm = nm.substring(nm.lastIndexOf('.') + 1);
    jp.add(new JLabel(nm, JLabel.CENTER),
      BorderLayout.CENTER);
    jp.setBorder(b);
    return jp;
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.setLayout(new GridLayout(2,4));
    cp.add(showBorder(new TitledBorder("Title")));
    cp.add(showBorder(new EtchedBorder()));
    cp.add(showBorder(new LineBorder(Color.blue)));
    cp.add(showBorder(
      new MatteBorder(5,5,30,30,Color.green)));
    cp.add(showBorder(
      new BevelBorder(BevelBorder.RAISED)));
    cp.add(showBorder(
      new SoftBevelBorder(BevelBorder.LOWERED)));
    cp.add(showBorder(new CompoundBorder(
      new EtchedBorder(),
      new LineBorder(Color.red))));
  }
  public static void main(String[] args) {
    Console.run(new Borders(), 500, 300);
  }
} ///:~

Most of the examples in this section use TitledBorder, but you can see that the rest of the borders are as easy to use. You can also create your own borders and put them inside buttons, labels, etc.—anything derived from JComponent.

Buttons

Swing adds a number of different types of buttons, and it also changes the organization of the selection components: all buttons, checkboxes, radio buttons, and even menu items are inherited from AbstractButton (which, since menu items are included, would probably have been better named “AbstractChooser” or something equally general). You’ll see the use of menu items shortly, but the following example shows the various types of buttons available:

//: c13:Buttons.java
// Various Swing buttons.
// <applet code=Buttons
//  width=350 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.plaf.basic.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;

public class Buttons extends JApplet {
  JButton jb = new JButton("JButton");
  BasicArrowButton
    up = new BasicArrowButton(
      BasicArrowButton.NORTH),
    down = new BasicArrowButton(
      BasicArrowButton.SOUTH),
    right = new BasicArrowButton(
      BasicArrowButton.EAST),
    left = new BasicArrowButton(
      BasicArrowButton.WEST);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(jb);
    cp.add(new JToggleButton("JToggleButton"));
    cp.add(new JCheckBox("JCheckBox"));
    cp.add(new JRadioButton("JRadioButton"));
    JPanel jp = new JPanel();
    jp.setBorder(new TitledBorder("Directions"));
    jp.add(up);
    jp.add(down);
    jp.add(left);
    jp.add(right);
    cp.add(jp);
  }
  public static void main(String[] args) {
    Console.run(new Buttons(), 350, 100);
  }
} ///:~

The JButton looks like the AWT button, but there’s more you can do to it (like add images, as you’ll see later). In javax.swing.plaf.basic, there is also a BasicArrowButton that is convenient.

When you run the example, you’ll see that the toggle button holds its last position, in or out. But the check boxes and radio buttons behave identically to each other, just clicking on or off (they are inherited from JToggleButton).

Button groups

If you want radio buttons to behave in an “exclusive or” fashion, you must add them to a button group, in a similar but less awkward way as the old AWT. But as the example below demonstrates, any AbstractButton can be added to a ButtonGroup.

To avoid repeating a lot of code, this example uses reflection to generate the groups of different types of buttons. This is seen in makeBPanel, which creates a button group and a JPanel, and for each String in the array that’s the second argument to makeBPanel( ), it adds an object of the class represented by the first argument:

//: c13:ButtonGroups.java
// Uses reflection to create groups
// of different types of AbstractButton.
// <applet code=ButtonGroups
//  width=500 height=300></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import java.lang.reflect.*;
import com.bruceeckel.swing.*;

public class ButtonGroups extends JApplet {
  static String[] ids = {
    "June", "Ward", "Beaver",
    "Wally", "Eddie", "Lumpy",
  };
  static JPanel
  makeBPanel(Class bClass, String[] ids) {
    ButtonGroup bg = new ButtonGroup();
    JPanel jp = new JPanel();
    String title = bClass.getName();
    title = title.substring(
      title.lastIndexOf('.') + 1);
    jp.setBorder(new TitledBorder(title));
    for(int i = 0; i < ids.length; i++) {
      AbstractButton ab = new JButton("failed");
      try {
        // Get the dynamic constructor method
        // that takes a String argument:
        Constructor ctor = bClass.getConstructor(
          new Class[] { String.class });
        // Create a new object:
        ab = (AbstractButton)ctor.newInstance(
          new Object[]{ids[i]});
      } catch(Exception ex) {
        System.out.println("can't create " +
          bClass);
      }
      bg.add(ab);
      jp.add(ab);
    }
    return jp;
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(makeBPanel(JButton.class, ids));
    cp.add(makeBPanel(JToggleButton.class, ids));
    cp.add(makeBPanel(JCheckBox.class, ids));
    cp.add(makeBPanel(JRadioButton.class, ids));
  }
  public static void main(String[] args) {
    Console.run(new ButtonGroups(), 500, 300);
  }
} ///:~

The title for the border is taken from the name of the class, stripping off all the path information. The AbstractButton is initialized to a JButton that has the label “Failed” so if you ignore the exception message, you’ll still see the problem on screen. The getConstructor( ) method produces a Constructor object that takes the array of arguments of the types in the Class array passed to getConstructor( ). Then all you do is call newInstance( ), passing it an array of Object containing your actual arguments—in this case, just the String from the ids array.

This adds a little complexity to what is a simple process. To get “exclusive or” behavior with buttons, you create a button group and add each button for which you want that behavior to the group. When you run the program, you’ll see that all the buttons except JButton exhibit this “exclusive or” behavior.

Icons

You can use an Icon inside a JLabel or anything that inherits from AbstractButton (including JButton, JCheckbox, JradioButton, and the different kinds of JMenuItem). Using Icons with JLabels is quite straightforward (you’ll see an example later). The following example explores all the additional ways you can use Icons with buttons and their descendants.

You can use any gif files you want, but the ones used in this example are part of this book’s code distribution, available at www.BruceEckel.com. To open a file and bring in the image, simply create an ImageIcon and hand it the file name. From then on, you can use the resulting Icon in your program.

//: c13:Faces.java
// Icon behavior in Jbuttons.
// <applet code=Faces
//  width=400 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Faces extends JApplet {
  // The following path information is necessary
  // to run via an applet directly from the disk:
  static String path =
    "C:/aaa-TIJ2-distribution/code/c13/";
  static Icon[] faces = {
    new ImageIcon(path + "face0.gif"),
    new ImageIcon(path + "face1.gif"),
    new ImageIcon(path + "face2.gif"),
    new ImageIcon(path + "face3.gif"),
    new ImageIcon(path + "face4.gif"),
  };
  JButton
    jb = new JButton("JButton", faces[3]),
    jb2 = new JButton("Disable");
  boolean mad = false;
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    jb.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        if(mad) {
          jb.setIcon(faces[3]);
          mad = false;
        } else {
          jb.setIcon(faces[0]);
          mad = true;
        }
        jb.setVerticalAlignment(JButton.TOP);
        jb.setHorizontalAlignment(JButton.LEFT);
      }
    });
    jb.setRolloverEnabled(true);
    jb.setRolloverIcon(faces[1]);
    jb.setPressedIcon(faces[2]);
    jb.setDisabledIcon(faces[4]);
    jb.setToolTipText("Yow!");
    cp.add(jb);
    jb2.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        if(jb.isEnabled()) {
          jb.setEnabled(false);
          jb2.setText("Enable");
        } else {
          jb.setEnabled(true);
          jb2.setText("Disable");
        }
      }
    });
    cp.add(jb2);
  }
  public static void main(String[] args) {
    Console.run(new Faces(), 400, 200);
  }
} ///:~

An Icon can be used in many constructors, but you can also use setIcon( ) to add or change an Icon. This example also shows how a JButton (or any AbstractButton) can set the various different sorts of icons that appear when things happen to that button: when it’s pressed, disabled, or “rolled over” (the mouse moves over it without clicking). You’ll see that this gives the button a rather animated feel.

Note that a tool tip is also added to the button.

Menus

Menus are much improved and more flexible in Swing—for example, you can use them just about anywhere, including panels and applets. The syntax for using them is much the same as it was in the old AWT, and this preserves the same problem present in the old AWT: you must hard-code your menus and there isn’t any support for menus as resources (which, among other things, would make them easier to change for other languages). In addition, menu code gets long-winded and sometimes messy. The following approach takes a step in the direction of solving this problem by putting all the information about each menu into a two-dimensional array of Object (that way you can put anything you want into the array). This array is organized so that the first row represents the menu name, and the remaining rows represent the menu items and their characteristics. You’ll notice the rows of the array do not have to be uniform from one to the next—as long as your code knows where everything should be, each row can be completely different.

//: c13:Menus.java
// A menu-building system; also demonstrates
// icons in labels and menu items.
// <applet code=Menus
//  width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Menus extends JApplet {
  static final Boolean
    bT = new Boolean(true),
    bF = new Boolean(false);
  // Dummy class to create type identifiers:
  static class MType { MType(int i) {} };
  static final MType
    mi = new MType(1), // Normal menu item
    cb = new MType(2), // Checkbox menu item
    rb = new MType(3); // Radio button menu item
  JTextField t = new JTextField(10);
  JLabel l = new JLabel("Icon Selected",
    Faces.faces[0], JLabel.CENTER);
  ActionListener a1 = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      t.setText(
        ((JMenuItem)e.getSource()).getText());
    }
  };
  ActionListener a2 = new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      JMenuItem mi = (JMenuItem)e.getSource();
      l.setText(mi.getText());
      l.setIcon(mi.getIcon());
    }
  };
  // Store menu data as "resources":
  public Object[][] fileMenu = {
    // Menu name and accelerator:
    { "File", new Character('F') },
    // Name type accel listener enabled
    { "New", mi, new Character('N'), a1, bT },
    { "Open", mi, new Character('O'), a1, bT },
    { "Save", mi, new Character('S'), a1, bF },
    { "Save As", mi, new Character('A'), a1, bF},
    { null }, // Separator
    { "Exit", mi, new Character('x'), a1, bT },
  };
  public Object[][] editMenu = {
    // Menu name:
    { "Edit", new Character('E') },
    // Name type accel listener enabled
    { "Cut", mi, new Character('t'), a1, bT },
    { "Copy", mi, new Character('C'), a1, bT },
    { "Paste", mi, new Character('P'), a1, bT },
    { null }, // Separator
    { "Select All", mi,new Character('l'),a1,bT},
  };
  public Object[][] helpMenu = {
    // Menu name:
    { "Help", new Character('H') },
    // Name type accel listener enabled
    { "Index", mi, new Character('I'), a1, bT },
    { "Using help", mi,new Character('U'),a1,bT},
    { null }, // Separator
    { "About", mi, new Character('t'), a1, bT },
  };
  public Object[][] optionMenu = {
    // Menu name:
    { "Options", new Character('O') },
    // Name type accel listener enabled
    { "Option 1", cb, new Character('1'), a1,bT},
    { "Option 2", cb, new Character('2'), a1,bT},
  };
  public Object[][] faceMenu = {
    // Menu name:
    { "Faces", new Character('a') },
    // Optinal last element is icon
    { "Face 0", rb, new Character('0'), a2, bT,
      Faces.faces[0] },
    { "Face 1", rb, new Character('1'), a2, bT,
      Faces.faces[1] },
    { "Face 2", rb, new Character('2'), a2, bT,
      Faces.faces[2] },
    { "Face 3", rb, new Character('3'), a2, bT,
      Faces.faces[3] },
    { "Face 4", rb, new Character('4'), a2, bT,
      Faces.faces[4] },
  };
  public Object[] menuBar = {
    fileMenu, editMenu, faceMenu,
    optionMenu, helpMenu,
  };
  static public JMenuBar
  createMenuBar(Object[] menuBarData) {
    JMenuBar menuBar = new JMenuBar();
    for(int i = 0; i < menuBarData.length; i++)
      menuBar.add(
        createMenu((Object[][])menuBarData[i]));
    return menuBar;
  }
  static ButtonGroup bgroup;
  static public JMenu
  createMenu(Object[][] menuData) {
    JMenu menu = new JMenu();
    menu.setText((String)menuData[0][0]);
    menu.setMnemonic(
      ((Character)menuData[0][1]).charValue());
    // Create redundantly, in case there are
    // any radio buttons:
    bgroup = new ButtonGroup();
    for(int i = 1; i < menuData.length; i++) {
      if(menuData[i][0] == null)
        menu.add(new JSeparator());
      else
        menu.add(createMenuItem(menuData[i]));
    }
    return menu;
  }
  static public JMenuItem
  createMenuItem(Object[] data) {
    JMenuItem m = null;
    MType type = (MType)data[1];
    if(type == mi)
      m = new JMenuItem();
    else if(type == cb)
      m = new JCheckBoxMenuItem();
    else if(type == rb) {
      m = new JRadioButtonMenuItem();
      bgroup.add(m);
    }
    m.setText((String)data[0]);
    m.setMnemonic(
      ((Character)data[2]).charValue());
    m.addActionListener(
      (ActionListener)data[3]);
    m.setEnabled(
      ((Boolean)data[4]).booleanValue());
    if(data.length == 6)
      m.setIcon((Icon)data[5]);
    return m;
  }
  public void init() {
    Container cp = getContentPane();
    cp.add(createMenuBar(menuBar),
      BorderLayout.NORTH);
    JPanel p = new JPanel();
    p.setLayout(new BorderLayout());
    p.add(t, BorderLayout.NORTH);
    p.add(l, BorderLayout.CENTER);
    cp.add(p, BorderLayout.CENTER);
  }
  public static void main(String[] args) {
    Console.run(new Menus(), 300, 200);
  }
} ///:~

The goal is to allow the programmer to simply create tables to represent each menu, rather than typing lines of code to build the menus. Each table produces one menu, and the first row in the table contains the menu name and its keyboard accelerator. The remaining rows contain the data for each menu item: the string to be placed on the menu item, what type of menu item it is, its keyboard accelerator, the actionlistener that is fired when this menu item is selected, and whether this menu item is enabled. If a row starts with null it is treated as a separator.

To prevent wasteful and tedious multiple creations of Boolean objects and type flags, these are created as static final values at the beginning of the class: bT and bF to represent Booleans and different objects of the dummy class MType to describe normal menu items (mi), checkbox menu items (cb), and radio button menu items (rb). Remember that an array of Object may hold only Object references and not primitive values.

This example also shows how JLabels and JMenuItems (and their descendants) may hold Icons. An Icon is placed into the JLabel via its constructor and changed when the corresponding menu item is selected.

The menuBar array contains the references to all the file menus in the order that you want them to appear on the menu bar. You pass this array to createMenuBar( ), which breaks it up into individual arrays of menu data, passing each to createMenu( ). This method, in turn, takes the first line of the menu data and creates a JMenu from it, then calls createMenuItem( ) for each of the remaining lines of menu data. Finally, createMenuItem( ) parses each line of menu data and determines the type of menu and its attributes, and creates that menu item appropriately. In the end, as you can see in the Menus( ) constructor, to create a menu from these tables say createMenuBar(menuBar) and everything is handled recursively.

This example does not take care of building cascading menus, but you should have enough of the concept that you can add that capability if you need it.

Popup menus

The most straightforward way to implement a JPopupMenu is to create an inner class that extends MouseAdapter, then add an object of that inner class to each component which should produce popup behavior:

//: c13:Popup.java
// Creating popup menus with Swing.
// <applet code=Popup
//  width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Popup extends JApplet {
  JPopupMenu popup = new JPopupMenu();
  JTextField t = new JTextField(10);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    ActionListener al = new ActionListener() {
      public void actionPerformed(ActionEvent e){
        t.setText(
          ((JMenuItem)e.getSource()).getText());
      }
    };
    JMenuItem m = new JMenuItem("Hither");
    m.addActionListener(al);
    popup.add(m);
    m = new JMenuItem("Yon");
    m.addActionListener(al);
    popup.add(m);
    m = new JMenuItem("Afar");
    m.addActionListener(al);
    popup.add(m);
    popup.addSeparator();
    m = new JMenuItem("Stay Here");
    m.addActionListener(al);
    popup.add(m);
    PopupListener pl = new PopupListener();
    addMouseListener(pl);
    t.addMouseListener(pl);
  }
  class PopupListener extends MouseAdapter {
    public void mousePressed(MouseEvent e) {
      maybeShowPopup(e);
    }
    public void mouseReleased(MouseEvent e) {
      maybeShowPopup(e);
    }
    private void maybeShowPopup(MouseEvent e) {
      if(e.isPopupTrigger()) {
        popup.show(
          e.getComponent(), e.getX(), e.getY());
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new Popup(), 300, 200);
  }
} ///:~

The same ActionListener is added to each JMenuItem, so that it fetches the text from the menu label and inserts it into the JTextField.

List boxes and combo boxes

List boxes and combo boxes in Swing work much as they do in the old AWT, but they also have increased functionality if you need it. In addition, some conveniences have been added. For example, the JList has a constructor that takes an array of Strings to display (oddly enough this same feature is not available in JComboBox). Here’s a simple example that shows the basic use of each:

//: c13:ListCombo.java
// List boxes & Combo boxes.
// <applet code=ListCombo
//  width=300 height=125></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class ListCombo extends JApplet {
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(2,1));
    JList list = new JList(ButtonGroups.ids);
    cp.add(new JScrollPane(list));
    JComboBox combo = new JComboBox();
    for(int i = 0; i < 100; i++)
      combo.addItem(Integer.toString(i));
    cp.add(combo);
  }
  public static void main(String[] args) {
    Console.run(new ListCombo(), 300, 125);
  }
} ///:~

Something else that seems a bit odd at first is that JLists do not automatically provide scrolling, even though that’s something you always expect. Adding support for scrolling turns out to be quite easy, as shown above—you simply wrap the JList in a JScrollPane and all the details are automatically managed for you.

Sliders and progress bars

A slider allows the user to input data by moving a point back and forth, which is intuitive in some situations (volume controls, for example). A progress bar displays data in a relative fashion from “full” to “empty” so the user gets a perspective. My favorite example for these is to simply hook the slider to the progress bar so when you move the slider the progress bar changes accordingly:

//: c13:Progress.java
// Using progress bars and sliders.
// <applet code=Progress
//  width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;

public class Progress extends JApplet {
  JProgressBar pb = new JProgressBar();
  JSlider sb =
    new JSlider(JSlider.HORIZONTAL, 0, 100, 60);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(2,1));
    cp.add(pb);
    sb.setValue(0);
    sb.setPaintTicks(true);
    sb.setMajorTickSpacing(20);
    sb.setMinorTickSpacing(5);
    sb.setBorder(new TitledBorder("Slide Me"));
    pb.setModel(sb.getModel()); // Share model
    cp.add(sb);
  }
  public static void main(String[] args) {
    Console.run(new Progress(), 300, 200);
  }
} ///:~

The JProgressBar is fairly straightforward, but the JSlider has a lot of options, such as the orientation and major and minor tick marks. Notice how straightforward it is to add a titled border.

Trees

Using a JTree can be as simple as saying:

add(new JTree(
  new Object[] {"this", "that", "other"}));

This displays a primitive tree. The API for trees is vast, however—certainly one of the largest in Swing. It appears that you can do just about anything with trees, but more sophisticated tasks might require quite a bit of research and experimentation.

Fortunately, there is a middle ground provided in the library: the “default” tree components, which generally do what you need. So most of the time you can use these components, and only in special cases will you need to delve in and understand trees more deeply.

The following example uses the “default” tree components to display a tree in an applet. When you press the button, a new subtree is added under the currently-selected node (if no node is selected, the root node is used):

//: c13:Trees.java
// Simple Swing tree example. Trees can
// be made vastly more complex than this.
// <applet code=Trees
//  width=250 height=250></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.tree.*;
import com.bruceeckel.swing.*;

// Takes an array of Strings and makes the first
// element a node and the rest leaves:
class Branch {
  DefaultMutableTreeNode r;
  public Branch(String[] data) {
    r = new DefaultMutableTreeNode(data[0]);
    for(int i = 1; i < data.length; i++)
      r.add(new DefaultMutableTreeNode(data[i]));
  }
  public DefaultMutableTreeNode node() {
    return r;
  }
}

public class Trees extends JApplet {
  String[][] data = {
    { "Colors", "Red", "Blue", "Green" },
    { "Flavors", "Tart", "Sweet", "Bland" },
    { "Length", "Short", "Medium", "Long" },
    { "Volume", "High", "Medium", "Low" },
    { "Temperature", "High", "Medium", "Low" },
    { "Intensity", "High", "Medium", "Low" },
  };
  static int i = 0;
  DefaultMutableTreeNode root, child, chosen;
  JTree tree;
  DefaultTreeModel model;
  public void init() {
    Container cp = getContentPane();
    root = new DefaultMutableTreeNode("root");
    tree = new JTree(root);
    // Add it and make it take care of scrolling:
    cp.add(new JScrollPane(tree),
      BorderLayout.CENTER);
    // Capture the tree's model:
    model =(DefaultTreeModel)tree.getModel();
    JButton test = new JButton("Press me");
    test.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        if(i < data.length) {
          child = new Branch(data[i++]).node();
          // What's the last one you clicked?
          chosen = (DefaultMutableTreeNode)
            tree.getLastSelectedPathComponent();
          if(chosen == null) chosen = root;
          // The model will create the
          // appropriate event. In response, the
          // tree will update itself:
          model.insertNodeInto(child, chosen, 0);
          // This puts the new node on the
          // currently chosen node.
        }
      }
    });
    // Change the button's colors:
    test.setBackground(Color.blue);
    test.setForeground(Color.white);
    JPanel p = new JPanel();
    p.add(test);
    cp.add(p, BorderLayout.SOUTH);
  }
  public static void main(String[] args) {
    Console.run(new Trees(), 250, 250);
  }
} ///:~

The first class, Branch, is a tool to take an array of String and build a DefaultMutableTreeNode with the first String as the root and the rest of the Strings in the array as leaves. Then node( ) can be called to produce the root of this “branch.”

The Trees class contains a two-dimensional array of Strings from which Branches can be made and a static int i to count through this array. The DefaultMutableTreeNode objects hold the nodes, but the physical representation on screen is controlled by the JTree and its associated model, the DefaultTreeModel. Note that when the JTree is added to the applet, it is wrapped in a JScrollPane—this is all it takes to provide automatic scrolling.

The JTree is controlled through its model. When you make a change to the model, the model generates an event that causes the JTree to perform any necessary updates to the visible representation of the tree. In init( ), the model is captured by calling getModel( ). When the button is pressed, a new “branch” is created. Then the currently selected component is found (or the root if nothing is selected) and the model’s insertNodeInto( ) method does all the work of changing the tree and causing it to be updated.

Most of the time an example like the one above will give you what you need in a tree. However, trees have the power to do just about anything you can imagine—everywhere you see the word “default” in the example above, you can substitute your own class to get different behavior. But beware: almost all of these classes have a large interface, so you could spend a lot of time struggling to understand the intricacies of trees.

Tables

Like trees, tables in Swing are vast and powerful. They are primarily intended to be the popular “grid” interface to databases via Java Database Connectivity (JDBC, discussed in Chapter 15) and thus they have a tremendous amount of flexibility, which you pay for in complexity. There’s easily enough here to be the basis of a full-blown spreadsheet and could probably justify an entire book. However, it is also possible to create a relatively simple JTable if you understand the basics.

The JTable controls how the data is displayed, but the TableModel controls the data itself. So to create a JTable you’ll typically create a TableModel first. You can fully implement the TableModel interface, but it’s usually simpler to inherit from the helper class AbstractTableModel:

//: c13:Table.java
// Simple demonstration of JTable.
// <applet code=Table
//  width=350 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.table.*;
import javax.swing.event.*;
import com.bruceeckel.swing.*;

// The TableModel controls all the data:
class DataModel extends AbstractTableModel {
  Object[][] data = {
    {"one", "two", "three", "four"},
    {"five", "six", "seven", "eight"},
    {"nine", "ten", "eleven", "twelve"},
  };
  // Prints data when table changes:
  class TML implements TableModelListener {
    public void tableChanged(TableModelEvent e) {
      for(int i = 0; i < data.length; i++) {
        for(int j = 0; j < data[0].length; j++)
          System.out.print(data[i][j] + " ");
        System.out.println();
      }
    }
  }
  public DataModel() {
    addTableModelListener(new TML());
  }
  public int getColumnCount() {
    return data[0].length;
  }
  public int getRowCount() {
    return data.length;
  }
  public Object getValueAt(int row, int col) {
    return data[row][col];
  }
  public void
  setValueAt(Object val, int row, int col) {
    data[row][col] = val;
    // Indicate the change has happened:
    fireTableDataChanged();
  }
  public boolean
  isCellEditable(int row, int col) {
    return true;
  }
};

public class Table extends JApplet {
  public void init() {
    Container cp = getContentPane();
    JTable table = new JTable(new DataModel());
    JScrollPane scrollpane =
      JTable.createScrollPaneForTable(table);
    cp.add(scrollpane, BorderLayout.CENTER);
  }
  public static void main(String[] args) {
    Console.run(new Table(), 350, 100);
  }
} ///:~

DataModel contains an array of data, but you could also get the data from some other source such as a database. The constructor adds a TableModelListener which prints the array every time the table is changed. The rest of the methods follow the Beans naming convention, and are used by JTable when it wants to present the information in DataModel. AbstractTableModel provides default methods for setValueAt( ) and isCellEditable( ) that prevent changes to the data, so if you want to be able to edit the data, you must override these methods.

Once you have a TableModel, you only need to hand it to the JTable constructor. All the details of displaying, editing and updating will be taken care of for you. Notice that this example also puts the JTable in a JScrollPane, which requires a special JTable method.

More to Swing

This section was meant only to give you an introduction to the power of Swing and to get you started so you could see how relatively simple it is to feel your way through the libraries. What you’ve seen so far will probably suffice for a good portion of your UI design needs. However, there’s a lot more to Swing—it’s intended to be a fully-powered UI design tool kit. If you don’t see what you need here, delve into the online documentation from Sun and search the Web. There’s probably a way to accomplish just about everything you can imagine.

Some of the topics that were not covered in this section include:

  • More specific components such as JColorChooser, JFileChooser, JPasswordField, JHTMLPane (which performs simple HTML formatting and display), and JTextPane (a text editor that supports formatting, word wrap, and images). These are fairly straightforward to use.
  • The new event types for Swing. In many ways, these are like exceptions: the type is what’s important, and the name can be used to infer just about everything else about them.
  • A new layout manager called BoxLayout.
  • Splitter control: a divider style splitter bar that allows you to dynamically manipulate the position of other components.
  • JLayeredPane and JInternalFrame, used together to create child frame windows inside parent frame windows, to produce multiple-document interface (MDI) applications.
  • Pluggable look and feel, so you can write a single program that can dynamically adapt to behave as expected under different platforms and operating systems.
  • Custom cursors.
  • Dockable floating toolbars with the JToolbar API.
  • Double-buffering and Automatic repaint batching for smoother screen redraws.
  • Built-in “undo” support.
  • Drag and drop support.

Using URLs from within an applet

It’s possible for an applet to cause the display of any URL through the Web browser the applet is running within. You can do this with the following line:

getAppletContext().showDocument(u);

in which u is the URL object. Here’s a simple example that redirects you to another Web page. The page happens to be the output of a CGI program, but you can as easily go to an ordinary HTML page, so you could build on this applet to produce a password-protected gateway to a particular portion of your Web site:

//: c13:ShowHTML.java
// <applet code=ShowHTML width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.io.*;
import com.bruceeckel.swing.*;

public class ShowHTML extends JApplet {
  static final String CGIProgram = "MyCGIProgram";
  JButton send = new JButton("Go");
  JLabel l = new JLabel();
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    send.addActionListener(new Al());
    cp.add(send);
    cp.add(l);
  }
  class Al implements ActionListener {
    public void actionPerformed(ActionEvent ae) {
      try {
        // This could be an HTML page instead of
        // a CGI program. Notice that this CGI
        // program doesn't use arguments, but
        // you can add them in the usual way.
        URL u = new URL(
          getDocumentBase(),
          "cgi-bin/" + CGIProgram);
        // Display the output of the URL using
        // the Web browser, as an ordinary page:
        getAppletContext().showDocument(u);
      } catch(Exception e) {
        l.setText(e.toString());
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new ShowHTML(), 100, 50);
  }
} ///:~

The beauty of the URL class is how much it shields you from. You can connect to Web servers without knowing much at all about what’s going on under the covers.

Reading a file from the server

A variation on the above program reads a file located on the server. The file is specified by the client:

//: c13:Fetcher.java
// <applet code=Fetcher width=500 height=300>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.io.*;
import com.bruceeckel.swing.*;

public class Fetcher extends JApplet {
  JButton fetchIt= new JButton("Fetch the Data");
  JTextField f =
    new JTextField("Fetcher.java", 20);
  JTextArea t = new JTextArea(10,40);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    fetchIt.addActionListener(new FetchL());
    cp.add(new JScrollPane(t));
    cp.add(f); cp.add(fetchIt);
  }
  public class FetchL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      try {
        URL url =
          new URL(getDocumentBase(),f.getText());
        t.setText(url + "\n");
        InputStream is = url.openStream();
        BufferedReader in = new BufferedReader(
          new InputStreamReader(is));
        String line;
        while ((line = in.readLine()) != null)
          t.append(line + "\n");
      } catch(Exception ex) {
        t.append(ex.toString());
      }
    }
  }
  public static void main(String[] args) {
    Console.run(new Fetcher(), 500, 300);
  }
} ///:~

A method lookup tool

To finish the chapter, we’ll develop a useful application that allows you to quickly look up methods for classes, as a programming aid.

Chapter 12 introduced the Java 1.1 concept of reflection and used that feature to look up methods for a particular class—either the entire list of methods or a subset of those whose names match a keyword you provide. The magic of this is that it can automatically show you all the methods for a class without forcing you to walk up the inheritance hierarchy examining the base classes at each level. Thus, it provides a valuable timesaving tool for programming: because the names of most Java method names are made nicely verbose and descriptive, you can search for the method names that contain a particular word of interest. When you find what you think you’re looking for, check the online documentation.

However, by Chapter 12 you hadn’t seen Swing, so that tool was developed as a command-line application. Here is the more useful GUI version, which dynamically updates the output as you type and also allows you to cut and paste from the output:

//: c13:DisplayMethods.java
// Display the methods of any class inside
// a window. Dynamically narrows your search.
// <applet code = DisplayMethods
// width=650 height=700></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.lang.reflect.*;
import java.io.*;
import com.bruceeckel.swing.*;

public class DisplayMethods extends JApplet {
  Class cl;
  Method[] m;
  Constructor[] ctor;
  String[] n = new String[0];
  JTextField
    name = new JTextField(40),
    searchFor = new JTextField(30);
  JCheckBox strip =
    new JCheckBox("Strip Qualifiers", true);
  JTextArea results = new JTextArea(40, 65);
  class NameL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      String nm = name.getText().trim();
      if(nm.length() == 0) {
        results.setText("No match");
        n = new String[0];
        return;
      }
      try {
        cl = Class.forName(nm);
      } catch (ClassNotFoundException ex) {
        results.setText("No match");
        return;
      }
      m = cl.getMethods();
      ctor = cl.getConstructors();
      // Convert to an array of Strings:
      n = new String[m.length + ctor.length];
      for(int i = 0; i < m.length; i++)
        n[i] = m[i].toString();
      for(int i = 0; i < ctor.length; i++)
        n[i + m.length] = ctor[i].toString();
      reDisplay();
    }
  }
  class StripL implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
      reDisplay();
    }
  }
  class SearchForL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      reDisplay();
    }
  }
  void reDisplay() {
    // Create the result set:
    String[] rs = new String[n.length];
    String find = searchFor.getText();
    int j = 0;
    // Select from the list if find exists:
    for (int i = 0; i < n.length; i++) {
      if(find == null)
        rs[j++] = n[i];
      else if(n[i].indexOf(find) != -1)
          rs[j++] = n[i];
    }
    results.setText("");
    if(strip.isSelected())
      for (int i = 0; i < j; i++)
        results.append(
          StripQualifiers.strip(rs[i]) + "\n");
    else // Leave qualifiers on
      for (int i = 0; i < j; i++)
        results.append(rs[i] + "\n");
  }
  public void init() {
    name.addActionListener(new NameL());
    searchFor.addActionListener(new SearchForL());
    strip.addItemListener(new StripL());
    JPanel
      top = new JPanel(),
      lower = new JPanel(),
      p = new JPanel(new BorderLayout());
    top.add(new JLabel("Qualified class name:"));
    top.add(name);
    lower.add(
      new JLabel("String to search for:"));
    lower.add(searchFor);
    lower.add(strip);
    p.add(top, BorderLayout.NORTH);
    p.add(lower, BorderLayout.SOUTH);
    Container cp = getContentPane();
    cp.add(p, BorderLayout.NORTH);
    cp.add(results, BorderLayout.CENTER);
  }
  public static void main(String[] args) {
    Console.run(new DisplayMethods(), 650, 700);
  }
}

class StripQualifiers {
  private StreamTokenizer st;
  public StripQualifiers(String qualified) {
      st = new StreamTokenizer(
        new StringReader(qualified));
      st.ordinaryChar(' ');
  }
  public String getNext() {
    String s = null;
    try {
      if(st.nextToken() !=
            StreamTokenizer.TT_EOF) {
        switch(st.ttype) {
          case StreamTokenizer.TT_EOL:
            s = null;
            break;
          case StreamTokenizer.TT_NUMBER:
            s = Double.toString(st.nval);
            break;
          case StreamTokenizer.TT_WORD:
            s = new String(st.sval);
            break;
          default: // single character in ttype
            s = String.valueOf((char)st.ttype);
        }
      }
    } catch(IOException e) {
      System.out.println(e);
    }
    return s;
  }
  public static String strip(String qualified) {
    StripQualifiers sq =
      new StripQualifiers(qualified);
    String s = "", si;
    while((si = sq.getNext()) != null) {
      int lastDot = si.lastIndexOf('.');
      if(lastDot != -1)
        si = si.substring(lastDot + 1);
      s += si;
    }
    return s;
  }
} ///:~

Some things you’ve seen before. As with many of the GUI programs in this book, this is created to perform both as an application and as an applet. Also, the StripQualifiers class is exactly the same as it was in Chapter 12.

The GUI contains a TextField name in which you can enter the fully-qualified class name you want to look up, and another one, searchFor, in which you can enter the optional text to search for within the list of methods. The Checkbox allows you to say whether you want to use the fully-qualified names in the output or if you want the qualification stripped off. Finally, the results are displayed in a TextArea.

You’ll notice that there are no buttons or other components by which to indicate that you want the search to start. That’s because both of the TextFields and the Checkbox are monitored by their listener objects. Whenever you make a change, the list is immediately updated. If you change the text within the name field, the new text is captured in class NameL. If the text isn’t empty, it is used inside Class.forName( ) to try to look up the class. As you’re typing, of course, the name will be incomplete and Class.forName( ) will fail, which means that it throws an exception. This is trapped and the TextArea is set to “No match”. But as soon as you type in a correct name (capitalization counts), Class.forName( ) is successful and getMethods( ) and getConstructors( ) will return arrays of Method and Constructor objects, respectively. Each of the objects in these arrays is turned into a String via toString( ) (this produces the complete method or constructor signature) and both lists are combined into n, a single String array. The array n is a member of class DisplayMethods and is used in updating the display whenever reDisplay( ) is called.

If you change the Checkbox or searchFor components, their listeners simply call reDisplay( ). reDisplay( ) creates a temporary array of String called rs (for “result set”). The result set is either copied directly from n if there is no find word, or conditionally copied from the Strings in n that contain the find word. Finally, the strip Checkbox is interrogated to see if the user wants the names to be stripped (the default is “yes”). If so, StripQualifiers.strip( ) does the job; if not, the list is simply displayed.

In init( ), you might think that there’s a lot of busy work involved in setting up the layout. In fact, it is possible to lay out the components with less work, but the advantage of using BorderLayouts this way is that it allows the user to resize the window and make—in particular—the TextArea larger, which means you can resize to allow you to see longer names without scrolling.

You might find that you’ll keep this tool running while you’re programming, since it provides one of the best “first lines of attack” when you’re trying to figure out what method to call.

Summary

Of all the libraries in Java, the GUI library has seen the most dramatic changes from Java 1.0 to Java 2. The Java 1.0 AWT was roundly criticized as being one of the worst designs seen, and while it would allow you to create portable programs, the resulting GUI was “equally mediocre on all platforms.” It was also limiting, awkward, and unpleasant to use compared with native application development tools on a particular platform.

When Java 1.1 introduced the new event model and Java Beans, the stage was set—now it was possible to create GUI components that could be easily dragged and dropped inside visual application builder tools. In addition, the design of the event model and Beans clearly shows strong consideration for ease of programming and maintainable code (something that was not evident in the 1.0 AWT). But it wasn’t until the GUI components—the JFC/Swing classes—appeared that the job was finished. With the Swing components, cross-platform GUI programming can be a civilized experience.

Actually, the only thing that’s missing is the application builder tool, and this is where the real revolution lies. Microsoft’s Visual Basic and Visual C++ require their application builder tools, as does Borland’s Delphi and C++ Builder. If you want the application builder tool to get better, you have to cross your fingers and hope the vendor will give you what you want. But Java is an open environment, and so not only does it allow for competing application builder environments, it encourages them. And for these tools to be taken seriously, they must support Java Beans. This means a leveled playing field: if a better application builder tool comes along, you’re not tied to the one you’ve been using—you can pick up and move to the new one and increase your productivity. This kind of competitive environment for GUI application builder tools has not been seen before, and the resulting competition can generate only positive results for the productivity of the programmer.

Exercises
  1. Create an applet with a text field and three buttons. When you press each button, make some different text appear in the text field.
  2. Add a check box to the applet created in Exercise 1, capture the event, and insert different text into the text field.
  3. Create an applet and add all the components that cause action( ) to be called, then capture their events and display an appropriate message for each inside a text field.
  4. Add to Exercise 3 the components that can be used only with events detected by handleEvent( ). Override handleEvent( ) and display appropriate messages for each inside a text field.
  5. Create an applet with a Button and a TextField. Write a referenceEvent( ) so that if the button has the focus, characters typed into it will appear in the TextField.
  6. Create an application and add to the main frame all the components described in this chapter, including menus and a dialog box.
  7. Modify TextNew.java so that the characters in t2 retain the original case that they were typed in, instead of automatically being forced to upper case.
  8. Add Frog.class to the manifest file shown in this chapter and run jar to create a JAR file containing both Frog and BangBean. Now either download and install the BDK from Sun or use your own Beans-enabled program builder tool and add the JAR file to your environment so you can test the two Beans.
  9. Create your own Java Bean called Valve that contains two properties: a boolean called “on” and an int called “level.” Create a manifest file, use jar to package your Bean, then load it into the beanbox or into your own Beans-enabled program builder tool so that you can test it.
  10. (Somewhat challenging) Change Menus.java so that it handles cascading menus.

[59] A varation on this is called “the principle of least astonishment,” which essentially says: “don’t surprise the user.”

[60] This is an example of the design pattern called the template method.

[61] It is assumed that the reader is familiar with the basics of HTML. It’s not too hard to figure out, and there are lots of books and resources.

[62] ShowStatus( ) is also a method of Applet, so you can call it directly, without calling getAppletContext( ).

[63] This behavior is apparently a bug and will be fixed in a later version of Java.

[64] There is no MouseMotionEvent even though it seems like there ought to be. Clicking and motion is combined into MouseEvent, so this second appearance of MouseEvent in the table is not an error.

[65] It also solves the problem of “callbacks” without adding any awkward “method pointer” feature to Java.

[66] At the time this section was written, the Swing library had been pronounced “frozen” by Sun, so this code should compile and run without problems as long as you’ve downloaded and installed the Swing library. (You should be able to compile one of Sun’s included demonstration programs to test your installation.) If you do encounter difficulties, check www.BruceEckel.com for updated code.


14: Multiple Threads

Objects provide a way to divide a program up into independent sections. Often, you also need to turn a program into separate, independently-running subtasks.

Each of these independent subtasks is called a thread, and you program as if each thread runs by itself and has the CPU to itself. Some underlying mechanism is actually dividing up the CPU time for you, but in general, you don’t have to think about it, which makes programming with multiple threads a much easier task.

Some definitions are useful at this point. A process is a self-contained running program with its own address space. A multitasking operating system is capable of running more than one process (program) at a time, while making it look like each one is chugging along by periodically providing CPU cycles to each process. A thread is a single sequential flow of control within a process. A single process can thus have multiple concurrently executing threads.

There are many possible uses for multithreading, but in general, you’ll have some part of your program tied to a particular event or resource, and you don’t want to hang up the rest of your program because of that. So you create a thread associated with that event or resource and let it run independently of the main program. A good example is a “quit” button—you don’t want to be forced to poll the quit button in every piece of code you write in your program and yet you want the quit button to be responsive, as if you were checking it regularly. In fact, one of the most immediately compelling reasons for multithreading is to produce a responsive user interface.

Responsive user interfaces

As a starting point, consider a program that performs some CPU-intensive operation and thus ends up ignoring user input and being unresponsive. This one, a combined applet/application, will simply display the result of a running counter:

//: c14:Counter1.java
// A non-responsive user interface.
// <applet code=Counter1 width=300 height=100>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;

public class Counter1 extends JApplet {
  private int count = 0;
  private JButton
    onOff = new JButton("Toggle"),
    start = new JButton("Start");
  private JTextField t = new JTextField(10);
  private boolean runFlag = true;
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    start.addActionListener(new StartL());
    cp.add(start);
    onOff.addActionListener(new OnOffL());
    cp.add(onOff);
  }
  public void go() {
    while (true) {
      try {
        Thread.sleep(100);
      } catch (InterruptedException e) {}
      if (runFlag)
        t.setText(Integer.toString(count++));
    }
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      go();
    }
  }
  class OnOffL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      runFlag = !runFlag;
    }
  }
  public static void main(String[] args) {
    Console.run(new Counter1(), 300, 100);
  }
} ///:~

At this point, the Swing and applet code should be reasonably familiar from Chapter 13. The go( ) method is where the program stays busy: it puts the current value of count into the TextField t, then increments count.

Part of the infinite loop inside go( ) is to call sleep( ). sleep( ) must be associated with a Thread object, and it turns out that every application has some thread associated with it. (Indeed, Java is based on threads and there are always some running along with your application.) So regardless of whether you’re explicitly using threads, you can produce the current thread used by your program with Thread and the static sleep( ) method.

Note that sleep( ) can throw InterruptedException, although throwing such an exception is considered a hostile way to break from a thread and should be discouraged. (Once again, exceptions are for exceptional conditions, not normal flow of control.) Interrupting a sleeping thread is included to support a future language feature.

When the start button is pressed, go( ) is invoked. And upon examining go( ), you might naively think (as I did) that it should allow multithreading because it goes to sleep. That is, while the method is asleep, it seems like the CPU could be busy monitoring other button presses. But it turns out that the real problem is that go( ) never returns, since it’s in an infinite loop, and this means that actionPerformed( ) never returns. Since you’re stuck inside actionPerformed( ) for the first keypress, the program can’t handle any other events. (To get out, you must somehow kill the process; the easiest way to do this is to press Control-C in the console window.)

The basic problem here is that go( ) needs to continue performing its operations, and at the same time it needs to return so actionPerformed( ) can complete and the user interface can continue responding to the user. But in a conventional method like go( ) it cannot continue and at the same time return control to the rest of the program. This sounds like an impossible thing to accomplish, as if the CPU must be in two places at once, but this is precisely the illusion that threading provides. The thread model (and programming support in Java) is a programming convenience to simplify juggling several operations at the same time within a single program. With threads, the CPU will pop around and give each thread some of its time. Each thread has the consciousness of constantly having the CPU to itself, but the CPU’s time is actually sliced between all the threads.

Threading reduces computing efficiency somewhat, but the net improvement in program design, resource balancing, and user convenience is often quite valuable. Of course, if you have more than one CPU, then the operating system can dedicate each CPU to a set of threads or even a single thread and the whole program can run much faster. Multitasking and multithreading tend to be the most reasonable ways to utilize multiprocessor systems.

Inheriting from Thread

The simplest way to create a thread is to inherit from class Thread, which has all the wiring necessary to create and run threads. The most important method for Thread is run( ), which you must override to make the thread do your bidding. Thus, run( ) is the code that will be executed “simultaneously” with the other threads in a program.

The following example creates any number of threads that it keeps track of by assigning each thread a unique number, generated with a static variable. The Thread’s run( ) method is overridden to count down each time it passes through its loop and to finish when the count is zero (at the point when run( ) returns, the thread is terminated).

//: c14:SimpleThread.java
// Very simple Threading example.

public class SimpleThread extends Thread {
  private int countDown = 5;
  private static int threadCount = 0;
  private int threadNumber = ++threadCount;
  public SimpleThread() {
    System.out.println("Making " + threadNumber);
  }
  public void run() {
    while(true) {
      System.out.println("Thread " +
        threadNumber + "(" + countDown + ")");
      if(--countDown == 0) return;
    }
  }
  public static void main(String[] args) {
    for(int i = 0; i < 5; i++)
      new SimpleThread().start();
    System.out.println("All Threads Started");
  }
} ///:~

A run( ) method virtually always has some kind of loop that continues until the thread is no longer necessary, so you must establish the condition on which to break out of this loop (or, in the case above, simply return from run( )). Often, run( ) is cast in the form of an infinite loop, which means that, barring some external call to stop( ) or destroy( ) for that thread, it will run forever (until the program completes).

In main( ) you can see a number of threads being created and run. The special method that comes with the Thread class is start( ), which performs special initialization for the thread and then calls run( ). So the steps are: the constructor is called to build the object, then start( ) configures the thread and calls run( ). If you don’t call start( ) (which you can do in the constructor, if that’s appropriate) the thread will never be started.

The output for one run of this program (it will be different every time) is:

Making 1
Making 2
Making 3
Making 4
Making 5
Thread 1(5)
Thread 1(4)
Thread 1(3)
Thread 1(2)
Thread 2(5)
Thread 2(4)
Thread 2(3)
Thread 2(2)
Thread 2(1)
Thread 1(1)
All Threads Started
Thread 3(5)
Thread 4(5)
Thread 4(4)
Thread 4(3)
Thread 4(2)
Thread 4(1)
Thread 5(5)
Thread 5(4)
Thread 5(3)
Thread 5(2)
Thread 5(1)
Thread 3(4)
Thread 3(3)
Thread 3(2)
Thread 3(1)

You’ll notice that nowhere in this example is sleep( ) called, and yet the output indicates that each thread gets a portion of the CPU’s time in which to execute. This shows that sleep( ), while it relies on the existence of a thread in order to execute, is not involved with either enabling or disabling threading. It’s simply another method.

You can also see that the threads are not run in the order that they’re created. In fact, the order that the CPU attends to an existing set of threads is indeterminate, unless you go in and adjust the priorities using Thread’s setPriority( ) method.

When main( ) creates the Thread objects it isn’t capturing the references for any of them. An ordinary object would be fair game for garbage collection, but not a Thread. Each Thread “registers” itself so there is actually a reference to it someplace and the garbage collector can’t clean it up.

Threading for a responsive interface

Now it’s possible to solve the problem in Counter1.java with a thread. The trick is to place the subtask—that is, the loop that’s inside go( )—inside the run( ) method of a thread. When the user presses the start button, the thread is started, but then the creation of the thread completes, so even though the thread is running, the main job of the program (watching for and responding to user-interface events) can continue. Here’s the solution:

//: c14:Counter2.java
// A responsive user interface with threads.
// <applet code=Counter2 width=300 height=100>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Counter2 extends JApplet {
  private class SeparateSubTask extends Thread {
    int count = 0;
    boolean runFlag = true;
    SeparateSubTask() { start(); }
    public void run() {
      while (true) {
       try {
        sleep(100);
       } catch (InterruptedException e){}
       if(runFlag)
         t.setText(Integer.toString(count++));
      }
    }
  }
  private SeparateSubTask sp = null;
  private JTextField t = new JTextField(10);
  private JButton
    onOff = new JButton("Toggle"),
    start = new JButton("Start");
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(sp == null)
        sp = new SeparateSubTask();
    }
  }
  class OnOffL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(sp != null)
        sp.runFlag = !sp.runFlag; // invertFlag();
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    start.addActionListener(new StartL());
    cp.add(start);
    onOff.addActionListener(new OnOffL());
    cp.add(onOff);
  }
  public static void main(String[] args) {
    Console.run(new Counter2 (), 300, 100);
  }
} ///:~

Counter2 is now a straightforward program, whose job is only to set up and maintain the user interface. But now, when the user presses the start button, a method is not called. Instead a thread of class SeparateSubTask is created (the constructor starts it, in this case), and then the Counter2 event loop continues. Note that the reference to the SeparateSubTask is stored so that when you press the onOff button it can toggle the runFlag inside the SeparateSubTask object. That thread (when it looks at the flag) can then start and stop itself. (This could also have been accomplished by making SeparateSubTask an inner class.)

The class SeparateSubTask is a simple extension of Thread with a constructor (that stores the Counter2 reference and then runs the thread by calling start( )) and a run( ) that essentially contains the code from inside go( ) in Counter1.java. Because SeparateSubTask knows that it holds a reference to a Counter2, it can reach in and access Counter2’s TextField when it needs to.

When you press the onOff button, you’ll see a virtually instant response. Of course, the response isn’t really instant, not like that of a system that’s driven by interrupts. The counter stops only when the thread has the CPU and notices that the flag has changed.

You can see that the inner class SeparateSubTask is private, which means that its fields and methods can be given default access (except for run( ), which must be public since it is public in the base class). The private inner class is not accessible to anyone but Counter2, and since the two classes are tightly coupled it’s convenient to loosen the access restrictions between them. In SeparateSubTask you can see that the invertFlag( ) method has been removed since Counter2 can now directly access runFlag.

The reference to the Counter2 object is being captured automatically by the inner class mechanism. In run( ), you can see that t is transparently accessed, as if it were a field of SeparateSubTask. The t field in the parent class is private since SeparateSubTask can access it without getting any special permission—and it’s always good to make fields “as private as possible” so they cannot be accidentally changed by forces outside your class.

Anytime you notice classes that appear to have high coupling with each other, consider the coding and maintenance improvements you might get by using inner classes.

Combining the thread
with the main class

In the example above you can see that the thread class is separate from the program’s main class. This makes a lot of sense and is relatively easy to understand. There is, however, an alternate form that you will often see used that is not so clear but is usually more concise (which probably accounts for its popularity). This form combines the main program class with the thread class by making the main program class a thread. Since for a GUI program the main program class must be inherited from either Frame or Applet, an interface must be used to paste on the additional functionality. This interface is called Runnable, and it contains the same basic method that Thread does. In fact, Thread also implements Runnable, which specifies only that there be a run( ) method.

The use of the combined program/thread is not quite so obvious. When you start the program, you create an object that’s Runnable, but you don’t start the thread. This must be done explicitly. You can see this in the following program, which reproduces the functionality of Counter2:

//: c14:Counter3.java
// Using the Runnable interface to turn the
// main class into a thread.
// <applet code=Counter3 width=300 height=100>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Counter3
    extends JApplet implements Runnable {
  private int count = 0;
  private boolean runFlag = true;
  private Thread selfThread = null;
  private JButton
    onOff = new JButton("Toggle"),
    start = new JButton("Start");
  private JTextField t = new JTextField(10);
  public void run() {
    while (true) {
      try {
        selfThread.sleep(100);
      } catch (InterruptedException e){}
      if(runFlag)
        t.setText(Integer.toString(count++));
    }
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(selfThread == null) {
        selfThread = new Thread(Counter3.this);
        selfThread.start();
      }
    }
  }
  class OnOffL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      runFlag = !runFlag;
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    start.addActionListener(new StartL());
    cp.add(start);
    onOff.addActionListener(new OnOffL());
    cp.add(onOff);
  }
  public static void main(String[] args) {
    Console.run(new Counter3(), 300, 100);
  }
} ///:~

Now the run( ) is inside the class, but it’s still dormant after init( ) completes. When you press the start button, the thread is created (if it doesn’t already exist) in the somewhat obscure expression:

new Thread(Counter3.this);

When something has a Runnable interface, it simply means that it has a run( ) method, but there’s nothing special about that—it doesn’t produce any innate threading abilities, like those of a class inherited from Thread. So to produce a thread from a Runnable object, you must create a thread separately and hand it the Runnable object; there’s a special constructor for this that takes a Runnable as its argument. You can then call start( ) for that thread:

selfThread.start();

This performs the usual initialization and then calls run( ).

The convenient aspect about the Runnable interface is that everything belongs to the same class. If you need to access something, you simply do it without going through a separate object. The penalty for this convenience is strict, though—you can have only a single thread running for that particular object (although you can create more objects of that type, or create other threads in different classes).

Note that the Runnable interface is not what imposes this restriction. It’s the combination of Runnable and your main class that does it, since you can have only one object of your main class per application.

Making many threads

Consider the creation of many different threads. You can’t do this with the previous example, so you must go back to having separate classes inherited from Thread to encapsulate the run( ). But this is a more general solution and easier to understand, so while the previous example shows a coding style you’ll often see, I can’t recommend it for most cases because it’s just a little bit more confusing and less flexible.

The following example repeats the form of the examples above with counters and toggle buttons. But now all the information for a particular counter, including the button and text field, is inside its own object that is inherited from Thread. All the fields in Ticker are private, which means that the Ticker implementation can be changed at will, including the quantity and type of data components to acquire and display information. When a Ticker object is created, the constructor requires a reference to an AWT Container, which Ticker fills with its visual components. This way, if you change the visual components, the code that uses Ticker doesn’t need to be modified.

//: c14:Counter4.java
// By keeping your thread as a distinct class,
// you can have as many threads as you want.
// <applet code=Counter4 width=200 height=600>
// <param name=size value="12"></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Counter4 extends JApplet {
  private JButton start = new JButton("Start");
  private boolean started = false;
  private Ticker[] s;
  private boolean isApplet = true;
  private int size;
  class Ticker extends Thread {
    private JButton b = new JButton("Toggle");
    private JTextField t = new JTextField(10);
    private int count = 0;
    private boolean runFlag = true;
    public Ticker() {
      b.addActionListener(new ToggleL());
      JPanel p = new JPanel();
      p.add(t);
      p.add(b);
      // Calls JApplet.getContentPane().add():
      getContentPane().add(p);
    }
    class ToggleL implements ActionListener {
      public void actionPerformed(ActionEvent e) {
        runFlag = !runFlag;
      }
    }
    public void run() {
      while (true) {
        if (runFlag)
          t.setText(Integer.toString(count++));
        try {
          sleep(100);
        } catch (InterruptedException e) {}
      }
    }
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(!started) {
        started = true;
        for (int i = 0; i < s.length; i++)
          s[i].start();
      }
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    // Get parameter "size" from Web page:
    if (isApplet)
      size =
        Integer.parseInt(getParameter("size"));
    s = new Ticker[size];
    for (int i = 0; i < s.length; i++)
      s[i] = new Ticker();
    start.addActionListener(new StartL());
    cp.add(start);
  }
  public static void main(String[] args) {
    Counter4 applet = new Counter4();
    // This isn't an applet, so set the flag and
    // produce the parameter values from args:
    applet.isApplet = false;
    applet.size =
      (args.length == 0 ? 12 :
        Integer.parseInt(args[0]));
    Console.run(applet, 200, applet.size * 50);
  }
} ///:~

Ticker contains not only its threading equipment but also the way to control and display the thread. You can create as many threads as you want without explicitly creating the windowing components.

In Counter4 there’s an array of Ticker objects called s. For maximum flexibility, the size of this array is initialized by reaching out into the Web page using applet parameters. Here’s what the size parameter looks like on the page, embedded inside the applet description:

<applet code=Counter4 width=600 height=600>
<param name=size value="20">
</applet>

The param, name, and value are all Web-page keywords. name is what you’ll be referring to in your program, and value can be any string, not just something that resolves to a number.

You’ll notice that the determination of the size of the array s is done inside init( ), and not as part of an inline definition of s. That is, you cannot say as part of the class definition (outside of any methods):

int size = Integer.parseInt(getParameter("size"));
Ticker[] s = new Ticker[size];

You can compile this, but you’ll get a strange “null-pointer exception” at run-time. It works fine if you move the getParameter( ) initialization inside of init( ). The applet framework performs the necessary startup to grab the parameters before entering init( ).

In addition, this code is set up to be either an applet or an application. When it’s an application the size argument is extracted from the command line (or a default value is provided).

Once the size of the array is established, new Ticker objects are created; as part of the Ticker constructor the button and text field for each Ticker is added to the applet.

Pressing the start button means looping through the entire array of Tickers and calling start( ) for each one. Remember, start( ) performs necessary thread initialization and then calls run( ) for that thread.

The ToggleL listener simply inverts the flag in Ticker and when the associated thread next takes note it can react accordingly.

One value of this example is that it allows you to easily create large sets of independent subtasks and to monitor their behavior. In this case, you’ll see that as the number of subtasks gets larger, your machine will probably show more divergence in the displayed numbers because of the way that the threads are served.

You can also experiment to discover how important the sleep(100) is inside Ticker.run( ). If you remove the sleep( ), things will work fine until you press a toggle button. Then that particular thread has a false runFlag and the run( ) is just tied up in a tight infinite loop, which appears difficult to break during multithreading, so the responsiveness and speed of the program really bogs down.

Daemon threads

A “daemon” thread is one that is supposed to provide a general service in the background as long as the program is running, but is not part of the essence of the program. Thus, when all of the non-daemon threads complete the program is terminated. Conversely, if there are any non-daemon threads still running the program doesn’t terminate. (There is, for instance, a thread that runs main( ).)

You can find out if a thread is a daemon by calling isDaemon( ), and you can turn the daemonhood of a thread on and off with setDaemon( ). If a thread is a daemon, then any threads it creates will automatically be daemons.

The following example demonstrates daemon threads:

//: c14:Daemons.java
// Daemonic behavior.
import java.io.*;

class Daemon extends Thread {
  private static final int SIZE = 10;
  private Thread[] t = new Thread[SIZE];
  public Daemon() {
    setDaemon(true);
    start();
  }
  public void run() {
    for(int i = 0; i < SIZE; i++)
      t[i] = new DaemonSpawn(i);
    for(int i = 0; i < SIZE; i++)
      System.out.println(
        "t[" + i + "].isDaemon() = "
        + t[i].isDaemon());
    while(true)
      yield();
  }
}

class DaemonSpawn extends Thread {
  public DaemonSpawn(int i) {
    System.out.println(
      "DaemonSpawn " + i + " started");
    start();
  }
  public void run() {
    while(true)
      yield();
  }
}

public class Daemons {
  public static void main(String[] args) {
    Thread d = new Daemon();
    System.out.println(
      "d.isDaemon() = " + d.isDaemon());
    // Allow the daemon threads to finish
    // their startup processes:
    BufferedReader stdin =
      new BufferedReader(
        new InputStreamReader(System.in));
    System.out.println("Waiting for CR");
    try {
      stdin.readLine();
    } catch(IOException e) {}
  }
} ///:~

The Daemon thread sets its daemon flag to “true” and then spawns a bunch of other threads to show that they are also daemons. Then it goes into an infinite loop that calls yield( ) to give up control to the other processes. In an earlier version of this program, the infinite loops would increment int counters, but this seemed to bring the whole program to a stop. Using yield( ) makes the program quite peppy.

There’s nothing to keep the program from terminating once main( ) finishes its job since there are nothing but daemon threads running. So that you can see the results of starting all the daemon threads, System.in is set up to read so the program waits for a carriage return before terminating. Without this you see only some of the results from the creation of the daemon threads. (Try replacing the readLine( ) code with sleep( ) calls of various lengths to see this behavior.)

Sharing limited resources

You can think of a single-threaded program as one lonely entity moving around through your problem space and doing one thing at a time. Because there’s only one entity, you never have to think about the problem of two entities trying to use the same resource at the same time, like two people trying to park in the same space, walk through a door at the same time, or even talk at the same time.

With multithreading, things aren’t lonely anymore, but you now have the possibility of two or more threads trying to use the same limited resource at once. Colliding over a resource must be prevented or else you’ll have two threads trying to access the same bank account at the same time, print to the same printer, or adjust the same valve, etc.

Improperly accessing resources

Consider a variation on the counters that have been used so far in this chapter. In the following example, each thread contains two counters that are incremented and displayed inside run( ). In addition, there’s another thread of class Watcher that is watching the counters to see if they’re always equivalent. This seems like a needless activity, since looking at the code it appears obvious that the counters will always be the same. But that’s where the surprise comes in. Here’s the first version of the program:

//: c14:Sharing1.java
// Problems with resource sharing while threading.
// <applet code=Sharing1 width=350 height=500>
// <param name=size value="12">
// <param name=observers value="15">
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Sharing1 extends JApplet {
  private static int accessCount = 0;
  private static JTextField aCount =
    new JTextField("0", 7);
  public static void incrementAccess() {
    accessCount++;
    aCount.setText(Integer.toString(accessCount));
  }
  private JButton
    start = new JButton("Start"),
    observer = new JButton("Observe");
  private boolean isApplet = true;
  private int numCounters = 0;
  private int numObservers = 0;
  private TwoCounter[] s;
  class TwoCounter extends Thread {
    private boolean started = false;
    private JTextField
      t1 = new JTextField(5),
      t2 = new JTextField(5);
    private JLabel l =
      new JLabel("count1 == count2");
    private int count1 = 0, count2 = 0;
    // Add the display components as a panel
    // to the given container:
    public TwoCounter() {
      JPanel p = new JPanel();
      p.add(t1);
      p.add(t2);
      p.add(l);
      getContentPane().add(p);
    }
    public void start() {
      if(!started) {
        started = true;
        super.start();
      }
    }
    public void run() {
      while (true) {
        t1.setText(Integer.toString(count1++));
        t2.setText(Integer.toString(count2++));
        try {
          sleep(500);
        } catch (InterruptedException e){}
      }
    }
    public void synchTest() {
      Sharing1.incrementAccess();
      if(count1 != count2)
        l.setText("Unsynched");
    }
  }
  class Watcher extends Thread {
    public Watcher() { start(); }
    public void run() {
      while(true) {
        for(int i = 0; i < s.length; i++)
          s[i].synchTest();
        try {
          sleep(500);
        } catch (InterruptedException e){}
      }
    }
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      for(int i = 0; i < s.length; i++)
        s[i].start();
    }
  }
  class ObserverL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      for(int i = 0; i < numObservers; i++)
        new Watcher();
    }
  }
  public void init() {
    if(isApplet) {
      numCounters =
        Integer.parseInt(getParameter("size"));
      numObservers =
        Integer.parseInt(
          getParameter("observers"));
    }
    s = new TwoCounter[numCounters];
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < s.length; i++)
      s[i] = new TwoCounter();
    JPanel p = new JPanel();
    start.addActionListener(new StartL());
    p.add(start);
    observer.addActionListener(new ObserverL());
    p.add(observer);
    p.add(new JLabel("Access Count"));
    p.add(aCount);
    cp.add(p);
  }
  public static void main(String[] args) {
    Sharing1 applet = new Sharing1();
    // This isn't an applet, so set the flag and
    // produce the parameter values from args:
    applet.isApplet = false;
    applet.numCounters =
      (args.length == 0 ? 12 :
        Integer.parseInt(args[0]));
    applet.numObservers =
      (args.length < 2 ? 15 :
        Integer.parseInt(args[1]));
    Console.run(applet, 350,
      applet.numCounters * 50);
  }
} ///:~

As before, each counter contains its own display components: two text fields and a label that initially indicates that the counts are equivalent. These components are added to the Container in the TwoCounter constructor. Because this thread is started via a button press by the user, it’s possible that start( ) could be called more than once. It’s illegal for Thread.start( ) to be called more than once for a thread (an exception is thrown). You can see that the machinery to prevent this in the started flag and the overridden start( ) method.

In run( ), count1 and count2 are incremented and displayed in a manner that would seem to keep them identical. Then sleep( ) is called; without this call the program balks because it becomes hard for the CPU to swap tasks.

The synchTest( ) method performs the apparently useless activity of checking to see if count1 is equivalent to count2; if they are not equivalent it sets the label to “Unsynched” to indicate this. But first, it calls a static member of the class Sharing1 that increments and displays an access counter to show how many times this check has occurred successfully. (The reason for this will become apparent in future variations of this example.)

The Watcher class is a thread whose job is to call synchTest( ) for all of the TwoCounter objects that are active. It does this by stepping through the array that’s kept in the Sharing1 object. You can think of the Watcher as constantly peeking over the shoulders of the TwoCounter objects.

Sharing1 contains an array of TwoCounter objects that it initializes in init( ) and starts as threads when you press the “start” button. Later, when you press the “Observe” button, one or more observers are created and freed upon the unsuspecting TwoCounter threads.

Note that to run this as an applet in a browser, your Web page will need to contain the lines:

<applet code=Sharing1 width=650 height=500>
<param name=size value="20">
<param name=observers value="1">
</applet>

You can change the width, height, and parameters to suit your experimental tastes. By changing the size and observers you’ll change the behavior of the program. You can also see that this program is set up to run as a stand-alone application by pulling the arguments from the command line (or providing defaults).

Here’s the surprising part. In TwoCounter.run( ), the infinite loop is just repeatedly passing over the adjacent lines:

t1.setText(Integer.toString(count1++));
t2.setText(Integer.toString(count2++));

(as well as sleeping, but that’s not important here). When you run the program, however, you’ll discover that count1 and count2 will be observed (by the Watcher) to be unequal at times! This is because of the nature of threads—they can be suspended at any time. So at times, the suspension occurs between the execution of the above two lines, and the Watcher thread happens to come along and perform the comparison at just this moment, thus finding the two counters to be different.

This example shows a fundamental problem with using threads. You never know when a thread might be run. Imagine sitting at a table with a fork, about to spear the last piece of food on your plate and as your fork reaches for it, the food suddenly vanishes (because your thread was suspended and another thread came in and stole the food). That’s the problem that you’re dealing with.

Sometimes you don’t care if a resource is being accessed at the same time you’re trying to use it (the food is on some other plate). But for multithreading to work, you need some way to prevent two threads from accessing the same resource, at least during critical periods.

Preventing this kind of collision is simply a matter of putting a lock on a resource when one thread is using it. The first thread that accesses a resource locks it, and then the other threads cannot access that resource until it is unlocked, at which time another thread locks and uses it, etc. If the front seat of the car is the limited resource, the child who shouts “Dibs!” asserts the lock.

How Java shares resources

Java has built-in support to prevent collisions over one kind of resource: the memory in an object. Since you typically make the data elements of a class private and access that memory only through methods, you can prevent collisions by making a particular method synchronized. Only one thread at a time can call a synchronized method for a particular object (although that thread can call more than one of the object’s synchronized methods). Here are simple synchronized methods:

synchronized void f() { /* ... */ }
synchronized void g(){ /* ... */ }

Each object contains a single lock (also called a monitor) that is automatically part of the object (you don’t have to write any special code). When you call any synchronized method, that object is locked and no other synchronized method of that object can be called until the first one finishes and releases the lock. In the example above, if f( ) is called for an object, g( ) cannot be called for the same object until f( ) is completed and releases the lock. Thus, there’s a single lock that’s shared by all the synchronized methods of a particular object, and this lock prevents common memory from being written by more than one method at a time (i.e. more than one thread at a time).

There’s also a single lock per class (as part of the Class object for the class), so that synchronized static methods can lock each other out from static data on a class-wide basis.

Note that if you want to guard some other resource from simultaneous access by multiple threads, you can do so by forcing access to that resource through synchronized methods.

Synchronizing the counters

Armed with this new keyword it appears that the solution is at hand: we’ll simply use the synchronized keyword for the methods in TwoCounter. The following example is the same as the previous one, with the addition of the new keyword:

//: c14:Sharing2.java
// Using the synchronized keyword to prevent
// multiple access to a particular resource.
// <applet code=Sharing2 width=350 height=500>
// <param name=size value="12">
// <param name=observers value="15">
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Sharing2 extends JApplet {
  TwoCounter[] s;
  private static int accessCount = 0;
  private static JTextField aCount =
    new JTextField("0", 7);
  public static void incrementAccess() {
    accessCount++;
    aCount.setText(Integer.toString(accessCount));
  }
  private JButton
    start = new JButton("Start"),
    observer = new JButton("Observe");
  private boolean isApplet = true;
  private int numCounters = 0;
  private int numObservers = 0;
  class TwoCounter extends Thread {
    private boolean started = false;
    private JTextField
      t1 = new JTextField(5),
      t2 = new JTextField(5);
    private JLabel l =
      new JLabel("count1 == count2");
    private int count1 = 0, count2 = 0;
    public TwoCounter() {
      JPanel p = new JPanel();
      p.add(t1);
      p.add(t2);
      p.add(l);
      getContentPane().add(p);
    }
    public void start() {
      if(!started) {
        started = true;
        super.start();
      }
    }
    public synchronized void run() {
      while (true) {
        t1.setText(Integer.toString(count1++));
        t2.setText(Integer.toString(count2++));
        try {
          sleep(500);
        } catch (InterruptedException e){}
      }
    }
    public synchronized void synchTest() {
      Sharing2.incrementAccess();
      if(count1 != count2)
        l.setText("Unsynched");
    }
  }

  class Watcher extends Thread {
    public Watcher() { start(); }
    public void run() {
      while(true) {
        for(int i = 0; i < s.length; i++)
          s[i].synchTest();
        try {
          sleep(500);
        } catch (InterruptedException e){}
      }
    }
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      for(int i = 0; i < s.length; i++)
        s[i].start();
    }
  }
  class ObserverL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      for(int i = 0; i < numObservers; i++)
        new Watcher();
    }
  }
  public void init() {
    if(isApplet) {
      numCounters =
        Integer.parseInt(getParameter("size"));
      numObservers =
        Integer.parseInt(
          getParameter("observers"));
    }
    s = new TwoCounter[numCounters];
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < s.length; i++)
      s[i] = new TwoCounter();
    JPanel p = new JPanel();
    start.addActionListener(new StartL());
    p.add(start);
    observer.addActionListener(new ObserverL());
    p.add(observer);
    p.add(new Label("Access Count"));
    p.add(aCount);
    cp.add(p);
  }
  public static void main(String[] args) {
    Sharing2 applet = new Sharing2();
    // This isn't an applet, so set the flag and
    // produce the parameter values from args:
    applet.isApplet = false;
    applet.numCounters =
      (args.length == 0 ? 12 :
        Integer.parseInt(args[0]));
    applet.numObservers =
      (args.length < 2 ? 15 :
        Integer.parseInt(args[1]));
    Console.run(applet, 350,
      applet.numCounters * 50);
  }
} ///:~

You’ll notice that both run( ) and synchTest( ) are synchronized. If you synchronize only one of the methods, then the other is free to ignore the object lock and can be called with impunity. This is an important point: Every method that accesses a critical shared resource must be synchronized or it won’t work right.

Now a new issue arises. The Watcher2 can never get a peek at what’s going on because the entire run( ) method has been synchronized, and since run( ) is always running for each object the lock is always tied up and synchTest( ) can never be called. You can see this because the accessCount never changes.

What we’d like for this example is a way to isolate only part of the code inside run( ). The section of code you want to isolate this way is called a critical section and you use the synchronized keyword in a different way to set up a critical section. Java supports critical sections with the synchronized block; this time synchronized is used to specify the object whose lock is being used to synchronize the enclosed code:

synchronized(syncObject) {
  // This code can be accessed
  // by only one thread at a time
}

Before the synchronized block can be entered, the lock must be acquired on syncObject. If some other thread already has this lock, then the block cannot be entered until the lock is given up.

The Sharing2 example can be modified by removing the synchronized keyword from the entire run( ) method and instead putting a synchronized block around the two critical lines. But what object should be used as the lock? The one that is already respected by synchTest( ), which is the current object (this)! So the modified run( ) looks like this:

  public void run() {
    while (true) {
      synchronized(this) {
        t1.setText(Integer.toString(count1++));
        t2.setText(Integer.toString(count2++));
      }
      try {
        sleep(500);
      } catch (InterruptedException e){}
    }
  }

This is the only change that must be made to Sharing2.java, and you’ll see that while the two counters are never out of synch (according to when the Watcher is allowed to look at them), there is still adequate access provided to the Watcher during the execution of run( ).

Of course, all synchronization depends on programmer diligence: every piece of code that can access a shared resource must be wrapped in an appropriate synchronized block.

Synchronized efficiency

Since having two methods write to the same piece of data never sounds like a particularly good idea, it might seem to make sense for all methods to be automatically synchronized and eliminate the synchronized keyword altogether. (Of course, the example with a synchronized run( ) shows that this wouldn’t work either.) But it turns out that acquiring a lock is not a cheap operation—it multiplies the cost of a method call (that is, entering and exiting from the method, not executing the body of the method) by a minimum of four times, and could be more depending on your implementation. So if you know that a particular method will not cause contention problems it is expedient to leave off the synchronized keyword.

Java Beans revisited

Now that you understand synchronization you can take another look at Java Beans. Whenever you create a Bean, you must assume that it will run in a multithreaded environment. This means that:

  1. Whenever possible, all the public methods of a Bean should be synchronized. Of course, this incurs the synchronized run-time overhead. If that’s a problem, methods that will not cause problems in critical sections can be left un-synchronized, but keep in mind that this is not always obvious. Methods that qualify tend to be small (such as getCircleSize( ) in the following example) and/or “atomic,” that is, the method call executes in such a short amount of code that the object cannot be changed during execution. Making such methods un-synchronized might not have a significant effect on the execution speed of your program. You might as well make all public methods of a Bean synchronized and remove the synchronized keyword only when you know for sure that it’s necessary and that it makes a difference.
  2. When firing a multicast event to a bunch of listeners interested in that event, you must assume that listeners might be added or removed while moving through the list.

The first point is fairly easy to deal with, but the second point requires a little more thought. Consider the BangBean.java example presented in the last chapter. That ducked out of the multithreading question by ignoring the synchronized keyword (which hadn’t been introduced yet) and making the event unicast. Here’s that example modified to work in a multithreaded environment and to use multicasting for events:

//: c14:BangBean2.java
// You should write your Beans this way so they
// can run in a multithreaded environment.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
import com.bruceeckel.swing.*;

public class BangBean2 extends JPanel
    implements Serializable {
  private int xm, ym;
  private int cSize = 20; // Circle size
  private String text = "Bang!";
  private int fontSize = 48;
  private Color tColor = Color.red;
  private ArrayList actionListeners =
    new ArrayList();
  public BangBean2() {
    addMouseListener(new ML());
    addMouseMotionListener(new MM());
  }
  public synchronized int getCircleSize() {
    return cSize;
  }
  public synchronized void
  setCircleSize(int newSize) {
    cSize = newSize;
  }
  public synchronized String getBangText() {
    return text;
  }
  public synchronized void
  setBangText(String newText) {
    text = newText;
  }
  public synchronized int getFontSize() {
    return fontSize;
  }
  public synchronized void
  setFontSize(int newSize) {
    fontSize = newSize;
  }
  public synchronized Color getTextColor() {
    return tColor;
  }
  public synchronized void
  setTextColor(Color newColor) {
    tColor = newColor;
  }
  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.black);
    g.drawOval(xm - cSize/2, ym - cSize/2,
      cSize, cSize);
  }
  // This is a multicast listener, which is
  // more typically used than the unicast
  // approach taken in BangBean.java:
  public synchronized void
    addActionListener(ActionListener l) {
    actionListeners.add(l);
  }
  public synchronized void
    removeActionListener(ActionListener l) {
    actionListeners.remove(l);
  }
  // Notice this isn't synchronized:
  public void notifyListeners() {
    ActionEvent a =
      new ActionEvent(BangBean2.this,
        ActionEvent.ACTION_PERFORMED, null);
    ArrayList lv = null;
    // Make a shallow copy of the vector in case
    // someone adds a listener while we're
    // calling listeners:
    synchronized(this) {
      lv = (ArrayList)actionListeners.clone();
    }
    // Call all the listener methods:
    for(int i = 0; i < lv.size(); i++) {
      ActionListener al =
        (ActionListener)lv.get(i);
      al.actionPerformed(a);
    }
  }
  class ML extends MouseAdapter {
    public void mousePressed(MouseEvent e) {
      Graphics g = getGraphics();
      g.setColor(tColor);
      g.setFont(
        new Font(
          "TimesRoman", Font.BOLD, fontSize));
      int width =
        g.getFontMetrics().stringWidth(text);
      g.drawString(text,
        (getSize().width - width) /2,
        getSize().height/2);
      g.dispose();
      notifyListeners();
    }
  }
  class MM extends MouseMotionAdapter {
    public void mouseMoved(MouseEvent e) {
      xm = e.getX();
      ym = e.getY();
      repaint();
    }
  }
  public static void main(String[] args) {
    BangBean2 bb = new BangBean2();
    bb.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        System.out.println("ActionEvent" + e);
      }
    });
    bb.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        System.out.println("BangBean2 action");
      }
    });
    bb.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e){
        System.out.println("More action");
      }
    });
    Console.run(bb, 300, 300);
  }
} ///:~

Adding synchronized to the methods is an easy change. However, notice in addActionListener( ) and removeActionListener( ) that the ActionListeners are now added to and removed from an ArrayList, so you can have as many as you want.

You can see that the method notifyListeners( ) is not synchronized. It can be called from more than one thread at a time. It’s also possible for addActionListener( ) or removeActionListener( ) to be called in the middle of a call to notifyListeners( ), which is a problem since it traverses the ArrayList actionListeners. To alleviate the problem, the ArrayList is cloned inside a synchronized clause and the clone is traversed. This way the original ArrayList can be manipulated without impact on notifyListeners( ).

The paint( ) method is also not synchronized. Deciding whether to synchronize overridden methods is not as clear as when you’re just adding your own methods. In this example it turns out that paint( ) seems to work OK whether it’s synchronized or not. But the issues you must consider are:

  1. Does the method modify the state of “critical” variables within the object? To discover whether the variables are “critical” you must determine whether they will be read or set by other threads in the program. (In this case, the reading or setting is virtually always accomplished via synchronized methods, so you can just examine those.) In the case of paint( ), no modification takes place.
  2. Does the method depend on the state of these “critical” variables? If a synchronized method modifies a variable that your method uses, then you might very well want to make your method synchronized as well. Based on this, you might observe that cSize is changed by synchronized methods and therefore paint( ) should be synchronized. Here, however, you can ask “What’s the worst thing that will happen if cSize is changed during a paint( )?” When you see that it’s nothing too bad, and a transient effect at that, it’s best to leave paint( ) un-synchronized to prevent the extra overhead from the synchronized method call.
  3. A third clue is to notice whether the base-class version of paint( ) is synchronized, which it isn’t. This isn’t an airtight argument, just a clue. In this case, for example, a field that is changed via synchronized methods (that is cSize) has been mixed into the paint( ) formula and might have changed the situation. Notice, however, that synchronized doesn’t inherit—that is, if a method is synchronized in the base class then it is not automatically synchronized in the derived class overridden version.

The test code in TestBangBean2 has been modified from that in the previous chapter to demonstrate the multicast ability of BangBean2 by adding extra listeners.

Blocking

A thread can be in any one of four states:

  1. New: the thread object has been created but it hasn’t been started yet so it cannot run.
  2. Runnable: This means that a thread can be run when the time-slicing mechanism has CPU cycles available for the thread. Thus, the thread might or might not be running, but there’s nothing to prevent it from being run if the scheduler can arrange it; it’s not dead or blocked.
  3. Dead: the normal way for a thread to die is by returning from its run( ) method. You can also call stop( ), but this throws an exception that’s a subclass of Error (which means you usually don’t catch it). Remember that throwing an exception should be a special event and not part of normal program execution; thus the use of stop( ) is discouraged (and it’s deprecated in Java 2). There’s also a destroy( ) method (which has never been implemented) that you should never call if you can avoid it since it’s drastic and doesn’t release object locks.
  4. Blocked: the thread could be run but there’s something that prevents it. While a thread is in the blocked state the scheduler will simply skip over it and not give it any CPU time. Until a thread re-enters the runnable state it won’t perform any operations.

Becoming blocked

The blocked state is the most interesting and is worth further examination. A thread can become blocked for five reasons:

  1. You’ve put the thread to sleep by calling sleep(milliseconds), in which case it will not be run for the specified time.
  2. You’ve suspended the execution of the thread with suspend( ). It will not become runnable again until the thread gets the resume( ) message.
  3. You’ve suspended the execution of the thread with wait( ). It will not become runnable again until the thread gets the notify( ) or notifyAll( ) message. (Yes, this looks just like number 2, but there’s a distinct difference that will be revealed.)
  4. The thread is waiting for some IO to complete.
  5. The thread is trying to call a synchronized method on another object and that object’s lock is not available.

You can also call yield( ) (a method of the Thread class) to voluntarily give up the CPU so that other threads can run. However, the same thing happens if the scheduler decides that your thread has had enough time and jumps to another thread. That is, nothing prevents the scheduler from restarting your thread. When a thread is blocked, there’s some reason that it cannot continue running.

The following example shows all five ways of becoming blocked. It all exists in a single file called Blocking.java, but it will be examined here in discrete pieces. (You’ll notice the “Continued” and “Continuing” tags that allow the code extraction tool to piece everything together.) First, the basic framework:

//: c14:Blocking.java
// Demonstrates the various ways a thread
// can be blocked.
// <applet code=Blocking width=350 height=550>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import com.bruceeckel.swing.*;

//////////// The basic framework ///////////
class Blockable extends Thread {
  private Peeker peeker;
  protected JTextField state = new JTextField(30);
  protected int i;
  public Blockable(Container c) {
    c.add(state);
    peeker = new Peeker(this, c);
  }
  public synchronized int read() { return i; }
  protected synchronized void update() {
    state.setText(getClass().getName()
      + " state: i = " + i);
  }
  public void stopPeeker() {
    // peeker.stop(); Deprecated in Java 1.2
    peeker.terminate(); // The preferred approach
  }
}

class Peeker extends Thread {
  private Blockable b;
  private int session;
  private JTextField status = new JTextField(30);
  private boolean stop = false;
  public Peeker(Blockable b, Container c) {
    c.add(status);
    this.b = b;
    start();
  }
  public void terminate() { stop = true; }
  public void run() {
    while (!stop) {
      status.setText(b.getClass().getName()
        + " Peeker " + (++session)
        + "; value = " + b.read());
       try {
        sleep(100);
      } catch (InterruptedException e){}
    }
  }
} ///:Continued

The Blockable class is meant to be a base class for all the classes in this example that demonstrate blocking. A Blockable object contains a TextField called state that is used to display information about the object. The method that displays this information is update( ). You can see it uses getClass( ).getName( ) to produce the name of the class instead of just printing it out; this is because update( ) cannot know the exact name of the class it is called for, since it will be a class derived from Blockable.

The indicator of change in Blockable is an int i, which will be incremented by the run( ) method of the derived class.

There’s a thread of class Peeker that is started for each Blockable object, and the Peeker’s job is to watch its associated Blockable object to see changes in i by calling read( ) and reporting them in its status TextField. This is important: Note that read( ) and update( ) are both synchronized, which means they require that the object lock be free.

Sleeping

The first test in this program is with sleep( ):

///:Continuing
///////////// Blocking via sleep() ///////////
class Sleeper1 extends Blockable {
  public Sleeper1(Container c) { super(c); }
  public synchronized void run() {
    while(true) {
      i++;
      update();
       try {
        sleep(1000);
      } catch (InterruptedException e){}
    }
  }
}

class Sleeper2 extends Blockable {
  public Sleeper2(Container c) { super(c); }
  public void run() {
    while(true) {
      change();
       try {
        sleep(1000);
      } catch (InterruptedException e){}
    }
  }
  public synchronized void change() {
      i++;
      update();
  }
} ///:Continued

In Sleeper1 the entire run( ) method is synchronized. You’ll see that the Peeker associated with this object will run along merrily until you start the thread, and then the Peeker stops cold. This is one form of blocking: since Sleeper1.run( ) is synchronized, and once the thread starts it’s always inside run( ), the method never gives up the object lock and the Peeker is blocked.

Sleeper2 provides a solution by making run un-synchronized. Only the change( ) method is synchronized, which means that while run( ) is in sleep( ), the Peeker can access the synchronized method it needs, namely read( ). Here you’ll see that the Peeker continues running when you start the Sleeper2 thread.

Suspending and resuming

The next part of the example introduces the concept of suspension. The Thread class has a method suspend( ) to temporarily halt the thread and resume( ) that restarts it at the point it was halted. Presumably, resume( ) is called by some thread outside the suspended one, and in this case there’s a separate class called Resumer that does just that. Each of the classes demonstrating suspend/resume has an associated resumer:

///:Continuing
/////////// Blocking via suspend() ///////////
class SuspendResume extends Blockable {
  public SuspendResume(Container c) {
    super(c);
    new Resumer(this);
  }
}

class SuspendResume1 extends SuspendResume {
  public SuspendResume1(Container c) { super(c);}
  public synchronized void run() {
    while(true) {
      i++;
      update();
      suspend(); // Deprecated in Java 1.2
    }
  }
}

class SuspendResume2 extends SuspendResume {
  public SuspendResume2(Container c) { super(c);}
  public void run() {
    while(true) {
      change();
      suspend(); // Deprecated in Java 1.2
    }
  }
  public synchronized void change() {
      i++;
      update();
  }
}

class Resumer extends Thread {
  private SuspendResume sr;
  public Resumer(SuspendResume sr) {
    this.sr = sr;
    start();
  }
  public void run() {
    while(true) {
       try {
        sleep(1000);
      } catch (InterruptedException e){}
      sr.resume(); // Deprecated in Java 1.2
    }
  }
} ///:Continued

SuspendResume1 also has a synchronized run( ) method. Again, when you start this thread you’ll see that its associated Peeker gets blocked waiting for the lock to become available, which never happens. This is fixed as before in SuspendResume2, which does not synchronize the entire run( ) method but instead uses a separate synchronized change( ) method.

You should be aware that Java 2 deprecates the use of suspend( ) and resume( ), because suspend( ) holds the object’s lock and is thus deadlock-prone. That is, you can easily get a number of locked objects waiting on each other, and this will cause your program to freeze. Although you might see them used in older programs you should not use suspend( ) and resume( ). The proper solution is described later in this chapter.

Wait and notify

The point with the first two examples is that both sleep( ) and suspend( ) do not release the lock as they are called. You must be aware of this when working with locks. On the other hand, the method wait( ) does release the lock when it is called, which means that other synchronized methods in the thread object could be called during a wait( ). In the following two classes, you’ll see that the run( ) method is fully synchronized in both cases, however, the Peeker still has full access to the synchronized methods during a wait( ). This is because wait( ) releases the lock on the object as it suspends the method it’s called within.

You’ll also see that there are two forms of wait( ). The first takes an argument in milliseconds that has the same meaning as in sleep( ): pause for this period of time. The difference is that in wait( ), the object lock is released and you can come out of the wait( ) because of a notify( ) as well as having the clock run out.

The second form takes no arguments, and means that the wait( ) will continue until a notify( ) comes along and will not automatically terminate after a time.

One fairly unique aspect of wait( ) and notify( ) is that both methods are part of the base class Object and not part of Thread as are sleep( ), suspend( ), and resume( ). Although this seems a bit strange at first—to have something that’s exclusively for threading as part of the universal base class—it’s essential because they manipulate the lock that’s also part of every object. As a result, you can put a wait( ) inside any synchronized method, regardless of whether there’s any threading going on inside that particular class. In fact, the only place you can call wait( ) is within a synchronized method or block. If you call wait( ) or notify( ) within a method that’s not synchronized, the program will compile, but when you run it you’ll get an IllegalMonitorStateException with the somewhat non-intuitive message “current thread not owner.” Note that sleep( ), suspend( ), and resume( ) can all be called within non-synchronized methods since they don’t manipulate the lock.

You can call wait( ) or notify( ) only for your own lock. Again, you can compile code that tries to use the wrong lock, but it will produce the same IllegalMonitorStateException message as before. You can’t fool with someone else’s lock, but you can ask another object to perform an operation that manipulates its own lock. So one approach is to create a synchronized method that calls notify( ) for its own object. However, in Notifier you’ll see the notify( ) call inside a synchronized block:

synchronized(wn2) {
  wn2.notify();
}

where wn2 is the object of type WaitNotify2. This method, which is not part of WaitNotify2, acquires the lock on the wn2 object, at which point it’s legal for it to call notify( ) for wn2 and you won’t get the IllegalMonitorStateException.

///:Continuing
/////////// Blocking via wait() ///////////
class WaitNotify1 extends Blockable {
  public WaitNotify1(Container c) { super(c); }
  public synchronized void run() {
    while(true) {
      i++;
      update();
       try {
        wait(1000);
      } catch (InterruptedException e){}
    }
  }
}

class WaitNotify2 extends Blockable {
  public WaitNotify2(Container c) {
    super(c);
    new Notifier(this);
  }
  public synchronized void run() {
    while(true) {
      i++;
      update();
       try {
        wait();
      } catch (InterruptedException e){}
    }
  }
}

class Notifier extends Thread {
  private WaitNotify2 wn2;
  public Notifier(WaitNotify2 wn2) {
    this.wn2 = wn2;
    start();
  }
  public void run() {
    while(true) {
       try {
        sleep(2000);
      } catch (InterruptedException e){}
      synchronized(wn2) {
        wn2.notify();
      }
    }
  }
} ///:Continued

wait( ) is typically used when you’ve gotten to the point where you’re waiting for some other condition, under the control of forces outside your thread, to change and you don’t want to idly wait by inside the thread. So wait( ) allows you to put the thread to sleep while waiting for the world to change, and only when a notify( ) or notifyAll( ) occurs does the thread wake up and check for changes. Thus, it provides a way to synchronize between threads.

Blocking on IO

If a stream is waiting for some IO activity, it will automatically block. In the following portion of the example, the two classes work with generic Reader and Writer objects (using the Java 1.1 Streams), but in the test framework a piped stream will be set up to allow the two threads to safely pass data to each other (which is the purpose of piped streams).

The Sender puts data into the Writer and sleeps for a random amount of time. However, Receiver has no sleep( ), suspend( ), or wait( ). But when it does a read( ) it automatically blocks when there is no more data.

///:Continuing
class Sender extends Blockable { // send
  private Writer out;
  public Sender(Container c, Writer out) {
    super(c);
    this.out = out;
  }
  public void run() {
    while(true) {
      for(char c = 'A'; c <= 'z'; c++) {
         try {
          i++;
          out.write(c);
          state.setText("Sender sent: "
            + (char)c);
          sleep((int)(3000 * Math.random()));
        } catch (InterruptedException e){}
          catch (IOException e) {}
      }
    }
  }
}

class Receiver extends Blockable {
  private Reader in;
  public Receiver(Container c, Reader in) {
    super(c);
    this.in = in;
  }
  public void run() {
    try {
      while(true) {
        i++; // Show peeker it's alive
        // Blocks until characters are there:
        state.setText("Receiver read: "
          + (char)in.read());
      }
    } catch(IOException e) { e.printStackTrace();}
  }
} ///:Continued

Both classes also put information into their state fields and change i so the Peeker can see that the thread is running.

Testing

The main applet class is surprisingly simple because most of the work has been put into the Blockable framework. Basically, an array of Blockable objects is created, and since each one is a thread, they perform their own activities when you press the “start” button. There’s also a button and actionPerformed( ) clause to stop all of the Peeker objects, which provides a demonstration of the alternative to the deprecated (in Java 2) stop( ) method of Thread.

To set up a connection between the Sender and Receiver objects, a PipedWriter and PipedReader are created. Note that the PipedReader in must be connected to the PipedWriter out via a constructor argument. After that, anything that’s placed in out can later be extracted from in, as if it passed through a pipe (hence the name). The in and out objects are then passed to the Receiver and Sender constructors, respectively, which treat them as Reader and Writer objects of any type (that is, they are upcast).

The array of Blockable references b is not initialized at its point of definition because the piped streams cannot be set up before that definition takes place (the need for the try block prevents this).

///:Continuing
/////////// Testing Everything ///////////
public class Blocking extends JApplet {
  private JButton
    start = new JButton("Start"),
    stopPeekers = new JButton("Stop Peekers");
  private boolean started = false;
  private Blockable[] b;
  private PipedWriter out;
  private PipedReader in;
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(!started) {
        started = true;
        for(int i = 0; i < b.length; i++)
          b[i].start();
      }
    }
  }
  class StopPeekersL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      // Demonstration of the preferred
      // alternative to Thread.stop():
      for(int i = 0; i < b.length; i++)
        b[i].stopPeeker();
    }
  }
  public void init() {
     Container cp = getContentPane();
     cp.setLayout(new FlowLayout());
     out = new PipedWriter();
    try {
      in = new PipedReader(out);
    } catch(IOException e) {}
    b = new Blockable[] {
      new Sleeper1(cp),
      new Sleeper2(cp),
      new SuspendResume1(cp),
      new SuspendResume2(cp),
      new WaitNotify1(cp),
      new WaitNotify2(cp),
      new Sender(cp, out),
      new Receiver(cp, in)
    };
    start.addActionListener(new StartL());
    cp.add(start);
    stopPeekers.addActionListener(
      new StopPeekersL());
    cp.add(stopPeekers);
  }
  public static void main(String[] args) {
    Console.run(new Blocking(), 350, 550);
  }
} ///:~

In init( ), notice the loop that moves through the entire array and adds the state and peeker.status text fields to the page.

When the Blockable threads are initially created, each one automatically creates and starts its own Peeker. So you’ll see the Peekers running before the Blockable threads are started. This is essential, as some of the Peekers will get blocked and stop when the Blockable threads start, and it’s essential to see this to understand that particular aspect of blocking.

Deadlock

Because threads can become blocked and because objects can have synchronized methods that prevent threads from accessing that object until the synchronization lock is released, it’s possible for one thread to get stuck waiting for another thread, which in turn waits for another thread, etc., until the chain leads back to a thread waiting on the first one. Thus, there’s a continuous loop of threads waiting on each other and no one can move. This is called deadlock. The claim is that it doesn’t happen that often, but when it happens to you it’s frustrating to debug.

There is no language support to help prevent deadlock; it’s up to you to avoid it by careful design. These are not comforting words to the person who’s trying to debug a deadlocking program.

The deprecation of stop( ), suspend( ),
resume( ), and destroy( ) in Java 2

One change that has been made in Java 2 to reduce the possibility of deadlock is the deprecation of Thread’s stop( ), suspend( ), resume( ), and destroy( ) methods.

The reason that the stop( ) method is deprecated is because it is unsafe. It releases all the locks that the thread had acquired, and if the objects are in an inconsistent state (“damaged”) other threads can view and modify them in that state. The resulting problems can be subtle and difficult to detect. Instead of using stop( ), you should follow the example in Blocking.java and use a flag to tell the thread when to terminate itself by exiting its run( ) method.

There are times when a thread blocks, such as when it is waiting for input, and it cannot poll a flag as it does in Blocking.java. In these cases, you still shouldn’t use stop( ), but instead you can use the interrupt( ) method in Thread to break out of the blocked code:

//: c14:Interrupt.java
// The alternative approach to using stop()
// when a thread is blocked.
// <applet code=Interrupt width=200 height=100>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

class Blocked extends Thread {
  public synchronized void run() {
    try {
      wait(); // Blocks
    } catch(InterruptedException e) {
      System.out.println("InterruptedException");
    }
    System.out.println("Exiting run()");
  }
}

public class Interrupt extends JApplet {
  private JButton
    interrupt = new JButton("Interrupt");
  private Blocked blocked = new Blocked();
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(interrupt);
    interrupt.addActionListener(
      new ActionListener() {
        public
        void actionPerformed(ActionEvent e) {
          System.out.println("Button pressed");
          if(blocked == null) return;
          Thread remove = blocked;
          blocked = null; // to release it
          remove.interrupt();
        }
      });
    blocked.start();
  }
  public static void main(String[] args) {
    Console.run(new Interrupt(), 200, 100);
  }
} ///:~

The wait( ) inside Blocked.run( ) produces the blocked thread. When you press the button, the blocked reference is set to null so the garbage collector will clean it up, and then the object’s interrupt( ) method is called. The first time you press the button you’ll see that the thread quits, but after that there’s no thread to kill so you just see that the button has been pressed.

The suspend( ) and resume( ) methods turn out to be inherently deadlock-prone. When you call suspend( ), the target thread stops but it still holds any locks that it has acquired up to that point. So no other thread can access the locked resources until the thread is resumed. Any thread that wants to resume the target thread and also tries to use any of the locked resources produces deadlock. You should not use suspend( ) and resume( ), but instead put a flag in your Thread class to indicate whether the thread should be active or suspended. If the flag indicates that the thread is suspended, the thread goes into a wait using wait( ). When the flag indicates that the thread should be resumed the thread is restarted with notify( ). An example can be produced by modifying Counter2.java. Although the effect is similar, you’ll notice that the code organization is quite different—anonymous inner classes are used for all of the listeners and the Thread is an inner class, which makes programming slightly more convenient since it eliminates some of the extra bookkeeping necessary in Counter2.java:

//: c14:Suspend.java
// The alternative approach to using suspend()
// and resume(), which are deprecated in Java 2.
// <applet code=Suspend width=300 height=100>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

public class Suspend extends JApplet {
  private JTextField t = new JTextField(10);
  private JButton
    suspend = new JButton("Suspend"),
    resume = new JButton("Resume");
  private Suspendable ss = new Suspendable();
  class Suspendable extends Thread {
    private int count = 0;
    private boolean suspended = false;
    public Suspendable() { start(); }
    public void fauxSuspend() {
      suspended = true;
    }
    public synchronized void fauxResume() {
      suspended = false;
      notify();
    }
    public void run() {
      while (true) {
        try {
          sleep(100);
          synchronized(this) {
            while(suspended)
              wait();
          }
        } catch (InterruptedException e){}
        t.setText(Integer.toString(count++));
      }
    }
  }
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    cp.add(t);
    suspend.addActionListener(
      new ActionListener() {
        public
        void actionPerformed(ActionEvent e) {
          ss.fauxSuspend();
        }
      });
    cp.add(suspend);
    resume.addActionListener(
      new ActionListener() {
        public
        void actionPerformed(ActionEvent e) {
          ss.fauxResume();
        }
      });
    cp.add(resume);
  }
  public static void main(String[] args) {
    Console.run(new Suspend(), 300, 100);
  }
} ///:~

The flag suspended inside Suspendable is used to turn suspension on and off. To suspend, the flag is set to true by calling fauxSuspend( ) and this is detected inside run( ). The wait( ), as described earlier in this chapter, must be synchronized so that it has the object lock. In fauxResume( ), the suspended flag is set to false and notify( ) is called—since this wakes up wait( ) inside a synchronized clause the fauxResume( ) method must also be synchronized so that it acquires the lock before calling notify( ) (thus the lock is available for the wait( ) to wake up with). If you follow the style shown in this program you can avoid using suspend( ) and resume( ).

The destroy( ) method of Thread has never been implemented; it’s like a suspend( ) that cannot resume, so it has the same deadlock issues as suspend( ). However, this is not a deprecated method and it might be implemented in a future version of Java (after 2) for special situations in which the risk of a deadlock is acceptable.

You might wonder why these methods, now deprecated, were included in Java in the first place. It seems a clear admission of a rather significant mistake to simply remove them outright (and pokes yet another hole in the arguments for Java’s exceptional design and infallibility touted by Sun marketing people). The heartening part about the change is that it clearly indicates that the technical people and not the marketing people are running the show—they discovered a problem and they are fixing it. I find this much more promising and hopeful than leaving the problem in because fixing it would admit an error. It means that Java will continue to improve, even if it means a little discomfort on the part of Java programmers. I’d rather deal with the discomfort than watch the language stagnate.

Priorities

The priority of a thread tells the scheduler how important this thread is. If there are a number of threads blocked and waiting to be run, the scheduler will run the one with the highest priority first. However, this doesn’t mean that threads with lower priority don’t get run (that is, you can’t get deadlocked because of priorities). Lower priority threads just tend to run less often.

You can read the priority of a thread with getPriority( ) and change it with setPriority( ). The form of the prior “counter” examples can be used to show the effect of changing the priorities. In this applet you’ll see that the counters slow down as the associated threads have their priorities lowered:

//: c14:Counter5.java
// Adjusting the priorities of threads.
// <applet code=Counter5 width=450 height=600>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

class Ticker2 extends Thread {
  private JButton
    b = new JButton("Toggle"),
    incPriority = new JButton("up"),
    decPriority = new JButton("down");
  private JTextField
    t = new JTextField(10),
    pr = new JTextField(3); // Display priority
  private int count = 0;
  private boolean runFlag = true;
  public Ticker2(Container c) {
    b.addActionListener(new ToggleL());
    incPriority.addActionListener(new UpL());
    decPriority.addActionListener(new DownL());
    JPanel p = new JPanel();
    p.add(t);
    p.add(pr);
    p.add(b);
    p.add(incPriority);
    p.add(decPriority);
    c.add(p);
  }
  class ToggleL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      runFlag = !runFlag;
    }
  }
  class UpL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      int newPriority = getPriority() + 1;
      if(newPriority > Thread.MAX_PRIORITY)
        newPriority = Thread.MAX_PRIORITY;
      setPriority(newPriority);
    }
  }
  class DownL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      int newPriority = getPriority() - 1;
      if(newPriority < Thread.MIN_PRIORITY)
        newPriority = Thread.MIN_PRIORITY;
      setPriority(newPriority);
    }
  }
  public void run() {
    while (true) {
      if(runFlag) {
        t.setText(Integer.toString(count++));
        pr.setText(
          Integer.toString(getPriority()));
      }
      yield();
    }
  }
}

public class Counter5 extends JApplet {
  private JButton
    start = new JButton("Start"),
    upMax = new JButton("Inc Max Priority"),
    downMax = new JButton("Dec Max Priority");
  private boolean started = false;
  private static final int SIZE = 10;
  private Ticker2[] s = new Ticker2[SIZE];
  private TextField mp = new TextField(3);
  public void init() {
    Container cp = getContentPane();
    cp.setLayout(new FlowLayout());
    for(int i = 0; i < s.length; i++)
      s[i] = new Ticker2(cp);
    cp.add(new JLabel(
      "MAX_PRIORITY = " + Thread.MAX_PRIORITY));
    cp.add(new JLabel("MIN_PRIORITY = "
      + Thread.MIN_PRIORITY));
    cp.add(new JLabel("Group Max Priority = "));
    cp.add(mp);
    cp.add(start);
    cp.add(upMax);
    cp.add(downMax);
    start.addActionListener(new StartL());
    upMax.addActionListener(new UpMaxL());
    downMax.addActionListener(new DownMaxL());
    showMaxPriority();
    // Recursively display parent thread groups:
    ThreadGroup parent =
      s[0].getThreadGroup().getParent();
    while(parent != null) {
      cp.add(new Label(
        "Parent threadgroup max priority = "
        + parent.getMaxPriority()));
      parent = parent.getParent();
    }
  }
  public void showMaxPriority() {
    mp.setText(Integer.toString(
      s[0].getThreadGroup().getMaxPriority()));
  }
  class StartL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      if(!started) {
        started = true;
        for(int i = 0; i < s.length; i++)
          s[i].start();
      }
    }
  }
  class UpMaxL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      int maxp =
        s[0].getThreadGroup().getMaxPriority();
      if(++maxp > Thread.MAX_PRIORITY)
        maxp = Thread.MAX_PRIORITY;
      s[0].getThreadGroup().setMaxPriority(maxp);
      showMaxPriority();
    }
  }
  class DownMaxL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      int maxp =
        s[0].getThreadGroup().getMaxPriority();
      if(--maxp < Thread.MIN_PRIORITY)
        maxp = Thread.MIN_PRIORITY;
      s[0].getThreadGroup().setMaxPriority(maxp);
      showMaxPriority();
    }
  }
  public static void main(String[] args) {
    Console.run(new Counter5(), 450, 600);
  }
} ///:~

Ticker2 follows the form established earlier in this chapter, but there’s an extra TextField for displaying the priority of the thread and two more buttons for incrementing and decrementing the priority.

Also notice the use of yield( ), which voluntarily hands control back to the scheduler. Without this the multithreading mechanism still works, but you’ll notice it runs slowly (try removing the call to yield( )!). You could also call sleep( ), but then the rate of counting would be controlled by the sleep( ) duration instead of the priority.

The init( ) in Counter5 creates an array of 10 Ticker2s; their buttons and fields are placed on the form by the Ticker2 constructor. Counter5 adds buttons to start everything up as well as increment and decrement the maximum priority of the threadgroup. In addition, there are labels that display the maximum and minimum priorities possible for a thread and a TextField to show the thread group’s maximum priority. (The next section will fully describe thread groups.) Finally, the priorities of the parent thread groups are also displayed as labels.

When you press an “up” or “down” button, that Ticker2’s priority is fetched and incremented or decremented accordingly.

When you run this program, you’ll notice several things. First of all, the thread group’s default priority is 5. Even if you decrement the maximum priority below 5 before starting the threads (or before creating the threads, which requires a code change), each thread will have a default priority of 5.

The simple test is to take one counter and decrement its priority to one, and observe that it counts much slower. But now try to increment it again. You can get it back up to the thread group’s priority, but no higher. Now decrement the thread group’s priority a couple of times. The thread priorities are unchanged, but if you try to modify them either up or down you’ll see that they’ll automatically pop to the priority of the thread group. Also, new threads will still be given a default priority, even if that’s higher than the group priority. (Thus the group priority is not a way to prevent new threads from having higher priorities than existing ones.)

Finally, try to increment the group maximum priority. It can’t be done. You can only reduce thread group maximum priorities, not increase them.

Thread groups

All threads belong to a thread group. This can be either the default thread group or a group you explicitly specify when you create the thread. At creation, the thread is bound to a group and cannot change to a different group. Each application has at least one thread that belongs to the system thread group. If you create more threads without specifying a group, they will also belong to the system thread group.

Thread groups must also belong to other thread groups. The thread group that a new one belongs to must be specified in the constructor. If you create a thread group without specifying a thread group for it to belong to, it will be placed under the system thread group. Thus, all thread groups in your application will ultimately have the system thread group as the parent.

The reason for the existence of thread groups is hard to determine from the literature, which tends to be confusing on this subject. It’s often cited as “security reasons.” According to Arnold & Gosling, [67] “Threads within a thread group can modify the other threads in the group, including any farther down the hierarchy. A thread cannot modify threads outside of its own group or contained groups.” It’s hard to know what “modify” is supposed to mean here. The following example shows a thread in a “leaf” subgroup modifying the priorities of all the threads in its tree of thread groups as well as calling a method for all the threads in its tree.

//: c14:TestAccess.java
// How threads can access other threads
// in a parent thread group.

public class TestAccess {
  public static void main(String[] args) {
    ThreadGroup
      x = new ThreadGroup("x"),
      y = new ThreadGroup(x, "y"),
      z = new ThreadGroup(y, "z");
    Thread
      one = new TestThread1(x, "one"),
      two = new TestThread2(z, "two");
  }
}

class TestThread1 extends Thread {
  private int i;
  TestThread1(ThreadGroup g, String name) {
    super(g, name);
  }
  void f() {
    i++; // modify this thread
    System.out.println(getName() + " f()");
  }
}

class TestThread2 extends TestThread1 {
  TestThread2(ThreadGroup g, String name) {
    super(g, name);
    start();
  }
  public void run() {
    ThreadGroup g =
      getThreadGroup().getParent().getParent();
    g.list();
    Thread[] gAll = new Thread[g.activeCount()];
    g.enumerate(gAll);
    for(int i = 0; i < gAll.length; i++) {
      gAll[i].setPriority(Thread.MIN_PRIORITY);
      ((TestThread1)gAll[i]).f();
    }
    g.list();
  }
} ///:~

In main( ), several ThreadGroups are created, leafing off from each other: x has no argument but its name (a String), so it is automatically placed in the “system” thread group, while y is under x and z is under y. Note that initialization happens in textual order so this code is legal.

Two threads are created and placed in different thread groups. TestThread1 doesn’t have a run( ) method but it does have an f( ) that modifies the thread and prints something so you can see it was called. TestThread2 is a subclass of TestThread1 and its run( ) is fairly elaborate. It first gets the thread group of the current thread, then moves up the heritage tree by two levels using getParent( ). (This is contrived since I purposely place the TestThread2 object two levels down in the hierarchy.) At this point, an array of references to Threads is created using the method activeCount( ) to ask how many threads are in this thread group and all the child thread groups. The enumerate( ) method places references to all of these threads in the array gAll, then I simply move through the entire array calling the f( ) method for each thread, as well as modifying the priority. Thus, a thread in a “leaf” thread group modifies threads in parent thread groups.

The debugging method list( ) prints all the information about a thread group to standard output and is helpful when investigating thread group behavior. Here’s the output of the program:

java.lang.ThreadGroup[name=x,maxpri=10]
    Thread[one,5,x]
    java.lang.ThreadGroup[name=y,maxpri=10]
        java.lang.ThreadGroup[name=z,maxpri=10]
            Thread[two,5,z]
one f()
two f()
java.lang.ThreadGroup[name=x,maxpri=10]
    Thread[one,1,x]
    java.lang.ThreadGroup[name=y,maxpri=10]
        java.lang.ThreadGroup[name=z,maxpri=10]
            Thread[two,1,z]

Not only does list( ) print the class name of ThreadGroup or Thread, but it also prints the thread group name and its maximum priority. For threads, the thread name is printed, followed by the thread priority and the group that it belongs to. Note that list( ) indents the threads and thread groups to indicate that they are children of the un-indented thread group.

You can see that f( ) is called by the TestThread2 run( ) method, so it’s obvious that all threads in a group are vulnerable. However, you can access only the threads that branch off from your own system thread group tree, and perhaps this is what is meant by “safety.” You cannot access anyone else’s system thread group tree.

Controlling thread groups

Putting aside the safety issue, one thing thread groups do seem to be useful for is control: you can perform certain operations on an entire thread group with a single command. The following example demonstrates this and the restrictions on priorities within thread groups. The commented numbers in parentheses provide a reference to compare to the output.

//: c14:ThreadGroup1.java
// How thread groups control priorities
// of the threads inside them.

public class ThreadGroup1 {
  public static void main(String[] args) {
    // Get the system thread & print its Info:
    ThreadGroup sys =
      Thread.currentThread().getThreadGroup();
    sys.list(); // (1)
    // Reduce the system thread group priority:
    sys.setMaxPriority(Thread.MAX_PRIORITY - 1);
    // Increase the main thread priority:
    Thread curr = Thread.currentThread();
    curr.setPriority(curr.getPriority() + 1);
    sys.list(); // (2)
    // Attempt to set a new group to the max:
    ThreadGroup g1 = new ThreadGroup("g1");
    g1.setMaxPriority(Thread.MAX_PRIORITY);
    // Attempt to set a new thread to the max:
    Thread t = new Thread(g1, "A");
    t.setPriority(Thread.MAX_PRIORITY);
    g1.list(); // (3)
    // Reduce g1's max priority, then attempt
    // to increase it:
    g1.setMaxPriority(Thread.MAX_PRIORITY - 2);
    g1.setMaxPriority(Thread.MAX_PRIORITY);
    g1.list(); // (4)
    // Attempt to set a new thread to the max:
    t = new Thread(g1, "B");
    t.setPriority(Thread.MAX_PRIORITY);
    g1.list(); // (5)
    // Lower the max priority below the default
    // thread priority:
    g1.setMaxPriority(Thread.MIN_PRIORITY + 2);
    // Look at a new thread's priority before
    // and after changing it:
    t = new Thread(g1, "C");
    g1.list(); // (6)
    t.setPriority(t.getPriority() -1);
    g1.list(); // (7)
    // Make g2 a child Threadgroup of g1 and
    // try to increase its priority:
    ThreadGroup g2 = new ThreadGroup(g1, "g2");
    g2.list(); // (8)
    g2.setMaxPriority(Thread.MAX_PRIORITY);
    g2.list(); // (9)
    // Add a bunch of new threads to g2:
    for (int i = 0; i < 5; i++)
      new Thread(g2, Integer.toString(i));
    // Show information about all threadgroups
    // and threads:
    sys.list(); // (10)
    System.out.println("Starting all threads:");
    Thread[] all = new Thread[sys.activeCount()];
    sys.enumerate(all);
    for(int i = 0; i < all.length; i++)
      if(!all[i].isAlive())
        all[i].start();
    // Suspends & Stops all threads in
    // this group and its subgroups:
    System.out.println("All threads started");
    sys.suspend(); // Deprecated in Java 2
    // Never gets here...
    System.out.println("All threads suspended");
    sys.stop(); // Deprecated in Java 2
    System.out.println("All threads stopped");
  }
} ///:~

The output that follows has been edited to allow it to fit on the page (the java.lang. has been removed) and to add numbers to correspond to the commented numbers in the listing above.

(1) ThreadGroup[name=system,maxpri=10]
      Thread[main,5,system]
(2) ThreadGroup[name=system,maxpri=9]
      Thread[main,6,system]
(3) ThreadGroup[name=g1,maxpri=9]
      Thread[A,9,g1]
(4) ThreadGroup[name=g1,maxpri=8]
      Thread[A,9,g1]
(5) ThreadGroup[name=g1,maxpri=8]
      Thread[A,9,g1]
      Thread[B,8,g1]
(6) ThreadGroup[name=g1,maxpri=3]
      Thread[A,9,g1]
      Thread[B,8,g1]
      Thread[C,6,g1]
(7) ThreadGroup[name=g1,maxpri=3]
      Thread[A,9,g1]
      Thread[B,8,g1]
      Thread[C,3,g1]
(8) ThreadGroup[name=g2,maxpri=3]
(9) ThreadGroup[name=g2,maxpri=3]
(10)ThreadGroup[name=system,maxpri=9]
      Thread[main,6,system]
      ThreadGroup[name=g1,maxpri=3]
        Thread[A,9,g1]
        Thread[B,8,g1]
        Thread[C,3,g1]
        ThreadGroup[name=g2,maxpri=3]
          Thread[0,6,g2]
          Thread[1,6,g2]
          Thread[2,6,g2]
          Thread[3,6,g2]
          Thread[4,6,g2]
Starting all threads:
All threads started

All programs have at least one thread running, and the first action in main( ) is to call the static method of Thread called currentThread( ). From this thread, the thread group is produced and list( ) is called for the result. The output is:

(1) ThreadGroup[name=system,maxpri=10]
      Thread[main,5,system]

You can see that the name of the main thread group is system, and the name of the main thread is main, and it belongs to the system thread group.

The second exercise shows that the system group’s maximum priority can be reduced and the main thread can have its priority increased:

(2) ThreadGroup[name=system,maxpri=9]
      Thread[main,6,system]

The third exercise creates a new thread group, g1, which automatically belongs to the system thread group since it isn’t otherwise specified. A new thread A is placed in g1. After attempting to set this group’s maximum priority to the highest level and A’s priority to the highest level, the result is:

(3) ThreadGroup[name=g1,maxpri=9]
      Thread[A,9,g1]

Thus, it’s not possible to change the thread group’s maximum priority to be higher than its parent thread group.

The fourth exercise reduces g1’s maximum priority by two and then tries to increase it up to Thread.MAX_PRIORITY. The result is:

(4) ThreadGroup[name=g1,maxpri=8]
      Thread[A,9,g1]

You can see that the increase in maximum priority didn’t work. You can only decrease a thread group’s maximum priority, not increase it. Also, notice that thread A’s priority didn’t change, and now it is higher than the thread group’s maximum priority. Changing a thread group’s maximum priority doesn’t affect existing threads.

The fifth exercise attempts to set a new thread to maximum priority:

(5) ThreadGroup[name=g1,maxpri=8]
      Thread[A,9,g1]
      Thread[B,8,g1]

The new thread cannot be changed to anything higher than the maximum thread group priority.

The default thread priority for this program is 6; that’s the priority a new thread will be created at and where it will stay if you don’t manipulate the priority. Exercise six lowers the maximum thread group priority below the default thread priority to see what happens when you create a new thread under this condition:

(6) ThreadGroup[name=g1,maxpri=3]
      Thread[A,9,g1]
      Thread[B,8,g1]
      Thread[C,6,g1]

Even though the maximum priority of the thread group is 3, the new thread is still created using the default priority of 6. Thus, maximum thread group priority does not affect default priority. (In fact, there appears to be no way to set the default priority for new threads.)

After changing the priority, attempting to decrement it by one, the result is:

(7) ThreadGroup[name=g1,maxpri=3]
      Thread[A,9,g1]
      Thread[B,8,g1]
      Thread[C,3,g1]

Only when you attempt to change the priority is the thread group’s maximum priority enforced.

A similar experiment is performed in (8) and (9), in which a new thread group g2 is created as a child of g1 and its maximum priority is changed. You can see that it’s impossible for g2’s maximum to go higher than g1’s:

(8) ThreadGroup[name=g2,maxpri=3]
(9) ThreadGroup[name=g2,maxpri=3]

Also notice that g2 is automatically set to the thread group maximum priority of g1 as g2 is created.

After all of these experiments, the entire system of thread groups and threads is printed:

(10)ThreadGroup[name=system,maxpri=9]
      Thread[main,6,system]
      ThreadGroup[name=g1,maxpri=3]
        Thread[A,9,g1]
        Thread[B,8,g1]
        Thread[C,3,g1]
        ThreadGroup[name=g2,maxpri=3]
          Thread[0,6,g2]
          Thread[1,6,g2]
          Thread[2,6,g2]
          Thread[3,6,g2]
          Thread[4,6,g2]

So because of the rules of thread groups, a child group must always have a maximum priority that’s less than or equal to its parent’s maximum priority.

The last part of this program demonstrates methods for an entire group of threads. First the program moves through the entire tree of threads and starts each one that hasn’t been started. For drama, the system group is then suspended and finally stopped. (Although it’s interesting to see that suspend( ) and stop( ) work on entire thread groups, you should keep in mind that these methods are deprecated in Java 2.) But when you suspend the system group you also suspend the main thread and the whole program shuts down, so it never gets to the point where the threads are stopped. Actually, if you do stop the main thread it throws a ThreadDeath exception, so this is not a typical thing to do. Since ThreadGroup is inherited from Object, which contains the wait( ) method, you can also choose to suspend the program for any number of seconds by calling wait(seconds * 1000). This must acquire the lock inside a synchronized block, of course.

The ThreadGroup class also has suspend( ) and resume( ) methods so you can stop and start an entire thread group and all of its threads and subgroups with a single command. (Again, suspend( ) and resume( ) are deprecated in Java 2.)

Thread groups can seem a bit mysterious at first, but keep in mind that you probably won’t be using them directly very often.

Runnable revisited

Earlier in this chapter, I suggested that you think carefully before making an applet or main Frame as an implementation of Runnable. If you take that approach, you can make only one of those threads in your program. This limits your flexibility if you decide that you want to have more than one thread of that type.

Of course, if you must inherit from a class and you want to add threading behavior to the class, Runnable is the correct solution. The final example in this chapter exploits this by making a Runnable Canvas class that paints different colors on itself. This application is set up to take values from the command line to determine how big the grid of colors is and how long to sleep( ) between color changes. By playing with these values you’ll discover some interesting and possibly inexplicable features of threads:

//: c14:ColorBoxes.java
// Using the Runnable interface.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;

class CBox extends JPanel implements Runnable {
  private Thread t;
  private int pause;
  private static final Color[] colors = {
    Color.black, Color.blue, Color.cyan,
    Color.darkGray, Color.gray, Color.green,
    Color.lightGray, Color.magenta,
    Color.orange, Color.pink, Color.red,
    Color.white, Color.yellow
  };
  private Color cColor = newColor();
  private static final Color newColor() {
    return colors[
      (int)(Math.random() * colors.length)
    ];
  }
  public void paintComponent(Graphics  g) {
    super.paintComponent(g);
    g.setColor(cColor);
    Dimension s = getSize();
    g.fillRect(0, 0, s.width, s.height);
  }
  public CBox(int pause) {
    this.pause = pause;
    t = new Thread(this);
    t.start();
  }
  public void run() {
    while(true) {
      cColor = newColor();
      repaint();
      try {
        t.sleep(pause);
      } catch(InterruptedException e) {}
    }
  }
}

public class ColorBoxes extends JFrame {
  public ColorBoxes(int pause, int grid) {
    setTitle("ColorBoxes");
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(grid, grid));
    for (int i = 0; i < grid * grid; i++)
      cp.add(new CBox(pause));
  }
  public static void main(String[] args) {
    int grid = 8;
    int pause = 50;
    if(args.length > 0)
      grid = Integer.parseInt(args[0]);
    if(args.length > 1)
      pause = Integer.parseInt(args[1]);
    Console.run(new ColorBoxes(pause, grid),
      500, 400);
  }
} ///:~

ColorBoxes is a typical application with a constructor that sets up the GUI. This constructor takes an argument of int grid to set up the GridLayout so that it has grid cells in each dimension. Then it adds the appropriate number of CBox objects to fill the grid, passing the pause value to each one. In main( ) you can see how pause and grid have default values that can be changed if you pass in command-line arguments.

CBox is where all the work takes place. This is inherited from Canvas and it implements the Runnable interface so each Canvas can also be a Thread. Remember that when you implement Runnable, you don’t make a Thread object, just a class that has a run( ) method. Thus, you must explicitly create a Thread object and hand the Runnable object to the constructor, then call start( ) (this happens in the constructor). In CBox this thread is called t.

Notice the array colors, which is an enumeration of all the colors in class Color. This is used in newColor( ) to produce a randomly-selected color. The current cell color is cColor.

paint( ) is quite simple—it just sets the color to cColor and fills the entire canvas with that color.

In run( ), you see the infinite loop that sets the cColor to a new random color and then calls repaint( ) to show it. Then the thread goes to sleep( ) for the amount of time specified on the command line.

Precisely because this design is flexible and threading is tied to each Canvas element, you can experiment by making as many threads as you want. (In reality, there is a restriction imposed by the number of threads your JVM can comfortably handle.)

This program also makes an interesting benchmark, since it can show dramatic speed differences between one JVM implementation and another.

Too many threads

At some point, you’ll find that ColorBoxes bogs down. On my machine, this occurred somewhere after a 10 x 10 grid. Why does this happen? You’re naturally suspicious that Swing might have something to do with it, so here’s an example that tests that premise by making fewer threads. The code is reorganized so that an ArrayList implements Runnable and that ArrayList holds a number of color blocks and randomly chooses ones to update. Then a number of these ArrayList objects are created, depending roughly on the grid dimension you choose. As a result, you have far fewer threads than color blocks, so if there’s a speedup we’ll know it was because there were too many threads in the previous example:

//: c14:ColorBoxes2.java
// Balancing thread use.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

class CBox2 extends JPanel {
  private static final Color[] colors = {
    Color.black, Color.blue, Color.cyan,
    Color.darkGray, Color.gray, Color.green,
    Color.lightGray, Color.magenta,
    Color.orange, Color.pink, Color.red,
    Color.white, Color.yellow
  };
  private Color cColor = newColor();
  private static final Color newColor() {
    return colors[
      (int)(Math.random() * colors.length)
    ];
  }
  void nextColor() {
    cColor = newColor();
    repaint();
  }
  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(cColor);
    Dimension s = getSize();
    g.fillRect(0, 0, s.width, s.height);
  }
}

class CBoxVector
  extends ArrayList implements Runnable {
  private Thread t;
  private int pause;
  public CBoxVector(int pause) {
    this.pause = pause;
    t = new Thread(this);
  }
  public void go() { t.start(); }
  public void run() {
    while(true) {
      int i = (int)(Math.random() * size());
      ((CBox2)get(i)).nextColor();
      try {
        t.sleep(pause);
      } catch(InterruptedException e) {}
    }
  }
  public Object last() { return get(size() - 1);}
}

public class ColorBoxes2 extends JFrame {
  private CBoxVector[] v;
  public ColorBoxes2(int pause, int grid) {
    setTitle("ColorBoxes2");
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(grid, grid));
    v = new CBoxVector[grid];
    for(int i = 0; i < grid; i++)
      v[i] = new CBoxVector(pause);
    for (int i = 0; i < grid * grid; i++) {
      v[i % grid].add(new CBox2());
      cp.add((CBox2)v[i % grid].last());
    }
    for(int i = 0; i < grid; i++)
      v[i].go();
  }
  public static void main(String[] args) {
    int grid = 8;
    // Shorter default pause than ColorBoxes:
    int pause = 5;
    if(args.length > 0)
      grid = Integer.parseInt(args[0]);
    if(args.length > 1)
      pause = Integer.parseInt(args[1]);
    Console.run(new ColorBoxes2(pause, grid),
      500, 400);
  }
} ///:~

In ColorBoxes2 an array of CBoxVector is created and initialized to hold grid CBoxVectors, each of which knows how long to sleep. An equal number of CBox2 objects is then added to each CBoxVector, and each vector is told to go( ), which starts its thread.

CBox2 is similar to CBox: it paints itself with a randomly-chosen color. But that’s all a CBox2 does. All of the threading has been moved into CBoxVector.

The CBoxVector could also have inherited Thread and had a member object of type ArrayList. That design has the advantage that the add( ) and get( ) methods could then be given specific argument and return value types instead of generic Objects. (Their names could also be changed to something shorter.) However, the design used here seemed at first glance to require less code. In addition, it automatically retains all the other behaviors of an ArrayList. With all the casting and parentheses necessary for get( ), this might not be the case as your body of code grows.

As before, when you implement Runnable you don’t get all of the equipment that comes with Thread, so you have to create a new Thread and hand yourself to its constructor in order to have something to start( ), as you can see in the CBoxVector constructor and in go( ). The run( ) method simply chooses a random element number within the vector and calls nextColor( ) for that element to cause it to choose a new randomly-selected color.

Upon running this program, you see that it does indeed run faster and respond more quickly (for instance, when you interrupt it, it stops more quickly), and it doesn’t seem to bog down as much at higher grid sizes. Thus, a new factor is added into the threading equation: you must watch to see that you don’t have “too many threads” (whatever that turns out to mean for your particular program and platform). If you do, you must try to use techniques like the one above to “balance” the number of threads in your program. If you see performance problems in a multithreaded program you now have a number of issues to examine:

  1. Do you have enough calls to sleep( ), yield( ), and/or wait( )?
  2. Are calls to sleep( ) long enough?
  3. Are you running too many threads?
  4. Have you tried different platforms and JVMs?

Issues like this are one reason that multithreaded programming is often considered an art.

Summary

It is vital to learn when to use multithreading and when to avoid it. The main reason to use it is to manage a number of tasks whose intermingling will make more efficient use of the computer or be more convenient for the user. The classic example of resource balancing is using the CPU during I/O waits. The classic example of user convenience is monitoring a “stop” button during long downloads.

The main drawbacks to multithreading are:

  1. Slowdown while waiting for shared resources
  2. Additional CPU overhead required to manage threads
  3. Unrewarded complexity, such as the silly idea of having a separate thread to update each element of an array
  4. Pathologies including starving, racing, and deadlock

An additional advantage to threads is that they substitute “light” execution context switches (of the order of 100 instructions) for “heavy” process context switches (of the order of 1000s of instructions). Since all threads in a given process share the same memory space, a light context switch changes only program execution and local variables. On the other hand, a process change, the heavy context switch, must exchange the full memory space.

Threading is like stepping into an entirely new world and learning a whole new programming language, or at least a new set of language concepts. With the appearance of thread support in most microcomputer operating systems, extensions for threads have also been appearing in programming languages or libraries. In all cases, thread programming (1) seems mysterious and requires a shift in the way you think about programming and (2) looks similar to thread support in other languages, so when you understand threads, you understand a common tongue. And although support for threads can make Java seem like a more complicated language, don’t blame Java. Threads are tricky.

One of the biggest difficulties with threads occurs because more than one thread might be sharing a resource, such as the memory in an object, and you must make sure that multiple threads don’t try to read and change that resource at the same time. This requires judicious use of the synchronized keyword, which is a helpful tool but must be understood thoroughly because it can quietly introduce deadlock situations.

In addition, there’s a certain art to the application of threads. Java is designed to allow you to create as many objects as you need to solve your problem—at least in theory. (Creating millions of objects for an engineering finite-element analysis, for example, might not be practical in Java.) However, it seems that there is an upper bound to the number of threads you’ll want to create because at some point a large number of threads seems to become unwieldy. This critical point is not in the many thousands as it might be with objects, but rather in the neighborhood of less than 100. As you often create only a handful of threads to solve a problem, this is typically not much of a limit, yet in a more general design it becomes a constraint.

A significant non-intuitive issue in threading is that, because of thread scheduling, you can typically make your applications run faster by inserting calls to sleep( ) inside run( )’s main loop. This definitely makes it feel like an art, in particular when the longer delays seem to speed up performance. Of course, the reason this happens is that shorter delays can cause the end-of-sleep( ) scheduler interrupt to happen before the running thread is ready to go to sleep, forcing the scheduler to stop it and restart it later so it can finish what it was doing and then go to sleep. It takes extra thought to realize how messy things can get.

One thing you might notice missing in this chapter is an animation example, which is one of the most popular things to do with applets. However, a complete solution (with sound) to this problem comes with the Java JDK (available at java.sun.com) in the demo section. In addition, we can expect better animation support to become part of future versions of Java, while completely different non-Java, non-programming solutions to animation for the Web are appearing that will probably be superior to traditional approaches. For explanations about how Java animation works, see Core Java by Cornell & Horstmann, Prentice-Hall, 1997. For more advanced discussions of threading, see Concurrent Programming in Java by Doug Lea, Addison-Wesley, 1997, or Java Threads by Oaks & Wong, O’Reilly, 1997.

Exercises
  1. Inherit a class from Thread and override the run( ) method. Inside run( ), print a message, then call sleep( ). Repeat this three times, then return from run( ). Put a start-up message in the constructor and override finalize( ) to print a shut-down message. Make a separate thread class that calls System.gc( ) and System.runFinalization( ) inside run( ), printing a message as it does so. Make several thread objects of both types and run them to see what happens.
  2. Modify Counter2.java so that the thread is an inner class and doesn’t need to explicitly store a reference to a Counter2.
  3. Modify Sharing2.java to add a synchronized block inside the run( ) method of TwoCounter instead of synchronizing the entire run( ) method.
  4. Create two Thread subclasses, one with a run( ) that starts up, captures the reference of the second Thread object and then calls wait( ). The other class’ run( ) should call notifyAll( ) for the first thread after some number of seconds have passed, so the first thread can print a message.
  5. In Counter5.java inside Ticker2, remove the yield( ) and explain the results. Replace the yield( ) with a sleep( ) and explain the results.
  6. In ThreadGroup1.java, replace the call to sys.suspend( ) with a call to wait( ) for the thread group, causing it to wait for two seconds. For this to work correctly you must acquire the lock for sys inside a synchronized block.
  7. Change Daemons.java so that main( ) has a sleep( ) instead of a readLine( ). Experiment with different sleep times to see what happens.
  8. (Intermediate) In Chapter 8, locate the GreenhouseControls.java example, which consists of three files. In Event.java, the class Event is based on watching the time. Change Event so that it is a Thread, and change the rest of the design so that it works with this new Thread-based Event.

[67] The Java Programming Language, by Ken Arnold and James Gosling, Addison-Wesley 1996 pp 179.


15: Distributed Computing

Historically, programming across multiple machines has been error-prone, difficult, and complex.

The programmer had to know many details about the network and sometimes even the hardware. You usually needed to understand the various “layers” of the networking protocol, and there were a lot of different functions in each different networking library concerned with connecting, packing, and unpacking blocks of information; shipping those blocks back and forth; and handshaking. It was a daunting task.

However, the concept of networking is not so difficult. You want to get some information from that machine over there and move it to this machine here, or vice versa. It’s quite similar to reading and writing files, except that the file exists on a remote machine and the remote machine can decide exactly what it wants to do about the information you’re requesting or sending.

One of Java’s great strengths is painless networking. As much as possible, the underlying details of networking have been abstracted away and taken care of within the JVM and local machine installation of Java. The programming model you use is that of a file; in fact, you actually wrap the network connection (a “socket”) with stream objects, so you end up using the same method calls as you do with all other streams. In addition, Java’s built-in multithreading is exceptionally handy when dealing with another networking issue: handling multiple connections at once.

This chapter introduces Java’s networking support using easy-to-understand examples.

Network programming

Identifying a machine

Of course, in order to tell one machine from another and to make sure that you are connected with the machine you want, there must be some way of uniquely identifying machines on a network. Early networks were satisfied to provide unique names for machines within the local network. However, Java works within the Internet, which requires a way to uniquely identify a machine from all the others in the world. This is accomplished with the IP (Internet Protocol) address that can exist in two forms:

  1. The familiar DNS (Domain Name System) form. My domain name is bruceeckel.com, so suppose I have a computer called Opus in my domain. Its domain name would be Opus.bruceeckel.com. This is exactly the kind of name that you use when you send email to people, and is often incorporated into a World-Wide-Web address.
  2. Alternatively, you can use the “dotted quad” form, which is four numbers separated by dots, such as 123.255.28.120.

In both cases, the IP address is represented internally as a 32-bit number [68] (so each of the quad numbers cannot exceed 255), and you can get a special Java object to represent this number from either of the forms above by using the static InetAddress.getByName( ) method that’s in java.net. The result is an object of type InetAddress that you can use to build a “socket” as you will see later.

As a simple example of using InetAddress.getByName( ), consider what happens if you have a dial-up Internet service provider (ISP). Each time you dial up, you are assigned a temporary IP address. But while you’re connected, your IP address has the same validity as any other IP address on the Internet. If someone connects to your machine using your IP address then they can connect to a Web server or FTP server that you have running on your machine. Of course, they need to know your IP address, and since it’s assigned each time you dial up, how can you find out what it is?

The following program uses InetAddress.getByName( ) to produce your IP address. To use it, you must know the name of your computer. It has been tested only on Windows 95, but there you can go to “Settings,” “Control Panel,” “Network,” and then select the “Identification” tab. “Computer name” is the name to put on the command line.

//: c15:WhoAmI.java
// Finds out your network address when
// you're connected to the Internet.
import java.net.*;

public class WhoAmI {
  public static void main(String[] args)
      throws Exception {
    if(args.length != 1) {
      System.err.println(
        "Usage: WhoAmI MachineName");
      System.exit(1);
    }
    InetAddress a =
      InetAddress.getByName(args[0]);
    System.out.println(a);
  }
} ///:~

In this case, the machine is called “peppy.” So, once I’ve connected to my ISP I run the program:

java WhoAmI peppy

I get back a message like this (of course, the address is different each time):

peppy/199.190.87.75

If I tell my friend this address, he can log onto my personal Web server by going to the URL http://199.190.87.75 (only as long as I continue to stay connected during that session). This can sometimes be a handy way to distribute information to someone else or to test out a Web site configuration before posting it to a “real” server.

Servers and clients

The whole point of a network is to allow two machines to connect and talk to each other. Once the two machines have found each other they can have a nice, two-way conversation. But how do they find each other? It’s like getting lost in an amusement park: one machine has to stay in one place and listen while the other machine says, “Hey, where are you?”

The machine that “stays in one place” is called the server, and the one that seeks is called the client. This distinction is important only while the client is trying to connect to the server. Once they’ve connected, it becomes a two-way communication process and it doesn’t matter anymore that one happened to take the role of server and the other happened to take the role of the client.

So the job of the server is to listen for a connection, and that’s performed by the special server object that you create. The job of the client is to try to make a connection to a server, and this is performed by the special client object you create. Once the connection is made, you’ll see that at both server and client ends, the connection is just magically turned into an IO stream object, and from then on you can treat the connection as if you were reading from and writing to a file. Thus, after the connection is made you will just use the familiar IO commands from Chapter 11. This is one of the nice features of Java networking.

Testing programs without a network

For many reasons, you might not have a client machine, a server machine, and a network available to test your programs. You might be performing exercises in a classroom situation, or you could be writing programs that aren’t yet stable enough to put onto the network. The creators of the Internet Protocol were aware of this issue, and they created a special address called localhost to be the “local loopback” IP address for testing without a network. The generic way to produce this address in Java is:

InetAddress addr = InetAddress.getByName(null);

If you hand getByName( ) a null, it defaults to using the localhost. The InetAddress is what you use to refer to the particular machine, and you must produce this before you can go any further. You can’t manipulate the contents of an InetAddress (but you can print them out, as you’ll see in the next example). The only way you can create an InetAddress is through one of that class’s static member methods getByName( ) (which is what you’ll usually use), getAllByName( ), or getLocalHost( ).

You can also produce the local loopback address by handing it the string localhost:

InetAddress.getByName("localhost");

or by using its dotted quad form to name the reserved IP number for the loopback:

InetAddress.getByName("127.0.0.1");

All three forms produce the same result.

Port: a unique place
within the machine

An IP address isn’t enough to identify a unique server, since many servers can exist on one machine. Each IP machine also contains ports, and when you’re setting up a client or a server you must choose a port where both client and server agree to connect; if you’re meeting someone, the IP address is the neighborhood and the port is the bar.

The port is not a physical location in a machine, but a software abstraction (mainly for bookkeeping purposes). The client program knows how to connect to the machine via its IP address, but how does it connect to a desired service (potentially one of many on that machine)? That’s where the port numbers come in as second level of addressing. The idea is that if you ask for a particular port, you’re requesting the service that’s associated with the port number. The time of day is a simple example of a service. Typically, each service is associated with a unique port number on a given server machine. It’s up to the client to know ahead of time which port number the desired service is running on.

The system services reserve the use of ports 1 through 1024, so you shouldn’t use those or any other port that you know to be in use. The first choice for examples in this book will be port 8080 (in memory of the venerable old 8-bit Intel 8080 chip in my first computer, a CP/M machine).

Sockets

The socket is the software abstraction used to represent the “terminals” of a connection between two machines. For a given connection, there’s a socket on each machine, and you can imagine a hypothetical “cable” running between the two machines with each end of the “cable” plugged into a socket. Of course, the physical hardware and cabling between machines is completely unknown. The whole point of the abstraction is that we don’t have to know more than is necessary.

In Java, you create a socket to make the connection to the other machine, then you get an InputStream and OutputStream (or, with the appropriate converters, Reader and Writer) from the socket in order to be able to treat the connection as an IO stream object. There are two stream-based socket classes: a ServerSocket that a server uses to “listen” for incoming connections and a Socket that a client uses in order to initiate a connection. Once a client makes a socket connection, the ServerSocket returns (via the accept( ) method) a corresponding server side Socket through which direct communications will take place. From then on, you have a true Socket to Socket connection and you treat both ends the same way because they are the same. At this point, you use the methods getInputStream( ) and getOutputStream( ) to produce the corresponding InputStream and OutputStream objects from each Socket. These must be wrapped inside buffers and formatting classes just like any other stream object described in Chapter 11.

The use of the term ServerSocket would seem to be another example of a confusing name scheme in the Java libraries. You might think ServerSocket would be better named “ServerConnector” or something without the word “Socket” in it. You might also think that ServerSocket and Socket should both be inherited from some common base class. Indeed, the two classes do have several methods in common but not enough to give them a common base class. Instead, ServerSocket’s job is to wait until some other machine connects to it, then to return an actual Socket. This is why ServerSocket seems to be a bit misnamed, since its job isn’t really to be a socket but instead to make a Socket object when someone else connects to it.

However, the ServerSocket does create a physical “server” or listening socket on the host machine. This socket listens for incoming connections and then returns an “established” socket (with the local and remote endpoints defined) via the accept( ) method. The confusing part is that both of these sockets (listening and established) are associated with the same server socket. The listening socket can accept only new connection requests and not data packets. So while ServerSocket doesn’t make much sense programmatically, it does “physically.”

When you create a ServerSocket, you give it only a port number. You don’t have to give it an IP address because it’s already on the machine it represents. When you create a Socket, however, you must give both the IP address and the port number where you’re trying to connect. (On the other hand, the Socket that comes back from ServerSocket.accept( ) already contains all this information.)

A simple server and client

This example makes the simplest use of servers and clients using sockets. All the server does is wait for a connection, then uses the Socket produced by that connection to create an InputStream and OutputStream. After that, everything it reads from the InputStream it echoes to the OutputStream until it receives the line END, at which time it closes the connection.

The client makes the connection to the server, then creates an OutputStream. Lines of text are sent through the OutputStream. The client also creates an InputStream to hear what the server is saying (which, in this case, is just the words echoed back).

Both the server and client use the same port number and the client uses the local loopback address to connect to the server on the same machine so you don’t have to test it over a network. (For some configurations, you might need to be connected to a network for the programs to work, even if you aren’t communicating over that network.)

Here is the server:

//: c15:JabberServer.java
// Very simple server that just
// echoes whatever the client sends.
import java.io.*;
import java.net.*;

public class JabberServer {
  // Choose a port outside of the range 1-1024:
  public static final int PORT = 8080;
  public static void main(String[] args)
      throws IOException {
    ServerSocket s = new ServerSocket(PORT);
    System.out.println("Started: " + s);
    try {
      // Blocks until a connection occurs:
      Socket socket = s.accept();
      try {
        System.out.println(
          "Connection accepted: "+ socket);
        BufferedReader in =
          new BufferedReader(
            new InputStreamReader(
              socket.getInputStream()));
        // Output is automatically flushed
        // by PrintWriter:
        PrintWriter out =
          new PrintWriter(
            new BufferedWriter(
              new OutputStreamWriter(
                socket.getOutputStream())),true);
        while (true) {
          String str = in.readLine();
          if (str.equals("END")) break;
          System.out.println("Echoing: " + str);
          out.println(str);
        }
      // Always close the two sockets...
      } finally {
        System.out.println("closing...");
        socket.close();
      }
    } finally {
      s.close();
    }
  }
} ///:~

You can see that the ServerSocket just needs a port number, not an IP address (since it’s running on this machine!). When you call accept( ), the method blocks until some client tries to connect to it. That is, it’s there waiting for a connection but other processes can run (see Chapter 14). When a connection is made, accept( ) returns with a Socket object representing that connection.

The responsibility for cleaning up the sockets is crafted carefully here. If the ServerSocket constructor fails, the program just quits (notice we must assume that the constructor for ServerSocket doesn’t leave any open network sockets lying around if it fails). For this case, main( ) throws IOException so a try block is not necessary. If the ServerSocket constructor is successful then all other method calls must be guarded in a try-finally block to ensure that, no matter how the block is left, the ServerSocket is properly closed.

The same logic is used for the Socket returned by accept( ). If accept( ) fails, then we must assume that the Socket doesn’t exist or hold any resources, so it doesn’t need to be cleaned up. If it’s successful, however, the following statements must be in a try-finally block so that if they fail the Socket will still be cleaned up. Care is required here because sockets use important non-memory resources, so you must be diligent in order to clean them up (since there is no destructor in Java to do it for you).

Both the ServerSocket and the Socket produced by accept( ) are printed to System.out. This means that their toString( ) methods are automatically called. These produce:

ServerSocket[addr=0.0.0.0,PORT=0,localport=8080]
Socket[addr=127.0.0.1,PORT=1077,localport=8080]

Shortly, you’ll see how these fit together with what the client is doing.

The next part of the program looks just like opening files for reading and writing except that the InputStream and OutputStream are created from the Socket object. Both the InputStream and OutputStream objects are converted to Java 1.1 Reader and Writer objects using the “converter” classes InputStreamReader and OutputStreamWriter, respectively. You could also have used the Java 1.0 InputStream and OutputStream classes directly, but with output there’s a distinct advantage to using the Writer approach. This appears with PrintWriter, which has an overloaded constructor that takes a second argument, a boolean flag that indicates whether to automatically flush the output at the end of each println( ) (but not print( )) statement. Every time you write to out, its buffer must be flushed so the information goes out over the network. Flushing is important for this particular example because the client and server each wait for a line from the other party before proceeding. If flushing doesn’t occur, the information will not be put onto the network until the buffer is full, which causes lots of problems in this example.

When writing network programs you need to be careful about using automatic flushing. Every time you flush the buffer a packet must be created and sent. In this case, that’s exactly what we want, since if the packet containing the line isn’t sent then the handshaking back and forth between server and client will stop. Put another way, the end of a line is the end of a message. But in many cases messages aren’t delimited by lines so it’s much more efficient to not use auto flushing and instead let the built-in buffering decide when to build and send a packet. This way, larger packets can be sent and the process will be faster.

Note that, like virtually all streams you open, these are buffered. There’s an exercise at the end of the chapter to show you what happens if you don’t buffer the streams (things get slow).

The infinite while loop reads lines from the BufferedReader in and writes information to System.out and to the PrintWriter out. Note that these could be any streams, they just happen to be connected to the network.

When the client sends the line consisting of “END” the program breaks out of the loop and closes the Socket.

Here’s the client:

//: c15:JabberClient.java
// Very simple client that just sends
// lines to the server and reads lines
// that the server sends.
import java.net.*;
import java.io.*;

public class JabberClient {
  public static void main(String[] args)
      throws IOException {
    // Passing null to getByName() produces the
    // special "Local Loopback" IP address, for
    // testing on one machine w/o a network:
    InetAddress addr =
      InetAddress.getByName(null);
    // Alternatively, you can use
    // the address or name:
    // InetAddress addr =
    //    InetAddress.getByName("127.0.0.1");
    // InetAddress addr =
    //    InetAddress.getByName("localhost");
    System.out.println("addr = " + addr);
    Socket socket =
      new Socket(addr, JabberServer.PORT);
    // Guard everything in a try-finally to make
    // sure that the socket is closed:
    try {
      System.out.println("socket = " + socket);
      BufferedReader in =
        new BufferedReader(
          new InputStreamReader(
            socket.getInputStream()));
      // Output is automatically flushed
      // by PrintWriter:
      PrintWriter out =
        new PrintWriter(
          new BufferedWriter(
            new OutputStreamWriter(
              socket.getOutputStream())),true);
      for(int i = 0; i < 10; i ++) {
        out.println("howdy " + i);
        String str = in.readLine();
        System.out.println(str);
      }
      out.println("END");
    } finally {
      System.out.println("closing...");
      socket.close();
    }
  }
} ///:~

In main( ) you can see all three ways to produce the InetAddress of the local loopback IP address: using null, localhost, or the explicit reserved address 127.0.0.1. Of course, if you want to connect to a machine across a network you substitute that machine’s IP address. When the InetAddress addr is printed (via the automatic call to its toString( ) method) the result is:

localhost/127.0.0.1

By handing getByName( ) a null, it defaulted to finding the localhost, and that produced the special address 127.0.0.1.

Note that the Socket called socket is created with both the InetAddress and the port number. To understand what it means when you print one of these Socket objects, remember that an Internet connection is determined uniquely by these four pieces of data: clientHost, clientPortNumber, serverHost, and serverPortNumber. When the server comes up, it takes up its assigned port (8080) on the localhost (127.0.0.1). When the client comes up, it is allocated to the next available port on its machine, 1077 in this case, which also happens to be on the same machine (127.0.0.1) as the server. Now, in order for data to move between the client and server, each side has to know where to send it. Therefore, during the process of connecting to the “known” server, the client sends a “return address” so the server knows where to send its data. This is what you see in the example output for the server side:

Socket[addr=127.0.0.1,port=1077,localport=8080]

This means that the server just accepted a connection from 127.0.0.1 on port 1077 while listening on its local port (8080). On the client side:

Socket[addr=localhost/127.0.0.1,PORT=8080,localport=1077]

which means that the client made a connection to 127.0.0.1 on port 8080 using the local port 1077.

You’ll notice that every time you start up the client anew, the local port number is incremented. It starts at 1025 (one past the reserved block of ports) and keeps going up until you reboot the machine, at which point it starts at 1025 again. (On UNIX machines, once the upper limit of the socket range is reached, the numbers will wrap around to the lowest available number again.)

Once the Socket object has been created, the process of turning it into a BufferedReader and PrintWriter is the same as in the server (again, in both cases you start with a Socket). Here, the client initiates the conversation by sending the string “howdy” followed by a number. Note that the buffer must again be flushed (which happens automatically via the second argument to the PrintWriter constructor). If the buffer isn’t flushed, the whole conversation will hang because the initial “howdy” will never get sent (the buffer isn’t full enough to cause the send to happen automatically). Each line that is sent back from the server is written to System.out to verify that everything is working correctly. To terminate the conversation, the agreed-upon “END” is sent. If the client simply hangs up, then the server throws an exception.

You can see that the same care is taken here to ensure that the network resources represented by the Socket are properly cleaned up, using a try-finally block.

Sockets produce a “dedicated” connection that persists until it is explicitly disconnected. (The dedicated connection can still be disconnected un-explicitly if one side, or an intermediary link, of the connection crashes.) This means the two parties are locked in communication and the connection is constantly open. This seems like a logical approach to networking, but it puts an extra load on the network. Later in the chapter you’ll see a different approach to networking, in which the connections are only temporary.

Serving multiple clients

The JabberServer works, but it can handle only one client at a time. In a typical server, you’ll want to be able to deal with many clients at once. The answer is multithreading, and in languages that don’t directly support multithreading this means all sorts of complications. In Chapter 14 you saw that multithreading in Java is about as simple as possible, considering that multithreading is a rather complex topic. Because threading in Java is reasonably straightforward, making a server that handles multiple clients is relatively easy.

The basic scheme is to make a single ServerSocket in the server and call accept( ) to wait for a new connection. When accept( ) returns, you take the resulting Socket and use it to create a new thread whose job is to serve that particular client. Then you call accept( ) again to wait for a new client.

In the following server code, you can see that it looks similar to the JabberServer.java example except that all of the operations to serve a particular client have been moved inside a separate thread class:

//: c15:MultiJabberServer.java
// A server that uses multithreading
// to handle any number of clients.
import java.io.*;
import java.net.*;

class ServeOneJabber extends Thread {
  private Socket socket;
  private BufferedReader in;
  private PrintWriter out;
  public ServeOneJabber(Socket s)
      throws IOException {
    socket = s;
    in =
      new BufferedReader(
        new InputStreamReader(
          socket.getInputStream()));
    // Enable auto-flush:
    out =
      new PrintWriter(
        new BufferedWriter(
          new OutputStreamWriter(
            socket.getOutputStream())), true);
    // If any of the above calls throw an
    // exception, the caller is responsible for
    // closing the socket. Otherwise the thread
    // will close it.
    start(); // Calls run()
  }
  public void run() {
    try {
      while (true) {
        String str = in.readLine();
        if (str.equals("END")) break;
        System.out.println("Echoing: " + str);
        out.println(str);
      }
      System.out.println("closing...");
    } catch (IOException e) {
    } finally {
      try {
        socket.close();
      } catch(IOException e) {}
    }
  }
}

public class MultiJabberServer {
  static final int PORT = 8080;
  public static void main(String[] args)
      throws IOException {
    ServerSocket s = new ServerSocket(PORT);
    System.out.println("Server Started");
    try {
      while(true) {
        // Blocks until a connection occurs:
        Socket socket = s.accept();
        try {
          new ServeOneJabber(socket);
        } catch(IOException e) {
          // If it fails, close the socket,
          // otherwise the thread will close it:
          socket.close();
        }
      }
    } finally {
      s.close();
    }
  }
} ///:~

The ServeOneJabber thread takes the Socket object that’s produced by accept( ) in main( ) every time a new client makes a connection. Then, as before, it creates a BufferedReader and auto-flushed PrintWriter object using the Socket. Finally, it calls the special Thread method start( ), which performs thread initialization and then calls run( ). This performs the same kind of action as in the previous example: reading something from the socket and then echoing it back until it reads the special “END” signal.

The responsibility for cleaning up the socket must again be carefully designed. In this case, the socket is created outside of the ServeOneJabber so the responsibility can be shared. If the ServeOneJabber constructor fails, it will just throw the exception to the caller, who will then clean up the thread. But if the constructor succeeds, then the ServeOneJabber object takes over responsibility for cleaning up the thread, in its run( ).

Notice the simplicity of the MultiJabberServer. As before, a ServerSocket is created and accept( ) is called to allow a new connection. But this time, the return value of accept( ) (a Socket) is passed to the constructor for ServeOneJabber, which creates a new thread to handle that connection. When the connection is terminated, the thread simply goes away.

If the creation of the ServerSocket fails, the exception is again thrown through main( ). But if it succeeds, the outer try-finally guarantees its cleanup. The inner try-catch guards only against the failure of the ServeOneJabber constructor; if the constructor succeeds, then the ServeOneJabber thread will close the associated socket.

To test that the server really does handle multiple clients, the following program creates many clients (using threads) that connect to the same server. Each thread has a limited lifetime, and when it goes away, that leaves space for the creation of a new thread. The maximum number of threads allowed is determined by the final int maxthreads. You’ll notice that this value is rather critical, since if you make it too high the threads seem to run out of resources and the program mysteriously fails.

//: c15:MultiJabberClient.java
// Client that tests the MultiJabberServer
// by starting up multiple clients.
import java.net.*;
import java.io.*;

class JabberClientThread extends Thread {
  private Socket socket;
  private BufferedReader in;
  private PrintWriter out;
  private static int counter = 0;
  private int id = counter++;
  private static int threadcount = 0;
  public static int threadCount() {
    return threadcount;
  }
  public JabberClientThread(InetAddress addr) {
    System.out.println("Making client " + id);
    threadcount++;
    try {
      socket =
        new Socket(addr, MultiJabberServer.PORT);
    } catch(IOException e) {
      // If the creation of the socket fails,
      // nothing needs to be cleaned up.
    }
    try {
      in =
        new BufferedReader(
          new InputStreamReader(
            socket.getInputStream()));
      // Enable auto-flush:
      out =
        new PrintWriter(
          new BufferedWriter(
            new OutputStreamWriter(
              socket.getOutputStream())), true);
      start();
    } catch(IOException e) {
      // The socket should be closed on any
      // failures other than the socket
      // constructor:
      try {
        socket.close();
      } catch(IOException e2) {}
    }
    // Otherwise the socket will be closed by
    // the run() method of the thread.
  }
  public void run() {
    try {
      for(int i = 0; i < 25; i++) {
        out.println("Client " + id + ": " + i);
        String str = in.readLine();
        System.out.println(str);
      }
      out.println("END");
    } catch(IOException e) {
    } finally {
      // Always close it:
      try {
        socket.close();
      } catch(IOException e) {}
      threadcount--; // Ending this thread
    }
  }
}

public class MultiJabberClient {
  static final int MAX_THREADS = 40;
  public static void main(String[] args)
      throws IOException, InterruptedException {
    InetAddress addr =
      InetAddress.getByName(null);
    while(true) {
      if(JabberClientThread.threadCount()
         < MAX_THREADS)
        new JabberClientThread(addr);
      Thread.currentThread().sleep(100);
    }
  }
} ///:~

The JabberClientThread constructor takes an InetAddress and uses it to open a Socket. You’re probably starting to see the pattern: the Socket is always used to create some kind of Reader and/or Writer (or InputStream and/or OutputStream) object, which is the only way that the Socket can be used. (You can, of course, write a class or two to automate this process instead of doing all the typing if it becomes painful.) Again, start( ) performs thread initialization and calls run( ). Here, messages are sent to the server and information from the server is echoed to the screen. However, the thread has a limited lifetime and eventually completes. Note that the socket is cleaned up if the constructor fails after the socket is created but before the constructor completes. Otherwise the responsibility for calling close( ) for the socket is relegated to the run( ) method.

The threadcount keeps track of how many JabberClientThread objects currently exist. It is incremented as part of the constructor and decremented as run( ) exits (which means the thread is terminating). In MultiJabberClient.main( ), you can see that the number of threads is tested, and if there are too many, no more are created. Then the method sleeps. This way, some threads will eventually terminate and more can be created. You can experiment with MAX_THREADS to see where your particular system begins to have trouble with too many connections.

Datagrams

The examples you’ve seen so far use the Transmission Control Protocol (TCP, also known as stream-based sockets), which is designed for ultimate reliability and guarantees that the data will get there. It allows retransmission of lost data, it provides multiple paths through different routers in case one goes down, and bytes are delivered in the order they are sent. All this control and reliability comes at a cost: TCP has a high overhead.

There’s a second protocol, called User Datagram Protocol (UDP), which doesn’t guarantee that the packets will be delivered and doesn’t guarantee that they will arrive in the order they were sent. It’s called an “unreliable protocol” (TCP is a “reliable protocol”), which sounds bad, but because it’s much faster it can be useful. There are some applications, such as an audio signal, in which it isn’t so critical if a few packets are dropped here or there but speed is vital. Or consider a time-of-day server, where it really doesn’t matter if one of the messages is lost. Also, some applications might be able to fire off a UDP message to a server and can then assume, if there is no response in a reasonable period of time, that the message was lost.

Typically, you’ll do most of your direct network programming with TCP, and only occasionally will you use UDP. There’s a more complete treatment of UDP in the first edition of this book (available on the CD ROM bound into this book, or as a free download from www.BruceEckel.com).

Enterprise programming concepts

Enterprise computing is about collecting and distributing information.

You do this by creating common repositories (single points of access) to that information, and allowing people to get at that information in multiple ways. So enterprise computing is creating and manipulating those common repositories, and providing ways for users to view and manipulate the information in those repositories.

In this chapter you’ll see that there are a number of different ways to achieve this goal. The varied mechanics needed for the collection and distribution of information has to do with the variety of clients we must deal with (different application protocols) and the variety of repositories holding our enterprise data (relational databases, hierarchical databases, files, newsgroups and Email).

Since we can loosely define the any enterprise as a community of individuals working together let's leverage this definition and create a "Community Information System" (CIS) that will allow members of the community to collect and distribute information about what is going on within their group. This group could be 20 people or 20,000 but some of the services that may be needed would include the following:

  • Community Calendar/Schedule of events
  • Community directory (phone/email listings)
  • Community interest lists
  • General Announcements/Notices/News items
  • Lost & Found
  • Ticket sales to community events (this would justify the security issues)
  • Classified ads
  • An anti-spamming system to prevent spam harvesters—either you have a password, or it actually emails the information back to you, after verifying your email address shows that you’re a community member.

So lets start by looking at an API set that provides a common interface to all those relational databases.

Java Database Connectivity (JDBC)

It has been estimated that half of all software development involves client/server operations. A great promise of Java has been the ability to build platform-independent client/server database applications. In Java 1.1 this has come to fruition with Java DataBase Connectivity (JDBC).

One of the major problems with databases has been the feature wars between the database companies. There is a “standard” database language, Structured Query Language (SQL-92), but usually you must know which database vendor you’re working with despite the standard. JDBC is designed to be platform-independent, so you don’t need to worry about the database you’re using while you’re programming. However, it’s still possible to make vendor-specific calls from JDBC so you aren’t restricted from doing what you must.

One place where programmers may need to use SQL type names is in the SQL TABLE CREATE statement when they are creating a new database table and defining the SQL type for each column. Unfortunately there are significant variations between SQL types supported by different database products. Different databases that support SQL types with the same semantics and structure may give those types different names. Most major databases support an SQL data type for large binary values, in Oracle this type is called a LONG RAW, Sybase calls it IMAGE, Informix calls it BYTE, and DB2 LONG VARCHAR FOR BIT DATA. Therefore, if database portability is a goal you should try to use only generic SQL type identifiers.

Portability is an issue when writing for a book where readers may be testing the examples with all kinds of unknown data stores. We have tried to write these examples to be as portable as possible. You should also notice that all the database specific code has been pulled out to a single class file to centralize any changes that may need to be made to get the examples operational in your environment.

JDBC, like many of the APIs in Java, is designed for simplicity. The method calls you make correspond to the logical operations you’d think of doing when gathering data from a database: connect to the database, create a statement and execute the query, and look at the result set.

To allow this platform independence, JDBC provides a driver manager that dynamically maintains all the driver objects that your database queries will need. So if you have three different kinds of vendor databases to connect to, you’ll need three different driver objects. The driver objects register themselves with the driver manager at the time of loading, and you can force the loading using Class.forName( ).

To open a database, you must create a “database URL” that specifies:

  1. That you’re using JDBC with “jdbc”
  2. The “subprotocol”: the name of the driver or the name of a database connectivity mechanism. Since the design of JDBC was inspired by ODBC, the first subprotocol available is the “jdbc-odbc bridge,” specified by “odbc”
  3. The database identifier. This varies with the database driver used, but it generally provides a logical name that is mapped by the database administration software to a physical directory where the database tables are located. For your database identifier to have any meaning, you must register the name using your database administration software. (The process of registration varies from platform to platform.)

All this information is combined into one string, the “database URL.” For example, to connect through the ODBC subprotocol to a database identified as “people,” the database URL could be:

String dbUrl = "jdbc:odbc:people";

If you’re connecting across a network, the database URL will also contain the information identifying the remote machine.

When you’re ready to connect to the database, you call the static method DriverManager.getConnection( ), passing it the database URL, the user name, and a password to get into the database. You get back a Connection object that you can then use to query and manipulate the database.

The following example opens a database of contact information and looks for a person’s last name as given on the command line. It selects only the names of people that have email addresses, then prints out all the ones that match the given last name:

//: c15:jdbc:Lookup.java
// Looks up email addresses in a
// local database using JDBC.
import java.sql.*;

public class Lookup {
  public static void main(String[] args) {
    String dbUrl = "jdbc:odbc:people";
    String user = "";
    String password = "";
    try {
      // Load the driver (registers itself)
      Class.forName(
        "sun.jdbc.odbc.JdbcOdbcDriver");
      Connection c = DriverManager.getConnection(
        dbUrl, user, password);
      Statement s = c.createStatement();
      // SQL code:
      ResultSet r =
        s.executeQuery(
          "SELECT FIRST, LAST, EMAIL " +
          "FROM people.csv people " +
          "WHERE " +
          "(LAST='" + args[0] + "') " +
          " AND (EMAIL Is Not Null) " +
          "ORDER BY FIRST");
      while(r.next()) {
        // Capitalization doesn't matter:
        System.out.println(
          r.getString("Last") + ", "
          + r.getString("fIRST")
          + ": " + r.getString("EMAIL") );
      }
      s.close(); // Also closes ResultSet
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

You can see the creation of the database URL as previously described. In this example, there is no password protection on the database so the user name and password are empty strings.

Once the connection is made with DriverManager.getConnection( ), you can use the resulting Connection object to create a Statement object using the createStatement( ) method. With the resulting Statement, you can call executeQuery( ), passing in a string containing an SQL-92 standard SQL statement. (You’ll see shortly how you can generate this statement automatically, so you don’t have to know much about SQL.)

The executeQuery( ) method returns a ResultSet object, which is quite a bit like an iterator: the next( ) method moves the iterator to the next record in the statement, or returns false if the end of the result set has been reached. You’ll always get a ResultSet object back from executeQuery( ) even if a query results in an empty set (that is, an exception is not thrown). Note that you must call next( ) once before trying to read any record data. If the result set is empty, this first call to next( ) will return false. For each record in the result set, you can select the fields using (among other approaches) the field name as a string. Also note that the capitalization of the field name is ignored—it doesn’t matter with an SQL database. You determine the type you’ll get back by calling getInt( ), getString( ), getFloat( ), etc. At this point, you’ve got your database data in Java native format and can do whatever you want with it using ordinary Java code.

Getting the example to work

With JDBC, understanding the code is relatively simple. The confusing part is making it work on your particular system. The reason this is confusing is that it requires you to figure out how to get your JDBC driver to load properly, and how to set up a database using your database administration software.

Of course, this process can vary radically from machine to machine, but the process I used to make it work under 32-bit Windows might give you clues to help you attack your own situation.

Step 1: Find the JDBC Driver

The program above contains the statement:

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

This implies a directory structure, which is deceiving. With this particular installation of JDK 1.1, there was no file called JdbcOdbcDriver.class, so if you looked at this example and went searching for it you’d be frustrated. Other published examples use a pseudo name, such as “myDriver.ClassName,” which is less than helpful. In fact, the load statement above for the jdbc-odbc driver (the only one that actually comes with JDK 1.1) appears in only a few places in the online documentation (in particular, a page labeled “JDBC-ODBC Bridge Driver”). If the load statement above doesn’t work, then the name might have been changed as part of a Java version change, so you should hunt through the documentation again.

If the load statement is wrong, you’ll get an exception at this point. To test whether your driver load statement is working correctly, comment out the code after the statement and up to the catch clause; if the program throws no exceptions it means that the driver is loading properly.

Step 2: Configure the database

Again, this is specific to 32-bit Windows; you might need to do some research to figure it out for your own platform.

First, open the control panel. You might find two icons that say “ODBC.” You must use the one that says “32bit ODBC,” since the other one is for backward compatibility with 16-bit ODBC software and will produce no results for JDBC. When you open the “32bit ODBC” icon, you’ll see a tabbed dialog with a number of tabs, including “User DSN,” “System DSN,” “File DSN,” etc., in which “DSN” means “Data Source Name.” It turns out that for the JDBC-ODBC bridge, the only place where it’s important to set up your database is “System DSN,” but you’ll also want to test your configuration and create queries, and for that you’ll also need to set up your database in “File DSN.” This will allow the Microsoft Query tool (that comes with Microsoft Office) to find the database. Note that other query tools are also available from other vendors.

The most interesting database is one that you’re already using. Standard ODBC supports a number of different file formats including such venerable workhorses as DBase. However, it also includes the simple “comma-separated ASCII” format, which virtually every data tool has the ability to write. In my case, I just took my “people” database that I’ve been maintaining for years using various contact-management tools and exported it as a comma-separated ASCII file (these typically have an extension of .csv). In the “File DSN” section I chose “Add,” chose the text driver to handle my comma-separated ASCII file, and then un-checked “use current directory” to allow me to specify the directory where I exported the data file.

You’ll notice when you do this that you don’t actually specify a file, only a directory. That’s because a database is typically represented as a collection of files under a single directory (although it could be represented in other forms as well). Each file usually contains a single table, and the SQL statements can produce results that are culled from multiple tables in the database (this is called a join). A database that contains only a single table (like this one) is usually called a flat-file database. Most problems that go beyond the simple storage and retrieval of data generally require multiple tables that must be related by joins to produce the desired results, and these are called relational databases.

Step 3: Test the configuration

To test the configuration you’ll need a way to discover whether the database is visible from a program that queries it. Of course, you can simply run the JDBC program example above up to and including the statement:

Connection c = DriverManager.getConnection(
  dbUrl, user, password);

If an exception is thrown, your configuration was incorrect.

However, it’s useful to get a query-generation tool involved at this point. I used Microsoft Query that came with Microsoft Office, but you might prefer something else. The query tool must know where the database is, and Microsoft Query required that I go to the ODBC Administrator’s “File DSN” tab and add a new entry there, again specifying the text driver and the directory where my database lives. You can name the entry anything you want, but it’s helpful to use the same name you used in “System DSN.”

Once you’ve done this, you will see that your database is available when you create a new query using your query tool.

Step 4: Generate your SQL query

The query that I created using Microsoft Query not only showed me that my database was there and in good order, but it also automatically created the SQL code that I needed to insert into my Java program. I wanted a query that would search for records that had the last name that was typed on the command line when starting the Java program. So as a starting point, I searched for a specific last name, ‘Eckel’. I also wanted to display only those names that had email addresses associated with them. The steps I took to create this query were:

  1. Start a new query and use the Query Wizard. Select the “people” database. (This is the equivalent of opening the database connection using the appropriate database URL.)
  2. Select the “people” table within the database. From within the table, choose the columns FIRST, LAST, and EMAIL.
  3. Under “Filter Data,” choose LAST and select “equals” with an argument of Eckel. Click the “And” radio button.
  4. Choose EMAIL and select “Is not Null.”
  5. Under “Sort By,” choose FIRST.

The result of this query will show you whether you’re getting what you want.

Now you can press the SQL button and without any research on your part, up will pop the correct SQL code, ready for you to cut and paste. For this query, it looked like this:

SELECT people.FIRST, people.LAST, people.EMAIL
FROM people.csv people
WHERE (people.LAST='Eckel') AND
(people.EMAIL Is Not Null)
ORDER BY people.FIRST

With more complicated queries it’s easy to get things wrong, but with a query tool you can interactively test your queries and automatically generate the correct code. It’s hard to argue the case for doing this by hand.

Step 5: Modify and paste in your query

You’ll notice that the code above looks different from what’s used in the program. That’s because the query tool uses full qualification for all of the names, even when there’s only one table involved. (When more than one table is involved, the qualification prevents collisions between columns from different tables that have the same names.) Since this query involves only one table, you can optionally remove the “people” qualifier from most of the names, like this:
SELECT FIRST, LAST, EMAIL
FROM people.csv people
WHERE (LAST='Eckel') AND
(EMAIL Is Not Null)
ORDER BY FIRST

In addition, you don’t want this program to be hard coded to look for only one name. Instead, it should hunt for the name given as the command-line argument. Making these changes and turning the SQL statement into a dynamically-created String produces:

"SELECT FIRST, LAST, EMAIL " +
"FROM people.csv people " +
"WHERE " +
"(LAST='" + args[0] + "') " +
" AND (EMAIL Is Not Null) " +
"ORDER BY FIRST");

SQL has another way to insert names into a query called stored procedures, which is used for speed. But for much of your database experimentation and for your first cut, building your own query strings in Java is fine.

You can see from this example that by using the tools currently available—in particular the query-building tool—database programming with SQL and JDBC can be quite straightforward.

A GUI version of the lookup program

It’s more useful to leave the lookup program running all the time and simply switch to it and type in a name whenever you want to look someone up. The following program creates the lookup program as an application/applet, and it also adds name completion so the data will show up without forcing you to type the entire last name:

//: c15:jdbc:VLookup.java
// GUI version of Lookup.java.
// <applet code=VLookup
// width=500 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.sql.*;
import com.bruceeckel.swing.*;

public class VLookup extends JApplet {
  String dbUrl = "jdbc:odbc:people";
  String user = "";
  String password = "";
  Statement s;
  JTextField searchFor = new JTextField(20);
  JLabel completion =
    new JLabel("                        ");
  JTextArea results = new JTextArea(40, 20);
  public void init() {
    searchFor.getDocument().addDocumentListener(
      new SearchL());
    JPanel p = new JPanel();
    p.add(new Label("Last name to search for:"));
    p.add(searchFor);
    p.add(completion);
    Container cp = getContentPane();
    cp.add(p, BorderLayout.NORTH);
    cp.add(results, BorderLayout.CENTER);
    try {
      // Load the driver (registers itself)
      Class.forName(
        "sun.jdbc.odbc.JdbcOdbcDriver");
      Connection c = DriverManager.getConnection(
        dbUrl, user, password);
      s = c.createStatement();
    } catch(Exception e) {
      results.setText(e.getMessage());
    }
  }
  class SearchL implements DocumentListener {
    public void changedUpdate(DocumentEvent e){}
    public void insertUpdate(DocumentEvent e){
      textValueChanged();
    }
    public void removeUpdate(DocumentEvent e){
      textValueChanged();
    }
  }
  public void textValueChanged() {
    ResultSet r;
    if(searchFor.getText().length() == 0) {
      completion.setText("");
      results.setText("");
      return;
    }
    try {
      // Name completion:
      r = s.executeQuery(
        "SELECT LAST FROM people.csv people " +
        "WHERE (LAST Like '" +
        searchFor.getText()  +
        "%') ORDER BY LAST");
      if(r.next())
        completion.setText(
          r.getString("last"));
      r = s.executeQuery(
        "SELECT FIRST, LAST, EMAIL " +
        "FROM people.csv people " +
        "WHERE (LAST='" +
        completion.getText() +
        "') AND (EMAIL Is Not Null) " +
        "ORDER BY FIRST");
    } catch(Exception e) {
      results.setText(
        searchFor.getText() + "\n");
      results.append(e.getMessage());
      return;
    }
    results.setText("");
    try {
      while(r.next()) {
        results.append(
          r.getString("Last") + ", "
          + r.getString("fIRST") +
          ": " + r.getString("EMAIL") + "\n");
      }
    } catch(Exception e) {
      results.setText(e.getMessage());
    }
  }
  public static void main(String[] args) {
    Console.run(new VLookup(), 500, 200);
  }
} ///:~

Much of the database logic is the same, but you can see that a TextListener is added to listen to the TextField, so that whenever you type a new character it first tries to do a name completion by looking up the last name in the database and using the first one that shows up. (It places it in the completion Label, and uses that as the lookup text.) This way, as soon as you’ve typed enough characters for the program to uniquely find the name you’re looking for, you can stop.

Why the JDBC API
seems so complex

When you browse the online documentation for JDBC it can seem daunting. In particular, in the DatabaseMetaData interface—which is just huge, contrary to most of the interfaces you see in Java—there are methods such as dataDefinitionCausesTransactionCommit( ), getMaxColumnNameLength( ), getMaxStatementLength( ), storesMixedCaseQuotedIdentifiers( ), supportsANSI92IntermediateSQL( ), supportsLimitedOuterJoins( ), and so on. What’s this all about?

As mentioned earlier, databases have seemed from their inception to be in a constant state of turmoil, primarily because the demand for database applications, and thus database tools, is so great. Only recently has there been any convergence on the common language of SQL (and there are plenty of other database languages in common use). But even with an SQL “standard” there are so many variations on that theme that JDBC must provide the large DatabaseMetaData interface so that your code can discover the capabilities of the particular “standard” SQL database that it’s currently connected to. In short, you can write simple, transportable SQL, but if you want to optimize speed your coding will multiply tremendously as you investigate the capabilities of a particular vendor’s database.

This, of course, is not Java’s fault. The discrepancies between database products are just something that JDBC tries to help compensate for. But bear in mind that your life will be easier if you can either write generic queries and not worry too much about performance, or, if you must tune for performance, know the platform you’re writing for so you don’t need to write all that investigation code.

There is more JDBC information available in the electronic documents that come as part of the Java 1.1 distribution from Sun. In addition, you can find more in the book JDBC Database Access with Java (Hamilton, Cattel, and Fisher, Addison-Wesley, 1997). Other JDBC books are appearing regularly.

A more sophisticated JDBC Example

If you’re connecting across a network, the database URL will contain the connection information and the dbUrl can become a bit intimidating. Here is an example from a CloudScape database being called from a remote client utilizing RMI:

jdbc:rmi://192.168.170.27:1099/jdbc:cloudscape:db

This database URL is really two jdbc calls in one. The first part "jdbc:rmi://192.168.170.27:1099/" uses RMI to make the connection to the remote database engine listening on port 1099 at IP Address 192.168.170.27. The second part of the URL, " jdbc:cloudscape:db" conveys the more typical settings using the subprotocol and database name but this will only happen after the first section has made the connection via RMI to the remote machine.

When you’re ready to connect to the database, you call the static method DriverManager.getConnection( ), passing it the database URL, the user name, and a password to get into the database. You get back a Connection object that you can then use to query and manipulate the database.

At this point we should dive into some example code. Our database specific code will reside in the class DBStuff. Our database URL, JDBC driver, username and password will have access methods, all other SQL statements will be public strings.

//: c15:jdbc:DBStuff.java
// A class to hold all our
// database specific code
// for the community
// interests database
import java.sql.*;

public class DBStuff {
  // All the database stuff
  // Specific for CloudScape.
  String dbDriver =
      "COM.cloudscape.core.JDBCDriver";
  String dbURL =
      "jdbc:cloudscape:d:/docs/_work/JSapienDB";
  String user = "";
  String password = "";
  public String dropMemTbl = "drop table MEMBERS";
  public String createMemTbl =
       "create table MEMBERS " +
       "(MEM_ID INTEGER primary key, " +
       "MEM_UNAME VARCHAR(12) not null unique, " +
       "MEM_LNAME VARCHAR(40), " +
       "MEM_FNAME VARCHAR(20), " +
       "ADDRESS VARCHAR(40), " +
       "CITY VARCHAR(20), " +
       "STATE CHAR(4), " +
       "ZIP CHAR(5), " +
       "PHONE CHAR(12), " +
       "EMAIL VARCHAR(30))";
  public String createMemIdx =
       "create unique index " +
       "LNAME_IDX on MEMBERS(MEM_LNAME)";

  public String dropEvtTbl = "drop table EVENTS";
  public String createEvtTbl =
       "create table EVENTS " +
       "(EVT_ID INTEGER primary key, " +
       "EVT_TITLE VARCHAR(30) not null, " +
       "EVT_TYPE VARCHAR(20), " +
       "LOC_ID INTEGER, " +
       "PRICE DECIMAL, " +
       "DATETIME TIMESTAMP)";
  public String createEvtIdx =
       "create unique index " +
       "TITLE_IDX on EVENTS(EVT_TITLE)";

  public String dropEMTbl = "drop table EVTMEMS";
  public String createEMTbl =
       "create table EVTMEMS " +
       "(MEM_ID INTEGER not null, " +
       "EVT_ID INTEGER not null, " +
       "MEM_ORD INTEGER)";
  public String createEMIdx =
       "create unique index " +
       "EVTMEM_IDX on EVTMEMS(MEM_ID, EVT_ID)";

  public String dropLocTbl = "drop table LOCATIONS";
  public String createLocTbl =
       "create table LOCATIONS " +
       "(LOC_ID INTEGER primary key, " +
       "LOC_NAME VARCHAR(30) not null, " +
       "CONTACT VARCHAR(50), " +
       "ADDRESS VARCHAR(40), " +
       "CITY VARCHAR(20), " +
       "STATE VARCHAR(4), " +
       "ZIP VARCHAR(5), " +
       "PHONE CHAR(12), " +
       "DIRECTIONS VARCHAR(4096))";
  public String createLocIdx =
       "create unique index " +
       "NAME_IDX on LOCATIONS(LOC_NAME)";

  public String getDriver() {
    return dbDriver;
  }
  public String getDbURL() {
    return dbURL;
  }
  public String getUser() {
    return user;
  }
  public String getPassword() {
    return password;
  }
} ///:~

Our DBStuff class will generate a set of tables that will have a structure as shown below. Certainly not elaborate but just right for the level of detail we would like to show. There are numerous books, seminars and software packages that will help you in the design and development of a database. We do not want to elaborate on those areas, we will focus primarily on Java. Our goal is to provide a simple example to test most of our Enterprise APIs and a structure that could be easily transferred to the database you use.

The following class, CreateTables uses the DBStuff class to load the JDBC driver, make a connection to the database, then create the table structure outlined above. Once the connection is made and SQL statements have been written there is not much to do except push the SQL to the database and or handle errors responsibily.

//: c15:jdbc:CreateTables.java
// Creates database tables for
// community interests database
import java.sql.*;

public class CreateTables {

  public static void main(String[] args) {
    DBStuff db = new DBStuff();
    try {
      // Load the driver (registers itself)
      Class.forName(db.getDriver());
    } catch(java.lang.ClassNotFoundException e) {
      System.err.print("ClassNotFoundException: ");
      e.printStackTrace();
    }
    try {
      Connection c = DriverManager.getConnection(
        db.getDbURL(),
        db.getUser(),
        db.getPassword());
      Statement s = c.createStatement();
      // SQL code:
      // Create the MEMBERS table
      try {
        s.executeUpdate( db.dropMemTbl );
      } catch(SQLException sqlEx) {
        String msg;
        msg = "Table MEMBERS not present. " +
              "Drop failed.";
        System.out.println(msg);
      }
      s.executeUpdate( db.createMemTbl );
      s.executeUpdate( db.createMemIdx );

      // Create the EVENTS table
      try {
        s.executeUpdate( db.dropEvtTbl );
      } catch(SQLException sqlEx) {
        String msg;
        msg = "Table EVENTS not present. " +
              "Drop failed.";
        System.out.println(msg);
      }
      s.executeUpdate( db.createEvtTbl );
      s.executeUpdate( db.createEvtIdx );

      // Create the EVTMEMS table
      try {
        s.executeUpdate( db.dropEMTbl );
      } catch(SQLException sqlEx) {
        String msg;
        msg = "Table EVTMEMS not present. " +
              "Drop failed.";
        System.out.println(msg);
      }
      s.executeUpdate( db.createEMTbl );

      // Create the LOCATIONS table
      try {
        s.executeUpdate( db.dropLocTbl );
      } catch(SQLException sqlEx) {
        String msg;
        msg = "Table LOCATIONS not present. " +
              "Drop failed.";
        System.out.println(msg);
      }
      s.executeUpdate( db.createLocTbl );
      s.executeUpdate( db.createLocIdx );

      s.close();
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

You can see the creation of the database URL as previously described. In this example, there is no password protection on the database so the user name and password are empty strings.

Once the connection is made with DriverManager.getConnection( ), you can use the resulting Connection object to create a Statement object using the createStatement( ) method. With the resulting Statement, you can call executeUpdate( ), passing in a string containing an SQL-92 standard SQL statement. (You’ll see shortly how you can generate this statement automatically, so you don’t have to know much about SQL.) executeUpdate( ) usually will return the number of rows that were affected by the SQL statement. executeUpdate( ) is more commonly used to execute INSERT, UPDATE, or DELETE statements that modify one or more rows. For statements such as CREATE TABLE, DROP TABLE and CREATE INDEX, executeUpdate( ) always returns zero.

The Statement interface provides two other methods for executing SQL statements: executeQuery( ) and execute( ). The correct method to use is determined by what the SQL statement produces.

The executeQuery( ) method returns a ResultSet object, which is quite a bit like an iterator: the next( ) method moves the iterator to the next record in the statement, or returns false if the end of the result set has been reached. You’ll always get a ResultSet object back from executeQuery( ) even if a query results in an empty set (that is, an exception is not thrown). Note that you must call next( ) once before trying to read any record data. If the result set is empty, this first call to next( ) will return false. For each record in the result set, you can select the fields using (among other approaches) the field name as a string. Also note that the capitalization of the field name is ignored—it doesn’t matter with an SQL database. You determine the type you’ll get back by calling getInt( ), getString( ), getFloat( ), etc. At this point, you’ve got your database data in Java native format and can do whatever you want with it using ordinary Java code.

The method execute( ) is used to execute statements that return more than one result set, more than one update count, or a combination of the two.

Let's now load the table we created above with data. This will require us to perform a series of INSERTS followed by a SELECT to see all the data in our table and to let use exercise a result set.

Servlets

Now that we understand JDBC and how it abstracts all those different database backends into a common API set, let’s turn our attention to another common task - handling HTTP requests and responses. It is taken for granted in today’s technical environment that client access from the Internet or corporate intranets is a sure way to allow many users to access data and resources easily. This type of access is predicated on the clients utilizing the World Wide Web standards of Hypertext Markup Language (HTML) and Hypertext Transfer Protocol (HTTP). Wouldn’t it be nice to have an API set that abstracted out this commonly used area of client access? Welcome Java Servlets!

Traditionally, the way to handle a problem such as allowing an Internet client to update their personal data is to create an HTML page with a text field and a “submit” button. The user can type whatever he or she wants into the text field, and it will be submitted to the server without question. As it submits the data, the Web page also tells the server what to do with the data by mentioning the Common Gateway Interface (CGI) program that the server should run after receiving this data. This CGI program is typically written in either Perl or C (and sometimes C++, if the server supports it), and it must handle everything. First it looks at the data and decides whether it’s in the correct format. If not, the CGI program must create an HTML page to describe the problem; this page is handed to the server, which sends it back to the user. The user must then back up a page and try again. If the data is correct, the CGI program opens the data file and either adds the email address to the file or discovers that the address is already in the file. In both cases it must format an appropriate HTML page for the server to return to the user.

As Java programmers, this seems like an awkward way for us to solve the problem, and naturally, we’d like to do the whole thing in Java. First, we’ll use a Java applet to take care of data validation at the client site, without all that tedious Web traffic and page formatting. Then let’s skip the Perl CGI script in favor of a Java application running on the server. In fact, let’s skip the Web server altogether and simply make our own network connection from the applet to the Java application on the server!

As you’ll see, there are a number of issues that make this a more complicated problem than it seems. It would be ideal to write the applet but applet’s, while a proven technology with plenty of support, have been problematic in the Wild World Web where different browsers handle applet’s differently. In a corporate intranet where there is some level of standardization this seems possible in the short term but what happens with the next acquisition or merger? What happens when employees want to start working from home? That’s easy - things start to look a lot like the Internet - you can’t depend on anything especially how applets are implemented in your client’s browsers. So to be on the safe side, what we really want to do is deal with straight HTML and HTTP within our java server. The client knows nothing of the implementation, they are only aware that they can get at their data and perform their work without installing, upgrading or calling tech support.

Sun has delivered on this need. The Servlet API wraps up the HTTP protocol so we can put Java on the server side of our HTTP connection and deal with our client in HTML and HTTP. Servlets are completely server side java for the Web. The client sees nothing except HTTP and HTML.

The basic servlet

The architecture of the Servlet API is that of a classic service provider with a service( ) method through which all client requests will drive and life cycle methods init( ) and destroy( ).

public interface Servlet {
    public void init(ServletConfig config)
      throws ServletException;
    public ServletConfig getServletConfig();
    public void service(ServletRequest req,
      ServletResponse res)
      throws ServletException, IOException;
    public String getServletInfo();
    public void destroy();
}

getServletConfig( ) sole purpose is to return a ServletConfig object which contains initialization and startup parameters for this servlet and getServletInfo( ) returns a string containing information about the servlet, such as author, version, and copyright.

The GenericServlet class is a shell implementation of this interface, nothing more. The HttpServlet class is an extension of GenericServlet and is designed specifically to handle the HTTP protocol.

Although you can derive your servlets from GenericServlet, Sun recommends that all servlets derive from HttpServlet. This makes sense since your servlet is designed to work with a servlet engine that is satisfying clients requests from within a Web server. Why not utilize the built-in parsing capabilities for POST and GET that come with HttpServlet? We will be getting into this shortly.

The most wonderful attribute of the Servlet API is the auxiliary objects that come along with the HttpServlet class to support it. Look at the service( ) method in the Servlet interface. It has two parameters ServletRequest and ServletResponse. With the HttpServlet class these two object are extended for HTTP as well—HttpServletRequest and HttpServletResponse. Let’s take a closer look.

//: c15:servlets:ServletsRule.java
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class ServletsRule extends HttpServlet {
  int i = 0; // Servlet "persistence"
  public void service(HttpServletRequest req,
  HttpServletResponse res) throws IOException {
    PrintWriter out = res.getWriter();
    out.print("<HEAD><TITLE>");
    out.print("A server-side strategy");
    out.print("</TITLE></HEAD><BODY>");
    out.print("<h1>Servlets Rule! " + i++);
    out.print("</h1></BODY>");
    out.close();
  }
} ///:~

ServletsRule is about as simple as a servlet can get. But that is the beauty of it all - just think how much stuff is being handled for us! Once the servlet is initialized - its init( ) method has run to completion - can clients enter the service( ) method. In the service method our main responsibility is to interact with the HTTP request the client sent us and build a HTTP response based upon the attributes contained within the request. In ServletsRule we only manipulate the response object without looking at what the client may has sent us. We call the getWriter( ) method of the response object to get a PrintWriter object. The PrintWriter is used for writing character-based response data.

As we dive more deeply into the HttpRequest and HttpResponse objects you should notice that a greater understanding of HTTP and HTML would be helpful. Servlets are designed for Web server-side development and it shows. Now let’s scratch a little deeper by getting some HTML form data that was passed to the servlet in the request object.

//: c15:servlets:EchoForm.java
// Dumps the name-value pairs of any HTML form
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;

public class EchoForm extends HttpServlet {
  public void doGet(HttpServletRequest req,
    HttpServletResponse res) throws IOException {
    res.setContentType("text/html");
    PrintWriter out = res.getWriter();
    out.print("<h1>Your form contained:</h1>");
    Enumeration flds = req.getParameterNames();
    while(flds.hasMoreElements()) {
      String field = (String)flds.nextElement();
      String value = req.getParameter(field);
      out.print(field + " = " + value + "<br>");
    }
    out.close();
  }
  public void doPost(HttpServletRequest req,
    HttpServletResponse res) throws IOException {
    doGet(req, res);
  }
} ///:~

The EchoForm servlet parrots back to our client the request fields that have been sent to the servlet as part of the request object. Since the request fields will be placed into a response we must set up the response object with calls to setContentType( ) and getWriter( ). In an HttpResponse object the setContentType( ) methods sets the Content-Type HTTP header. This is most commonly "text/html".

Notice that in EchoForm the service( ) method has been replaced by doGet( ). This automatic HTTP method name parsing is a feature of HttpServlet. HTTP was designed for the Web and has been made more general than necessary. The first word on the full request line is simply the name of the method (command) to be executed on the Web page. The built-in methods are GET, POST, HEAD, PUT, DELETE, LINK and UNLINK. The HttpServlet class is written to parse these methods and let the programmer react differently for a GET than a POST or even a HEAD request method. If you don't care you can just override service( ). Since most HTTP methods are POST or GET many times you just implement one and direct the other to it.

In The HTTP request object has the potential to come with request parameters and generally does. The parameters are typically name-value pairs sent as part of its query string (for GET requests) or as encoded post data (for POST requests). EchoForm uses the request object's getParameterNames( ) method to loop through the parameter list. The getParameter( ) method is used to pull the value. The pair is then written to the PrintWriter of the response object with the appropriate HTML tags. The response object is sent to the client when the servlet is finished.

Servlets and Multithreading

Now that you understand the basics you should be realizing that servlets are excellent for server-side Web development. Elegant and straight forward, they do just about everything for you - right? Well, almost. Remember early we said all client requests drive through the service method? This is the well-used, high traffic corridor of the servlet and more than one client request may come through at the same time. The servlet engine has a pool of threads that it will dispatch to handle client requests. It is quite likely that two clients arriving at the same time could beprocessing through your service( ) or doGet( ) or doPost( ) methods at the same time. Therefore the service( ) methods and other methods called by HttpServlet.service( ) (e.g., doGet( ), doPost( ), doHead( ),etc.) need to be written in a thread-safe manner. Any common resources (files, databases) that will be used by your client requests will need to be synchronized.

ThreadServlet is a simple example that simply synchronizes around the threads sleep( ) method. This will hold up all threads until the allotted time (5000 ms) is all used up. When testing this you should start several browsers instances and hit this servlet as quickly as possible.

//: c15:servlets:ThreadServlet.java
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class ThreadServlet extends HttpServlet {
  int i;
  public void service(HttpServletRequest req,
    HttpServletResponse res) throws IOException {
    res.setContentType("text/html");
    PrintWriter out = res.getWriter();
    synchronized(this) {
      try {
        Thread.currentThread().sleep(5000);
      } catch(InterruptedException e) {}
    }
    out.print("<h1>Finished " + i++ + "</h1>");
    out.close();
  }
} ///:~

Handling Sessions with Servlets

The servlet API comes with more than just the classes that implement the servlet interface, GenericServlet and HttpServlet and the Request and Response objects. The design of HTTP is such that it is a 'sessionless' protocol. A great deal of effort has gone into mechanisms that will allow Web developers to track sessions. How could companies do e-commerce if you couldn't keep track of client and the items they have put into their shopping cart? You couldn't! This may be great for privacy advocates but it does little to help create robust, commerce driven Web sites.

There are several methods of session tracking but the most common method is with persistant 'cookies'. The term cookie sounds cute and could be perceived as a session tracking solution that was baked up in someone's garage. The fact is that cookies are an integral part of the Internet standards. The HTTP Working Group of the Internet Engineering Task Force has written cookies in the official standard in RFC 2109 (http://ds.internic.net/rfc/rfc2109.txt or check http://www.cookiecentral.com).

A cookie is nothing more than a small piece of information sent by a Web server to a browser. The browser stores the cookie locally and all calls to the server from that browser will contain the cookie as an identifier. The cookie therefore acts to uniquely identify the client with each hit of this Web server. It should be noted that clients can turn off the browsers ability to accept cookies. If your site still need to be able to session track this type of client then another method of session tracking (URL rewriting or hidden form fields) will have to be incorporated. The session tracking capabilities built into the Servlet API are designed around cookies.

The Cookie Class

The Servlet API (version 2.0 and up) provides the javax.servlet.http.Cookie class. This class incorporates all the HTTP header details and allows the setting of various cookie attributes. Using the cookie is simply a matter of creating it using the constructor and adding it to the response object. The constructor takes a cookie name as the first argument and a value as the second. Cookies are added to the response object before you send any content.

Cookie oreo = new Cookie("TIJava", "2000");
res.addCookie(cookie);

Cookies are then received by calling the getCookies( ) method of the HttpServletRequest object which returns an array of cookie objects.

Cookie[] cookies = req.getCookies();

The Session Class

A session in the world of HTTP and the Internet is one or more page requests by a client to a Web site during a defined period of time. If I am buying my groceries on-line, I want a session to be confined to the period from when I first add an item to my shopping cart to the point where I checkout. Each item I add to the shopping cart will be a new connection in the HTTP world, they have no knowledge of previous connections or items in the shopping cart. The mechanics supplied by the Cookie specification allows us to perform 'session tracking'.

You should understand that a cookie is an object that encapsulates that small bit of information that will be stored on the client side. A Servlet Session object lives on the server side of the communication channel and its goal is to capture data about this client that would be useful as the client moves through and interacts with your Web site. This data may be pertinent for the present session, such as items in the shopping cart or it may be information you asked the client to enter such as authentication information entered when the client first entered your Web site and which should not have to be re-enter before a set time of inactivity.

The Session class of the Servlet API uses the Cookie class but really all the session object needs is a unique identifier stored on the client and passed to the server. Usually this is a cookie and that is the mechanism we will cover here. Web sites may also use the other types of session tracking but these mechanisms will be more difficult to implement as they are not encapsulated into the Servlet API.

Let's take a look at implementing session tracking with the Servlet API:

//: c15:servlets:SessionPeek.java
// Using the HttpSession class.
import java.io.*;
import java.util.Date;
import javax.servlet.*;
import javax.servlet.http.*;

public class SessionPeek extends HttpServlet {
  public void doGet(HttpServletRequest req,
  HttpServletResponse res)
  throws ServletException, IOException {
    // Retrieve Seesion Object before any
    // output is sent to the client.
    HttpSession session = req.getSession();
    // Get the output stream
    ServletOutputStream out =
      res.getOutputStream();
    res.setContentType("text/html");
    out.println("<HEAD><TITLE> SessionPeek ");
    out.println(" </TITLE></HEAD><BODY>");
    out.println("<h1> SessionPeek </h1>");
    // A simple hit counter for this session.
    Integer ival = (Integer)
      session.getValue("sesspeek.cntr");
    if(ival==null)
      ival = new Integer(1);
    else
      ival = new Integer(ival.intValue() + 1);
    session.putValue("sesspeek.cntr", ival);
    out.println("You have hit this page <b>"
      + ival + "</b> times.<p>");
    // Session Data
    out.println("<h2>");
    out.println(" Saved Session Data </h2>");
    // loop through all data in the session
    // and spit is out.
    String[] sesNames = session.getValueNames();
    for(int i = 0; i < sesNames.length; i++) {
      String name = sesNames[i];
      String value =
        session.getValue(name).toString();
      out.println(name + " = " + value + "<br>");
    }
    // Session Statistics
    out.println("<h3> Session Statistics </h3>");
    out.println("Session ID: "
      + session.getId() + "<br>");
    out.println("New Session: " + session.isNew()
      + "<br>");
    out.println("Creation Time: "
      + session.getCreationTime());
    out.println("<I>(" +
      new Date(session.getCreationTime())
      + ")</I><br>");
    out.println("Last Accessed Time: " +
      session.getLastAccessedTime());
    out.println("<I>(" +
      new Date(session.getLastAccessedTime())
      + ")</I><br>");
    out.println("Session Inactive Interval: "
      + session.getMaxInactiveInterval());
    out.println("Session ID in Request: "
      + req.getRequestedSessionId() + "<br>");
    out.println("Is session id from Cookie: "
      + req.isRequestedSessionIdFromCookie()
      + "<br>");
    out.println("Is session id from URL: "
      + req.isRequestedSessionIdFromURL()
      + "<br>");
    out.println("Is session id valid: "
      + req.isRequestedSessionIdValid()
      + "<br>");
    out.println("</BODY>");
    out.close();
  }
  public String getServletInfo() {
    return "A session tracking servlet";
  }
} ///:~

The very first thing we do when we enter the doGet( ) method is to call getSession( ) on the request object. getSession will return the session object associated with this request. Do not be misled into thinking that the session object is returned with the request. The session object does not travel across the network it lives on the server and is associated with a client and its requests.

getSession( ) now comes in two versions—no parameter as used here and getSession(boolean). getSession(true) is equivalent to getSession( ). The only reason for the boolean is to state whether you want to the session object created if it is not found. getSession(true) is the most likely call hence getSession( ).

The session object, if it is not new, will give us details about our client on their previous visits. If the session object is new then we will start with this visit to gather information about this client’s activities. Capturing this client information is done through the setAttribute( ) and getAttribute( ) methods of the session object.

java.lang.Object getAttribute(java.lang.String)
void setAttribute(java.lang.String name,
                  java.lang.Object value)

The session object uses a simple name-value pairing for loading information. The values must be derived from java.lang.Object and the name is a string. In ServletPeek we are keeping track of how many times the client has been back here during this session. This is done with an Integer object that is named sesspeek.cntr. You should notice in the if-else statement that if the name is not found we create a new Integer with value of 1, otherwise we create a new Integer with a value equal to the incremented value of the previously held Integer. We stick the new Integer into the session object and let the garbage collector handle the old Integer. You should realize that if you are using the same key the new object would overwrite the old one. Lastly, we use our incremented counter to display how many times the client has visited during this session.

Related to getAttribute( ) and setAttribute( ) is getAttributeNames( ). getAttributeNames( ) returns an enumeration of all the names of objects that are bound to the session object. This is quite handy and a small while loop has been added to SessionPeek to show this method in action.

This brings us to the question “Just how long does a session object hang around for?” The answer depends upon the servlet engine you are using although I think they usually default to 30 minutes (1800 seconds), which is what you should see from the ServletPeek call to getMaxInactiveInterval( ). We have tested this and found mixed results between servlet engines. Sometimes the session object can hang around overnight. I have never seen a case where the session object disappears before the Inactive Interval. You can try this by setting the Inactive Interval with setMaxInactiveInterval( ) to 5 seconds and see if your session object hangs around or if it is cleaned up at the appropriate time. This may be an attribute you will want to investigate while choosing a servlet engine.

Getting the Servlet examples to work

If you are not already working with an application server that handles Sun's Servlet and JSP technologies for you, then you may want to reevaluate your choice of application server or you may want to download the Tomcat implementation of Java Servlets and JSP’s. This can be found at http://jakarta.apache.org.

First, you should follow the instructions for decompressing the version specific to your environment. This will install the Tomcat implementation into a directory structure under where you unzipped it. Second, edit the server.xml so that you have a new Web application. Lastly, you will then want to load the servlet examples into that new Web application directory where the Tomcat server can find them.

Java Server Pages

Servlets are found by the servlet engine looking for the .class file along a Web application path defined by the configuration of the Web server. That makes sense, no problem there. But what if we would like the Web server to compile the servlet at the time it is invoked, thereby insuring that the lastest and greatest code is delivered to the client. This is the nature of JavaServer Pages or JSPs.

You can think of JSPs as special Java tags inside the HTML page that will result in a servlet being generated, then compiled by the Web server at the time the client invokes that page. This allows the separation of the dynamic content and the static content in the HTML page. You get support for scriping and tags plus the reuse associated with those tags and the JSP components that provide the functionality. Essentially, your previously static HTML pages have become dynamic in some of it parts.

The structure of a JSP page is a cross between a servlet and an HTML page. The JSP page is a test-based document that describes how to process a request and create a response. The text based description of the page intermixes template data with some dynamic actions and leverages the Java platform. JavaServer Pages is a standard extension that is defined on top of the Servlet Standard Extension. JSP 1.1 uses class from Servlet 2.2 which relies on JRE 1.1.

Here’s an extremely simple JSP example that uses a standard Java library call to get the current time in milliseconds, which is then divided by 1000 to produce the time in seconds. Since a JSP expression (the <%= ) is used, the result of the calculation is coerced into a String so it can be printed to the out object (which puts it in the Web page):

//:! c15:jsp:ShowSeconds.jsp
<html><body>
<H1>The time in seconds is:
<%= System.currentTimeMillis()/1000 %></H1>
</body></html>
///:~

There is a great deal more going on here than meets the eye. If we follow the route of the request and response we can get an idea how much many layers the request and response are moving through. The client creates the request for the JSP page and sends it off to the Web Server. The Web Server must be able to find the JSP page and forward the request to the page. The JSP page has associated with it a compiled component that is created from the Java code embedded within. This JSP component is the ultimate destination of the request and the creator of the response. The response then bubbles up the same path that the request followed picking up pieces to pass back to the client along the way.

This is important as Sun describes a JSP page as “a text-based document that describes how to process a request and create a response.” That about a broad as you can get so let’s dig deeper.

Basic operations

We know that the server automatically creates, compiles, loads and runs a special servlet to generate the page’s content. The static portions of the HTML page are generated by the servlet using the equivalent of out.println( ) calls within the servlet. The dynamic portions are included directly into the servlet.

There is good and bad in everything and JSPs are no different. The downside to all this dynamism is poor performance for first time access. Try it—it is obvious. The first access is slow and subsequent accesses are excellent.

Implicit Objects

When we looked at servlets there were several objects already built into the API—response, request, session, etc. These objects are very conveniently built into the JSP specification and they provide the same robust foundation for manipulating HTTP and HTML in a Web application.

JSP writers have access to these implicit objects within the JSP page just as you would within a servlet. The implicit objects in a JSP are detailed in the table below. Each of the variables has a class or interface that is defined in the core Java technology or the Java Servlet API. Scope of each object can vary significantly. For example, a Session object would have a scope exceeding that of a page as it many span several client requests and pages and an application object would provide service to a group of jsp pages that together would represent a Web application.

Implicit variable | Of Type (javax.servlet) | Description | Scope
request | protocol dependent subtype of : HttpServletRequest. | The request that triggers the service invocation. | request
response | protocol dependent subtype of : HttpServletResponse. | The response to the request. | page
pageContext | jsp.PageContext | The page context encapsulates implementation-dependent features and provides convenience methods and namespace access for this JSP. | page
session | protocol dependent subtype of: http.HttpSession | The session object created for the requesting client. See Servlet Session object. | session
application | ServletContext | The servlet context obtained from the servlet configuration object (e.g., getServletConfig( ).getContext( ) | app
out | jsp.JspWriter | The object that writes into the output stream. | page
config | ServletConfig | The ServletConfig for this JSP. | page
page | java.lang.Object | The instance of this page’s implementation class processing the current request. | page

JSP Scripting Elements

The implicit objects and the power of Java are all brought together with JSP actions. Actions affect the current out stream and use, modify or create objects. The actions to be performed will be determined by the details of the request object received by the JSP page. The JSP specification includes actions types that are standard and must be implemented by conforming engines. The syntax for action elements is based on XML.

The actions all start with Directives. Directives are messages to the JSP engine and the syntax is:

<%@ directive {attr=”value”}*%>

Directives do not produce any output into the current out stream but they are important in setting up your JSP pages attributes and dependencies with the JSP engine. As an example the line:

<%@ page language=”java” %>

says that the scripting language being used within the JSP page is Java. In fact the specification only describes the semantics of scripts for the language attribute equal to Java. This should give you an idea of the flexibility that is being built into the JSP technology. In the future, if you were to choose another language, say Python (a good scripting choice), then that language would have to support the Java Run-time Environment by exposing the Java technology object model to the script environment, especially the implicit variables defined above, JavaBeans properties, and public methods.

The most important directive is the page directive. It defines a number of page dependent attributes and communications these attributes to the JSP engine. These attributes include: language, extends, import, session, buffer, autoFlush, isThreadSafe, info and errorPage. For example:

<%@ page session=”true” import=”java.util.*” %>

This line indicates that the page requires participation in an (HTTP) session. Since we have not set the language directive the JSP engine defaults to java and the implicit script language variable named “session” is of type javax.servlet.http.HttpSession. If the directive had been false then the implicit variable “session” would be unavailable, the default is true.

The import attribute describes the types that are available to the scripting environment. This attribute is used just as it would be in the Java programming language i.e. a (comma separated) list of either a fully qualified Java type name denoting that type, or of a package named followed by the “.*” string denoting all the public types declared in that package. The import list is imported by the translated JSP page implementation and is available to the scripting environment. Again, this is currently only defined for when the value of the language directive is “java”.

Once the directives have been used to set the scripting environment we can utilize the scripting language elements. JSP 1.1 has three scripting language elements—declarations, scriptlets, and expressions. A declaration will declare elements, a scriptlet is a statement fragment, and an expression is a complete language expression. In JSP each scripting element begins with a “<%”. The exact syntax for each is:

<%! declaration %>
<%  scriptlet   %>
<%= expression  %>

White space is optional after “<%!”, “<%”, “<%=”, and before “%>”.

As mentioned earlier, all these tags are based upon XML. More accurately you could state that a JSP page could be mapped to a XML document and although this is a little touted section of the specification, I suspect you will be hearing more and more about this aspect of JSP as Java, XML, and server-side Java become more intertwined. I will not go into these mapping details here but you should be aware of them and if you need more details you should refer to the JSP specification. Therefore, you should realize that the XML equivalent syntax for the scripting elements above would be:

<jsp:declaration> declaration </jsp:declaration>
<jsp:scriptlet>   scriptlet   </jsp:scriptlet>
<jsp:expression>  expression  </jsp:expression>

Declarations are used to declare variables and methods in the scripting language used in a JSP page—Java at this time. The declaration should be a complete Java statement and should not produce any output in the current out stream. In the Hello.jsp example below the variables loadTime, loadDate and hitCount are all complete Java statements declares new variables and initializes them.

//:! c15:jsp:Hello.jsp
<%-- This JSP comment will not appear in the
generated html --%>
<%-- This is a JSP directive: --%>
<%@ page import="java.util.*" %>
<%-- These are declarations: --%>
<%!
    long loadTime= System.currentTimeMillis();
    Date loadDate = new Date();
    int hitCount = 0;
%>
<html><body>
<%-- The next several lines are the result of a
JSP expression inserted in the generated html;
the '=' indicates a JSP expression --%>
<H1>This page was loaded at <%= loadDate %> </H1>
<H1>Hello, world! It's <%= new Date() %></H1>
<H2>Here's an object: <%= new Object() %></H2>
<H2>This page has been up
<%= (System.currentTimeMillis()-loadTime)/1000 %>
seconds</H2>
<H3>Page has been accessed <%= ++hitCount %>
times since <%= loadDate %></H3>
<%-- A "scriptlet" which writes to the server
console. Note that a ';' is required: --%>
<%
   System.out.println("Goodbye");
   out.println(“Cheerio”);
%>
</body></html>
///:~

At the tail end of Hello.jsp is a scriptlet that writes “Goodbye” to the Web server console and “Cheerio” to the implicit out JspWriter object. Scriptlets can contain any code fragments that are valid Java statements. Scriptlets are executed at request-processing time. When all the scriptlets fragments in a given JSP are combined in the order they appear in the JSP page, they should yield a valid statement as defined by the Java programming language. Whether or not they produce any output into the out stream depends upon the actual code in the scriptlet. You should be careful as scriptlets can have side effects through their modification of the objects visible within them.

JSP expressions can found intermingled with the HTML in the middle section of Hello.jsp. Expressions are interesting because they must be complete Java statements, which are then evaluated. The result of the JSP expression is coerced to a java.lang.String which is emitted into the current implicit out JspWriter object. If the result of the expression cannot be coerced to a java.lang.String then a ClassCastException is thrown.

Extracting fields and values

//:! c15:jsp:DisplayFormData.jsp
<%-- Fetching the data from an HTML form. --%>
<%-- This JSP also generates the form. --%>
<%@ page import="java.util.*" %>
<html><body>
<H1>DisplayFormData</H1><H3>
<%
Enumeration f = request.getParameterNames();
if(!f.hasMoreElements()) { // No fields
%>
<form method="POST" action="DisplayFormData.jsp">
<%
  for(int i = 0; i < 10; i++) {
%>
    Field<%=i%>:
    <input type="text" size="20"
      name="Field<%=i%>" value="Value<%=i%>"><br>
<% } %>
  <INPUT TYPE=submit name=submit Value="Submit">
</form>
<% } %>
<%
Enumeration flds = request.getParameterNames();
while(flds.hasMoreElements()) {
  String field = (String)flds.nextElement();
  String value = request.getParameter(field);
%>
  <li><%= field %> = <%= value %></li>
<%
}
%></H3></body></html>
///:~

JSP Page Attributes and Scope

I have spent some time trying to get my code editor to view a .jsp page with syntax highlighting. This is bit more difficult than it would seem. Is a .jsp page Java or is it HTML? That's easy - it's both. So setting up my color coding was like melding the HTML section with the Java section. The real pint is that a .jsp page provides a new set of tags that allows you to separate the passive display code (HTML) from the dynamic programming code (Java).

There is no reason you can't have a whole block of code that performs some action that will provide content for you HTML. This action could be a database call or a call to some other resource. PageContext.jsp below calls getAttributeNamesInScope( ) method of pageContext to get all the attributes in the scope passed in (1 refers to page).

//:! c15:jsp:PageContext.jsp
<%--Viewing the attributes in the pageContext--%>
<%-- Note that you can include any amount of code
inside the scriptlet tags --%>
<%@ page import="java.util.*" %>
<html><body>
<%
  session.setAttribute("My dog", "Ralph");
  for(int scope = 1; scope <= 4; scope++) {
    out.println("<H3>Scope: " +
                 scope + "</H3><BR>");
    Enumeration e =
      pageContext.getAttributeNamesInScope(scope);
    while(e.hasMoreElements()) {
      out.println("\t<li>" +
        e.nextElement() + "</li>");
    }
  }
%>
<H4>End of list</H4>
</body></html>
///:~

The output looks like this:

Scope: 1

• javax.servlet.jsp.jspOut

• javax.servlet.jsp.jspPage

• javax.servlet.jsp.jspSession

• javax.servlet.jsp.jspApplication

• javax.servlet.jsp.jspPageContext

• javax.servlet.jsp.jspConfig

• javax.servlet.jsp.jspResponse

• javax.servlet.jsp.jspRequest

Scope: 2

• org.apache.tomcat.servlet.resolved

Scope: 3

• My dog

Scope: 4

• sun.servlet.workdir

• javax.servlet.context.tempdir

End of list

Scope 1 is the page scope and all objects reference available in this scope will be discarded upon completion of the current request by the page body. Scope 2 refers to the request scope and will be discarded upon completion of the current client request. As you can see I am using the Apache Tomcat implementation of the Servlets and JSP. (I am not sure what org.apache.tomcat.servlet.resolved is I will try to find out.) Scope 3 will be our session scope and the only object we have with session scope is the one that we added right before the for loop - "My dog". Scope 4 is the scope of our application and is based upon the ServletContext object. There is one ServletContext per "Web application" per Java Virtual Machine. (A "Web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace such as /catalog. In the Tomcat release this information is set via server.xml file.) At the application scope level we have to objects that represent paths for working directory and temporary directory.

Manipulating sessions in JSP

Let's take a closer look at sessions within the JSP model. The next example will exercise the session object a little bit and allow you to manipulate the amount of time before your session becomes invalid. First we must capture some information about this session object. I make a call to getID( ), getCreationTime( ) and getMaxInactiveInterval( ) and display these attributes about our session. When I first bring this session up in the Tomcat implementation the MaxInactiveInterval is 1800 seconds or 30 minutes. Now I know a bit about my session, so lets change its behavior by shortening the MaxInactiveInterval to 5 seconds. Now we should see some action. Next, I check to see if the object "My dog" is attached to the session object giving it session scope. The first time through this should be null but right afterwards we do create a String object "Ralph" and attach it to the session object by call setAttribute( ). Now Ralph should hang around for at least 5 seconds. The invalidate button at the bottom calls a second .jsp page SessionObject2.jsp that simply asks the session if it has the object tagged "My dog" then kills the session by calling invalidate( ) on the session object. "Ralph" is gone. The other button on the bottom of SessionObject.jsp is "Keep Around". This calls a third page, SessionObject3.jsp, that does NOT invalidate the session and you can see that "Ralph" in fact does hang around as long as your 5 second time interval does not expire. Try the refresh button on SessionObject.jsp or move back and forth between SessionObject and SessionObject3.jsp (Keep Around button) a couple of times using different intervals to get a feel for how long "My dog" stays around. (For those of you who have kids this is like the Tomagotchi pets - as long as you play with "Ralph" he will stick around otherwise he packs it up :-)

//:! c15:jsp:SessionObject.jsp
<%--Setting and getting session object values--%>
<html><body>
<H1>Session id: <%= session.getId() %></H1>
<H3><li>This session was created at
<%= session.getCreationTime() %></li></H1>
<H3><li>MaxInactiveInterval=
  <%= session.getMaxInactiveInterval() %></li></H3>
<% session.setMaxInactiveInterval(5); %>
<H3><li>Reset MaxInactiveInterval=
  <%= session.getMaxInactiveInterval() %></li></H3>
<H2>If this session object "My dog" is still around <H2>
<H3><li>Session value for "My dog" =
<%=
    session.getAttribute("My dog")
%></li></H3>
<%-- Now add the session object "My dog" --%>
<%
    session.setAttribute("My dog",
                    new String("Ralph"));
%>
<H1>My dog's name is
<%= session.getAttribute("My dog") %></H1>
<%-- See if "My dog" wanders to another form --%>
<FORM TYPE=POST ACTION=SessionObject2.jsp>
<INPUT TYPE=submit name=submit Value="Invalidate">
</FORM>
<FORM TYPE=POST ACTION=SessionObject3.jsp>
<INPUT TYPE=submit name=submit Value="Keep Around">
</FORM>
</body></html>
///:~
//:! c15:jsp:SessionObject2.jsp
<%--The session object carries through--%>
<html><body>
<H1>Session id: <%= session.getId() %></H1>
<H1>Session value for "My dog"
<%= session.getValue("My dog") %></H1>
<% session.invalidate(); %>
</body></html>
///:~
//:! c15:jsp:SessionObject3.jsp
<%--The session object carries through--%>
<html><body>
<H1>Session id: <%= session.getId() %></H1>
<H1>Session value for "My dog"
<%= session.getValue("My dog") %></H1>
<FORM TYPE=POST ACTION=SessionObject.jsp>
<INPUT TYPE=submit name=submit Value="Return">
</FORM>
</body></html>
///:~

Creating and modifying Cookies

//:! c15:jsp:Cookies.jsp
<%--This program has different behaviors under
 different browsers! --%>
<html><body>
<H1>Session id: <%= session.getId() %></H1>
<%
  Cookie[] cookies = request.getCookies();
  for(int i = 0; i < cookies.length; i++) { %>
Cookie name: <%= cookies[i].getName() %> <br>
value: <%= cookies[i].getValue() %><br>
Max age in seconds:
  <%= cookies[i].getMaxAge() %><br>
<% cookies[i].setMaxAge(3); %>
Max age in seconds:
  <%= cookies[i].getMaxAge() %><br>
<% response.addCookie(cookies[i]); %>
<% } %>
<%-- <% response.addCookie(
    new Cookie("Bob", "Car salesman")); %> --%>
</body></html>
///:~

RMI (Remote Method Invocation)

Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and you can send a message to that object and get a result as if the object lived on your local machine. This simplification is exactly what Java 1.1 Remote Method Invocation (RMI) allows you to do. This section walks you through the steps necessary to create your own RMI objects.

Remote interfaces

RMI makes heavy use of interfaces. When you want to create a remote object, you mask the underlying implementation by passing around an interface. Thus, when the client gets a reference to a remote object, what they really get is an interface reference, which happens to connect to some local stub code that talks across the network. But you don’t think about this, you just send messages via your interface reference.

When you create a remote interface, you must follow these guidelines:

  1. The remote interface must be public (it cannot have “package access,” that is, it cannot be “friendly”). Otherwise, a client will get an error when attempting to load a remote object that implements the remote interface.
  2. The remote interface must extend the interface java.rmi.Remote.
  3. Each method in the remote interface must declare java.rmi.RemoteException in its throws clause in addition to any application-specific exceptions.
  4. A remote object passed as an argument or return value (either directly or embedded within a local object) must be declared as the remote interface, not the implementation class.

Here’s a simple remote interface that represents an accurate time service:

//: c15:rmi:PerfectTimeI.java
// The PerfectTime remote interface.
package c15.rmi;
import java.rmi.*;

interface PerfectTimeI extends Remote {
  long getPerfectTime() throws RemoteException;
} ///:~

It looks like any other interface except that it extends Remote and all of its methods throw RemoteException. Remember that an interface and all of its methods are automatically public.

Implementing the remote interface

The server must contain a class that extends UnicastRemoteObject and implements the remote interface. This class can also have additional methods, but only the methods in the remote interface will be available to the client, of course, since the client will get only a reference to the interface, not the class that implements it.

You must explicitly define the constructor for the remote object even if you’re only defining a default constructor that calls the base-class constructor. You must write it out since it must throw RemoteException.

Here’s the implementation of the remote interface PerfectTimeI:

//: c15:rmi:PerfectTime.java
// The implementation of
// the PerfectTime remote object.
package c15.rmi;
import java.rmi.*;
import java.rmi.server.*;
import java.rmi.registry.*;
import java.net.*;

public class PerfectTime
    extends UnicastRemoteObject
    implements PerfectTimeI {
  // Implementation of the interface:
  public long getPerfectTime()
      throws RemoteException {
    return System.currentTimeMillis();
  }
  // Must implement constructor
  // to throw RemoteException:
  public PerfectTime() throws RemoteException {
    // super(); // Called automatically
  }
  // Registration for RMI serving:
  public static void main(String[] args) {
    System.setSecurityManager(
      new RMISecurityManager());
    try {
      PerfectTime pt = new PerfectTime();
      Naming.bind(
        "//peppy:2005/PerfectTime", pt);
      System.out.println("Ready to do time");
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

Here, main( ) handles all the details of setting up the server. When you’re serving RMI objects, at some point in your program you must:

  1. Create and install a security manager that supports RMI. The only one available for RMI as part of the Java distribution is RMISecurityManager.
  2. Create one or more instances of a remote object. Here, you can see the creation of the PerfectTime object.
  3. Register at least one of the remote objects with the RMI remote object registry for bootstrapping purposes. One remote object can have methods that produce references to other remote objects. This allows you to set it up so the client must go to the registry only once, to get the first remote object.

Setting up the registry

Here, you see a call to the static method Naming.bind( ). However, this call requires that the registry be running as a separate process on the computer. The name of the registry server is rmiregistry, and under 32-bit Windows you say:

start rmiregistry

to start it in the background. On Unix, it is:

rmiregistry &

Like many network programs, the rmiregistry is located at the IP address of whatever machine started it up, but it must also be listening at a port. If you invoke the rmiregistry as above, with no argument, the registry’s port will default to 1099. If you want it to be at some other port, you add an argument on the command line to specify the port. For this example, the port will be located at 2005, so the rmiregistry should be started like this under 32-bit Windows:

start rmiregistry 2005

or for Unix:

rmiregistry 2005 &

The information about the port must also be given to the bind( ) command, as well as the IP address of the machine where the registry is located. But this brings up what can be a frustrating problem if you’re expecting to test RMI programs locally the way the network programs have been tested so far in this chapter. In the JDK 1.1.1 release, there are a couple of problems: [69]

  1. localhost does not work with RMI. Thus, to experiment with RMI on a single machine, you must provide the name of the machine. To find out the name of your machine under 32-bit Windows, go to the control panel and select “Network.” Select the “Identification” tab, and you’ll see your computer name. In my case, I called my computer “Peppy.” It appears that capitalization is ignored.
  2. RMI will not work unless your computer has an active TCP/IP connection, even if all your components are just talking to each other on the local machine. This means that you must connect to your Internet service provider before trying to run the program or you’ll get some obscure exception messages.

With all this in mind, the bind( ) command becomes:

Naming.bind("//peppy:2005/PerfectTime", pt);

If you are using the default port 1099, you don’t need to specify a port, so you could say:

Naming.bind("//peppy/PerfectTime", pt);

You should be able to perform local testing by leaving off the IP address and using only the identifier:

Naming.bind("PerfectTime", pt);

The name for the service is arbitrary; it happens to be PerfectTime here, just like the name of the class, but you could call it anything you want. The important thing is that it’s a unique name in the registry that the client knows to look for to procure the remote object. If the name is already in the registry, you’ll get an AlreadyBoundException. To prevent this, you can always use rebind( ) instead of bind( ), since rebind( ) either adds a new entry or replaces the one that’s already there.

Even though main( ) exits, your object has been created and registered so it’s kept alive by the registry, waiting for a client to come along and request it. As long as the rmiregistry is running and you don’t call Naming.unbind( ) on your name, the object will be there. For this reason, when you’re developing your code you need to shut down the rmiregistry and restart it when you compile a new version of your remote object.

You aren’t forced to start up rmiregistry as an external process. If you know that your application is the only one that’s going to use the registry, you can start it up inside your program with the line:

LocateRegistry.createRegistry(2005);

Like before, 2005 is the port number we happen to be using in this example. This is the equivalent of running rmiregistry 2005 from a command line, but it can often be more convenient when you’re developing RMI code since it eliminates the extra steps of starting and stopping the registry. Once you’ve executed this code, you can bind( ) using Naming as before.

Creating stubs and skeletons

If you compile and run PerfectTime.java, it won’t work even if you have the rmiregistry running correctly. That’s because the framework for RMI isn’t all there yet. You must first create the stubs and skeletons that provide the network connection operations and allow you to pretend that the remote object is just another local object on your machine.

What’s going on behind the scenes is complex. Any objects that you pass into or return from a remote object must implement Serializable (if you want to pass remote references instead of the entire objects, the object arguments can implement Remote), so you can imagine that the stubs and skeletons are automatically performing serialization and deserialization as they “marshal” all of the arguments across the network and return the result. Fortunately, you don’t have to know any of this, but you do have to create the stubs and skeletons. This is a simple process: you invoke the rmic tool on your compiled code, and it creates the necessary files. So the only requirement is that another step be added to your compilation process.

The rmic tool is particular about packages and classpaths, however. PerfectTime.java is in the package c15.rmi, and even if you invoke rmic in the same directory in which PerfectTime.class is located, rmic won’t find the file, since it searches the classpath. So you must specify the location off the class path, like so:

rmic c15.rmi.PerfectTime

You don’t have to be in the directory containing PerfectTime.class when you execute this command, but the results will be placed in the current directory.

When rmic runs successfully, you’ll have two new classes in the directory:

PerfectTime_Stub.class
PerfectTime_Skel.class

corresponding to the stub and skeleton. Now you’re ready to get the server and client to talk to each other.

Using the remote object

The whole point of RMI is to make the use of remote objects simple. The only extra thing that you must do in your client program is to look up and fetch the remote interface from the server. From then on, it’s just regular Java programming: sending messages to objects. Here’s the program that uses PerfectTime:

//: c15:rmi:DisplayPerfectTime.java
// Uses remote object PerfectTime.
package c15.rmi;
import java.rmi.*;
import java.rmi.registry.*;

public class DisplayPerfectTime {
  public static void main(String[] args) {
    System.setSecurityManager(
      new RMISecurityManager());
    try {
      PerfectTimeI t =
        (PerfectTimeI)Naming.lookup(
          "//peppy:2005/PerfectTime");
      for(int i = 0; i < 10; i++)
        System.out.println("Perfect time = " +
          t.getPerfectTime());
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

The ID string is the same as the one used to register the object with Naming, and the first part represents the URL and port number. Since you’re using a URL, you can also specify a machine on the Internet.

What comes back from Naming.lookup( ) must be cast to the remote interface, not to the class. If you use the class instead, you’ll get an exception.

You can see in the method call

t.getPerfectTime( )

that once you have a reference to the remote object, programming with it is indistinguishable from programming with a local object (with one difference: remote methods throw RemoteException).

Introduction to CORBA

In large, distributed applications, your needs might not be satisfied by the preceding approaches. For example, you might want to interface with legacy datastores, or you might need services from a server object regardless of its physical location. These situations require some form of Remote Procedure Call (RPC), and possibly language independence. This is where CORBA can help.

CORBA is not a language feature; it’s an integration technology. It’s a specification that vendors can follow to implement CORBA-compliant integration products. CORBA is part of the OMG’s effort to define a standard framework for distributed, language-independent object interoperability.

CORBA supplies the ability to make remote procedure calls into Java objects and non-Java objects, and to interface with legacy systems in a location-transparent way. Java adds networking support and a nice object-oriented language for building graphical and non-graphical applications. The Java and OMG object model map nicely to each other; for example, both Java and CORBA implement the interface concept and a reference object model.

CORBA Fundamentals

The object interoperability specification developed by the OMG is commonly referred to as the Object Management Architecture (OMA). The OMA defines two components: the Core Object Model and the OMA Reference Architecture. The Core Object Model states the basic concepts of object, interface, operation, and so on. (CORBA is a refinement of the Core Object Model.) The OMA Reference Architecture defines an underlying infrastructure of services and mechanisms that allow objects to interoperate. The OMA Reference Architecture includes the Object Request Broker (ORB), Object Services (also known as CORBAservices), and common facilities.

The ORB is the communication bus by which objects can request services from other objects, regardless of their physical location. This means that what looks like a method call in the client code is actually a complex operation. First, a connection with the server object must exist, and to create a connection the ORB must know where the server implementation code resides. Once the connection is established, the method arguments must be marshaled, i.e. converted in a binary stream to be sent across a network. Other information that must be sent are the server machine name, the server process, and the identity of the server object inside that process. Finally, this information is sent through a low-level wire protocol, the information is decoded on the server side, and the call is executed. The ORB hides all of this complexity from the programmer and makes the operation almost as simple as calling a method on local object.

There is no specification for how an ORB Core should be implemented, but to provide a basic compatibility among different vendors’ ORBs, the OMG defines a set of services that are accessible through standard interfaces.

CORBA Interface Definition Language (IDL)

CORBA is designed for language transparency: a client object can call methods on a server object of different class, regardless of the language they are implemented with. Of course, the client object must know the names and signatures of methods that the server object exposes. This is where IDL comes in. The CORBA IDL is a language-neutral way to specify data types, attributes, operations, interfaces, and more. The IDL syntax is similar to the C++ or Java syntax. The following table shows the correspondence between some of the concepts common to three languages that can be specified through CORBA IDL:

CORBA IDL | Java | C++
Module | Package | Namespace
Interface | Interface | Pure abstract class
Method | Method | Member function

The inheritance concept is supported as well, using the colon operator as in C++. The programmer writes an IDL description of the attributes, methods, and interfaces that will be implemented and used by the server and clients. The IDL is then compiled by a vendor-provided IDL/Java compiler, which reads the IDL source and generates Java code.

The IDL compiler is an extremely useful tool: it doesn’t just generate a Java source equivalent of the IDL, it also generates the code that will be used to marshal method arguments and to make remote calls. This code, called the stub and skeleton code, is organized in multiple Java source files and is usually part of the same Java package.

The naming service

The naming service is one of the fundamental CORBA services. A CORBA object is accessed through a reference, a piece of information that’s not meaningful for the human reader. But references can be assigned programmer-defined, string names. This operation is known as stringifying the reference, and one of the OMA components, the Naming Service, is devoted to performing string-to-object and object-to-string conversion and mapping. Since the Naming Service acts as a telephone directory that both servers and clients can consult and manipulate, it runs as a separate process. Creating an object-to-string mapping is called binding an object, and removing the mapping is called unbinding. Getting an object reference passing a string is called resolving the name.

For example, on startup, a server application could create a server object, bind the object into the name service, and then wait for clients to make requests. A client first obtains a server object reference, resolving the string name, and then can make calls into the server using the reference.

Again, the Naming Service specification is part of CORBA, but the application that implements it is provided by the ORB vendor. The way you get access to the Naming Service functionality can vary from vendor to vendor.

An example

The code shown here will not be elaborate because different ORBs have different ways to access CORBA services, so examples are vendor specific. (The example below uses JavaIDL, a free product from Sun that comes with a light-weight ORB, a naming service, and an IDL-to-Java compiler.) In addition, since Java is young and still evolving, not all CORBA features are present in the various Java/CORBA products.

We want to implement a server, running on some machine, that can be queried for the exact time. We also want to implement a client that asks for the exact time. In this case we’ll be implementing both programs in Java, but we could also use two different languages (which often happens in real situations).

Writing the IDL source

The first step is to write an IDL description of the services provided. This is usually done by the server programmer, who is then free to implement the server in any language in which a CORBA IDL compiler exists. The IDL file is distributed to the client side programmer and becomes the bridge between languages.

The example below shows the IDL description of our ExactTime server:

//: c15:corba:ExactTime.idl
//# You must install idltojava.exe from
//# java.sun.com and adjust the settings to use
//# your local C preprocessor in order to compile
//# This file. See docs at java.sun.com.
module remotetime {
   interface ExactTime {
      string getTime();
   };
}; ///:~

This is a declaration of the ExactTime interface inside the remotetime namespace. The interface is made up of one single method that gives back the current time in string format.

Creating stubs and skeletons

The second step is to compile the IDL to create the Java stub and skeleton code that we’ll use for implementing the client and the server. The tool that comes with the JavaIDL product is idltojava:

idltojava remotetime.idl

This will automatically generate code for both the stub and the skeleton. Idltojava generates a Java package named after the IDL module, remotetime, and the generated Java files are put in the remotetime subdirectory. _ExactTimeImplBase.java is the skeleton that we’ll use to implement the server object, and _ExactTimeStub.java will be used for the client. There are Java representations of the IDL interface in ExactTime.java and a couple of other support files used, for example, to facilitate access to the naming service operations.

Implementing the server and the client

Below you can see the code for the server side. The server object implementation is in the ExactTimeServer class. The RemoteTimeServer is the application that creates a server object, registers it with the ORB, gives a name to the object reference, and then sits quietly waiting for client requests.

//: c15:corba:RemoteTimeServer.java
import remotetime.*;
import org.omg.CosNaming.*;
import org.omg.CosNaming.NamingContextPackage.*;
import org.omg.CORBA.*;
import java.util.*;
import java.text.*;

// Server object implementation
class ExactTimeServer extends _ExactTimeImplBase {
  public String getTime(){
    return DateFormat.
        getTimeInstance(DateFormat.FULL).
          format(new Date(
              System.currentTimeMillis()));
  }
}

// Remote application implementation
public class RemoteTimeServer {
  public static void main(String[] args) {
    try {
      // ORB creation and initialization:
      ORB orb = ORB.init(args, null);
      // Create the server object and register it:
      ExactTimeServer timeServerObjRef =
        new ExactTimeServer();
      orb.connect(timeServerObjRef);
      // Get the root naming context:
      org.omg.CORBA.Object objRef =
        orb.resolve_initial_references(
          "NameService");
      NamingContext ncRef =
        NamingContextHelper.narrow(objRef);
      // Assign a string name to the
      // object reference (binding):
      NameComponent nc =
        new NameComponent("ExactTime", "");
      NameComponent[] path = { nc };
      ncRef.rebind(path, timeServerObjRef);
      // Wait for client requests:
      java.lang.Object sync =
        new java.lang.Object();
      synchronized(sync){
        sync.wait();
      }
    }
    catch (Exception e) {
      System.out.println(
         "Remote Time server error: " + e);
      e.printStackTrace(System.out);
    }
  }
} ///:~

As you can see, implementing the server object is simple; it’s a regular Java class that inherits from the skeleton code generated by the IDL compiler. Things get a bit more complicated when it comes to interacting with the ORB and other CORBA services.

Some CORBA services

This is a short description of what the JavaIDL-related code is doing (primarily ignoring the part of the CORBA code that is vendor dependent). The first line in main( ) starts up the ORB, and of course, this is because our server object will need to interact with it. Right after the ORB initialization, a server object is created. Actually, the right term would be a transient servant object: an object that receives requests from clients, and whose lifetime is the same as the process that creates it. Once the transient servant object is created, it is registered with the ORB, which means that the ORB knows of its existence and can now forward requests to it.

Up to this point, all we have is timeServerObjRef, an object reference that is known only inside the current server process. The next step will be to assign a stringified name to this servant object; clients will use that name to locate the servant object. We accomplish this operation using the Naming Service. First, we need an object reference to the Naming Service; the call to resolve_initial_references( ) takes the stringified object reference of the Naming Service that is “NameService,” in JavaIDL, and returns an object reference. This is cast to a specific NamingContext reference using the narrow( ) method. We can use now the naming services.

To bind the servant object with a stringified object reference, we first create a NameComponent object, initialized with “ExactTime,” the name string we want to bind to the servant object. Then, using the rebind( ) method, the stringified reference is bound to the object reference. We use rebind( ) to assign a reference, even if it already exists, whereas bind( ) raises an exception if the reference already exists. A name is made up in CORBA by a sequence of NameContexts—that’s why we use an array to bind the name to the object reference.

The servant object is finally ready for use by clients. At this point, the server process enters a wait state. Again, this is because it is a transient servant, so its lifetime is confined to the server process. JavaIDL does not currently support persistent objects—objects that survive the execution of the process that creates them.

Now that we have an idea of what the server code is doing, let’s look at the client code:

//: c15:corba:RemoteTimeClient.java
import remotetime.*;
import org.omg.CosNaming.*;
import org.omg.CORBA.*;

public class RemoteTimeClient {
  public static void main(String[] args) {
    try {
      // ORB creation and initialization:
      ORB orb = ORB.init(args, null);
      // Get the root naming context:
      org.omg.CORBA.Object objRef =
        orb.resolve_initial_references(
          "NameService");
      NamingContext ncRef =
        NamingContextHelper.narrow(objRef);
      // Get (resolve) the stringified object
      // reference for the time server:
      NameComponent nc =
        new NameComponent("ExactTime", "");
      NameComponent[] path = { nc };
      ExactTime timeObjRef =
        ExactTimeHelper.narrow(
          ncRef.resolve(path));
      // Make requests to the server object:
      String exactTime = timeObjRef.getTime();
      System.out.println(exactTime);
    } catch (Exception e) {
      System.out.println(
         "Remote Time server error: " + e);
      e.printStackTrace(System.out);
    }
  }
} ///:~

The first few lines do the same as they do in the server process: the ORB is initialized and a reference to the naming service server is resolved. Next, we need an object reference for the servant object, so we pass the stringified object reference to the resolve( ) method, and we cast the result into an ExactTime interface reference using the narrow( ) method. Finally, we call getTime( ).

Activating the name service process

Finally we have a server and a client application ready to interoperate. You’ve seen that both need the naming service to bind and resolve stringified object references. You must start the naming service process before running either the server or the client. In JavaIDL, the naming service is a Java application that comes with the product package, but it can be different with other products. The JavaIDL naming service runs inside an instance of the JVM and listens by default to network port 900.

Activating the server and the client

Now you are ready to start your server and client application (in this order, since our server is transient). If everything is set up correctly, what you’ll get is a single output line on the client console window, giving you the current time. Of course, this might be not very exciting by itself, but you should take one thing into account: even if they are on the same physical machine, the client and the server application are running inside different virtual machines and they can communicate via an underlying integration layer, the ORB and the Naming Service.

This is a simple example, designed to work without a network, but an ORB is usually configured for location transparency. When the server and the client are on different machines, the ORB can resolve remote stringified references using a component known as the Implementation Repository. Although the Implementation Repository is part of CORBA, there is almost no specification, so it differs from vendor to vendor.

As you can see, there is much more to CORBA than what has been covered here, but you should get the basic idea. If you want more information about CORBA, the place to start is the OMG Web site, at http://www.omg.org. There you’ll find documentation, white papers, proceedings, and references to other CORBA sources and products.

Java Applets and CORBA

Java applets can act as CORBA clients. This way, an applet can access remote information and services exposed as CORBA objects. But an applet can connect only with the server from which it was downloaded, so all the CORBA objects the applet interacts with must be on that server. This is the opposite of what CORBA tries to do: give you complete location transparency.

This is an issue of network security. If you’re on an intranet, one solution is to loosen the security restrictions on the browser. Or, set up a firewall policy for connecting with external servers.

Some Java ORB products offer proprietary solutions to this problem. For example, some implement what is called HTTP Tunneling, while others have their special firewall features.

This is too complex a topic to be covered in an appendix, but it is definitely something you should be aware of.

CORBA vs. RMI

You saw that one of the main CORBA features is RPC support, which allows your local objects to call methods in remote objects. Of course, there already is a native Java feature that does exactly the same thing: RMI (see Chapter 15). While RMI makes RPC possible between Java objects, CORBA makes RPC possible between objects implemented in any language. It’s a big difference.

However, RMI can be used to call services on remote, non-Java code. All you need is some kind of wrapper Java object around the non-Java code on the server side. The wrapper object connects externally to Java clients via RMI, and internally connects to the non-Java code using one of the techniques shown above, such as JNI or J/Direct.

This approach requires you to write a kind of integration layer, which is exactly what CORBA does for you, but then you don’t need a third-party ORB.

Enterprise Java Beans

[70] By now, you've been introduced to CORBA and RMI. But could you imagine trying to develop a large-scale application using CORBA and/or RMI? Your manager has asked you to develop a multi-tiered application to view and update records in a database through a Web interface. You sit back and think of what that really means. Sure, you can write a database application using JDBC, a Web interface using JSP/Servlets, and a distributed system using CORBA/RMI. But what extra considerations must you make when developing a distributed object based system rather than just knowing API's? Here are the issues:

Performance: The new distributed objects that you are creating are going to have to perform, as they potentially could service many clients at a time. So you'll want to think of optimization techniques such as caching, pooling of resources (e.g., JDBC database connections). You'll also have to manage the lifecycle of your distributed object.

Scalability: The distributed objects must also be scalable. Scalable in a distributed application sense means that the number of instances of your distributed object can be increased and moved over to a different machine without the modification of any code. Take for example a system that you develop internally as a small lookup of clients inside your organization from a database. The application works well when you use it, but your manager has seen it and has said "Robert, that is an excellent system, get it on our public Web-site now!!!"-Will my distributed object be able to handle the load of a potentially limitless demand?

Security: Does my distributed object manage the authorization of the clients that accesses it? Can I add new users and roles to it without recompilation?

Distributed Transactions: Can my distributed object reference distributed-transactions transparently? Can I update my Oracle and Sybase databases simultaneously within the same transaction and roll them both back if a certain criteria is not met?

Reusability: Have I created my distributed object so that I can move it into another vendors' application server? Can I resell my distributed object (component) to somebody else? Can I buy somebody else's component and use it without having to recompile and 'hack it into shape'?

Availability: If one of the machines in my system was to go down, are my clients able to automatically fail-over to back up copies of my objects running on other machines?

As you can see from the above, the considerations that a developer must make when developing a distributed system and we haven't even mentioned solving the problem that we were originally trying to solve!

So you now have your list of extra problems that you must solve. So how do you go about doing it? Surely somebody must have done this before? Could I use some well-known design patterns to help me solve these problems? Then an idea flashes in your head... "I could create a framework that handles all of these issues and write my components on top of the framework!".... This is where Enterprise JavaBeans comes into the picture.

Sun, along with other leading distributed object vendors (listed at URL HERE!) realized that sooner or later every development team would be reinventing the wheel. So they created the Enterprise JavaBeans specification (EJB). EJB is a specification for a server-side component model that tackles all of the considerations mentioned above using a defined, standard approach that allows developers to create components-which are actually called Enterprise JavaBeans (EJB's)-that are isolated from low-level 'plumbing' code and focus solely on providing business logic. Because EJB's are defined as a standard they can be used without being vendor dependent.

What’s defined in the EJB specification?

The Enterprise JavaBeans specification, currently at version 1.1 - public release 2 - defines a server side component model. It defines 6 roles that are used to perform the tasks in development and deployment as well as defining the components of the system.

Roles

The EJB specification defines roles that are used during in the development, deployment and running of a distributed system. Vendors, administrators and developers play the various roles. They allow the partitioning of technical and domain knowledge. This allows the vendor to provide a technically sound framework and the developers to create domain specific components e.g., an Accounting component. The same party can perform one or many roles. The roles defined in the EJB specification have been summarized in the following table:

Role | Responsibility
Enterprise Bean Provider | The developer who is responsible for creating reusable EJB components. These components are packaged into a special jar file (ejb-jar file).
Application Assembler | Create and assemble applications from a collection of ejb-jar files. This includes writing applications that utilize the collection of EJB’s (e.g., Servlets, JSP, Swing etc. etc.)
Deployer | The Deployer’s role of the to take the collection of ejb-jar files from the Assembler and/or Bean Provider and deploy them into a run-time environment - one or many EJB Container(s).
EJB Container/Server Provider | Provide a run-time environment and tools that are used to deploy, administer and “run” EJB components.
System Administrator | Over see the most important goal of the entire system - That it is up and running. Management of a distributed application can consist of many different components and services all configured and interacting together correctly.

Components of EJB

EJB components are reusable business logic. EJB components adhere to strict standards and design patterns as defined in the EJB specification. This allows the components to be portable and also allow other services—such as security, caching and distributed transactions - to be performed on the components’ behalf. An Enterprise Bean Provider is responsible for developing EJB components. The internals of an EJB component are covered in - “What makes up an EJB component?”

EJB Container

The EJB Container is a run-time environment that contains -or runs—EJB components and provides a set of standard services to the components. The EJB Containers responsibilities are tightly defined by the specification to allow for vendor neutrality. The EJB container provides the low-level “plumbing” of EJB, including distributed transactions, security, life cycle management of beans, caching, threading and session management. The EJB Container Provider is responsible for providing an EJB Container.

EJB Server

An EJB Server is defined as an Application Server that contains and runs 1 or more EJB Containers. that both the Container and Server are the same vendor. The EJB Server Provider is responsible for providing an EJB Server. The specification suggests and you can assume for this introduction, that the EJB Container and EJB Server are the same.

Java Naming and Directory Interface (JNDI)

Java Naming and Directory Interface (JNDI) is used in Enterprise JavaBeans as the naming service for EJB Components on the network and other container services such as transactions. JNDI maps very closely to other naming and directory standards such as CORBA CosNaming and can actually be implemeted as a wrapper on top of it.

Java Transaction API / Java Transaction Service (JTA/JTS)

JTA/JTS is used in Enterprise JavaBeans as the transactional API. An Enterprise Bean Provider can use the JTS to create transactional code although the EJB Container commonly implements transactions in EJB on the EJB components’ behalf. The Deployer can define the transactional attributes of an EJB component at deployment time. The EJB Container is responsible for handling the transaction whether it is local or distributed. The JTS specification is the Java mapping to the CORBA OTS (Object Transaction Service)

CORBA and RMI/IIOP

The EJB specification defines interoperablility with CORBA. The 1.1 specification quotes “The Enterprise JavaBeans architecture will be compatible with the CORBA protocols.” CORBA interoperability is achieved through the mapping of EJB services such as JTS and JNDI to corresponding CORBA services and the implementation of RMI on top of the CORBA protocol IIOP.

Use of CORBA and RMI/IIOP in Enterprise JavaBeans is implemented in the EJB Container and is the responsibility of the EJB Container provider. Use of CORBA and RMI/IIOP in the EJB Container is hidden from the EJB Component itself. This means that the Enterprise Bean Provider can write their EJB Component and deploy it into any EJB Container without any regard of which communication protocol is being used.

The components and available services of EJB.

What makes up an EJB component?

Enterprise Bean

The Enterprise Bean is a Java class that the Enterprise Bean Provider develops. It implements an EnterpriseBean interface (more detail in a later section) and provides the implementation of the business methods that the component is to perform. The class does not implement any authorization/authentication code, multithreading, transactional code.

Home Interface

Every Enterprise Bean that is created must have an associated Home interface. The Home interface is used as a factory for your EJB. Clients use the Home interface to find an instance of your EJB or create a new instance of your EJB.

Remote Interface

The Remote interface is a Java Interface that reflects the methods of your Enterprise Bean that you wish to expose to the outside world. The Remote interface plays a similar role to a CORBA IDL interface.

Deployment Descriptor

The Deployment Descriptor is an XML file that contains information about your EJB. Using XML allows the Deployer to easily change attributes about your EJB. The configurable attributes defined in the Deployment Descriptor include:

  • The Home and Remote interface names that are required by your EJB
  • The name to publish into JNDI for your EJB’s Home interface
  • Transactional attributes for each method of your EJB
  • Access Control Lists for authentication

EJB-Jar File

The EJB-Jar file is a normal java jar file that contains your EJB, Home and Remote interfaces, as well as the Deployment Descriptor.

How does EJB work?

Now that we have our EJB-Jar file containing our Bean, Home and Remote interfaces and Deployment Descriptor, Let’s take a look at how all of these pieces fit together and why Home and Remote interfaces are needed and how the EJB Container uses them.

Who implements the Home and Remote Interfaces?

The EJB Container implements the Home and Remote interfaces that are in our EJB-Jar file. Because the EJB Container implements the Home interface that - as mentioned earlier - provides methods to create and find your EJB. This means that the EJB Container is responsible for the lifecycle management of your EJB. This level of indirection allows for optimizations to occur. For example 5 clients simultaneously request to create an EJB through a Home Interface, the EJB Container could create only one and share that EJB between all 5 clients. This is achieved through the Remote Interface, which is again implemented by the EJB Container. The implemented Remote object plays the role of a proxy object to the EJB.

The following diagram show the level of indirection achieved by this approach and that all calls to the EJB are ‘proxied’ through the EJB Container via the Home and Remote interfaces. This level of indirection is also the reason why the EJB Container can control security and transactional behavior.

Types of EJB’s

There should be one question in your head from the previous section, “Surely sharing the same EJB between clients can improve performance, but what If I want to maintain state on my server?”

The Enterprise JavaBeans specification defines different types of EJB’s that have different characteristics and exhibit different behavior. Two categories of EJB’s have been defined in the specification. Session Beans and Entity Beans, and each of these categories has variations. A hierarchy of the various types of EJB components is shown in the following figure.

Session Beans

Session Beans are used to represent Use-Cases or Workflow on behalf of a client. They represent operations on persistent data, not persistent data itself. There are two types of Session Beans, Stateless and Stateful. All Session Beans must implement the javax.ejb.SessionBean interface. The EJB Container governs the life of a Session Bean. If the EJB Container crashes, data for all Stateful Session Beans could be lost. Some high-end EJB Containers provide recovery for Stateful Session Beans.

Stateless Session Beans

Stateless Session Beans are the simplest type of EJB component to implement. They do not maintain any conversational state with clients between method invocations so they are easily reusable on the server side and because they can be cached, they scale well on demand. When using Stateless Session Beans, all state must be stored outside of the EJB.

Stateful Session Beans

Stateful Session Beans - as you could probably guess - maintain state between invocations. They have a 1 to 1 logical mapping to a client and can maintain state within themselves. The EJB Container is responsible for pooling and caching of Stateful Session Beans, which is achieved through Passivation and Activation.

Entity Beans

Entity Beans are components that represent persistent data and behavior of this data. Entity Beans can be shared amongst multiple clients, the same as data in a database. The EJB Container is responsible for caching Entity Beans and for maintaining the integrity of the Entity Beans. The life of an Entity Bean outlives the EJB Container, so if an EJB Container crashes, the Entity Bean is expected to still be available when the EJB Container becomes available.

There are two types of Entity Beans, those that have Bean-Managed persistence and Container Managed persistence.

Container Managed Persistence (CMP)

A CMP Entity Bean has its’ persistence implemented on its behalf by the EJB Container. Through attributes specified in the Deployment Descriptor, the EJB Container will map the Entity Bean’s attributes to some persistent store (usually -but not always—a database). CMP reduces the time to develop and dramatically reduces the amount of code required for the EJB.

Bean Managed Persistence (BMP)

A BMP Entity Bean has its’ persistence implemented by the Enterprise Bean Provider. The Enterprise Bean Provider is responsible for implementing the logic required to create a new EJB, update some attributes of the EJBS, delete an EJB and find an EJB from persistent store. This usually involves writing JDBC code to interact with a database or other persistent store. With BMP, the developer is in full control of how the Entity Bean persistence is managed.

BMP also gives flexibility where a CMP implementation may not be available e.g., if you wanted to create an EJB that wrapped some code on an existing mainframe system, you could write your persistence using CORBA.

How do I put the ‘E’ in my existing JavaBeans?

There is much confusion about the relationship between the JavaBeans component model and the Enterprise JavaBeans specification. Whilst both the JavaBeans and Enterprise JavaBeans specifications share the same objectives in promoting reuse and portability of Java code between development and deployment tools with the use of standard design patterns, the motives behind each specification are geared to solve different problems.

The standards defined in the JavaBeans component model are designed for creating reusable components that are typically used in IDE development tools and are commonly, although not exclusively visual components.

The Enterprise JavaBeans specification defines a component model for developing server side java code. Because EJB’s can potentially run on many different server-side platforms -including mainframes that do not have visual displays - An EJB cannot make use of the java.awt package.

Developing an Enterprise Java Bean

We will now implement the “Perfect Time” example from the previous RMI section as an Enterprise JavaBean component. The example will be a simple Stateless Session Bean. Enterprise JavaBean components will consist of at least one class and two interfaces.

The first interface defined is the Remote Interface to our Enterprise JavaBean component. When you create a Remote interface for an EJB , you must follow these guidelines:

  1. The remote interface must be public.
  2. The remote interface must extend the interface javax.ejb.EJBObject.
  3. Each method in the remote interface must declare java.rmi.RemoteException in its throws clause in addition to any application-specific exceptions.
  4. Any object passed as an argument or return value (either directly or embedded within a local object) must be a valid RMI-IIOP data type (this includes other EJB objects)

Here is the simple remote interface for our PerfectTime EJB:

//: c15:ejb:PerfectTime.java
//# You must install the J2EE Java Enterprise
//# Edition from java.sun.com and add j2ee.jar
//# To your CLASSPATH in order to compile
//# This file. See details at java.sun.com.
// Remote Interface of PerfectTimeBean
import java.rmi.*;
import javax.ejb.*;

public interface PerfectTime extends EJBObject {
  public long getPerfectTime()
    throws RemoteException;
} ///:~

The second interface defined is the Home Interface to our Enterprise JavaBean component. The Home interface is the factory where our component will be created. The Home interface can define create or finder methods. Create methods create instances of EJB’s, finder methods locate existing EJB’s and are used for Entity Beans only. When you create a Home interface for an EJB , you must follow these guidelines:

  1. The Home interface must be public.
  2. The Home interface must extend the interface javax.ejb.EJBHome.
  3. Each method in the Home interface must declare java.rmi.RemoteException in its throws clause as well as a javax.ejb.CreateException
  4. The return value of a create method must be a Remote Interface
  5. The return value of a finder method (Entity Beans only) must be a Remote Interface or java.util.Enumeration or java.util.Collection
  6. Any object passed as an argument (either directly or embedded within a local object) must be a valid RMI-IIOP data type (this includes other EJB objects)

The standard naming convention for Home interfaces is to take the Remote interface name and append “Home” to the end. Following is the Home interface for our PerfectTime EJB:

//: c15:ejb:PerfectTimeHome.java
// Home Interface of PerfectTimeBean.
import java.rmi.*;
import javax.ejb.*;

public interface PerfectTimeHome extends EJBHome {
  public PerfectTime create()
    throws CreateException, RemoteException;
} ///:~

Now that we have defined the interfaces of our component, we can now implement the business logic behind it. When you create your EJB implementation class, you must follow these guidelines, (note that you should consult the EJB specification for a complete list of guidelines when developing Enterprise JavaBeans):

  1. The class must be public.
  2. The class must implement an EJB interface (either javax.ejb.SessionBean or javax.ejb.EntityBean).
  3. The class should define methods that map directly to the methods in the Remote interface, Note that the class does not implement the Remote interface, it mirrors the methods in the Remote interface but does NOT throw java.rmi.RemoteException.
  4. Define one or more ejbCreate( ) methods to initialize your EJB.
  5. The return value and arguments of all methods must be valid RMI-IIOP data types.
//: c15:ejb:PerfectTimeBean.java
// Simple Stateless Session Bean
// that returns current system time.
import java.rmi.*;
import javax.ejb.*;

public class PerfectTimeBean
  implements SessionBean {
  private SessionContext sessionContext;
  //return current time
  public long getPerfectTime() {
     return System.currentTimeMillis();
  }
  // EJB methods
  public void
  ejbCreate() throws CreateException {}
  public void ejbRemove() {}
  public void ejbActivate() {}
  public void ejbPassivate() {}
  public void
  setSessionContext(SessionContext ctx) {
    sessionContext = ctx;
  }
}///:~

Notice that the EJB methods (ejbCreate( ), ejbRemove( ), ejbActivate( ), ejbPassivate( ) ) are all empty. These methods are invoked by the EJB Container and are used to control the state of our component. As this is a simple example, we can leave these empty. The setSessionContext( ) method passes a javax.ejb.SessionContext object which contains information about context that the component is in, such as the current transaction and security information.

After we have created our Enterprise Java Bean, we then need to create a Deployment Descriptor. In EJB 1.1, the Deployment Descriptor is an XML file that describes the EJB component. The Deployment Descriptor should be stored in a file called ejb-jar.xml.

<?xml version="1.0" encoding="Cp1252"?>
<!DOCTYPE ejb-jar PUBLIC '-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 1.1//EN' 'http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd'>

<ejb-jar>
  <description>example for chapter 15</description>
  <display-name></display-name>
  <small-icon></small-icon>
  <large-icon></large-icon>
  <enterprise-beans>
    <session>
      <ejb-name>PerfectTime</ejb-name>
      <home>PerfectTimeHome</home>
      <remote>PerfectTime</remote>
      <ejb-class>PerfectTimeBean</ejb-class>
      <session-type>Stateless</session-type>
      <transaction-type>Container</transaction-type>
    </session>
  </enterprise-beans>
  <ejb-client-jar></ejb-client-jar>
</ejb-jar>

Inside the <session> tag of our deployment descriptor we can see that our Component, the Remote interface and Home interfaces are begin defined. Deployment Descriptors can easily be automatically generated inside tools such as JBuilder.

Along with the standard ejb-jar.xml deployment descriptor the EJB 1.1 specification states that any vendor specific tags should be stored in a separate file, this is to achieve high portability between components and different brands EJB containers.

Now that we have created our component, and defined it’s composition in the Deployment Descriptor we then need to archive the files inside a standard Java Archive (JAR) file. The Deployment Descriptors should be placed inside the /META-INF sub-directory of the Jar file.

Once we have defined our EJB component in the Deployment Descriptor, the Deployer should then deploy the EJB component into the EJB Container. At the time of writing, this process was quite "gui intensive" and specific to each individual EJB Container, so we decided not to document the entire deployment process in this overview. Every EJB Container, however will have a documented process for deploying an EJB.

Because an EJB component is a distributed object, the deployment process should also create some client stubs for calling the EJB component. These classes whould be placed on the classpath of the client application. Because EJB components can be implemented on top of RMI-IIOP (CORBA) or RMI-JRMP, the stubs generated could vary between EJB Containers, nevertheless they are generated classes.

When a client program wishes to invoke an EJB, it must lookup the EJB component inside JNDI and obtain a reference to the Home interface of the EJB component. The Home Interface can then be invoked to create an instance of the EJB, which can then be invoked.

In this example the Client program is a simple Java program, but you should remember that it could just as easily be a Servlet, a JSP even a CORBA or RMI distributed object.

The PerfectTimeClient code is as follows.

//: c15:ejb:PerfectTimeClient.java
// Client program for PerfectTimeBean

public class PerfectTimeClient {
public static void
main(String[] args) throws Exception {
  // Get a JNDI context using the
  // JNDI Naming service:
  javax.naming.Context context =
    new javax.naming.InitialContext();
  // Look up the home interface in the
  // JNDI Naming service:
  Object ref = context.lookup("perfectTime");
  // Cast the remote object to the home interface:
  PerfectTimeHome home = (PerfectTimeHome)
    javax.rmi.PortableRemoteObject.narrow(
      ref, PerfectTimeHome.class);
  // Create a remote object from the home interface:
  PerfectTime pt = home.create();
  // Invoke  getPerfectTime()
  System.out.println(
    "Perfect Time EJB invoked, time is: " +
    pt.getPerfectTime() );
  }
} ///:~

In Summary

The Enterprise JavaBeans specification - although intially seems very daunting - is a dramatic step forward in the standardization and simplification or distributed object computing. It is a major piece of the Java 2, Enterprise Edition Platform and is receiving much support from the Distributed Object community. Many tools are currently available or will be in the near future to help accelerate the development of EJB components.

This overview was aimed at giving you a brief tour as to what EJB is all about. For more information about the Enterprise JavaBeans specification you should see the Official Enterprise JavaBeans Home Page at http://java.sun.com/products/ejb/. Here you can download the latest specification as well as the Java 2, Enterprise Edition Reference Implementation, which you can use to develop and deploy your own EJB components.

Jini: distributed services

This section [71] gives an overview of Sun Microsystems's Jini technology. It describes some Jini nuts and bolts and shows how Jini's architecture helps to raise the level of abstraction in distributed systems programming, effectively turning network programming into object-oriented programming.

Jini in context

Traditionally, operating systems have been designed with the assumption that a computer will have a processor, some memory, and a disk. When you boot a computer, the first thing it does is look for a disk. If it doesn't find a disk, it can't function as a computer. Increasingly, however, computers are appearing in a different guise: as embedded devices that have a processor, some memory, and a network connection—but no disk. The first thing a cellphone does when you boot it up, for example, is look for the telephone network. If it doesn't find the network, it can't function as a cellphone. This trend in the hardware environment, from disk-centric to network-centric, will affect how we organize our software—and that's where Jini comes in.

Jini is an attempt to rethink computer architecture, given the rising importance of the network and the proliferation of processors in devices that have no disk drive. These devices, which will come from many different vendors, will need to interact over a network. The network itself will be very dynamic—devices and services will be added and removed regularly. Jini provides mechanisms to enable smooth adding, removal, and finding of devices and services on the network. In addition, Jini provides a programming model that makes it easier for programmers to get their devices talking to each other.

Building on top of Java, object serialization, and RMI (which enable objects to move around the network from virtual machine to virtual machine) Jini attempts to extend the benefits of object-oriented programming to the network. Instead of requiring device vendors to agree on the network protocols through which their devices can interact, Jini enables the devices to talk to each other through interfaces to objects.

What is Jini?

Jini is a set of APIs and network protocols that can help you build and deploy distributed systems that are organized as federations of services. A service can be anything that sits on the network and is ready to perform a useful function. Hardware devices, software, communications channels—even human users themselves—can be services. A Jini-enabled disk drive, for example, could offer a “storage” service. A Jini-enabled printer could offer a “printing” service. A federation of services, then, is a set of services, currently available on the network, that a client (meaning a program, service, or user) can bring together to help it accomplish some goal.

To perform a task, a client enlists the help of services. For example, a client program might upload pictures from the image storage service in a digital camera, download the pictures to a persistent storage service offered by a disk drive, and send a page of thumbnail-sized versions of the images to the printing service of a color printer. In this example, the client program builds a distributed system consisting of itself, the image storage service, the persistent storage service, and the color-printing service. The client and services of this distributed system work together to perform the task: to offload and store images from a digital camera and print a page of thumbnails.

The idea behind the word federation is that the Jini view of the network doesn't involve a central controlling authority. Because no one service is in charge, the set of all services available on the network form a federation—a group composed of equal peers. Instead of a central authority, Jini's run-time infrastructure merely provides a way for clients and services to find each other (via a lookup service, which stores a directory of currently available services). After services locate each other, they are on their own. The client and its enlisted services perform their task independently of the Jini run-time infrastructure. If the Jini lookup service crashes, any distributed systems brought together via the lookup service before it crashed can continue their work. Jini even includes a network protocol that clients can use to find services in the absence of a lookup service.

How Jini works

Jini defines a run-time infrastructure that resides on the network and provides mechanisms that enable you to add, remove, locate, and access services. The run-time infrastructure resides in three places: in lookup services that sit on the network, in the service providers (such as Jini-enabled devices), and in clients. Lookup services are the central organizing mechanism for Jini-based systems. When new services become available on the network, they register themselves with a lookup service. When clients wish to locate a service to assist with some task, they consult a lookup service.

The run-time infrastructure uses one network-level protocol, called discovery, and two object-level protocols, called join and lookup. Discovery enables clients and services to locate lookup services. Join enables a service to register itself in a lookup service. Lookup enables a client to query for services that can help accomplish its goals.

The discovery process

Discovery works like this: Imagine you have a Jini-enabled disk drive that offers a persistent storage service. As soon as you connect the drive to the network, it broadcasts a presence announcement by dropping a multicast packet onto a well-known port. Included in the presence announcement is an IP address and port number where the disk drive can be contacted by a lookup service.

Lookup services monitor the well-known port for presence announcement packets. When a lookup service receives a presence announcement, it opens and inspects the packet. The packet contains information that enables the lookup service to determine whether or not it should contact the sender of the packet. If so, it contacts the sender directly by making a TCP connection to the IP address and port number extracted from the packet. Using RMI, the lookup service sends an object, called a service registrar, across the network to the originator of the packet. The purpose of the service registrar object is to facilitate further communication with the lookup service. By invoking methods on this object, the sender of the announcement packet can perform join and lookup on the lookup service. In the case of the disk drive, the lookup service would make a TCP connection to the disk drive and would send it a service registrar object, through which the disk drive would then register its persistent storage service via the join process.

The join process

Once a service provider has a service registrar object, the end product of discovery, it is ready to do a join—to become part of the federation of services that are registered in the lookup service. To do a join, the service provider invokes the register( ) method on the service registrar object, passing as a parameter an object called a service item, a bundle of objects that describe the service. The register( ) method sends a copy of the service item up to the lookup service, where the service item is stored. Once this has completed, the service provider has finished the join process: its service has become registered in the lookup service.

The service item is a container for several objects, including an object called a service object, which clients can use to interact with the service. The service item can also include any number of attributes, which can be any object. Some potential attributes are icons, classes that provide GUIs for the service, and objects that give more information about the service.

Service objects usually implement one or more interfaces through which clients interact with the service. For example, a lookup service is a Jini service, and its service object is the service registrar. The register( ) method invoked by service providers during join is declared in the ServiceRegistrar interface (a member of the net.jini.core.lookup package), which all service registrar objects implement. Clients and service providers talk to the lookup service through the service registrar object by invoking methods declared in the ServiceRegistrar interface. Likewise, a disk drive would provide a service object that implemented some well-known storage service interface. Clients would look up and interact with the disk drive by this storage service interface.

The lookup process

Once a service has registered with a lookup service via the join process, that service is available for use by clients who query that lookup service. To build a distributed system of services that will work together to perform some task, a client must locate and enlist the help of the individual services. To find a service, clients query lookup services via a process called lookup.

To perform a lookup, a client invokes the lookup( ) method on a service registrar object. (A client, like a service provider, gets a service registrar through the previously-described process of discovery.) The client passes as an argument to lookup( ) a service template, an object that serves as search criteria for the query. The service template can include a reference to an array of Class objects. These Class objects indicate to the lookup service the Java type (or types) of the service object desired by the client. The service template can also include a service ID, which uniquely identifies a service, and attributes, which must exactly match the attributes uploaded by the service provider in the service item. The service template can also contain wildcards for any of these fields. A wildcard in the service ID field, for example, will match any service ID. The lookup( ) method sends the service template to the lookup service, which performs the query and sends back zero to any matching service objects. The client gets a reference to the matching service objects as the return value of the lookup( ) method.

In the general case, a client looks up a service by Java type, usually an interface. For example, if a client needed to use a printer, it would compose a service template that included a Class object for a well-known interface to printer services. All printer services would implement this well-known interface. The lookup service would return a service object (or objects) that implemented this interface. Attributes can be included in the service template to narrow the number of matches for such a type-based search. The client would use the printer service by invoking methods from the well-known printer service interface on the service object.

Separation of interface and implementation

Jini's architecture brings object-oriented programming to the network by enabling network services to take advantage of one of the fundamentals of objects: the separation of interface and implementation. For example, a service object can grant clients access to the service in many ways. The object can actually represent the entire service, which is downloaded to the client during lookup and then executed locally. Alternatively, the service object can serve merely as a proxy to a remote server. Then when the client invokes methods on the service object, it sends the requests across the network to the server, which does the real work. A third option is for the local service object and a remote server to each do part of the work.

One important consequence of Jini's architecture is that the network protocol used to communicate between a proxy service object and a remote server does not need to be known to the client. As illustrated in the figure below, the network protocol is part of the service's implementation. This protocol is a private matter decided upon by the developer of the service. The client can communicate with the service via this private protocol because the service injects some of its own code (the service object) into the client's address space. The injected service object could communicate with the service via RMI, CORBA, DCOM, some home-brewed protocol built on top of sockets and streams, or anything else. The client simply doesn't need to care about network protocols, because it can talk to the well-known interface that the service object implements. The service object takes care of any necessary communication on the network.

The client talks to the service through a well-known interface

Different implementations of the same service interface can use completely different approaches and network protocols. A service can use specialized hardware to fulfill client requests, or it can do all its work in software. In fact, the implementation approach taken by a single service can evolve over time. The client can be sure it has a service object that understands the current implementation of the service, because the client receives the service object (by way of the lookup service) from the service provider itself. To the client, a service looks like the well-known interface, regardless of how the service is implemented.

Abstracting distributed systems

Jini attempts to raise the level of abstraction for distributed systems programming, from the network protocol level to the object interface level. In the emerging proliferation of embedded devices connected to networks, many pieces of a distributed system may come from different vendors. Jini makes it unnecessary for vendors of devices to agree on network level protocols that allow their devices to interact. Instead, vendors must agree on Java interfaces through which their devices can interact. The processes of discovery, join, and lookup, provided by the Jini run-time infrastructure, will enable devices to locate each other on the network. Once they locate each other, devices will be able to communicate with each other through Java interfaces.

JavaSpaces

Summary

There’s actually a lot more to networking than can be covered in this introductory treatment. Java networking also provides fairly extensive support for URLs, including protocol handlers for different types of content that can be discovered at an Internet site. You can find other Java networking features fully and carefully described in Java Network Programming by Elliotte Rusty Harold (O’Reilly, 1997).

Exercises
  1. Compile and run the JabberServer and JabberClient programs in this chapter. Now edit the files to remove all of the buffering for the input and output, then compile and run them again to observe the results.
  2. Create a server that asks for a password, then opens a file and sends the file over the network connection. Create a client that connects to this server, gives the appropriate password, then captures and saves the file. Test the pair of programs on your machine using the localhost (the local loopback IP address 127.0.0.1 produced by calling InetAddress.getByName(null)).
  3. Modify the server in Exercise 2 so that it uses multithreading to handle multiple clients.
  4. Modify JabberClient so that output flushing doesn’t occur and observe the effect.
  5. Modify MultiJabberServer so that it uses thread pooling. Instead of throwing away a thread each time a client disconnects, the thread should put itself into an “available pool” of threads. When a new client wants to connect, the server will look in the available pool for a thread to handle the request, and if one isn’t available, make a new one. This way the number of threads necessary will naturally grow to the required quantity. The value of thread pooling is that it doesn’t require the overhead of creating and destroying a new thread for each new client.
  6. Build on ShowHTML.java to produce an applet that is a password-protected gateway to a particular portion of your Web site.
  7. (More challenging) Create a client/server pair of programs that use datagrams to transmit a file from one machine to the other. (See the description at the end of the datagram section of this chapter.)
  8. (More challenging) Take the VLookup.java program and modify it so that when you click on the resulting name it automatically takes that name and copies it to the clipboard (so you can simply paste it into your email). You’ll need to look back at the IO stream chapter to remember how to use the Java 1.1 clipboard.

[68] This means a maximum of just over four billion numbers, which is rapidly running out. The new standard for IP addresses will use a 128-bit number, which should produce enough unique IP addresses for the foreseeable future.

[69] Many brain cells died in agony to discover this information.

[70] This section was contributed by Robert Castaneda, with help from Dave Bartlett.

[71] This section was contributed by Bill Venners (www.artima.com).


A: Passing & Returning Objects

By now you should be reasonably comfortable with the idea that when you’re “passing” an object, you’re actually passing a reference.

In many programming languages you can use that language’s “regular” way to pass objects around and most of the time everything works fine. But it always seems that there comes a point at which you must do something irregular and suddenly things get a bit more complicated (or in the case of C++, quite complicated). Java is no exception, and it’s important that you understand exactly what’s happening as you pass objects around and manipulate them. This appendix will provide that insight.

Another way to pose the question of this appendix, if you’re coming from a programming language so equipped, is “Does Java have pointers?” Some have claimed that pointers are hard and dangerous and therefore bad, and since Java is all goodness and light and will lift your earthly programming burdens, it cannot possibly contain such things. However, it’s more accurate to say that Java has pointers; indeed, every object identifier in Java (except for primitives) is one of these pointers, but their use is restricted and guarded not only by the compiler but by the run-time system. Or to put it another way, Java has pointers, but no pointer arithmetic. These are what I’ve been calling “references,” and you can think of them as “safety pointers,” not unlike the safety scissors of elementary school—they aren’t sharp so you cannot hurt yourself without great effort, but they can sometimes be slow and tedious.

Passing references around

When you pass a reference into a method, you’re still pointing to the same object. A simple experiment demonstrates this:

//: appendixa:PassReferences.java
// Passing references around.

public class PassReferences {
  static void f(PassReferences h) {
    System.out.println("h inside f(): " + h);
  }
  public static void main(String[] args) {
    PassReferences p = new PassReferences();
    System.out.println("p inside main(): " + p);
    f(p);
  }
} ///:~

The method toString( ) is automatically invoked in the print statements, and PassReferences inherits directly from Object with no redefinition of toString( ). Thus, Object’s version of toString( ) is used, which prints out the class of the object followed by the address where that object is located (not the reference, but the actual object storage). The output looks like this:

p inside main(): PassReferences@1653748
h inside f(): PassReferences@1653748

You can see that both p and h refer to the same object. This is far more efficient than duplicating a new PassReferences object just so that you can send an argument to a method. But it brings up an important issue.

Aliasing

Aliasing means that more than one reference is tied to the same object, as in the above example. The problem with aliasing occurs when someone writes to that object. If the owners of the other references aren’t expecting that object to change, they’ll be surprised. This can be demonstrated with a simple example:

//: appendixa:Alias1.java
// Aliasing two references to one object.

public class Alias1 {
  int i;
  Alias1(int ii) { i = ii; }
  public static void main(String[] args) {
    Alias1 x = new Alias1(7);
    Alias1 y = x; // Assign the reference
    System.out.println("x: " + x.i);
    System.out.println("y: " + y.i);
    System.out.println("Incrementing x");
    x.i++;
    System.out.println("x: " + x.i);
    System.out.println("y: " + y.i);
  }
} ///:~

In the line:

Alias1 y = x; // Assign the reference

a new Alias1 reference is created, but instead of being assigned to a fresh object created with new, it’s assigned to an existing reference. So the contents of reference x, which is the address of the object x is pointing to, is assigned to y, and thus both x and y are attached to the same object. So when x’s i is incremented in the statement:

x.i++;

y’s i will be affected as well. This can be seen in the output:

x: 7
y: 7
Incrementing x
x: 8
y: 8

One good solution in this case is to simply not do it: don’t consciously alias more than one reference to an object at the same scope. Your code will be much easier to understand and debug. However, when you’re passing a reference in as an argument—which is the way Java is supposed to work—you automatically alias because the local reference that’s created can modify the “outside object” (the object that was created outside the scope of the method). Here’s an example:

//: appendixa:Alias2.java
// Method calls implicitly alias their
// arguments.

public class Alias2 {
  int i;
  Alias2(int ii) { i = ii; }
  static void f(Alias2 reference) {
    reference.i++;
  }
  public static void main(String[] args) {
    Alias2 x = new Alias2(7);
    System.out.println("x: " + x.i);
    System.out.println("Calling f(x)");
    f(x);
    System.out.println("x: " + x.i);
  }
} ///:~

The output is:

x: 7
Calling f(x)
x: 8

The method is changing its argument, the outside object. When this kind of situation arises, you must decide whether it makes sense, whether the user expects it, and whether it’s going to cause problems.

In general, you call a method in order to produce a return value and/or a change of state in the object that the method is called for. (A method is how you “send a message” to that object.) It’s much less common to call a method in order to manipulate its arguments; this is referred to as “calling a method for its side effects.” Thus, when you create a method that modifies its arguments the user must be clearly instructed and warned about the use of that method and its potential surprises. Because of the confusion and pitfalls, it’s much better to avoid changing the argument.

If you need to modify an argument during a method call and you don’t intend to modify the outside argument, then you should protect that argument by making a copy inside your method. That’s the subject of much of this appendix.

Making local copies

To review: all argument passing in Java is performed by passing references. That is, when you pass “an object,” you’re really passing only a reference to an object that lives outside the method, so if you perform any modifications with that reference, you modify the outside object. In addition:

  • Aliasing happens automatically during argument passing.
  • There are no local objects, only local references.
  • References have scopes, objects do not.
  • Object lifetime is never an issue in Java.
  • There is no language support (e.g., ‘const’) to prevent objects from being modified (that is, to prevent the negative effects of aliasing).

If you’re only reading information from an object and not modifying it, passing a reference is the most efficient form of argument passing. This is nice; the default way of doing things is also the most efficient. However, sometimes it’s necessary to be able to treat the object as if it were “local” so that changes you make affect only a local copy and do not modify the outside object. Many programming languages support the ability to automatically make a local copy of the outside object, inside the method [72] . Java does not, but it allows you to produce this effect.

Pass by value

This brings up the terminology issue, which always seems good for an argument. The term is “pass by value,” and the meaning depends on how you perceive the operation of the program. The general meaning is that you get a local copy of whatever you’re passing, but the real question is how you think about what you’re passing. When it comes to the meaning of “pass by value,” there are two fairly distinct camps:

  1. Java passes everything by value. When you’re passing primitives into a method, you get a distinct copy of the primitive. When you’re passing a reference into a method, you get a copy of the reference. Ergo, everything is pass by value. Of course, the assumption is that you’re always thinking (and caring) that references are being passed, but it seems like the Java design has gone a long way toward allowing you to ignore (most of the time) that you’re working with a reference. That is, it seems to allow you to think of the reference as “the object,” since it implicitly dereferences it whenever you make a method call.
  2. Java passes primitives by value (no argument there), but objects are passed by reference. This is the world view that the reference is an alias for the object, so you don’t think about passing references, but instead say “I’m passing the object.” Since you don’t get a local copy of the object when you pass it into a method, objects are clearly not passed by value. There appears to be some support for this view within Sun, since one of the “reserved but not implemented” keywords was byvalue. (There’s no knowing, however, whether that keyword will ever see the light of day.)

Having given both camps a good airing and after saying “It depends on how you think of a reference,” I will attempt to sidestep the issue. In the end, it isn’t that important—what is important is that you understand that passing a reference allows the caller’s object to be changed unexpectedly.

Cloning objects

The most likely reason for making a local copy of an object is if you’re going to modify that object and you don’t want to modify the caller’s object. If you decide that you want to make a local copy, you simply use the clone( ) method to perform the operation. This is a method that’s defined as protected in the base class Object and which you must override as public in any derived classes that you want to clone. For example, the standard library class ArrayList overrides clone( ), so we can call clone( ) for ArrayList:

//: appendixa:Cloning.java
// The clone() operation works for only a few
// items in the standard Java library.
import java.util.*;

class Int {
  private int i;
  public Int(int ii) { i = ii; }
  public void increment() { i++; }
  public String toString() {
    return Integer.toString(i);
  }
}

public class Cloning {
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 10; i++ )
      v.add(new Int(i));
    System.out.println("v: " + v);
    ArrayList v2 = (ArrayList)v.clone();
    // Increment all v2's elements:
    for(Iterator e = v2.iterator();
        e.hasNext(); )
      ((Int)e.next()).increment();
    // See if it changed v's elements:
    System.out.println("v: " + v);
  }
} ///:~

The clone( ) method produces an Object, which must then be recast to the proper type. This example shows how ArrayList’s clone( ) method does not automatically try to clone each of the objects that the ArrayList contains—the old ArrayList and the cloned ArrayList are aliased to the same objects. This is often called a shallow copy, since it’s copying only the “surface” portion of an object. The actual object consists of this “surface” plus all the objects that the references are pointing to, plus all the objects those objects are pointing to, etc. This is often referred to as the “web of objects.” Copying the entire mess is called a deep copy.

You can see the effect of the shallow copy in the output, where the actions performed on v2 affect v:

v: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
v: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Not trying to clone( ) the objects contained in the ArrayList is probably a fair assumption because there’s no guarantee that those objects are cloneable [73] .

Adding cloneability to a class

Even though the clone method is defined in the base-of-all-classes Object, cloning is not automatically available in every class [74] . This would seem to be counterintuitive to the idea that base-class methods are always available in derived classes. Cloning in Java goes against this idea; if you want it to exist for a class, you must specifically add code to make cloning work.

Using a trick with protected

To prevent default clonability in every class you create, the clone( ) method is protected in the base class Object. Not only does this mean that it’s not available by default to the client programmer who is simply using the class (not subclassing it), but it also means that you cannot call clone( ) via a reference to the base class. (Although that might seem to be useful in some situations, such as to polymorphically clone a bunch of Objects.) It is in effect a way to give you, at compile-time, the information that your object is not cloneable—and oddly enough most classes in the standard Java library are not cloneable. Thus, if you say:

    Integer x = new Integer(1);
    x = x.clone();

You will get, at compile-time, an error message that says clone( ) is not accessible (since Integer doesn’t override it and it defaults to the protected version).

If, however, you’re in a class derived from Object (as all classes are), then you have permission to call Object.clone( ) because it’s protected and you’re an inheritor. The base class clone( ) has useful functionality—it performs the actual bitwise duplication of the derived-class object, thus acting as the common cloning operation. However, you then need to make your clone operation public for it to be accessible. So two key issues when you clone are:

  • Virtually always call super.clone( )
  • Make your clone public

You’ll probably want to override clone( ) in any further derived classes, otherwise your (now public) clone( ) will be used, and that might not do the right thing (although, since Object.clone( ) makes a copy of the actual object, it might). The protected trick works only once, the first time you inherit from a class that has no clonability and you want to make a class that’s cloneable. In any classes inherited from your class the clone( ) method is available since it’s not possible in Java to reduce the access of a method during derivation. That is, once a class is cloneable, everything derived from it is cloneable unless you use provided mechanisms (described later) to “turn off” cloning.

Implementing the Cloneable interface

There’s one more thing you need to do to complete the clonability of an object: implement the Cloneable interface. This interface is a bit strange because it’s empty!

interface Cloneable {}

The reason for implementing this empty interface is obviously not because you are going to upcast to Cloneable and call one of its methods. The use of interface here is considered by some to be a “hack” because it’s using a feature for something other than its original intent. Implementing the Cloneable interface acts as a kind of a flag, wired into the type of the class.

There are two reasons for the existence of the Cloneable interface. First, you might have an upcast reference to a base type and not know whether it’s possible to clone that object. In this case, you can use the instanceof keyword (described in Chapter 12) to find out whether the reference is connected to an object that can be cloned:

if(myReference instanceof Cloneable) // ...

The second reason is that mixed into this design for clonability was the thought that maybe you didn’t want all types of objects to be clonable. So Object.clone( ) verifies that a class implements the Cloneable interface. If not, it throws a CloneNotSupportedException exception. So in general, you’re forced to implement Cloneable as part of support for cloning.

Successful cloning

Once you understand the details of implementing the clone( ) method, you’re able to create classes that can be easily duplicated to provide a local copy:

//: appendixa:LocalCopy.java
// Creating local copies with clone().
import java.util.*;

class MyObject implements Cloneable {
  int i;
  MyObject(int ii) { i = ii; }
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {
      System.out.println("MyObject can't clone");
    }
    return o;
  }
  public String toString() {
    return Integer.toString(i);
  }
}

public class LocalCopy {
  static MyObject g(MyObject v) {
    // Passing a reference, modifies outside object:
    v.i++;
    return v;
  }
  static MyObject f(MyObject v) {
    v = (MyObject)v.clone(); // Local copy
    v.i++;
    return v;
  }
  public static void main(String[] args) {
    MyObject a = new MyObject(11);
    MyObject b = g(a);
    // Testing reference equivalence,
    // not object equivalence:
    if(a == b)
      System.out.println("a == b");
    else
      System.out.println("a != b");
    System.out.println("a = " + a);
    System.out.println("b = " + b);
    MyObject c = new MyObject(47);
    MyObject d = f(c);
    if(c == d)
      System.out.println("c == d");
    else
      System.out.println("c != d");
    System.out.println("c = " + c);
    System.out.println("d = " + d);
  }
} ///:~

First of all, clone( ) must be accessible so you must make it public. Second, for the initial part of your clone( ) operation you should call the base-class version of clone( ). The clone( ) that’s being called here is the one that’s predefined inside Object, and you can call it because it’s protected and thereby accessible in derived classes.

Object.clone( ) figures out how big the object is, creates enough memory for a new one, and copies all the bits from the old to the new. This is called a bitwise copy, and is typically what you’d expect a clone( ) method to do. But before Object.clone( ) performs its operations, it first checks to see if a class is Cloneable, that is, whether it implements the Cloneable interface. If it doesn’t, Object.clone( ) throws a CloneNotSupportedException to indicate that you can’t clone it. Thus, you’ve got to surround your call to super.clone( ) with a try-catch block, to catch an exception that should never happen (because you’ve implemented the Cloneable interface).

In LocalCopy, the two methods g( ) and f( ) demonstrate the difference between the two approaches for argument passing. g( ) shows passing by reference in which it modifies the outside object and returns a reference to that outside object, while f( ) clones the argument, thereby decoupling it and leaving the original object alone. It can then proceed to do whatever it wants, and even to return a reference to this new object without any ill effects to the original. Notice the somewhat curious-looking statement:

v = (MyObject)v.clone();

This is where the local copy is created. To prevent confusion by such a statement, remember that this rather strange coding idiom is perfectly feasible in Java because every object identifier is actually a reference. So the reference v is used to clone( ) a copy of what it refers to, and this returns a reference to the base type Object (because it’s defined that way in Object.clone( )) that must then be cast to the proper type.

In main( ), the difference between the effects of the two different argument-passing approaches in the two different methods is tested. The output is:

a == b
a = 12
b = 12
c != d
c = 47
d = 48

It’s important to notice that the equivalence tests in Java do not look inside the objects being compared to see if their values are the same. The == and != operators are simply comparing the references. If the addresses inside the references are the same, the references are pointing to the same object and are therefore “equal.” So what the operators are really testing is whether the references are aliased to the same object!

The effect of Object.clone( )

What actually happens when Object.clone( ) is called that makes it so essential to call super.clone( ) when you override clone( ) in your class? The clone( ) method in the root class is responsible for creating the correct amount of storage and making the bitwise copy of the bits from the original object into the new object’s storage. That is, it doesn’t just make storage and copy an Object—it actually figures out the size of the precise object that’s being copied and duplicates that. Since all this is happening from the code in the clone( ) method defined in the root class (that has no idea what’s being inherited from it), you can guess that the process involves RTTI to determine the actual object that’s being cloned. This way, the clone( ) method can create the proper amount of storage and do the correct bitcopy for that type.

Whatever you do, the first part of the cloning process should normally be a call to super.clone( ). This establishes the groundwork for the cloning operation by making an exact duplicate. At this point you can perform other operations necessary to complete the cloning.

To know for sure what those other operations are, you need to understand exactly what Object.clone( ) buys you. In particular, does it automatically clone the destination of all the references? The following example tests this:

//: appendixa:Snake.java
// Tests cloning to see if destination
// of references are also cloned.

public class Snake implements Cloneable {
  private Snake next;
  private char c;
  // Value of i == number of segments
  Snake(int i, char x) {
    c = x;
    if(--i > 0)
      next = new Snake(i, (char)(x + 1));
  }
  void increment() {
    c++;
    if(next != null)
      next.increment();
  }
  public String toString() {
    String s = ":" + c;
    if(next != null)
      s += next.toString();
    return s;
  }
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {}
    return o;
  }
  public static void main(String[] args) {
    Snake s = new Snake(5, 'a');
    System.out.println("s = " + s);
    Snake s2 = (Snake)s.clone();
    System.out.println("s2 = " + s2);
    s.increment();
    System.out.println(
      "after s.increment, s2 = " + s2);
  }
} ///:~

A Snake is made up of a bunch of segments, each of type Snake. Thus, it’s a singly-linked list. The segments are created recursively, decrementing the first constructor argument for each segment until zero is reached. To give each segment a unique tag, the second argument, a char, is incremented for each recursive constructor call.

The increment( ) method recursively increments each tag so you can see the change, and the toString( ) recursively prints each tag. The output is:

s = :a:b:c:d:e
s2 = :a:b:c:d:e
after s.increment, s2 = :a:c:d:e:f

This means that only the first segment is duplicated by Object.clone( ), therefore it does a shallow copy. If you want the whole snake to be duplicated—a deep copy—you must perform the additional operations inside your overridden clone( ).

You’ll typically call super.clone( ) in any class derived from a cloneable class to make sure that all of the base-class operations (including Object.clone( )) take place. This is followed by an explicit call to clone( ) for every reference in your object; otherwise those references will be aliased to those of the original object. It’s analogous to the way constructors are called—base-class constructor first, then the next-derived constructor, and so on to the most-derived constructor. The difference is that clone( ) is not a constructor so there’s nothing to make it happen automatically. You must make sure to do it yourself.

Cloning a composed object

There’s a problem you’ll encounter when trying to deep copy a composed object. You must assume that the clone( ) method in the member objects will in turn perform a deep copy on their references, and so on. This is quite a commitment. It effectively means that for a deep copy to work you must either control all of the code in all of the classes, or at least have enough knowledge about all of the classes involved in the deep copy to know that they are performing their own deep copy correctly.

This example shows what you must do to accomplish a deep copy when dealing with a composed object:

//: appendixa:DeepCopy.java
// Cloning a composed object.

class DepthReading implements Cloneable {
  private double depth;
  public DepthReading(double depth) {
    this.depth = depth;
  }
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return o;
  }
}

class TemperatureReading implements Cloneable {
  private long time;
  private double temperature;
  public TemperatureReading(double temperature) {
    time = System.currentTimeMillis();
    this.temperature = temperature;
  }
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return o;
  }
}

class OceanReading implements Cloneable {
  private DepthReading depth;
  private TemperatureReading temperature;
  public OceanReading(double tdata, double ddata){
    temperature = new TemperatureReading(tdata);
    depth = new DepthReading(ddata);
  }
  public Object clone() {
    OceanReading o = null;
    try {
      o = (OceanReading)super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    // Must clone references:
    o.depth = (DepthReading)o.depth.clone();
    o.temperature =
      (TemperatureReading)o.temperature.clone();
    return o; // Upcasts back to Object
  }
}

public class DeepCopy {
  public static void main(String[] args) {
    OceanReading reading =
      new OceanReading(33.9, 100.5);
    // Now clone it:
    OceanReading r =
      (OceanReading)reading.clone();
  }
} ///:~

DepthReading and TemperatureReading are quite similar; they both contain only primitives. Therefore, the clone( ) method can be quite simple: it calls super.clone( ) and returns the result. Note that the clone( ) code for both classes is identical.

OceanReading is composed of DepthReading and TemperatureReading objects and so, to produce a deep copy, its clone( ) must clone the references inside OceanReading. To accomplish this, the result of super.clone( ) must be cast to an OceanReading object (so you can access the depth and temperature references).

A deep copy with ArrayList

Let’s revisit the ArrayList example from earlier in this appendix. This time the Int2 class is cloneable so the ArrayList can be deep copied:

//: appendixa:AddingClone.java
// You must go through a few gyrations
// to add cloning to your own class.
import java.util.*;

class Int2 implements Cloneable {
  private int i;
  public Int2(int ii) { i = ii; }
  public void increment() { i++; }
  public String toString() {
    return Integer.toString(i);
  }
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {
      System.out.println("Int2 can't clone");
    }
    return o;
  }
}

// Once it's cloneable, inheritance
// doesn't remove cloneability:
class Int3 extends Int2 {
  private int j; // Automatically duplicated
  public Int3(int i) { super(i); }
}

public class AddingClone {
  public static void main(String[] args) {
    Int2 x = new Int2(10);
    Int2 x2 = (Int2)x.clone();
    x2.increment();
    System.out.println(
      "x = " + x + ", x2 = " + x2);
    // Anything inherited is also cloneable:
    Int3 x3 = new Int3(7);
    x3 = (Int3)x3.clone();

    ArrayList v = new ArrayList();
    for(int i = 0; i < 10; i++ )
      v.add(new Int2(i));
    System.out.println("v: " + v);
    ArrayList v2 = (ArrayList)v.clone();
    // Now clone each element:
    for(int i = 0; i < v.size(); i++)
      v2.set(i, ((Int2)v2.get(i)).clone());
    // Increment all v2's elements:
    for(Iterator e = v2.iterator();
        e.hasNext(); )
      ((Int2)e.next()).increment();
    // See if it changed v's elements:
    System.out.println("v: " + v);
    System.out.println("v2: " + v2);
  }
} ///:~

Int3 is inherited from Int2 and a new primitive member int j is added. You might think that you’d need to override clone( ) again to make sure j is copied, but that’s not the case. When Int2’s clone( ) is called as Int3’s clone( ), it calls Object.clone( ), which determines that it’s working with an Int3 and duplicates all the bits in the Int3. As long as you don’t add references that need to be cloned, the one call to Object.clone( ) performs all of the necessary duplication, regardless of how far down in the hierarchy clone( ) is defined.

You can see what’s necessary in order to do a deep copy of an ArrayList: after the ArrayList is cloned, you have to step through and clone each one of the objects pointed to by the ArrayList. You’d have to do something similar to this to do a deep copy of a HashMap.

The remainder of the example shows that the cloning did happen by showing that, once an object is cloned, you can change it and the original object is left untouched.

Deep copy via serialization

When you consider Java’s object serialization (introduced in Chapter 11), you might observe that an object that’s serialized and then deserialized is, in effect, cloned.

So why not use serialization to perform deep copying? Here’s an example that compares the two approaches by timing them:

//: appendixa:Compete.java
import java.io.*;

class Thing1 implements Serializable {}
class Thing2 implements Serializable {
  Thing1 o1 = new Thing1();
}

class Thing3 implements Cloneable {
  public Object clone() {
    Object o = null;
    try {
      o = super.clone();
    } catch (CloneNotSupportedException e) {
      System.out.println("Thing3 can't clone");
    }
    return o;
  }
}

class Thing4 implements Cloneable {
  Thing3 o3 = new Thing3();
  public Object clone() {
    Thing4 o = null;
    try {
      o = (Thing4)super.clone();
    } catch (CloneNotSupportedException e) {
      System.out.println("Thing4 can't clone");
    }
    // Clone the field, too:
    o.o3 = (Thing3)o3.clone();
    return o;
  }
}

public class Compete {
  static final int SIZE = 5000;
  public static void main(String[] args) {
    Thing2[] a = new Thing2[SIZE];
    for(int i = 0; i < a.length; i++)
      a[i] = new Thing2();
    Thing4[] b = new Thing4[SIZE];
    for(int i = 0; i < b.length; i++)
      b[i] = new Thing4();
    try {
      long t1 = System.currentTimeMillis();
      ByteArrayOutputStream buf =
        new ByteArrayOutputStream();
      ObjectOutputStream o =
        new ObjectOutputStream(buf);
      for(int i = 0; i < a.length; i++)
        o.writeObject(a[i]);
      // Now get copies:
      ObjectInputStream in =
        new ObjectInputStream(
          new ByteArrayInputStream(
            buf.toByteArray()));
      Thing2[] c = new Thing2[SIZE];
      for(int i = 0; i < c.length; i++)
        c[i] = (Thing2)in.readObject();
      long t2 = System.currentTimeMillis();
      System.out.println(
        "Duplication via serialization: " +
        (t2 - t1) + " Milliseconds");
      // Now try cloning:
      t1 = System.currentTimeMillis();
      Thing4[] d = new Thing4[SIZE];
      for(int i = 0; i < d.length; i++)
        d[i] = (Thing4)b[i].clone();
      t2 = System.currentTimeMillis();
      System.out.println(
        "Duplication via cloning: " +
        (t2 - t1) + " Milliseconds");
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~

Thing2 and Thing4 contain member objects so that there’s some deep copying going on. It’s interesting to notice that while Serializable classes are easy to set up, there’s much more work going on to duplicate them. Cloning involves a lot of work to set up the class, but the actual duplication of objects is relatively simple. The results really tell the tale. Here is the output from three different runs:

Duplication via serialization: 940 Milliseconds
Duplication via cloning: 50 Milliseconds

Duplication via serialization: 710 Milliseconds
Duplication via cloning: 60 Milliseconds

Duplication via serialization: 770 Milliseconds
Duplication via cloning: 50 Milliseconds

Despite the significant time difference between serialization and cloning, you’ll also notice that the serialization technique seems to vary more in its duration, while cloning tends to be more stable.

Adding cloneability
further down a hierarchy

If you create a new class, its base class defaults to Object, which defaults to non-clonability (as you’ll see in the next section). As long as you don’t explicitly add clonability, you won’t get it. But you can add it in at any layer and it will then be cloneable from that layer downward, like this:

//: appendixa:HorrorFlick.java
// You can insert Cloneability
// at any level of inheritance.
import java.util.*;

class Person {}
class Hero extends Person {}
class Scientist extends Person
    implements Cloneable {
  public Object clone() {
    try {
      return super.clone();
    } catch (CloneNotSupportedException e) {
      // this should never happen:
      // It's Cloneable already!
      throw new InternalError();
    }
  }
}
class MadScientist extends Scientist {}

public class HorrorFlick {
  public static void main(String[] args) {
    Person p = new Person();
    Hero h = new Hero();
    Scientist s = new Scientist();
    MadScientist m = new MadScientist();

    // p = (Person)p.clone(); // Compile error
    // h = (Hero)h.clone(); // Compile error
    s = (Scientist)s.clone();
    m = (MadScientist)m.clone();
  }
} ///:~

Before clonability was added, the compiler stopped you from trying to clone things. When clonability is added in Scientist, then Scientist and all its descendants are cloneable.

Why this strange design?

If all this seems to be a strange scheme, that’s because it is. You might wonder why it worked out this way. What is the meaning behind this design?

Originally, Java was designed as a language to control hardware boxes, and definitely not with the Internet in mind. In a general-purpose language like this, it makes sense that the programmer be able to clone any object. Thus, clone( ) was placed in the root class Object, but it was a public method so you could always clone any object. This seemed to be the most flexible approach, and after all, what could it hurt?

Well, when Java was seen as the ultimate Internet programming language, things changed. Suddenly, there are security issues, and of course, these issues are dealt with using objects, and you don’t necessarily want anyone to be able to clone your security objects. So what you’re seeing is a lot of patches applied on the original simple and straightforward scheme: clone( ) is now protected in Object. You must override it and implement Cloneable and deal with the exceptions.

It’s worth noting that you must use the Cloneable interface only if you’re going to call Object’s clone( ), method, since that method checks at run-time to make sure that your class implements Cloneable. But for consistency (and since Cloneable is empty anyway) you should implement it.

Controlling cloneability

You might suggest that, to remove clonability, the clone( ) method simply be made private, but this won’t work since you cannot take a base-class method and make it less accessible in a derived class. So it’s not that simple. And yet, it’s necessary to be able to control whether an object can be cloned. There are actually a number of attitudes you can take to this in a class that you design:

  1. Indifference. You don’t do anything about cloning, which means that your class can’t be cloned but a class that inherits from you can add cloning if it wants. This works only if the default Object.clone( ) will do something reasonable with all the fields in your class.
  2. Support clone( ). Follow the standard practice of implementing Cloneable and overriding clone( ). In the overridden clone( ), you call super.clone( ) and catch all exceptions (so your overridden clone( ) doesn’t throw any exceptions).
  3. Support cloning conditionally. If your class holds references to other objects that might or might not be cloneable (a container class, for example), your clone( ) can try to clone all of the objects for which you have references, and if they throw exceptions just pass those exceptions out to the programmer. For example, consider a special sort of ArrayList that tries to clone all the objects it holds. When you write such an ArrayList, you don’t know what sort of objects the client programmer might put into your ArrayList, so you don’t know whether they can be cloned.
  4. Don’t implement Cloneable but override clone( ) as protected, producing the correct copying behavior for any fields. This way, anyone inheriting from this class can override clone( ) and call super.clone( ) to produce the correct copying behavior. Note that your implementation can and should invoke super.clone( ) even though that method expects a Cloneable object (it will throw an exception otherwise), because no one will directly invoke it on an object of your type. It will get invoked only through a derived class, which, if it is to work successfully, implements Cloneable.
  5. Try to prevent cloning by not implementing Cloneable and overriding clone( ) to throw an exception. This is successful only if any class derived from this calls super.clone( ) in its redefinition of clone( ). Otherwise, a programmer may be able to get around it.
  6. Prevent cloning by making your class final. If clone( ) has not been overridden by any of your ancestor classes, then it can’t be. If it has, then override it again and throw CloneNotSupportedException. Making the class final is the only way to guarantee that cloning is prevented. In addition, when dealing with security objects or other situations in which you want to control the number of objects created you should make all constructors private and provide one or more special methods for creating objects. That way, these methods can restrict the number of objects created and the conditions in which they’re created. (A particular case of this is the singleton pattern shown in Thinking in Patterns with Java, downloadable at www.BruceEckel.com.)

Here’s an example that shows the various ways cloning can be implemented and then, later in the hierarchy, “turned off:”

//: appendixa:CheckCloneable.java
// Checking to see if a reference can be cloned.

// Can't clone this because it doesn't
// override clone():
class Ordinary {}

// Overrides clone, but doesn't implement
// Cloneable:
class WrongClone extends Ordinary {
  public Object clone()
      throws CloneNotSupportedException {
    return super.clone(); // Throws exception
  }
}

// Does all the right things for cloning:
class IsCloneable extends Ordinary
    implements Cloneable {
  public Object clone()
      throws CloneNotSupportedException {
    return super.clone();
  }
}

// Turn off cloning by throwing the exception:
class NoMore extends IsCloneable {
  public Object clone()
      throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }
}

class TryMore extends NoMore {
  public Object clone()
      throws CloneNotSupportedException {
    // Calls NoMore.clone(), throws exception:
    return super.clone();
  }
}

class BackOn extends NoMore {
  private BackOn duplicate(BackOn b) {
    // Somehow make a copy of b
    // and return that copy. This is a dummy
    // copy, just to make the point:
    return new BackOn();
  }
  public Object clone() {
    // Doesn't call NoMore.clone():
    return duplicate(this);
  }
}

// Can't inherit from this, so can't override
// the clone method like in BackOn:
final class ReallyNoMore extends NoMore {}

public class CheckCloneable {
  static Ordinary tryToClone(Ordinary ord) {
    String id = ord.getClass().getName();
    Ordinary x = null;
    if(ord instanceof Cloneable) {
      try {
        System.out.println("Attempting " + id);
        x = (Ordinary)((IsCloneable)ord).clone();
        System.out.println("Cloned " + id);
      } catch(CloneNotSupportedException e) {
        System.out.println(
          "Could not clone " + id);
      }
    }
    return x;
  }
  public static void main(String[] args) {
    // Upcasting:
    Ordinary[] ord = {
      new IsCloneable(),
      new WrongClone(),
      new NoMore(),
      new TryMore(),
      new BackOn(),
      new ReallyNoMore(),
    };
    Ordinary x = new Ordinary();
    // This won't compile, since clone() is
    // protected in Object:
    //! x = (Ordinary)x.clone();
    // tryToClone() checks first to see if
    // a class implements Cloneable:
    for(int i = 0; i < ord.length; i++)
      tryToClone(ord[i]);
  }
} ///:~

The first class, Ordinary, represents the kinds of classes we’ve seen throughout this book: no support for cloning, but as it turns out, no prevention of cloning either. But if you have a reference to an Ordinary object that might have been upcast from a more derived class, you can’t tell if it can be cloned or not.

The class WrongClone shows an incorrect way to implement cloning. It does override Object.clone( ) and makes that method public, but it doesn’t implement Cloneable, so when super.clone( ) is called (which results in a call to Object.clone( )), CloneNotSupportedException is thrown so the cloning doesn’t work.

In IsCloneable you can see all the right actions performed for cloning: clone( ) is overridden and Cloneable is implemented. However, this clone( ) method and several others that follow in this example do not catch CloneNotSupportedException, but instead pass it through to the caller, who must then put a try-catch block around it. In your own clone( ) methods you will typically catch CloneNotSupportedException inside clone( ) rather than passing it through. As you’ll see, in this example it’s more informative to pass the exceptions through.

Class NoMore attempts to “turn off” cloning in the way that the Java designers intended: in the derived class clone( ), you throw CloneNotSupportedException. The clone( ) method in class TryMore properly calls super.clone( ), and this resolves to NoMore.clone( ), which throws an exception and prevents cloning.

But what if the programmer doesn’t follow the “proper” path of calling super.clone( ) inside the overridden clone( ) method? In BackOn, you can see how this can happen. This class uses a separate method duplicate( ) to make a copy of the current object and calls this method inside clone( ) instead of calling super.clone( ). The exception is never thrown and the new class is cloneable. You can’t rely on throwing an exception to prevent making a cloneable class. The only sure-fire solution is shown in ReallyNoMore, which is final and thus cannot be inherited. That means if clone( ) throws an exception in the final class, it cannot be modified with inheritance and the prevention of cloning is assured. (You cannot explicitly call Object.clone( ) from a class that has an arbitrary level of inheritance; you are limited to calling super.clone( ), which has access to only the direct base class.) Thus, if you make any objects that involve security issues, you’ll want to make those classes final.

The first method you see in class CheckCloneable is tryToClone( ), which takes any Ordinary object and checks to see whether it’s cloneable with instanceof. If so, it casts the object to an IsCloneable, calls clone( ) and casts the result back to Ordinary, catching any exceptions that are thrown. Notice the use of run-time type identification (see Chapter 12) to print the class name so you can see what’s happening.

In main( ), different types of Ordinary objects are created and upcast to Ordinary in the array definition. The first two lines of code after that create a plain Ordinary object and try to clone it. However, this code will not compile because clone( ) is a protected method in Object. The remainder of the code steps through the array and tries to clone each object, reporting the success or failure of each. The output is:

Attempting IsCloneable
Cloned IsCloneable
Attempting NoMore
Could not clone NoMore
Attempting TryMore
Could not clone TryMore
Attempting BackOn
Cloned BackOn
Attempting ReallyNoMore
Could not clone ReallyNoMore

So to summarize, if you want a class to be cloneable:

  1. Implement the Cloneable interface.
  2. Override clone( ).
  3. Call super.clone( ) inside your clone( ).
  4. Capture exceptions inside your clone( ).

This will produce the most convenient effects.

The copy-constructor

Cloning can seem to be a complicated process to set up. It might seem like there should be an alternative. One approach that might occur to you (especially if you’re a C++ programmer) is to make a special constructor whose job it is to duplicate an object. In C++, this is called the copy constructor. At first, this seems like the obvious solution. Here’s an example:

//: appendixa:CopyConstructor.java
// A constructor for copying an object of the same
// type, as an attempt to create a local copy.

class FruitQualities {
  private int weight;
  private int color;
  private int firmness;
  private int ripeness;
  private int smell;
  // etc.
  FruitQualities() { // Default constructor
    // do something meaningful...
  }
  // Other constructors:
  // ...
  // Copy constructor:
  FruitQualities(FruitQualities f) {
    weight = f.weight;
    color = f.color;
    firmness = f.firmness;
    ripeness = f.ripeness;
    smell = f.smell;
    // etc.
  }
}

class Seed {
  // Members...
  Seed() { /* Default constructor */ }
  Seed(Seed s) { /* Copy constructor */ }
}

class Fruit {
  private FruitQualities fq;
  private int seeds;
  private Seed[] s;
  Fruit(FruitQualities q, int seedCount) {
    fq = q;
    seeds = seedCount;
    s = new Seed[seeds];
    for(int i = 0; i < seeds; i++)
      s[i] = new Seed();
  }
  // Other constructors:
  // ...
  // Copy constructor:
  Fruit(Fruit f) {
    fq = new FruitQualities(f.fq);
    seeds = f.seeds;
    // Call all Seed copy-constructors:
    for(int i = 0; i < seeds; i++)
      s[i] = new Seed(f.s[i]);
    // Other copy-construction activities...
  }
  // To allow derived constructors (or other
  // methods) to put in different qualities:
  protected void addQualities(FruitQualities q) {
    fq = q;
  }
  protected FruitQualities getQualities() {
    return fq;
  }
}

class Tomato extends Fruit {
  Tomato() {
    super(new FruitQualities(), 100);
  }
  Tomato(Tomato t) { // Copy-constructor
    super(t); // Upcast for base copy-constructor
    // Other copy-construction activities...
  }
}

class ZebraQualities extends FruitQualities {
  private int stripedness;
  ZebraQualities() { // Default constructor
    // do something meaningful...
  }
  ZebraQualities(ZebraQualities z) {
    super(z);
    stripedness = z.stripedness;
  }
}

class GreenZebra extends Tomato {
  GreenZebra() {
    addQualities(new ZebraQualities());
  }
  GreenZebra(GreenZebra g) {
    super(g); // Calls Tomato(Tomato)
    // Restore the right qualities:
    addQualities(new ZebraQualities());
  }
  void evaluate() {
    ZebraQualities zq =
      (ZebraQualities)getQualities();
    // Do something with the qualities
    // ...
  }
}

public class CopyConstructor {
  public static void ripen(Tomato t) {
    // Use the "copy constructor":
    t = new Tomato(t);
    System.out.println("In ripen, t is a " +
      t.getClass().getName());
  }
  public static void slice(Fruit f) {
    f = new Fruit(f); // Hmmm... will this work?
    System.out.println("In slice, f is a " +
      f.getClass().getName());
  }
  public static void main(String[] args) {
    Tomato tomato = new Tomato();
    ripen(tomato); // OK
    slice(tomato); // OOPS!
    GreenZebra g = new GreenZebra();
    ripen(g); // OOPS!
    slice(g); // OOPS!
    g.evaluate();
  }
} ///:~

This seems a bit strange at first. Sure, fruit has qualities, but why not just put data members representing those qualities directly into the Fruit class? There are two potential reasons. The first is that you might want to easily insert or change the qualities. Note that Fruit has a protected addQualities( ) method to allow derived classes to do this. (You might think the logical thing to do is to have a protected constructor in Fruit that takes a FruitQualities argument, but constructors don’t inherit so it wouldn’t be available in second or greater level classes.) By making the fruit qualities into a separate class, you have greater flexibility, including the ability to change the qualities midway through the lifetime of a particular Fruit object.

The second reason for making FruitQualities a separate object is in case you want to add new qualities or to change the behavior via inheritance and polymorphism. Note that for GreenZebra (which really is a type of tomato—I’ve grown them and they’re fabulous), the constructor calls addQualities( ) and passes it a ZebraQualities object, which is derived from FruitQualities so it can be attached to the FruitQualities reference in the base class. Of course, when GreenZebra uses the FruitQualities it must downcast it to the correct type (as seen in evaluate( )), but it always knows that type is ZebraQualities.

You’ll also see that there’s a Seed class, and that Fruit (which by definition carries its own seeds [75] ) contains an array of Seeds.

Finally, notice that each class has a copy constructor, and that each copy constructor must take care to call the copy constructors for the base class and member objects to produce a deep copy. The copy constructor is tested inside the class CopyConstructor. The method ripen( ) takes a Tomato argument and performs copy-construction on it in order to duplicate the object:

t = new Tomato(t);

while slice( ) takes a more generic Fruit object and also duplicates it:

f = new Fruit(f);

These are tested with different kinds of Fruit in main( ). Here’s the output:

In ripen, t is a Tomato
In slice, f is a Fruit
In ripen, t is a Tomato
In slice, f is a Fruit

This is where the problem shows up. After the copy-construction that happens to the Tomato inside slice( ), the result is no longer a Tomato object, but just a Fruit. It has lost all of its tomato-ness. Further, when you take a GreenZebra, both ripen( ) and slice( ) turn it into a Tomato and a Fruit, respectively. Thus, unfortunately, the copy constructor scheme is no good to us in Java when attempting to make a local copy of an object.

Why does it work in C++ and not Java?

The copy constructor is a fundamental part of C++, since it automatically makes a local copy of an object. Yet the example above proves that it does not work for Java. Why? In Java everything that we manipulate is a reference, while in C++ you can have reference-like entities and you can also pass around the objects directly. That’s what the C++ copy constructor is for: when you want to take an object and pass it in by value, thus duplicating the object. So it works fine in C++, but you should keep in mind that this scheme fails in Java, so don’t use it.

Read-only classes

While the local copy produced by clone( ) gives the desired results in the appropriate cases, it is an example of forcing the programmer (the author of the method) to be responsible for preventing the ill effects of aliasing. What if you’re making a library that’s so general purpose and commonly used that you cannot make the assumption that it will always be cloned in the proper places? Or more likely, what if you want to allow aliasing for efficiency—to prevent the needless duplication of objects—but you don’t want the negative side effects of aliasing?

One solution is to create immutable objects which belong to read-only classes. You can define a class such that no methods in the class cause changes to the internal state of the object. In such a class, aliasing has no impact since you can read only the internal state, so if many pieces of code are reading the same object there’s no problem.

As a simple example of immutable objects, Java’s standard library contains “wrapper” classes for all the primitive types. You might have already discovered that, if you want to store an int inside a container such as an ArrayList (which takes only Object references), you can wrap your int inside the standard library Integer class:

//: appendixa:ImmutableInteger.java
// The Integer class cannot be changed.
import java.util.*;

public class ImmutableInteger {
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 10; i++)
      v.add(new Integer(i));
    // But how do you change the int
    // inside the Integer?
  }
} ///:~

The Integer class (as well as all the primitive “wrapper” classes) implements immutability in a simple fashion: they have no methods that allow you to change the object.

If you do need an object that holds a primitive type that can be modified, you must create it yourself. Fortunately, this is trivial:

//: appendixa:MutableInteger.java
// A changeable wrapper class.
import java.util.*;

class IntValue {
  int n;
  IntValue(int x) { n = x; }
  public String toString() {
    return Integer.toString(n);
  }
}

public class MutableInteger {
  public static void main(String[] args) {
    ArrayList v = new ArrayList();
    for(int i = 0; i < 10; i++)
      v.add(new IntValue(i));
    System.out.println(v);
    for(int i = 0; i < v.size(); i++)
      ((IntValue)v.get(i)).n++;
    System.out.println(v);
  }
} ///:~

Note that n is friendly to simplify coding.

IntValue can be even simpler if the default initialization to zero is adequate (then you don’t need the constructor) and you don’t care about printing it out (then you don’t need the toString( )):

class IntValue { int n; }

Fetching the element out and casting it is a bit awkward, but that’s a feature of ArrayList, not of IntValue.

Creating read-only classes

It’s possible to create your own read-only class. Here’s an example:

//: appendixa:Immutable1.java
// Objects that cannot be modified
// are immune to aliasing.

public class Immutable1 {
  private int data;
  public Immutable1(int initVal) {
    data = initVal;
  }
  public int read() { return data; }
  public boolean nonzero() { return data != 0; }
  public Immutable1 quadruple() {
    return new Immutable1(data * 4);
  }
  static void f(Immutable1 i1) {
    Immutable1 quad = i1.quadruple();
    System.out.println("i1 = " + i1.read());
    System.out.println("quad = " + quad.read());
  }
  public static void main(String[] args) {
    Immutable1 x = new Immutable1(47);
    System.out.println("x = " + x.read());
    f(x);
    System.out.println("x = " + x.read());
  }
} ///:~

All data is private, and you’ll see that none of the public methods modify that data. Indeed, the method that does appear to modify an object is quadruple( ), but this creates a new Immutable1 object and leaves the original one untouched.

The method f( ) takes an Immutable1 object and performs various operations on it, and the output of main( ) demonstrates that there is no change to x. Thus, x’s object could be aliased many times without harm because the Immutable1 class is designed to guarantee that objects cannot be changed.

The drawback to immutability

Creating an immutable class seems at first to provide an elegant solution. However, whenever you do need a modified object of that new type you must suffer the overhead of a new object creation, as well as potentially causing more frequent garbage collections. For some classes this is not a problem, but for others (such as the String class) it is prohibitively expensive.

The solution is to create a companion class that can be modified. Then when you’re doing a lot of modifications, you can switch to using the modifiable companion class and switch back to the immutable class when you’re done.

The example above can be modified to show this:

//: appendixa:Immutable2.java
// A companion class for making
// changes to immutable objects.

class Mutable {
  private int data;
  public Mutable(int initVal) {
    data = initVal;
  }
  public Mutable add(int x) {
    data += x;
    return this;
  }
  public Mutable multiply(int x) {
    data *= x;
    return this;
  }
  public Immutable2 makeImmutable2() {
    return new Immutable2(data);
  }
}

public class Immutable2 {
  private int data;
  public Immutable2(int initVal) {
    data = initVal;
  }
  public int read() { return data; }
  public boolean nonzero() { return data != 0; }
  public Immutable2 add(int x) {
    return new Immutable2(data + x);
  }
  public Immutable2 multiply(int x) {
    return new Immutable2(data * x);
  }
  public Mutable makeMutable() {
    return new Mutable(data);
  }
  public static Immutable2 modify1(Immutable2 y){
    Immutable2 val = y.add(12);
    val = val.multiply(3);
    val = val.add(11);
    val = val.multiply(2);
    return val;
  }
  // This produces the same result:
  public static Immutable2 modify2(Immutable2 y){
    Mutable m = y.makeMutable();
    m.add(12).multiply(3).add(11).multiply(2);
    return m.makeImmutable2();
  }
  public static void main(String[] args) {
    Immutable2 i2 = new Immutable2(47);
    Immutable2 r1 = modify1(i2);
    Immutable2 r2 = modify2(i2);
    System.out.println("i2 = " + i2.read());
    System.out.println("r1 = " + r1.read());
    System.out.println("r2 = " + r2.read());
  }
} ///:~

Immutable2 contains methods that, as before, preserve the immutability of the objects by producing new objects whenever a modification is desired. These are the add( ) and multiply( ) methods. The companion class is called Mutable, and it also has add( ) and multiply( ) methods, but these modify the Mutable object rather than making a new one. In addition, Mutable has a method to use its data to produce an Immutable2 object and vice versa.

The two static methods modify1( ) and modify2( ) show two different approaches to producing the same result. In modify1( ), everything is done within the Immutable2 class and you can see that four new Immutable2 objects are created in the process. (And each time val is reassigned, the previous object becomes garbage.)

In the method modify2( ), you can see that the first action is to take the Immutable2 y and produce a Mutable from it. (This is just like calling clone( ) as you saw earlier, but this time a different type of object is created.) Then the Mutable object is used to perform a lot of change operations without requiring the creation of many new objects. Finally, it’s turned back into an Immutable2. Here, two new objects are created (the Mutable and the result Immutable2) instead of four.

This approach makes sense, then, when:

  1. You need immutable objects and
  2. You often need to make a lot of modifications or
  3. It’s expensive to create new immutable objects

Immutable Strings

Consider the following code:

//: appendixa:Stringer.java

public class Stringer {
  static String upcase(String s) {
    return s.toUpperCase();
  }
  public static void main(String[] args) {
    String q = new String("howdy");
    System.out.println(q); // howdy
    String qq = upcase(q);
    System.out.println(qq); // HOWDY
    System.out.println(q); // howdy
  }
} ///:~

When q is passed in to upcase( ) it’s actually a copy of the reference to q. The object this reference is connected to stays put in a single physical location. The references are copied as they are passed around.

Looking at the definition for upcase( ), you can see that the reference that’s passed in has the name s, and it exists for only as long as the body of upcase( ) is being executed. When upcase( ) completes, the local reference s vanishes. upcase( ) returns the result, which is the original string with all the characters set to uppercase. Of course, it actually returns a reference to the result. But it turns out that the reference that it returns is for a new object, and the original q is left alone. How does this happen?

Implicit constants

If you say:

String s = "asdf";
String x = Stringer.upcase(s);

do you really want the upcase( ) method to change the argument? In general, you don’t, because an argument usually looks to the reader of the code as a piece of information provided to the method, not something to be modified. This is an important guarantee, since it makes code easier to write and understand.

In C++, the availability of this guarantee was important enough to put in a special keyword, const, to allow the programmer to ensure that a reference (pointer or reference in C++) could not be used to modify the original object. But then the C++ programmer was required to be diligent and remember to use const everywhere. It can be confusing and easy to forget.

Overloading ‘+’ and the StringBuffer

Objects of the String class are designed to be immutable, using the technique shown previously. If you examine the online documentation for the String class (which is summarized a little later in this appendix), you’ll see that every method in the class that appears to modify a String really creates and returns a brand new String object containing the modification. The original String is left untouched. Thus, there’s no feature in Java like C++’s const to make the compiler support the immutability of your objects. If you want it, you have to wire it in yourself, like String does.

Since String objects are immutable, you can alias to a particular String as many times as you want. Because it’s read-only there’s no possibility that one reference will change something that will affect the other references. So a read-only object solves the aliasing problem nicely.

It also seems possible to handle all the cases in which you need a modified object by creating a brand new version of the object with the modifications, as String does. However, for some operations this isn’t efficient. A case in point is the operator ‘+’ that has been overloaded for String objects. Overloading means that it has been given an extra meaning when used with a particular class. (The ‘+’ and ‘+=‘ for String are the only operators that are overloaded in Java and Java does not allow the programmer to overload any others [76] ).

When used with String objects, the ‘+’ allows you to concatenate Strings together:

String s = "abc" + foo + "def" + Integer.toString(47);

You could imagine how this might work: the String “abc” could have a method append( ) that creates a new String object containing “abc” concatenated with the contents of foo. The new String object would then create another new String that added “def” and so on.

This would certainly work, but it requires the creation of a lot of String objects just to put together this new String, and then you have a bunch of the intermediate String objects that need to be garbage-collected. I suspect that the Java designers tried this approach first (which is a lesson in software design—you don’t really know anything about a system until you try it out in code and get something working). I also suspect they discovered that it delivered unacceptable performance.

The solution is a mutable companion class similar to the one shown previously. For String, this companion class is called StringBuffer, and the compiler automatically creates a StringBuffer to evaluate certain expressions, in particular when the overloaded operators + and += are used with String objects. This example shows what happens:

//: appendixa:ImmutableStrings.java
// Demonstrating StringBuffer.

public class ImmutableStrings {
  public static void main(String[] args) {
    String foo = "foo";
    String s = "abc" + foo +
      "def" + Integer.toString(47);
    System.out.println(s);
    // The "equivalent" using StringBuffer:
    StringBuffer sb =
      new StringBuffer("abc"); // Creates String!
    sb.append(foo);
    sb.append("def"); // Creates String!
    sb.append(Integer.toString(47));
    System.out.println(sb);
  }
} ///:~

In the creation of String s, the compiler is doing the rough equivalent of the subsequent code that uses sb: a StringBuffer is created and append( ) is used to add new characters directly into the StringBuffer object (rather than making new copies each time). While this is more efficient, it’s worth noting that each time you create a quoted character string such as “abc” and “def”, the compiler turns those into String objects. So there can be more objects created than you expect, despite the efficiency afforded through StringBuffer.

The String and
StringBuffer classes

Here is an overview of the methods available for both String and StringBuffer so you can get a feel for the way they interact. These tables don’t contain every single method, but rather the ones that are important to this discussion. Methods that are overloaded are summarized in a single row.

First, the String class:

Method | Arguments, Overloading | Use
Constructor | Overloaded: Default, String, StringBuffer, char arrays, byte arrays. | Creating String objects.
length( ) |  | Number of characters in String.
charAt() | int Index | The char at a location in the String.
getChars( ), getBytes( ) | The beginning and end from which to copy, the array to copy into, an index into the destination array. | Copy chars or bytes into an external array.
toCharArray( ) |  | Produces a char[] containing the characters in the String.
equals( ), equals-IgnoreCase( ) | A String to compare with. | An equality check on the contents of the two Strings.
compareTo( ) | A String to compare with. | Result is negative, zero, or positive depending on the lexicographical ordering of the String and the argument. Uppercase and lowercase are not equal!
regionMatches( ) | Offset into this String, the other String and its offset and length to compare. Overload adds “ignore case.” | boolean result indicates whether the region matches.
startsWith( ) | String that it might start with. Overload adds offset into argument. | boolean result indicates whether the String starts with the argument.
endsWith( ) | String that might be a suffix of this String. | boolean result indicates whether the argument is a suffix.
indexOf( ), lastIndexOf( ) | Overloaded: char, char and starting index, String, String, and starting index | Returns -1 if the argument is not found within this String, otherwise returns the index where the argument starts. lastIndexOf( ) searches backward from end.
substring( ) | Overloaded: Starting index, starting index, and ending index. | Returns a new String object containing the specified character set.
concat( ) | The String to concatenate | Returns a new String object containing the original String’s characters followed by the characters in the argument.
replace( ) | The old character to search for, the new character to replace it with. | Returns a new String object with the replacements made. Uses the old String if no match is found.
toLowerCase( ) toUpperCase( ) |  | Returns a new String object with the case of all letters changed. Uses the old String if no changes need to be made.
trim( ) |  | Returns a new String object with the white space removed from each end. Uses the old String if no changes need to be made.
valueOf( ) | Overloaded: Object, char[], char[] and offset and count, boolean, char, int, long, float, double. | Returns a String containing a character representation of the argument.
intern( ) |  | Produces one and only one String reference for each unique character sequence.

You can see that every String method carefully returns a new String object when it’s necessary to change the contents. Also notice that if the contents don’t need changing the method will just return a reference to the original String. This saves storage and overhead.

Here’s the StringBuffer class:

Method | Arguments, overloading | Use
Constructor | Overloaded: default, length of buffer to create, String to create from. | Create a new StringBuffer object.
toString( ) |  | Creates a String from this StringBuffer.
length( ) |  | Number of characters in the StringBuffer.
capacity( ) |  | Returns current number of spaces allocated.
ensure- Capacity( ) | Integer indicating desired capacity. | Makes the StringBuffer hold at least the desired number of spaces.
setLength( ) | Integer indicating new length of character string in buffer. | Truncates or expands the previous character string. If expanding, pads with nulls.
charAt( ) | Integer indicating the location of the desired element. | Returns the char at that location in the buffer.
setCharAt( ) | Integer indicating the location of the desired element and the new char value for the element. | Modifies the value at that location.
getChars( ) | The beginning and end from which to copy, the array to copy into, an index into the destination array. | Copy chars into an external array. There’s no getBytes( ) as in String.
append( ) | Overloaded: Object, String, char[], char[] with offset and length, boolean, char, int, long, float, double. | The argument is converted to a string and appended to the end of the current buffer, increasing the buffer if necessary.
insert( ) | Overloaded, each with a first argument of the offset at which to start inserting: Object, String, char[], boolean, char, int, long, float, double. | The second argument is converted to a string and inserted into the current buffer beginning at the offset. The buffer is increased if necessary.
reverse( ) |  | The order of the characters in the buffer is reversed.

The most commonly used method is append( ), which is used by the compiler when evaluating String expressions that contain the ‘+’ and ‘+=‘ operators. The insert( ) method has a similar form, and both methods perform significant manipulations to the buffer instead of creating new objects.

Strings are special

By now you’ve seen that the String class is not just another class in Java. There are a lot of special cases in String, not the least of which is that it’s a built-in class and fundamental to Java. Then there’s the fact that a quoted character string is converted to a String by the compiler and the special overloaded operators + and +=. In this appendix you’ve seen the remaining special case: the carefully-built immutability using the companion StringBuffer and some extra magic in the compiler.

Summary

Because everything is a reference in Java, and because every object is created on the heap and garbage collected only when it is no longer used, the flavor of object manipulation changes, especially when passing and returning objects. For example, in C or C++, if you wanted to initialize some piece of storage in a method, you’d probably request that the user pass the address of that piece of storage into the method. Otherwise you’d have to worry about who was responsible for destroying that storage. Thus, the interface and understanding of such methods is more complicated. But in Java, you never have to worry about responsibility or whether an object will still exist when it is needed, since that is always taken care of for you. Your can create an object at the point that it is needed, and no sooner, and never worry about the mechanics of passing around responsibility for that object: you simply pass the reference. Sometimes the simplification that this provides is unnoticed, other times it is staggering.

The downside to all this underlying magic is twofold:

  1. You always take the efficiency hit for the extra memory management (although this can be quite small), and there’s always a slight amount of uncertainty about the time something can take to run (since the garbage collector can be forced into action whenever you get low on memory). For most applications, the benefits outweigh the drawbacks, and particularly time-critical sections can be written using native methods (see Appendix B).
  2. Aliasing: sometimes you can accidentally end up with two references to the same object, which is a problem only if both references are assumed to point to a distinct object. This is where you need to pay a little closer attention and, when necessary, clone( ) an object to prevent the other reference from being surprised by an unexpected change. Alternatively, you can support aliasing for efficiency by creating immutable objects whose operations can return a new object of the same type or some different type, but never change the original object so that anyone aliased to that object sees no change.

Some people say that cloning in Java is a botched design, and to heck with it, so they implement their own version of cloning [77] and never call the Object.clone( ) method, thus eliminating the need to implement Cloneable and catch the CloneNotSupportedException. This is certainly a reasonable approach and since clone( ) is supported so rarely within the standard Java library, it is apparently a safe one as well. But as long as you don’t call Object.clone( ) you don’t need to implement Cloneable or catch the exception, so that would seem acceptable as well.

Exercises
  1. Demonstrate a second level of aliasing. Create a method that takes a reference to an object but doesn’t modify that reference’s object. However, the method calls a second method, passing it the reference, and this second method does modify the object.
  2. Create a class myString containing a String object that you initialize in the constructor using the constructor’s argument. Add a toString( ) method and a method concatenate( ) that appends a String object to your internal string. Implement clone( ) in myString. Create two static methods that each take a myString x reference as an argument and call x.concatenate(“test”), but in the second method call clone( ) first. Test the two methods and show the different effects.
  3. Create a class called Battery containing an int that is a battery number (as a unique identifier). Make it cloneable and give it a toString( ) method. Now create a class called Toy that contains an array of Battery and a toString( ) that prints out all the batteries. Write a clone( ) for Toy that automatically clones all of its Battery objects. Test this by cloning Toy and printing the result.
  4. Change CheckCloneable.java so that all of the clone( ) methods catch the CloneNotSupportedException rather than passing it to the caller.
  5. Using the mutable-companion-class technique, make an immutable class containing an int, a double and an array of char.
  6. Modify Compete.java to add more member objects to classes Thing2 and Thing4 and see if you can determine how the timings vary with complexity—whether it’s a simple linear relationship or if it seems more complicated.
  7. Starting with Snake.java, create a deep-copy version of the snake.
  8. Inherit an ArrayList and make its clone( ) perform a deep copy.

[72] In C, which generally handles small bits of data, the default is pass-by-value. C++ had to follow this form, but with objects pass-by-value isn’t usually the most efficient way. In addition, coding classes to support pass-by-value in C++ is a big headache.

[73] This is not the dictionary spelling of the word, but it’s what is used in the Java library, so I’ve used it here, too, in some hopes of reducing confusion.

[74] You can apparently create a simple counter-example to this statement, like this:

public class Cloneit implements Cloneable {
  public static void main (String[] args)
  throws CloneNotSupportedException {
    Cloneit a = new Cloneit();
    Cloneit b = (Cloneit)a.clone();
  }
}

However, this only works because main( ) is a method of Cloneit and thus has permission to call the protected base-class method clone( ). If you call it from a different class, it won’t compile.

[75] Except for the poor avocado, which has been reclassified to simply “fat.”

[76] C++ allows the programmer to overload operators at will. Because this can often be a complicated process (see Chapter 10 of Thinking in C++, 2nd edition, Prentice-Hall, 2000), the Java designers deemed it a “bad” feature that shouldn’t be included in Java. It wasn’t so bad that they didn’t end up doing it themselves, and ironically enough, operator overloading would be much easier to use in Java than in C++. This can be seen in Python (see www.Python.org) which has garbage collection and straightforward operator overloading.

[77] Doug Lea, who was helpful in resolving this issue, suggested this to me, saying that he simply creates a function called duplicate( ) for each class.


B: The Java Native Interface (JNI)

[78] The Java language and its standard API are rich enough to write full-fledged applications. But in some cases you must call non-Java code; for example, if you want to access operating-system-specific features, interface with special hardware devices, reuse a preexisting, non-Java code base, or implement time-critical sections of code.

Interfacing with non-Java code requires dedicated support in the compiler and in the Virtual Machine, and additional tools to map the Java code to the non-Java code. The standard solution for calling non-Java code that is provided by JavaSoft is called the Java Native Interface, which will be introduced in this appendix. This is not an in-depth treatment, and in some cases you’re assumed to have partial knowledge of the related concepts and techniques.

The Java Native Interface

JNI is a fairly rich programming interface that allows you to call native methods from a Java application. It was added in Java 1.1, maintaining a certain degree of compatibility with its Java 1.0 equivalent, the native method interface (NMI). NMI has design characteristics that make it unsuitable for adoption across all virtual machines. For this reason, future versions of the language might no longer support NMI, and it will not be covered here.

Currently, JNI is designed to interface with native methods written only in C or C++. Using JNI, your native methods can:

  • Create, inspect, and update Java objects (including arrays and Strings)
  • Call Java methods
  • Catch and throw exceptions
  • Load classes and obtain class information
  • Perform run-time type checking

Thus, virtually everything you can do with classes and objects in ordinary Java you can also do in native methods.

Calling a native method

We’ll start with a simple example: a Java program that calls a native method, which in turn calls the standard C library function printf( ).

The first step is to write the Java code declaring a native method and its arguments:

//: appendixb:ShowMessage.java
public class ShowMessage {
  private native void ShowMessage(String msg);
  static {
    System.loadLibrary("MsgImpl");
    // Linux hack, if you can't get your library
    // path set in your environment:
    // System.load(
    //  "/home/bruce/tij2/appendixb/MsgImpl.so");
  }
  public static void main(String[] args) {
    ShowMessage app = new ShowMessage();
    app.ShowMessage("Generated with JNI");
  }
} ///:~

The native method declaration is followed by a static block that calls System.loadLibrary( ) (which you could call at any time, but this style is more appropriate). System.loadLibrary( ) loads a DLL in memory and links to it. The DLL must be in your system library path. The file name extension is automatically added by the JVM depending on the platform.

In the above code you can also see a call to the System.load( ) method which is commented out. The path specified here is an absolute path, rather than relying on an environment variable. Using an environment variable is naturally the better and more portable solution, but if you can’t figure that out you can comment out the loadLibrary( ) call and uncomment this one, adjusting the path to your own directory.

The header file generator: javah

Now compile your Java source file and run javah on the resulting .class file, specifying the –jni switch (this is done automatically for you by the makefile in the source code distribution for this book):

javah –jni ShowMessage

javah reads the Java class file and for each native method declaration it generates a function prototype in a C or C++ header file. Here’s the output: the ShowMessage.h source file (edited slightly to fit into this book):

/* DO NOT EDIT THIS FILE
   - it is machine generated */
#include <jni.h>
/* Header for class ShowMessage */

#ifndef _Included_ShowMessage
#define _Included_ShowMessage
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     ShowMessage
 * Method:    ShowMessage
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL
Java_ShowMessage_ShowMessage
  (JNIEnv *, jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

As you can see by the #ifdef __cplusplus preprocessor directive, this file can be compiled either by a C or a C++ compiler. The first #include directive includes jni.h, a header file that, among other things, defines the types that you can see used in the rest of the file. JNIEXPORT and JNICALL are macros that expand to match platform-specific directives. JNIEnv, jobject and jstring are JNI data type definitions which will be explained shortly.

Name mangling and function signatures

JNI imposes a naming convention (called name mangling) on native methods. This is important, since it’s part of the mechanism by which the virtual machine links Java calls to native methods. Basically, all native methods start with the word “Java,” followed by the name of the class in which the Java native declaration appears, followed by the name of the Java method. The underscore character is used as a separator. If the Java native method is overloaded, then the function signature is appended to the name as well; you can see the native signature in the comments preceding the prototype. For more information about name mangling and native method signatures, please refer to the JNI documentation.

Implementing your DLL

At this point, all you have to do is write a C or C++ source code file that includes the javah-generated header file and implements the native method, then compile it and generate a dynamic link library. This part is platform-dependent. The code below is compiled and linked into a file called MsgImpl.dll for Windows or MsgImpl.so for Unix/Linux (the makefile packaged with the code listings contains the commands to do this—it is available on the CD ROM bound into this book, or as a free download from www.BruceEckel.com):

//: appendixb:MsgImpl.cpp
//# Tested with VC++ & BC++. Include path must
//# be adjusted to find the JNI headers. See
//# the makefile for this chapter (in the
//# downloadable source code) for an example.
#include <jni.h>
#include <stdio.h>
#include "ShowMessage.h"

extern "C" JNIEXPORT void JNICALL
Java_ShowMessage_ShowMessage(JNIEnv* env,
jobject, jstring jMsg) {
  const char* msg=env->GetStringUTFChars(jMsg,0);
  printf("Thinking in Java, JNI: %s\n", msg);
  env->ReleaseStringUTFChars(jMsg, msg);
} ///:~

The arguments that are passed into the native method are the gateway back into Java. The first, of type JNIEnv, contains all the hooks that allow you to call back into the JVM. (We’ll look at this in the next section.) The second argument has a different meaning depending on the type of method. For non-static methods like the example above, the second argument is the equivalent of the “this” pointer in C++ and similar to this in Java: it’s a reference to the object that called the native method. For static methods, it’s a reference to the Class object where the method is implemented.

The remaining arguments represent the Java objects passed into the native method call. Primitives are also passed in this way, but they come in by value.

In the following sections we’ll explain this code by looking at the ways that you access and control the JVM from inside a native method.

Accessing JNI functions:
The JNIEnv argument

JNI functions are those that the programmer uses to interact with the JVM from inside a native method. As you can see in the example above, every JNI native method receives a special argument as its first parameter: the JNIEnv argument, which is a pointer to a special JNI data structure of type JNIEnv_. One element of the JNI data structure is a pointer to an array generated by the JVM. Each element of this array is a pointer to a JNI function. The JNI functions can be called from the native method by dereferencing these pointers (it’s simpler than it sounds). Every JVM provides its own implementation of the JNI functions, but their addresses will always be at predefined offsets.

Through the JNIEnv argument, the programmer has access to a large set of functions. These functions can be grouped into the following categories:

  • Obtaining version information
  • Performing class and object operations
  • Handling global and local references to Java objects
  • Accessing instance fields and static fields
  • Calling instance methods and static methods
  • Performing string and array operations
  • Generating and handling Java exceptions

The number of JNI functions is quite large and won’t be covered here. Instead, I’ll show the rationale behind the use of these functions. For more detailed information, consult your compiler’s JNI documentation.

If you take a look at the jni.h header file, you’ll see that inside the #ifdef __cplusplus preprocessor conditional, the JNIEnv_ structure is defined as a class when compiled by a C++ compiler. This class contains a number of inline functions that let you access the JNI functions with an easy and familiar syntax. For example, the line of C++ code in the preceding example:

env->ReleaseStringUTFChars(jMsg, msg);

could also be called from C like this:

(*env)->ReleaseStringUTFChars(env, jMsg, msg);

You’ll notice that the C style is (naturally) more complicated—you need a double dereferencing of the env pointer, and you must also pass the same pointer as the first parameter to the JNI function call. The examples in this appendix use the C++ style.

Accessing Java Strings

As an example of accessing a JNI function, consider the code in MsgImpl.cpp. Here, the JNIEnv argument env is used to access a Java String. Java Strings are in Unicode format, so if you receive one and want to pass it to a non-Unicode function (printf( ), for example), you must first convert it into ASCII characters with the JNI function GetStringUTFChars( ). This function takes a Java String and converts it to UTF-8 characters. (These are 8 bits wide to hold ASCII values or 16 bits wide to hold Unicode. If the content of the original string was composed only of ASCII, the resulting string will be ASCII as well.)

GetStringUTFChars( ) is one of the member functions in JNIEnv. To access the JNI function, we use the typical C++ syntax for calling a member function though a pointer. You use the form above to access all of the JNI functions.

Passing and using Java objects

In the previous example we passed a String to the native method. You can also pass Java objects of your own creation to a native method. Inside your native method, you can access the fields and methods of the object that was received.

To pass objects, use the ordinary Java syntax when declaring the native method. In the example below, MyJavaClass has one public field and one public method. The class UseObjects declares a native method that takes an object of class MyJavaClass. To see if the native method manipulates its argument, the public field of the argument is set, the native method is called, and then the value of the public field is printed.

//: appendixb:UseObjects.java
class MyJavaClass {
  public int aValue;
  public void divByTwo() { aValue /= 2; }
}

public class UseObjects {
  private native void
    changeObject(MyJavaClass obj);
  static {
    System.loadLibrary("UseObjImpl");
    // Linux hack, if you can't get your library
    // path set in your environment:
    // System.load(
    //"/home/bruce/tij2/appendixb/UseObjImpl.so");
  }
  public static void main(String[] args) {
    UseObjects app = new UseObjects();
    MyJavaClass anObj = new MyJavaClass();
    anObj.aValue = 2;
    app.changeObject(anObj);
    System.out.println("Java: " + anObj.aValue);
  }
} ///:~

After compiling the code and running javah, you can implement the native method. In the example below, once the field and method ID are obtained, they are accessed through JNI functions.

//: appendixb:UseObjImpl.cpp
//# Tested with VC++ & BC++. Include path must
//# be adjusted to find the JNI headers. See
//# the makefile for this chapter (in the
//# downloadable source code) for an example.
#include <jni.h>
extern "C" JNIEXPORT void JNICALL
Java_UseObjects_changeObject(
JNIEnv* env, jobject, jobject obj) {
  jclass cls = env->GetObjectClass(obj);
  jfieldID fid = env->GetFieldID(
    cls, "aValue", "I");
  jmethodID mid = env->GetMethodID(
    cls, "divByTwo", "()V");
  int value = env->GetIntField(obj, fid);
  printf("Native: %d\n", value);
  env->SetIntField(obj, fid, 6);
  env->CallVoidMethod(obj, mid);
  value = env->GetIntField(obj, fid);
  printf("Native: %d\n", value);
} ///:~

Ignoring the “this” equivalent, the C++ function receives a jobject, which is the native side of the Java object reference we pass from the Java code. We simply read aValue, print it out, change the value, call the object’s divByTwo( ) method, and print the value out again.

To access a Java field or method, you must first obtain its identifier using GetFieldID( ) for fields and GetMethodID( ) for methods. These functions take the class object, a string containing the element name, and a string that gives type information: the data type of the field, or signature information for a method (details can be found in the JNI documentation). These functions return an identifier that you use to access the element. This approach might seem convoluted, but your native method has no knowledge of the internal layout of the Java object. Instead, it must access fields and methods through indexes returned by the JVM. This allows different JVMs to implement different internal object layouts with no impact on your native methods.

If you run the Java program, you’ll see that the object that’s passed from the Java side is manipulated by your native method. But what exactly is passed? A pointer or a Java reference? And what is the garbage collector doing during native method calls?

The garbage collector continues to operate during native method execution, but it’s guaranteed that your objects will not be garbage collected during a native method call. To ensure this, local references are created before, and destroyed right after, the native method call. Since their lifetime wraps the call, you know that the objects will be valid throughout the native method call.

Since these references are created and subsequently destroyed every time the function is called, you cannot make local copies in your native methods, in static variables. If you want a reference that lasts across function invocations, you need a global reference. Global references are not created by the JVM, but the programmer can make a global reference out of a local one by calling specific JNI functions. When you create a global reference, you become responsible for the lifetime of the referenced object. The global reference (and the object it refers to) will be in memory until the programmer explicitly frees the reference with the appropriate JNI function. It’s similar to malloc( ) and free( ) in C.

JNI and Java exceptions

With JNI, Java exceptions can be thrown, caught, printed, and rethrown just as they are inside a Java program. But it’s up to the programmer to call dedicated JNI functions to deal with exceptions. Here are the JNI functions for exception handling:

  • Throw( )
    Throws an existing exception object. Used in native methods to rethrow an exception.
  • ThrowNew( )
    Generates a new exception object and throws it.
  • ExceptionOccurred( )
    Determines if an exception was thrown and not yet cleared.
  • ExceptionDescribe( )
    Prints an exception and the stack trace.
  • ExceptionClear( )
    Clears a pending exception.
  • FatalError( )
    Raises a fatal error. Does not return.

Among these, you can’t ignore ExceptionOccurred( ) and ExceptionClear( ). Most JNI functions can generate exceptions, and there is no language feature that you can use in place of a Java try block, so you must call ExceptionOccurred( ) after each JNI function call to see if an exception was thrown. If you detect an exception, you may choose to handle it (and possibly rethrow it). You must make certain, however, that the exception is eventually cleared. This can be done in your function using ExceptionClear( ) or in some other function if the exception is rethrown, but it must be done.

You must ensure that the exception is cleared, because otherwise the results will be unpredictable if you call a JNI function while an exception is pending. There are few JNI functions that are safe to call during an exception; among these, of course, are all the exception handling functions.

JNI and threading

Since Java is a multithreaded language, several threads can call a native method concurrently. (The native method might be suspended in the middle of its operation when a second thread calls it.) It’s entirely up to the programmer to guarantee that the native call is thread-safe, i.e. it does not modify shared data in an unmonitored way. Basically, you have two options: declare the native method as synchronized or implement some other strategy within the native method to ensure correct, concurrent data manipulation.

Also, you should never pass the JNIEnv pointer across threads, since the internal structure it points to is allocated on a per-thread basis and contains information that makes sense only in that particular thread.

Using a preexisting code base

The easiest way to implement JNI native methods is to start writing native method prototypes in a Java class, compile that class, and run the .class file through javah. But what if you have a large, preexisting code base that you want to call from Java? Renaming all the functions in your DLLs to match the JNI name mangling convention is not a viable solution. The best approach is to write a wrapper DLL “outside” your original code base. The Java code calls functions in this new DLL, which in turn calls your original DLL functions. This solution is not just a work-around; in most cases you must do this anyway because you must call JNI functions on the object references before you can use them.

Additional information

You can find further introductory material, including a C (rather than C++) example and discussion of Microsoft issues, in Appendix A of the first edition of this book, which can be found on the CD ROM bound in with this book, or freely downloaded from www.BruceEckel.com. More extensive information is available at java.sun.com (in the search engine, select “training & tutorials” for keywords “native methods”). Chapter 11 of Core Java 2, Volume II, by Horstmann & Cornell (Prentice Hall, 2000) gives excellent coverage of native methods.

[78] This appendix was contributed by and used with the permission of Andrea Provaglio (www.AndreaProvaglio.com).


C: Java Programming Guidelines

This appendix contains suggestions to help guide you while performing low-level program design, and also while writing code.

Naturally, these are guidelines and not rules. The idea is to use them as inspirations, but to remember that there are occasional situations where you need to bend or break a rule.

Design
  1. Elegance always pays off. In the short term it might seem like it takes much longer to come up with a truly graceful solution to a problem, but when it works the first time and easily adapts to new situations instead of requiring hours, days, or months of struggle, you’ll see the rewards (even if no one can measure them). Not only does it give you a program that’s easier to build and debug, but it’s also easier to understand and maintain, and that’s where the financial value lies. This point can take some experience to believe, because it can appear that you’re not being productive while you’re making a piece of code elegant. Resist the urge to hurry; it will only slow you down.
  2. First make it work, then make it fast. This is true even if you are certain that a piece of code is really important and that it will be a principal bottleneck in your system. Don’t do it. Get the system going first with as simple a design as possible. Then if it isn’t going fast enough, profile it. You’ll almost always discover that “your” bottleneck isn’t the problem. Save your time for the really important stuff.
  3. Remember the “divide and conquer” principle. If the problem you’re looking at is too confusing, try to imagine what the basic operation of the program would be, given the existence of a magic “piece” that handles the hard parts. That “piece” is an object—write the code that uses the object, then look at the object and encapsulate its hard parts into other objects, etc.
  4. Separate the class creator from the class user (client programmer). The class user is the “customer” and doesn’t need or want to know what’s going on behind the scenes of the class. The class creator must be the expert in class design and write the class so that it can be used by the most novice programmer possible, yet still work robustly in the application. Library use will be easy only if it’s transparent.
  5. When you create a class, attempt to make your names so clear that comments are unnecessary. Your goal should be to make the client programmer’s interface conceptually simple. To this end, use method overloading when appropriate to create an intuitive, easy-to-use interface.
  6. Your analysis and design must produce, at minimum, the classes in your system, their public interfaces, and their relationships to other classes, especially base classes. If your design methodology produces more than that, ask yourself if all the pieces produced by that methodology have value over the lifetime of the program. If they do not, maintaining them will cost you. Members of development teams tend not to maintain anything that does not contribute to their productivity; this is a fact of life that many design methods don’t account for.
  7. Write the test code first (before you write the class), and keep it with the class. Automate the running of your tests through a makefile or similar tool. This way, any changes can be automatically verified by running the test code, and you’ll immediately discover errors. Because you know that you have the safety net of your test framework, you will be bolder about making sweeping changes when you discover the need. Remember that the greatest improvements in languages come from the built-in testing that type checking, exception handling, etc., provide, but those features take you only so far. You must go the rest of the way in creating a robust system by filling in the tests that verify features that are specific to your class or program.
  8. Write the test code first (before you write the class) in order to verify that your class design is complete. If you can’t write test code, you don’t know what your class looks like. In addition, the act of writing the test code will often flush out additional features or constraints that you need in the class—these features or constraints don’t always appear during analysis and design.
  9. Remember a fundamental rule of software engineering [79] : All software design problems can be simplified by introducing an extra level of conceptual indirection. This one idea is the basis of abstraction, the primary feature of object-oriented programming.
  10. In concert with guideline 9, an indirection should have a meaning. This meaning can be something as simple as “putting commonly used code in a single method.” If you add levels of indirection (abstraction, encapsulation, etc.) that don’t have meaning, it can be as bad as not having adequate indirection.
  11. Make classes as atomic as possible; that is, give each class a single, clear purpose. If your classes or your system design grows too complicated, break complex classes into simpler ones. The most obvious indicator of this is sheer size: if a class is big, chances are it’s doing too much and should be broken up.
  12. Watch for long argument lists. Method calls then become difficult to write, read and maintain. Instead, try to move the method to a class where it is (more) appropriate, and/or pass objects in as arguments.
  13. Don’t repeat yourself. If a piece of code is recurring in many methods in derived classes, put that code into a single method in the base class and call it from the derived-class methods. Not only do you save code space, you provide for easy propagation of changes. Sometimes the discovery of this common code will add valuable functionality to your interface.
  14. Watch for switch statements or chained if-else clauses. This is typically an indicator of type-check coding, which means you are choosing what code to execute based on some kind of type information (the exact type may not be obvious at first). You can usually replace this kind of code with inheritance and polymorphism; a polymorphic method call will perform the type checking for you, and allow for more reliable and easier extensibility.
  15. From a design standpoint, look for and separate things that change from things that stay the same. That is, search for the elements in a system that you might want to change without forcing a redesign, then encapsulate those elements in classes. You can learn significantly more about this concept in the Thinking in Patterns with Java, downloadable at www.BruceEckel.com.
  16. Don’t extend fundamental functionality by subclassing. If an interface element is essential to a class it should be in the base class, not added during derivation. If you’re adding methods by inheriting, perhaps you should rethink the design.
  17. Less is more. Start with a minimal interface to a class, as small and simple as you need to solve the problem at hand, but don’t try to anticipate all the ways that your class might be used. As the class is used, you’ll discover ways you must expand the interface. However, once a class is in use you cannot shrink the interface without disturbing client code. If you need to add more methods, that’s fine; it won’t disturb code, other than forcing recompiles. But even if new methods replace the functionality of old ones, leave the existing interface alone (you can combine the functionality in the underlying implementation if you want). If you need to expand the interface of an existing method by adding more arguments, create an overloaded method with the new arguments; this way you won’t disturb any existing calls to the existing method.
  18. Read your classes aloud to make sure they’re logical, referring to the relationship between a base class and derived class as “is-a” and member objects as “has-a.”
  19. When deciding between inheritance and composition, ask if you need to upcast to the base type. If not, prefer composition (member objects) to inheritance. This can eliminate the perceived need for multiple base types. If you inherit, users will think they are supposed to upcast.
  20. Use data members for variation in value and method overriding for variation in behavior. That is, if you find a class that uses state variables along with methods that switch behavior based on those variables, you should probably redesign it to express the differences in behavior within subclasses and overridden methods.
  21. Watch for overloading. A method should not conditionally execute code based on the value of an argument. In this case, you should create two or more overloaded methods instead.
  22. Use exception hierarchies, preferably derived from specific appropriate classes in the standard Java exception hierarchy. The person catching the exceptions can then catch the specific types of exceptions, followed by the base type. If you add new derived exceptions, existing client code will still catch the exception through the base type.
  23. Sometimes simple aggregation does the job. A “passenger comfort system” on an airline consists of disconnected elements: seat, air conditioning, video, etc., and yet you need to create many of these in a plane. Do you make private members and build a whole new interface? No—in this case, the components are also part of the public interface, so you should create public member objects. Those objects have their own private implementations, which are still safe. Be aware that simple aggregation is not a solution to be used often, but it does happen.
  24. When you design a class, think about the client programmer’s perspective (the class should be fairly obvious to use) and the perspective of the person maintaining the code (anticipate the kind of changes that will be made, to make them easy).
  25. Watch out for “giant object syndrome.” This is often an affliction of procedural programmers who are new to OOP and who end up writing a procedural program and sticking it inside one or two giant objects. With the exception of application frameworks, objects represent concepts in your application, not the application.
  26. If you must do something ugly, at least localize the ugliness inside a class.
  27. If you must do something nonportable, make an abstraction for that service and localize it within a class. This extra level of indirection prevents the non-portability from being distributed throughout your program.
  28. Objects should not simply hold some data; they should also have well-defined behaviors.
  29. Choose composition first when creating new classes from existing classes. You should only used inheritance if it is required by your design. If you use inheritance where composition will work, your designs will become needlessly complicated.
  30. Use inheritance and method overriding to express differences in behavior, and fields to express variations in state. An extreme example of what not to do is inheriting different classes to represent colors instead of using a “color” field.
  31. Watch out for variance. Two semantically different objects may have identical actions, or responsibilities, and there is a natural temptation to try to make one a subclass of the other just to benefit from inheritance. This is called variance, but there’s no real justification to force a superclass/subclass relationship where it doesn’t exist. A better solution is to create a general base class that produces an interface for both as derived classes—it requires a bit more space, but you still benefit from inheritance and will probably make an important discovery about the design.
  32. Watch out for limitation during inheritance. The clearest designs add new capabilities to inherited ones. A suspicious design removes old capabilities during inheritance without adding new ones. But rules are made to be broken, and if you are working from an old class library, it may be more efficient to restrict an existing class in its subclass than it would be to restructure the hierarchy so your new class fits in where it should, above the old class.
  33. Use design patterns to eliminate “naked functionality.” That is, if only one object of your class should be created, don’t bolt ahead to the application and write a comment “Make only one of these.” Wrap it in a singleton. If you have a lot of messy code in your main program that creates your objects, look for a creational pattern like a factory method in which you can encapsulate that creation. Eliminating “naked functionality” will not only make your code much easier to understand and maintain, it will also make it more bulletproof against the well-intentioned maintainers that come after you.
  34. Watch out for “analysis paralysis.” Remember that you must usually move forward in a project before you know everything, and that often the best and fastest way to learn about some of your unknown factors is to go to the next step rather than trying to figure it out in your head. Java has built-in firewalls; let them work for you. Your mistakes in a class or set of classes won’t destroy the integrity of the whole system.
  35. When you think you’ve got a good analysis, design, or implementation, do a walkthrough. Bring someone in from outside your group—this doesn’t have to be a consultant, but can be someone from another group within your company. Reviewing your work with a pair of fresh eyes can reveal problems at a stage where it’s much easier to fix them and more than pays for the time and money “lost” to the walkthrough process.

Implementation
  1. In general, follow the Sun coding conventions at
    http://java.sun.com/docs/codeconv/index.html (the code in this book follows these conventions as much as I was able). These are used for what constitutes arguably the largest body of code that the largest number of Java programmers will be exposed to. If you doggedly stick to the coding style you’ve always used then you will make it harder for your reader. Whatever coding conventions you decide upon, they should be consistent throughout the project. There is a free tool to automatically reformat Java code at: http://home.wtal.de/software-solutions/jindent.
  2. Whatever coding style you use, it really does make a difference if your team (and even better, your company) standardizes on it. This means to the point that everyone considers it fair game to fix someone else’s coding style if it doesn’t conform. The value of standardization is that it takes less brain cycles to parse the code, so that you can focus more on what the code means.
  3. Capitalize the first letter of class names. The first letter of fields, methods, and objects (references) should be lowercase. All identifiers should run their words together, and capitalize the first letter of all intermediate words. For example:
    ThisIsAClassName
    thisIsAMethodOrFieldName
    Capitalize all the letters of static final primitive identifiers that have constant initializers in their definitions. This indicates they are compile-time constants.
    Packages are a special case: they are all lowercase letters, even for intermediate words. The domain extension (com, org, net, edu, etc.) should also be lowercase. (This was a change between Java 1.1 and Java 2.)
  4. Don’t create your own “decorated” private data member names (prepending underscores, Hungarian notation, etc.). This is confusing, difficult to read, and difficult to maintain. Let classes and packages do the name scoping for you.
  5. When creating a class for general-purpose use, follow a “canonical form” and include definitions for equals( ), hashCode( ), toString( ), clone( ) (implement Cloneable), and implement Comparable and Serializable.
  6. Use the JavaBeans “get,” “set,” and “is” naming conventions for methods that read and change private fields, even if you don’t think you’re making a JavaBean at the time. Not only does it make it easy to use your class as a Bean, but it’s a standard way to name these kinds of methods and so will be more easily understood by the reader.
  7. For each class you create, consider including a static public test( ) that contains code to test that class. You don’t need to remove the test code to use the class in a project, and if you make any changes you can easily rerun the tests. This code also provides examples of how to use your class.
  8. Sometimes you need to inherit in order to access protected members of the base class. This can lead to a perceived need for multiple base types. If you don’t need to upcast, first derive a new class to perform the protected access. Then make that new class a member object inside any class that needs to use it, rather than inheriting.
  9. Avoid the use of final methods for efficiency purposes. Use final only when the program is running, but not fast enough, and your profiler has shown you that a method invocation is the bottleneck.
  10. If two classes are associated with each other in some functional way (such as containers and iterators), try to make one an inner class of the other. This not only emphasizes the association between the classes, but it allows the class name to be reused within a single package by nesting it within another class. The Java containers library does this by defining an inner Iterator class inside each container class, thereby providing the containers with a common interface. The other reason you’ll want to use an inner class is as part of the private implementation. Here, the inner class beneficial for implementation hiding rather than the class association and prevention of namespace pollution noted above.
  11. Anytime you notice classes that appear to have high coupling with each other, consider the coding and maintenance improvements you might get by using inner classes (see “Improving the code with an inner class” on page 303).
  12. Don’t fall prey to premature optimization. That way lies madness. In particular, don’t worry about writing (or avoiding) native methods, making some methods final, or tweaking code to be efficient when you are first constructing the system. Your primary goal should be to prove the design, unless the design requires a certain efficiency.
  13. Keep scopes as small as possible so the visibility and lifetime of your objects are as small as possible. This reduces the chance of using an object in the wrong context and hiding a difficult-to-find bug. For example, suppose you have a container and a piece of code that iterates through it. If you copy that code to use with a new container, you may accidentally end up using the size of the old container as the upper bound of the new one. If, however, the old container is out of scope, the error will be caught at compile-time.
  14. Use the containers in the standard Java library. Become proficient with their use and you’ll greatly increase your productivity. Prefer ArrayList for sequences, HashSet for sets, HashMap for associative arrays, and LinkedList for stacks (rather than Stack) and queues.
  15. For a program to be robust, each component must be robust. Use all the tools provided by Java: access control, exceptions, type checking, and so on in each class you create. That way you can safely move to the next level of abstraction when building your system.
  16. Prefer compile-time errors to run-time errors. Try to handle an error as close to the point of its occurrence as possible. Prefer dealing with the error at that point to throwing an exception. Catch any exceptions in the nearest handler that has enough information to deal with them. Do what you can with the exception at the current level; if that doesn’t solve the problem, rethrow the exception.
  17. Watch for long method definitions. Methods should be brief, functional units that describe and implement a discrete part of a class interface. A method that is long and complicated is difficult and expensive to maintain, and is probably trying to do too much all by itself. If you see such a method, it indicates that, at the least, it should be broken up into multiple methods. It may also suggest the creation of a new class. Small methods will also foster reuse within your class. (Sometimes methods must be large, but they should still do just one thing.)
  18. Make classes as atomic as possible. That is, give each class a single, clear purpose. If your classes or your system design grows too complicated, break complex classes into simpler ones. The most obvious indicator of this is sheer size: if a class is big, chances are it’s doing too much and should be broken up. Clues to suggest redesign of a class are:
    1) A complicated switch statement: consider using polymorphism
    2) A large number of methods that cover broadly different types of operations: consider using several classes
    3) A large number of member variables that concern broadly different characteristics: consider using several classes
  19. Keep things as “private as possible.” Once you publicize an aspect of your library (a method, a class, a field), you can never take it out. If you do, you’ll wreck somebody’s existing code, forcing them to rewrite and redesign. If you publicize only what you must, you can change everything else with impunity, and since designs tend to evolve this is an important freedom. In this way, implementation changes will have minimal impact on derived classes. Privacy is especially important when dealing with multithreading—only private fields can be protected against un-synchronized use.
  20. Use comments liberally, and use the javadoc comment-documentation syntax to produce your program documentation. However, the comments should add geniune meaning to the code; comments that only reiterate what the code is clearly expressing are annoying. Note that the typical verbose detail of Java class and method names reduce the need for as many comments.
  21. Avoid using “magic numbers,” which are numbers hard-wired into code. These are a nightmare if you need to change them, since you never know if “100” means “the array size” or “something else entirely.” Instead, create a constant with a descriptive name and use the constant identifier throughout your program. This makes the program easier to understand and much easier to maintain.
  22. When creating constructors, consider exceptions. In the best case, the constructor won’t do anything that throws an exception. In the next-best scenario, the class will be composed and inherited from robust classes only, so they will need no cleanup if an exception is thrown. Otherwise, you must clean up composed classes inside a finally clause. If a constructor must fail, the appropriate action is to throw an exception, so the caller doesn’t continue blindly, thinking that the object was created correctly.
  23. If your class requires any cleanup when the client programmer is finished with the object, place the cleanup code in a single, well- defined method with a name like cleanup( ) that clearly suggests its purpose. In addition, place a boolean flag in the class to indicate whether the object has been cleaned up so that finalize( ) can check for “the death condition” (see Chapter 4).
  24. The responsibility of finalize( ) can only be to verify “the death condition” (see Chapter 4) of an object for debugging. In special cases, it might be needed to release memory that would not otherwise be released by the garbage collector. Since the garbage collector might not get called for your object, you cannot use finalize( ) to perform necessary cleanup. For that you must create your own “cleanup” method. In the finalize( ) method for the class, check to make sure that the object has been cleaned up and throw a class derived from RuntimeException if it hasn’t, to indicate a programming error. Before relying on such a scheme, ensure that finalize( ) works on your system. (You might need to call System.gc( ) to ensure this behavior.)
  25. If an object must be cleaned up (other than by garbage collection) within a particular scope, use the following approach: Initialize the object and, if successful, immediately enter a try block with a finally clause that performs the cleanup.
  26. When overriding finalize( ) during inheritance, remember to call super.finalize( ) (this is not necessary if Object is your immediate superclass). You should call super.finalize( ) as the final act of your overridden finalize( ) rather than the first, to ensure that base-class components are still valid if you need them.
  27. When you are creating a fixed-size container of objects, transfer them to an array (especially if you’re returning this container from a method). This way you get the benefit of the array’s compile-time type checking, and the recipient of the array might not need to cast the objects in the array in order to use them. Note that the base-class of the containers library, java.util.Collection, has two toArray( ) methods to accomplish this.
  28. Choose interfaces over abstract classes. If you know something is going to be a base class, your first choice should be to make it an interface, and only if you’re forced to have method definitions or member variables should you change it to an abstract class. An interface talks about what the client wants to do, while a class tends to focus on (or allow) implementation details.
  29. Inside constructors, do only what is necessary to set the object into the proper state. Actively avoid calling other methods (except for final methods) since those methods can be overridden by someone else to produce unexpected results during construction. (See Chapter 7 for details.) Smaller, simpler constructors are less likely to throw exceptions or cause problems.
  30. To avoid a highly frustrating experience, make sure that there’s only one unpackaged class of each name anywhere in your classpath. Otherwise, the compiler can find the identically-named other class first, and report error messages that make no sense. If you suspect that you are having a classpath problem, try looking for .class files with the same names at each of the starting points in your classpath. Ideally, put all your classes within packages.
  31. Watch out for accidental overloading. If you attempt to override a base-class method and you don’t quite get the spelling right, you’ll end up adding a new method rather than overriding an existing method. However, this is perfectly legal, so you won’t get any error message from the compiler or run-time system—your code simply won’t work correctly.
  32. Watch out for premature optimization. First make it work, then make it fast—but only if you must, and only if it’s proven that there is a performance bottleneck in a particular section of your code. Unless you have used a profiler to discover a bottleneck, you will probably be wasting your time. The hidden cost of performance tweaks is that your code becomes less understandable and maintainable.
  33. Remember that code is read much more than it is written. Clean designs make for easy-to-understand programs, but comments, detailed explanations, and examples are invaluable. They will help both you and everyone who comes after you. If nothing else, the frustration of trying to ferret out useful information from the online Java documentation should convince you.

[79] Explained to me by Andrew Koenig.


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D: Resources

Software

The JDK from http://java.sun.com. Even if you choose to use a third-party development environment, it’s always a good idea to have the JDK on hand in case you come up against what might be a compiler error. The JDK is the touchstone, and if there is a bug in it, chances are it will be well-known.

The HTML Java documentation from http://java.sun.com. I have never found a reference book on the standard Java libraries that wasn’t out of date or missing information. Although the HTML documentation from Sun is shot through with small bugs and is sometimes unusably terse, all the classes and methods are at least there. People are sometimes uncomfortable at first using an online resource rather than a printed book, but it’s worth your while to get over this and open the HTML docs first, so you can at least get the big picture. If you can’t figure it out at that point, then reach for the printed books.

[[Parasoft’s jtest (this is only a placeholder, this product is still in testing).]]

Books

Thinking in Java, 1st edition. Available as fully-indexed, color-syntax-highlighted HTML on the CD ROM bound in with this book, or as a free download from www.BruceEckel.com. Includes older material and material that was not considered interesting enough to carry through to the 2nd edition.

Core Java 2, by Horstmann & Cornell, Prentice-Hall. Volume I—Fundamentals, 1999. Volume II—Advanced Features, 2000. Huge, comprehensive, and the first place I go when I’m hunting for answers. The books I recommend when you’ve completed Thinking in Java and need to cast a bigger net.

Java in a Nutshell: A Desktop Quick Reference, 2nd Edition, by David Flanagan, O’Reilly & Assoc. 1997. A compact summary of the online documentation of Java 1.1. Personally, I prefer to browse the docs from java.sun.com online, especially since they change so often. However, many folks still like printed documentation and this fits the bill; it also provides more discussion than the online documents.

The Java Class Libraries: An Annotated Reference, by Patrick Chan and Rosanna Lee, Addison-Wesley, 1997. What the online reference should have been: enough description to make it usable. One of the technical reviewers for Thinking in Java said, “If I had only one Java book, this would be it (well, in addition to yours, of course).” I’m not as thrilled with it as he is. It’s big, it’s expensive, and the quality of the examples doesn’t satisfy me. But it’s a place to look when you’re stuck and it seems to have more depth (and sheer size) than Java in a Nutshell.

Java Network Programming, by Elliotte Rusty Harold, O’Reilly, 1997. I didn’t begin to understand Java networking until I found this book. I also find his Web site, Café au Lait, to be a stimulating, opinionated, and up-to-date perspective on Java developments, unencumbered by allegiances to any vendors. His regular updates keep up with fast-changing news about Java. See http://metalab.unc.edu/javafaq/.

JDBC Database Access with Java, by Hamilton, Cattell & Fisher (Addison-Wesley, 1997). If you know nothing about SQL and databases, this is a nice, gentle introduction. It also contains some of the details as well as an “annotated reference” to the API (again, what the online reference should have been). The drawback, as with all books in The Java Series (“The ONLY Books Authorized by JavaSoft”) is that it’s been whitewashed so that it says only wonderful things about Java—you won’t find out about any dark corners in this series.

Java Programming with CORBA, by Andreas Vogel & Keith Duddy (John Wiley & Sons, 1997). A serious treatment of the subject with code examples for the three main Java ORBs (Visibroker, Orbix, Joe).

Design Patterns, by Gamma, Helm, Johnson & Vlissides (Addison-Wesley, 1995). The seminal book that started the patterns movement in programming.

UML Toolkit, by Hans-Erik Eriksson & Magnus Penker, (John Wiley & Sons, 1997). Explains UML and how to use it, and has a case study in Java. An
accompanying CD-ROM contains the Java code and a cut-down version of Rational Rose. An excellent introduction to UML and how to use it to build a real system.

Practical Algorithms for Programmers, by Binstock & Rex (Addison-Wesley, 1995). The algorithms are in C, so they’re fairly easy to translate into Java. Each algorithm is thoroughly explained.

Analysis & design

Extreme Programming Explained by Kent Beck (Addison-Wesley, 2000). I love this book. Yes, I tend to take a radical approach to things but I've always felt that there could be a much different, much better program development process, and I think XP comes pretty darn close. The only book that has had a similar impact on me was PeopleWare (described below), which talks primarily about the environment and dealing with corporate culture. Extreme Programming Explained talks about programming, and turns most things, even recent “findings,” on their ear. They even go so far as to say that pictures are OK as long as you don’t spend too much time on them and are willing to throw them away. (You’ll notice that this book does not have the “UML stamp of approval” on its cover.) I could see deciding whether to work for a company based solely on whether they used XP. Small book, small chapters, effortless to read, exciting to think about. You start imagining yourself working in such an atmosphere and it brings visions of a whole new world.

UML Distilled by Martin Fowler (2nd edition, Addison-Wesley, 2000). When you first encounter UML, it is daunting because there are so many diagrams and details. According to Fowler, most of this stuff is unnecessary so he cuts through to the essentials. For most projects, you only need to know a few diagramming tools, and Fowler’s goal is to come up with a good design rather than worry about all the artifacts of getting there. A nice, thin, readable book; the first one you should get if you need to understand UML.

The Unified Software Development Process by Ivar Jacobsen, Grady Booch, and James Rumbaugh (Addison-Wesley, 1999). I went in fully prepared to dislike this book. It seemed to have all the makings of a boring college text. I was pleasantly surprised—only pockets of the book contain explanations that seem as if those concepts aren’t clear to the authors. The bulk of the book is not only clear, but enjoyable. And best of all, the process makes a lot of practical sense. It’s not Extreme Programming (and does not have their clarity about testing) but it’s also part of the UML juggernaut—even if you can’t get XP adopted, most people have climbed aboard the “UML is good” bandwagon (regardless of their actual level of experience with it) and so you can probably get it adopted. I think this book should be the flagship of UML, and the one you can read after Fowler’s UML Distilled when you want more detail.

Before you choose any method, it’s helpful to gain perspective from those who are not trying to sell one. It’s easy to adopt a method without really understanding what you want out of it or what it will do for you. Others are using it, which seems a compelling reason. However, humans have a strange little psychological quirk: If they want to believe something will solve their problems, they’ll try it. (This is experimentation, which is good.) But if it doesn’t solve their problems, they may redouble their efforts and begin to announce loudly what a great thing they’ve discovered. (This is denial, which is not good.) The assumption here may be that if you can get other people in the same boat, you won’t be lonely, even if it’s going nowhere (or sinking).

This is not to suggest that all methodologies go nowhere, but that you should be armed to the teeth with mental tools that help you stay in experimentation mode (“It’s not working; let’s try something else”) and out of denial mode (“No, that’s not really a problem. Everything’s wonderful, we don’t need to change”). I think the following books, read before you choose a method, will provide you with these tools.

Software Creativity, by Robert Glass (Prentice-Hall, 1995). This is the best book I’ve seen that discusses perspective on the whole methodology issue. It’s a collection of short essays and papers that Glass has written and sometimes acquired (P.J. Plauger is one contributor), reflecting his many years of thinking and study on the subject. They’re entertaining and only long enough to say what’s necessary; he doesn’t ramble and bore you. He’s not just blowing smoke, either; there are hundreds of references to other papers and studies. All programmers and managers should read this book before wading into the methodology mire.

Software Runaways: Monumental Software Disasters, by Robert Glass (Prentice-Hall, 1997). The great thing about this book is that it brings to the forefront what we don’t talk about: how many projects not only fail, but fail spectacularly. I find that most of us still think “That can’t happen to me” (or “That can’t happen again”) and I think this puts us at a disadvantage. By keeping in mind that things can always go wrong, you’re in a much better position to make them go right.

Peopleware, by Tom Demarco and Timothy Lister (Dorset House, 2nd edition, 1999). Although they have backgrounds in software development, this book is about projects and teams in general. But the focus is on the people and their needs rather than the technology and its needs. They talk about creating an environment where people will be happy and productive, rather than deciding what rules those people should follow to be adequate components of a machine. This latter attitude, I think, is the biggest contributor to programmers smiling and nodding when XYZ method is adopted and then quietly doing whatever they’ve always done.

Complexity, by M. Mitchell Waldrop (Simon & Schuster, 1992). This chronicles the coming together of a group of scientists from different disciplines in Santa Fe, New Mexico, to discuss real problems that the individual disciplines couldn’t solve (the stock market in economics, the initial formation of life in biology, why people do what they do in sociology, etc.). By crossing physics, economics, chemistry, math, computer science, sociology, and others, a multidisciplinary approach to these problems is developing. But more important, a different way of thinking about these ultra-complex problems is emerging: Away from mathematical determinism and the illusion that you can write an equation that predicts all behavior and toward first observing and looking for a pattern and trying to emulate that pattern by any means possible. (The book chronicles, for example, the emergence of genetic algorithms.) This kind of thinking, I believe, is useful as we observe ways to manage more and more complex software projects.

My own list of books

Listed in order of publication. Not all of these are currently available.

Computer Interfacing with Pascal & C (Self-published via the Eisys imprint, 1988. Only available via www.BruceEckel.com). An introduction to electronics from back when CP/M was still king and DOS was an upstart. I used high-level languages and often the parallel port of the computer to drive various electronic projects. Adapted from my columns in the first and best magazine I wrote for, Micro Cornucopia (To paraphrase Larry O’Brien, long-time editor of Software Development Magazine: the best computer magazine ever published—they even had plans for building a robot in a flower pot!) Alas, Micro C became lost long before the Internet appeared. Creating this book was an extremely satisfying publishing experience.

Using C++ (Osborne/McGraw-Hill, 1989). One of the first books out on C++. This is out of print and replaced by its second edition, the renamed C++ Inside & Out.

C++ Inside & Out (Osborne/McGraw-Hill, 1993). As noted, actually the 2nd edition of Using C++. The C++ in this book is reasonably accurate, but it's circa 1992 and Thinking in C++ is intended to replace it. You can find out more about this book and download the source code at www.BruceEckel.com.

Thinking in C++, 1st edition (Prentice-Hall, 1995).

Thinking in C++, 2nd edition, Volume 1 (Prentice-Hall, 2000). Downloadable from www.BruceEckel.com.

Black Belt C++, the Master’s Collection, Bruce Eckel, editor (M&T Books, 1994). Out of print. A collection of chapters by various C++ luminaries based on their presentations in the C++ track at the Software Development Conference, which I chaired. The cover on this book stimulated me to gain control over all future cover designs.

Thinking in Java, 1st edition (Prentice-Hall, 1998). The first edition of this book won the Software Development Magazine Productivity Award and the Java Developer’s Journal Editor’s Choice Award in 1999. Downloadable from www.BruceEckel.com.

[ Previous Chapter ] [ Short TOC ] [ Table of Contents ] [ Index ]


Index

Please note that some names will be duplicated in capitalized form. Following Java style, the capitalized names refer to Java classes, while lowercase names refer to a general concept.

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Thinking in Java, 2nd ed. Revision 9 - Contents

Thinking in Java, 2nd ed. Revision 9 - Title Page

    Preface
      Preface to the 2nd edition
        Java 2
      The CD ROM
    Introduction
      Prerequisites
      Learning Java
      Goals
      Online documentation
      Chapters
      Exercises
      Multimedia CD ROM
      Source code
        Coding standards
      Java versions
      Seminars and mentoring
      Errors
      Note on the cover design
      Acknowledgements
        Internet contributors
    1: Introduction to Objects
      The progress of abstraction
      An object has an interface
      The hidden implementation
      Reusing the implementation
      Inheritance: reusing the interface
        Is-a vs. is-like-a relationships
      Interchangeable objects with polymorphism
        Abstract base classes and interfaces
      Object landscapes and lifetimes
        Collections and iterators
        The singly rooted hierarchy
        Collection libraries and support for easy collection use
          Downcasting vs. templates/generics
        The housekeeping dilemma: who should clean up?
          Garbage collectors vs. efficiency and flexibility
      Exception handling: dealing with errors
      Multithreading
      Persistence
      Java and the Internet
        What is the Web?
          Client/Server computing
          The Web as a giant server
        Client-side programming
          Plug-ins
          Scripting languages
          Java
          ActiveX
          Security
          Internet vs. intranet
        Server-side programming
        A separate arena: applications
      Analysis and design
        Phase 0: Make a plan
          The mission statement
        Phase 1: What are we making?
        Phase 2: How will we build it?
          Five stages of object design
          Guidelines for object development
        Phase 3: Build the core
        Phase 4: Iterate the use cases
        Phase 5: Evolution
        Plans pay off
      Extreme programming
        Write tests first
        Pair programming
      Why Java succeeds
        Systems are easier to express and understand
        Maximal leverage with libraries
        Error handling
        Programming in the large
      Strategies for transition
        Guidelines
          1. Training
          2. Low-risk project
          3. Model from success
          4. Use existing class libraries
          5. Don’t rewrite existing code in Java
        Management obstacles
          Startup costs
          Performance issues
          Common design errors
      Java vs. C++?
      Summary
    2: Everything is an Object
      You manipulate objects with references
      You must create all the objects
        Where storage lives
        Special case: primitive types
          High-precision numbers
        Arrays in Java
      You never need to destroy an object
        Scoping
        Scope of objects
      Creating new data types: class
        Fields and methods
          Default values for primitive members
      Methods, arguments, and return values
        The argument list
      Building a Java program
        Name visibility
        Using other components
        The static keyword
      Your first Java program
        Compiling and running
      Comments and embedded documentation
        Comment documentation
        Syntax
        Embedded HTML
        @see: referring to other classes
        Class documentation tags
          @version
          @author
          @since
        Variable documentation tags
        Method documentation tags
          @param
          @return
          @throws
          @deprecated
        Documentation example
      Coding style
      Summary
      Exercises
    3: Controlling Program Flow
      Using Java operators
        Precedence
        Assignment
          Aliasing during method calls
        Mathematical operators
          Unary minus and plus operators
        Auto increment and decrement
        Relational operators
          Testing object equivalence
        Logical operators
          Short-circuiting
        Bitwise operators
        Shift operators
        Ternary if-else operator
        The comma operator
        String operator +
        Common pitfalls when using operators
        Casting operators
          Literals
          Promotion
        Java has no “sizeof”
        Precedence revisited
        A compendium of operators
      Execution control
        true and false
        if-else
          return
        Iteration
        do-while
        for
          The comma operator
        break and continue
          The infamous “goto”
        switch
          Calculation details
      Summary
      Exercises
    4: Initialization & Cleanup
      Guaranteed initialization with the constructor
      Method overloading
        Distinguishing overloaded methods
        Overloading with primitives
        Overloading on return values
        Default constructors
        The this keyword
          Calling constructors from constructors
          The meaning of static
      Cleanup: finalization and garbage collection
        What is finalize( ) for?
        You must perform cleanup
        The death condition
        How a garbage collector works
      Member initialization
        Specifying initialization
        Constructor initialization
          Order of initialization
          Static data initialization
          Explicit static initialization
          Non-static instance initialization
      Array initialization
        Multidimensional arrays
      Summary
      Exercises
    5: Hiding the Implementation
      package: the library unit
        Creating unique package names
          Collisions
        A custom tool library
        Using imports to change behavior
        Package caveat
      Java access specifiers
        “Friendly”
        public: interface access
          The default package
        private: you can’t touch that!
        protected: “sort of friendly”
      Interface and implementation
      Class access
      Summary
      Exercises
    6: Reusing Classes
      Composition syntax
      Inheritance syntax
        Initializing the base class
          Constructors with arguments
          Catching base constructor exceptions
      Combining composition and inheritance
        Guaranteeing proper cleanup
          Order of garbage collection
        Name hiding
      Choosing composition vs. inheritance
      protected
      Incremental development
      Upcasting
        Why “upcasting”?
          Composition vs. inheritance revisited
      The final keyword
        Final data
          Blank finals
          Final arguments
        Final methods
          final and private
        Final classes
        Final caution
      Initialization and class loading
        Initialization with inheritance
      Summary
      Exercises
    7: Polymorphism
      Upcasting
        Why upcast?
      The twist
        Method call binding
        Producing the right behavior
        Extensibility
      Overriding vs. overloading
      Abstract classes and methods
      Constructors and polymorphism
        Order of constructor calls
        Inheritance and finalize( )
        Behavior of polymorphic methods inside constructors
      Designing with inheritance
        Pure inheritance vs. extension
        Downcasting and run-time type identification
      Summary
      Exercises
    8: Interfaces & Inner Classes
      Interfaces
        “Multiple inheritance” in Java
          Name collisions when combining interfaces
        Extending an interface with inheritance
        Grouping constants
        Initializing fields in interfaces
      Inner classes
        Inner classes and upcasting
        Inner classes in methods and scopes
        Anonymous inner classes
        The link to the outer class
        static inner classes
        Referring to the outer class object
        Inheriting from inner classes
        Can inner classes be overridden?
        Inner class identifiers
        Why inner classes?
          Closures & Callbacks
        Inner classes & control frameworks
      Summary
      Exercises
    9: Holding Your Objects
      Arrays
        Arrays are first-class objects
          Containers of primitives
        Returning an array
        The Arrays class
        Filling an array
        Copying an array
        Comparing arrays
        Array element comparisons
        Sorting an array
        Searching a sorted array
        Array summary
      Introduction to containers
        Printing containers
        Filling containers
      Container disadvantage: unknown type
        Sometimes it works anyway
        Making a type-conscious ArrayList
          Parameterized types
      Iterators
        Unintended recursion
      Container taxonomy
      Collection functionality
      List functionality
        Making a stack from a LinkedList
        Making a queue from a LinkedList
      Set functionality
        SortedSet
      Map functionality
        SortedMap
        Hashing and hash codes
          Understanding hashCode( )
          HashMap performance factors
        Overriding hashCode( )
      Holding references
        The WeakHashMap
      Iterators revisited
      Choosing an implementation
        Choosing between Lists
        Choosing between Sets
        Choosing between Maps
      Sorting and searching Lists
      Utilities
        Making a Collection or Map unmodifiable
        Synchronizing a Collection or Map
          Fail fast
      Unsupported operations
      Java 1.0/1.1 containers
        Vector & Enumeration
        Hashtable
        Stack
        BitSet
      Summary
      Exercises
    10: Error Handling with Exceptions
      Basic exceptions
        Exception arguments
      Catching an exception
        The try block
        Exception handlers
          Termination vs. resumption
        The exception specification
        Catching any exception
        Rethrowing an exception
      Standard Java exceptions
        The special case of RuntimeException
      Creating your own exceptions
      Exception restrictions
      Performing cleanup with finally
        What’s finally for?
        Pitfall: the lost exception
      Constructors
      Exception matching
        Exception guidelines
      Summary
      Exercises
    11: The Java IO System
      Input and output
        Types of InputStream
        Types of OutputStream
      Adding attributes and useful interfaces
        Reading from an InputStream with FilterInputStream
        Writing to an OutputStream with FilterOutputStream
      Off by itself: RandomAccessFile
      The File class
        A directory lister
          Anonymous inner classes
        Checking for and creating directories
      Typical uses of IO streams
        Input streams
          1. Buffered input file
          2. Input from memory
          3. Formatted memory input
          4. Line numbering and file output
        Output streams
          5. Storing and recovering data
          6. Reading and writing random access files
        Shorthand for file manipulation
          7. File input shorthand
          8. Formatted file output shorthand
          9. Data file output shorthand
        Reading from standard input
        Piped streams
      StreamTokenizer
        StringTokenizer
      Java 1.1 IO streams
        Sources and sinks of data
        Modifying stream behavior
        Unchanged Classes
        An example
        Redirecting standard IO
      Compression
        Simple compression with GZIP
        Multi-file storage with Zip
        The Java archive (jar) utility
      Object serialization
        Finding the class
        Controlling serialization
          The transient keyword
          An alternative to Externalizable
          Versioning
        Using persistence
      Checking capitalization style
      Summary
      Exercises
    12: Run-time Type Identification
      The need for RTTI
        The Class object
          Class literals
        Checking before a cast
          Using class literals
          A dynamic instanceof
          instanceof vs. Class equivalence
      RTTI syntax
      Reflection: run-time class information
        A class method extractor
      Summary
      Exercises
    13: Creating Windows & Applets
      The basic applet
        Running applets inside a Web browser
        Using Appletviewer
        Testing applets
      Making windows and applets
        A display framework
        Closing the window
        Using the Windows Explorer
      Making a button
      Capturing an event
      Text fields
      Text areas
      Labels
        HTML text on Swing components
      Check boxes
      Radio buttons
      Drop-down lists
      List boxes
      Tabbed panes
      Message boxes
      Menus
      Dialog Boxes
      Controlling layout
        FlowLayout
        BorderLayout
        GridLayout
        GridBagLayout
        BoxLayout
        Absolute positioning
      Alternatives to action
      Applet restrictions
        Applet advantages
      Windowed applications
        Menus
        Dialog boxes
          File dialogs
      The event model
        Event and listener types
          Using listener adapters for simplicity
      Packaging the applet into a JAR file
      Revisiting the earlier examples
        Demonstrating the framework methods
        Text fields
        Text areas
        Check boxes and radio buttons
        Drop-down lists
        Lists
        Menus
        Dialog boxes
      Selecting Look & Feel
      Binding events dynamically
      Separating business logic from UI logic
      Recommended coding approaches
        Baseline: the good way to do it
      JFC APIs
        Desktop colors
        The clipboard
      Visual programming and Beans
        What is a Bean?
        Extracting BeanInfo with the Introspector
        A more sophisticated Bean
        Packaging a Bean
        More complex Bean support
        More to Beans
      Introduction to Swing
        Benefits of Swing
        Easy conversion
        Tool tips
        Borders
        Buttons
        Button groups
        Icons
        Menus
        Popup menus
        List boxes and combo boxes
        Sliders and progress bars
        Trees
        Tables
        More to Swing
      Using URLs from within an applet
        Reading a file from the server
      A method lookup tool
      Summary
      Exercises
    14: Multiple Threads
      Responsive user interfaces
        Inheriting from Thread
        Threading for a responsive interface
        Combining the thread with the main class
        Making many threads
        Daemon threads
      Sharing limited resources
        Improperly accessing resources
        How Java shares resources
          Synchronizing the counters
          Synchronized efficiency
        Java Beans revisited
      Blocking
        Becoming blocked
          Sleeping
          Suspending and resuming
          Wait and notify
          Blocking on IO
          Testing
        Deadlock
          The deprecation of stop( ), suspend( ), resume( ), and destroy( ) in Java 2
      Priorities
        Thread groups
          Controlling thread groups
      Runnable revisited
        Too many threads
      Summary
      Exercises
    15: Distributed Computing
      Network programming
        Identifying a machine
          Servers and clients
            Testing programs without a network
          Port: a unique place within the machine
        Sockets
          A simple server and client
        Serving multiple clients
        Datagrams
      Enterprise programming concepts
      Java Database Connectivity (JDBC)
        Getting the example to work
          Step 1: Find the JDBC Driver
          Step 2: Configure the database
          Step 3: Test the configuration
          Step 4: Generate your SQL query
          Step 5: Modify and paste in your query
        A GUI version of the lookup program
        Why the JDBC API seems so complex
        A more sophisticated JDBC Example
      Servlets
      The basic servlet
        Servlets and Multithreading
        Handling Sessions with Servlets
          The Cookie Class
          The Session Class
        Getting the Servlet examples to work
      Java Server Pages
        Basic operations
          Implicit Objects
          JSP Scripting Elements
        Extracting fields and values
        JSP Page Attributes and Scope
          Scope: 1
          Scope: 2
          Scope: 3
          Scope: 4
          End of list
        Manipulating sessions in JSP
        Creating and modifying Cookies
      RMI (Remote Method Invocation)
        Remote interfaces
        Implementing the remote interface
          Setting up the registry
        Creating stubs and skeletons
        Using the remote object
      Introduction to CORBA
        CORBA Fundamentals
          CORBA Interface Definition Language (IDL)
          The naming service
        An example
          Writing the IDL source
          Creating stubs and skeletons
          Implementing the server and the client
          Some CORBA services
          Activating the name service process
          Activating the server and the client
        Java Applets and CORBA
        CORBA vs. RMI
      Enterprise Java Beans
        What’s defined in the EJB specification?
          Roles
          Components of EJB
            EJB Container
            EJB Server
            Java Naming and Directory Interface (JNDI)
            Java Transaction API / Java Transaction Service (JTA/JTS)
            CORBA and RMI/IIOP
        What makes up an EJB component?
          Enterprise Bean
          Home Interface
          Remote Interface
          Deployment Descriptor
          EJB-Jar File
        How does EJB work?
          Who implements the Home and Remote Interfaces?
        Types of EJB’s
          Session Beans
            Stateless Session Beans
            Stateful Session Beans
          Entity Beans
            Container Managed Persistence (CMP)
            Bean Managed Persistence (BMP)
        How do I put the ‘E’ in my existing JavaBeans?
        Developing an Enterprise Java Bean
        In Summary
      Jini: distributed services
        Jini in context
        What is Jini?
        How Jini works
        The discovery process
        The join process
        The lookup process
        Separation of interface and implementation
        Abstracting distributed systems
      JavaSpaces
      Summary
      Exercises
    A: Passing & Returning Objects
      Passing references around
        Aliasing
      Making local copies
        Pass by value
        Cloning objects
        Adding cloneability to a class
          Using a trick with protected
          Implementing the Cloneable interface
        Successful cloning
        The effect of Object.clone( )
        Cloning a composed object
        A deep copy with ArrayList
        Deep copy via serialization
        Adding cloneability further down a hierarchy
        Why this strange design?
      Controlling cloneability
        The copy-constructor
          Why does it work in C++ and not Java?
      Read-only classes
        Creating read-only classes
        The drawback to immutability
        Immutable Strings
          Implicit constants
          Overloading ‘+’ and the StringBuffer
        The String and StringBuffer classes
        Strings are special
      Summary
      Exercises
    B: The Java Native Interface (JNI)
      The Java Native Interface
        Calling a native method
          The header file generator: javah
          Name mangling and function signatures
          Implementing your DLL
        Accessing JNI functions: The JNIEnv argument
          Accessing Java Strings
        Passing and using Java objects
        JNI and Java exceptions
        JNI and threading
        Using a preexisting code base
      Additional information
    C: Java Programming Guidelines
      Design
      Implementation
    D: Resources
      Software
      Books
        Analysis & design
        My own list of books
    Index


Thinking in Java, 2nd ed. Revision 9 - Index

, ! , ‘ , & , > , < , + , - , 1 , = , @ , A , B , C , D , E , F , G , H , I , J , K , L , M , N , O , P , Q , R , S , T , U , V , W , X , Y , Z , [ , ^ , |

boolean
operators that won’t work with boolean, 1
javac , 1
!

!, 1
!=, 1
operator, 1
‘

‘+’
operator + for String, 1
&

&, 1
&&, 1
&=, 1
>

>, 1
>>, 1
>>=, 1
>=, 1
<

<, 1
<<, 1
<<=, 1
<=, 1
+

+, 1
-

-, 1
1

1.localhost
and RMI, 1
=

==, 1
operator, 1
vs. equals( ), 1
@

@deprecated, 1
A

abstract
class, 1
inheriting from an abstract class, 1
vs. interface, 1
abstract keyword, 1
Abstract Window Toolkit (AWT), 1
AbstractButton, 1
abstraction, 1
AbstractSequentialList, 1
AbstractSet, 1
accept( ), 1
access
class, 1
control, 1, 2
inner classes & access rights, 1
package access and friendly, 1
specifiers, 1, 2, 3
within a directory, via the default package, 1
action( ), 1, 2
cannot combine with listeners, 1
ActionEvent, 1, 2, 3, 4
ActionListener, 1
actionPerformed( ), 1
actor, in use cases, 1
adapters
listener adapters, 1
add( ), 1
add( ), ArrayList, 1
addActionListener( ), 1, 2, 3
addAdjustmentListener( ), 1
addComponentListener( ), 1
addContainerListener( ), 1
addFocusListener( ), 1
addItemListener( ), 1
addition , 1
addKeyListener( ), 1
addListener, 1
addMouseListener( ), 1
addMouseMotionListener( ), 1
addTextListener( ), 1
addWindowListener( ), 1
AdjustmentEvent, 1, 2
AdjustmentListener, 1
adjustmentValueChanged( ), 1
Adler32, 1
aggregate array initialization, 1
aggregation, 1
aliasing, 1
and String, 1
during a method call, 1
align, 1
alphabetic vs. lexicographic sorting, 1
AlreadyBoundException, 1
analysis
and design, object-oriented, 1
paralysis, 1
requirements analysis, 1
AND
bitwise, 1
logical (&&), 1
anonymous inner class, 1, 2, 3
and constructors, 1
anonymous inner class, and table-driven code, 1
appendText( ), 1
applet, 1, 2
advantages for client/server systems, 1
align, 1
and packages, 1
archive tag, for HTML and JAR files, 1
classpath, 1
codebase, 1
combined applets and applications, 1
combined with application, 1
destroy( ), 1
displaying a Web page from within an applet, 1
init( ), 1, 2
initialization parameters, 1
name, 1
packaging applets in a JAR file to optimize loading, 1
parameter, 1
placing inside a Web page, 1
restrictions, 1
start( ), 1, 2
stop( ), 1, 2
appletviewer, 1
application
application builder, 1
application framework, 1
combined applets and applications, 1
combined with Applet, 1
standalone windowed application, 1
windowed applications, 1
application framework, 1
archive tag, for HTML and JAR files, 1
argument
constructor, 1
final, 1, 2
passing a reference into a method, 1
variable argument lists (unknown quantity and type of arguments), 1
array, 1
associative array, 1
associative array, Map, 1
bounds checking, 1
comparing arrays, 1
copying an array, 1
dynamic aggregate initialization syntax, 1
element comparisons, 1
first-class objects, 1
initialization, 1
length, 1, 2
multidimensional, 1
of Object, 1
of objects, 1
of primitives, 1
return an array, 1
ArrayList, 1, 2, 3, 4, 5
add( ), 1
get( ), 1, 2
size( ), 1
type-conscious ArrayList, 1
Arrays class, container utility, 1
Arrays.asList( ), 1
Arrays.binarySearch( ), 1
Arrays.fill( ), 1
assigning objects, 1
assignment, 1
associative array, 1, 2
associative arrays (Maps), 1
auto-decrement operator, 1
auto-increment operator, 1
automatic type conversion, 1
available( ), 1
AWT
Abstract Window Toolkit, 1
action( ), 1, 2
action( ), cannot combine with listeners, 1
ActionEvent, 1, 2, 3, 4
ActionListener, 1
actionPerformed( ), 1
add( ), 1
addActionListener( ), 1, 2
addAdjustmentListener( ), 1
addComponentListener( ), 1
addContainerListener( ), 1
addFocusListener( ), 1
addItemListener( ), 1
addKeyListener( ), 1
addListener, 1
addMouseListener( ), 1
addMouseMotionListener( ), 1
addTextListener( ), 1
addWindowListener( ), 1
AdjustmentEvent, 1, 2
AdjustmentListener, 1
adjustmentValueChanged( ), 1
appendText( ), 1
Applet, 1
applet advantages for client/server systems, 1
applet restrictions, 1
archive tag, for HTML and JAR files, 1
BorderLayout, 1
Button, 1, 2
Button, creating your own, 1
Canvas, 1
CardLayout, 1
Checkbox, 1, 2
Checkbox, Java 1.1, 1
CheckboxGroup, 1
CheckboxMenuItem, 1
CheckboxMenuItem, Java 1.1, 1
Choice, 1
Choice, Java 1.1, 1
Component, 1, 2
ComponentAdapter, 1
componentAdded( ), 1
ComponentEvent, 1, 2
componentHidden( ), 1
ComponentListener, 1
componentMoved( ), 1
componentRemoved( ), 1
componentResized( ), 1
componentShown( ), 1
consume( ) , 1
Container, 1
ContainerAdapter, 1
ContainerEvent, 1, 2
ContainerListener, 1
controlling layout, 1
DataFlavor, 1
desktop colors, 1
Dialog, 1
dialog box, 1
Dialog, Java 1.1, 1
dispatchEvent( ), 1
dispatching messages, 1
dispose( ), 1
drop-down list, 1
East, 1
event listener, 1
Event object, 1
event target, 1
event-driven programming, 1
FileDialog, 1
flavor, clipboard, 1
FlowLayout, 1
Focus traversal, 1
FocusAdapter, 1
FocusEvent, 1, 2
focusGained( ), 1
FocusListener, 1
focusLost( ), 1
Frame, 1, 2
getAppletContext( ), 1
getContents( ), 1
getDirectory( ), 1
getFile( ), 1
getSelectedItems( ), 1
getState( ), 1
getText( ), 1
getTransferData( ), 1
getTransferDataFlavors( ), 1
gotFocus( ), 1
graphics, 1
GridBagLayout, 1
GridLayout, 1
handleEvent( ), 1, 2
inner classes, 1
isDataFlavorSupported( ), 1
isFocusTraversable( ), 1
ItemEvent, 1, 2, 3
ItemListener, 1, 2
itemStateChanged( ), 1
JAR, packaging applets to optimize loading, 1
KeyAdapter, 1
keyDown( ), 1
KeyEvent, 1, 2
KeyListener, 1
keyPressed( ), 1
keyReleased( ), 1
keyTyped( ), 1
keyUp( ), 1
Label, 1
List, 1
list boxes, 1
List, Java 1.1, 1
listener adapters, 1
lostFocus( ), 1
Menu, 1
menu shortcuts, 1
Menu, Java 1.1, 1
MenuBar, 1, 2
MenuComponent, 1
MenuItem, 1, 2
MenuItem, Java 1.1, 1
MouseAdapter, 1
mouseClicked( ), 1
mouseDown( ), 1, 2
mouseDrag( ), 1
mouseDragged( ), 1
mouseEnter( ), 1
mouseEntered( ), 1
MouseEvent, 1, 2
mouseExit( ), 1
mouseExited( ), 1
MouseListener, 1
MouseMotionAdapter, 1
MouseMotionListener, 1
mouseMove( ), 1
mouseMoved( ), 1
mousePressed( ), 1
mouseReleased( ), 1
mouseUp( ), 1
multicast, 1
multicast events, 1
North, 1
paint( ), 1, 2
radio button, 1
removeActionListener( ), 1, 2
removeAdjustmentListener( ), 1
removeComponentListener( ), 1
removeContainerListener( ), 1
removeFocusListener( ), 1
removeItemListener( ), 1
removeKeyListener( ), 1
removeMouseListener( ), 1
removeMouseMotionListener( ), 1
removeTextListener( ), 1
removeWindowListener( ), 1
requestFocus( ), 1
setActionCommand( ), 1
setAlignment( ), 1
setCheckboxGroup( ), 1
setContents( ), 1
setDirectory( ), 1
setEditable( ), 1
setFile( ), 1
setLayout( ), 1
setText( ), 1
show( ), 1
showStatus( ), 1
South, 1
standalone windowed application, 1
StringSelection, 1
super.action( ), 1
system clipboard, 1
target of an event, 1
TextArea, 1, 2
TextArea, Java 1.1, 1
TextComponent, 1
TextComponent, Java 1.1, 1
TextEvent, 1, 2
TextField, 1
TextField, Java 1.1, 1
TextListener, 1
textValueChanged( ), 1
TooManyListenersException, 1, 2
Transferable, 1
unicast, 1
unicast events, 1
Web browser window status line, 1
West, 1
windowActivated( ), 1
WindowAdapter, 1
windowClosed( ), 1
windowClosing( ), 1
windowDeactivated( ), 1
windowDeiconified( ), 1
windowed applications, 1
WindowEvent, 1, 2, 3
windowIconified( ), 1
WindowListener, 1
windowOpened( ), 1
WINDOW_CLOSING, 1
WINDOW_DESTROY, 1
B

bag, 1
base
types, 1
base 16, 1
base 8, 1
base class, 1, 2, 3
abstract base class, 1
base-class interface, 1
constructor, 1
constructors and exceptions, 1
initialization, 1
Basic
Microsoft Visual Basic, 1
basic concepts of object-oriented programming (OOP), 1
BASIC language, 1
BasicArrowButton, 1
beanbox Bean testing tool, 1
BeanInfo
custom BeanInfo, 1
Beans
and Borland’s Delphi, 1
and Microsoft’s Visual Basic, 1
and multithreading, 1
application builder, 1
beanbox Bean testing tool, 1
bound properties, 1
component, 1
constrained properties, 1
custom BeanInfo, 1
custom property editor, 1
custom property sheet, 1
events, 1
EventSetDescriptors, 1
FeatureDescriptor, 1
getBeanInfo( ), 1
getEventSetDescriptors( ), 1
getMethodDescriptors( ), 1
getName( ), 1
getPropertyDescriptors( ), 1
getPropertyType( ), 1
getReadMethod( ), 1
getWriteMethod( ), 1
indexed property, 1
Introspector, 1
JAR files for packaging, 1
manifest file, 1
Method, 1
MethodDescriptors, 1
naming convention, 1
properties, 1
PropertyChangeEvent, 1
PropertyDescriptors, 1
ProptertyVetoException, 1
reflection, 1, 2
Serializable, 1
visual programming, 1
Beck, Kent, 1
Bill Joy, 1
binary
numbers, 1
operators, 1
binary numbers, printing, 1
binarySearch( ), 1
bind( ), 1
binding
dynamic binding, 1
dynamic, late, or run-time binding, 1
early, 1
late, 1
late binding, 1
method call binding, 1
run-time binding, 1
BitSet, 1
bitwise
AND, 1
AND operator (&), 1
EXCLUSIVE OR XOR (^), 1
NOT ~, 1
operators, 1
OR, 1
OR operator (|), 1
bitwise copy, 1
blank final, 1
blocking
and available( ), 1
and threads, 1
on IO, 1
Booch, Grady, 1
book
errors, reporting, 1
updates of the book, 1
Boolean, 1
algebra, 1
and casting, 1
vs. C and C++, 1
BorderLayout, 1
Borland, 1
Delphi, 1
bound properties, 1
bounds checking, array, 1
BoxLayout, 1
break keyword, 1
browser
class browser, 1
BufferedInputStream, 1, 2
BufferedOutputStream, 1, 2
BufferedReader, 1, 2
BufferedWriter, 1
business objects/logic, 1
button, 1, 2
creating your own, 1
radio button, 1
ButtonGroup, 1
ByteArrayInputStream, 1
ByteArrayOutputStream, 1
C

C++, 1
copy constructor, 1
Standard Container Library aka STL, 1
strategies for transition to, 1
templates, 1
vector class, vs. array and ArrayList, 1
why it succeeds, 1
C/C++, interfacing with, 1
callback, 1, 2
callbacks
and inner classes, 1
Canvas, 1, 2
capacity, of a HashMap or HashSet, 1
capitalization
Java capitalization style source-code checking tool, 1
of package names, 1
CardLayout, 1
case statement, 1
cast, 1, 2, 3
and containers, 1
and primitive types, 1
from float or double to integral, truncation, 1
operators, 1
catch
catching an exception, 1
catching any exception, 1
keyword, 1
CD ROM for book, 1
CGI
Common-Gateway Interface, 1
change
vector of change, 1
CharArrayReader, 1
CharArrayWriter, 1
Checkbox, 1, 2
Java 1.1, 1
CheckboxGroup, 1
CheckboxMenuItem, 1
Java 1.1, 1
CheckedInputStream, 1
CheckedOutputStream, 1
Checksum, 1
Choice, 1
Java 1.1, 1
class, 1, 2, 3
abstract class, 1
access, 1
anonymous inner class, 1, 2, 3
anonymous inner class and constructors, 1
base class, 1, 2, 3
browser, 1
class hierarchies and exception handling, 1
class literal, 1, 2
Class object, 1, 2, 3
creators, 1
defining the interface, 1
derived class, 1
equivalence, and instanceof/isInstance( ), 1
final classes, 1
forName( ), 1, 2
getClass( ), 1
getConstructors( ), 1
getInterfaces( ), 1
getMethods( ), 1
getName( ), 1
getSuperclass( ), 1
inheritance diagrams, 1
inheriting from an abstract class, 1
inheriting from inner classes, 1
initialization & class loading, 1
initialization of data members, 1
initializing members at point of definition, 1
initializing the base class, 1
inner class, 1, 2
inner class nesting within any arbitrary scope, 1
inner classes, 1
inner classes & access rights, 1
inner classes and overriding, 1
inner classes and super, 1
inner classes and the AWT, 1
inner classes and upcasting, 1
inner classes in methods & scopes, 1
inner classes, identifiers and .class files, 1
instance of, 1
intializing the derived class, 1
isInstance, 1
isInterface( ), 1
keyword, 1
loading, 1
member initialization, 1
newInstance( ), 1
order of initialization, 1
printInfo( ), 1
private inner classes, 1
public class, and compilation units, 1
read-only classes, 1
referring to the outer class object in an inner class, 1
RTTI using the Class object, 1
static inner classes, 1
style of creating classes, 1
subobject, 1
Class object, 1
class-responsibility-collaboration (CRC) cards, 1
ClassCastException, 1, 2
classpath, 1, 2
and rmic, 1
cleanup
and garbage collector, 1
performing, 1
with finally, 1
cleanup, guaranteeing with finalize( ), 1
client programmer, 1
vs. library creator, 1
client, network, 1
clipboard
system clipboard, 1
clone( ), 1
and composition, 1
and inheritance, 1
Object.clone( ), 1
removing/turning off clonability, 1
super.clone( ), 1, 2
supporting cloning in derived classes, 1
Cloneable interface, 1
CloneNotSupportedException, 1
close( ), 1
closure, and inner classes, 1
code
calling non-Java code, 1
coding standards, 1, 2
organization, 1
re-use, 1
codebase, 1
Collection, 1
collection class, 1
Collections, 1
Collections.enumeration( ), 1
Collections.fill( ), 1
Collections.reverseOrder(), 1
collision
name, 1
collisions, during hashing, 1
colors
desktop colors, 1
combo box, 1
comma operator, 1, 2
comments
and embedded documentation, 1
common interface, 1
common pitfalls when using operators, 1
Common-Gateway Interface (CGI), 1
Comparable, 1, 2
Comparator, 1, 2
compareTo( ), in java.lang.Comparable, 1
comparing arrays, 1
compilation unit, 1
compile-time constant, 1
compiling a Java program, 1
Component, 1, 2
component, and Java Beans, 1
ComponentAdapter, 1
componentAdded( ), 1
ComponentEvent, 1, 2
componentHidden( ), 1
ComponentListener, 1
componentMoved( ), 1
componentRemoved( ), 1
componentResized( ), 1
componentShown( ), 1
composition, 1, 2
and cloning, 1
and design, 1
and dynamic behavior change, 1
choosing composition vs. inheritance, 1
combining composition & inheritance, 1
vs. inheritance, 1, 2
compression
Java 1.1 compression library, 1
concept, high, 1
ConcurrentModificationException, 1
conditional operator, 1
conference, Software Development Conference, 1
console input, 1
const, in C++, 1
constant
compile-time constant, 1
folding, 1
groups of constant values, 1
implicit constants, and String, 1
constrained properties, 1
constructor, 1, 2
and anonymous inner classes, 1
and exception handling, 1
and exceptions, 1
and finally, 1
and overloading, 1
and polymorphism, 1
arguments, 1
base-class constructor, 1
base-class constructors and exceptions, 1
behavior of polymorphic methods inside constructors, 1
C++ copy constructor, 1
calling base-class constructors with arguments, 1
calling from other constructors, 1
default, 1
default constructors, 1
for reflection, 1
initialization during inheritance and composition, 1
name, 1
no-arg constructors, 1
order of constructor calls with inheritance, 1
return value, 1
static construction clause, 1
synthesized default constructor access, 1
consulting & mentoring provided by Bruce Eckel , 1
consume( ), 1
Container, 1
class, 1, 2
of primitives, 1
container classes, utilities for, 1
ContainerAdapter, 1
ContainerEvent, 1, 2
ContainerListener, 1
continue keyword, 1
control
access, 1
control framework, and inner classes, 1
controlling access, 1
conversion
automatic, 1
narrowing conversion, 1, 2
widening conversion, 1
copy
deep copy, 1
shallow copy, 1
copying an array, 1
CORBA, 1
costs, startup, 1
coupling, 1
CRC, class-responsibility-collaboration cards, 1
CRC32, 1
createStatement( ), 1, 2
critical section, and synchronized block, 1
D

daemon threads, 1
data
final , 1
primitive data types and use with operators, 1
static initialization, 1
data type
equivalence to class, 1
database
flat-file database, 1
Java DataBase Connectivity (JDBC), 1
relational database, 1
URL, 1
DatabaseMetaData, 1
DataFlavor, 1
Datagram, 1
User Datagram Protocol (UDP), 1
DataInput, 1
DataInputStream, 1, 2, 3, 4, 5, 6
DataOutput, 1
DataOutputStream, 1, 2, 3
dead, Thread, 1
deadlock, multithreading, 1, 2
death condition, and finalize( ), 1
decorator design pattern, 1
decoupling
via polymorphism, 1
decoupling through polymorphism, 1
decrement operator, 1
deep copy, 1, 2
and Vector, 1
using serialization to perform deep copying, 1
default constructor, 1, 2
synthesizing a default constructor, 1
default constructor, access the same as the class, 1
default keyword, in a switch statement, 1
default package, 1
DefaultMutableTreeNode, 1
defaultReadObject( ), 1
DefaultTreeModel, 1
defaultWriteObject( ), 1
DeflaterOutputStream, 1
Delphi, from Borland, 1
Demarco, Tom, 1
dequeue, 1
derived
derived class, 1
derived class, initializing, 1
types, 1
design, 1
adding more methods to a design, 1
analysis and design, object-oriented, 1
and composition, 1
and inheritance, 1
and mistakes, 1
five stages of object design, 1
library design, 1
of object hierarchies, 1
patterns, 1, 2
design patterns, 1
decorator, 1
singleton, 1
desktop colors, 1
destroy( ), 1, 2
destructor, 1, 2, 3
Java doesn’t have one, 1
development, incremental, 1
diagram
inheritance, 1
use case, 1
diagram, class inheritance diagrams, 1
Dialog, 1
Java 1.1, 1
dialog box, 1
dictionary, 1
directory
and packages, 1
creating directories and paths, 1
lister, 1
dispatchEvent( ), 1
dispatching messages, AWT, 1
dispose( ), 1
division, 1
do-while, 1
documentation
comments & embedded documentation, 1
Domain Name System (DNS), 1
dotted quad, 1
double, literal value marker (D), 1
downcast, 1, 2, 3
type-safe downcast in run-time type identification, 1
drop-down list, 1
dynamic
behavior change with composition, 1
binding, 1, 2
dynamic aggregate initialization syntax for arrays, 1
E

early binding, 1, 2
East, 1
efficiency
and arrays, 1
and final, 1
and threads, 1
when using the synchronized keyword, 1
elegance, in programming, 1
else keyword, 1
encapsulation, 1
enum, groups of constant values in C & C++, 1
Enumeration, 1
equals( )
and hashed data structures, 1
equals( ), 1, 2, 3
overriding for HashMap, 1
vs. ==, 1
equivalence
==, 1
object equivalence, 1
error
handling with exceptions, 1
recovery, 1
reporting errors in book, 1
event
Event object, AWT, 1
event-driven programming, 1
event-driven system, 1
Java Beans, 1
listener, 1
multicast, 1
multicast event and Java Beans, 1
target, 1
unicast, 1
event listener
order of execution, 1
EventSetDescriptors, 1
evolution, in program development, 1
exception
and base-class constructors, 1
and constructors, 1
and inheritance, 1, 2
catching an exception, 1
catching any exception, 1
changing the point of origin of the exception, 1
class hierarchies, 1
constructors, 1
creating your own, 1
design issues, 1
Error class, 1
Exception class, 1
exception handler, 1
exception handling, 1
exception matching, 1
FileNotFoundException, 1
fillInStackTrace( ), 1
finally, 1
guarded region, 1
handler, 1
handling, 1
losing an exception, pitfall, 1
NullPointerException, 1
printStackTrace( ), 1
re-throwing an exception, 1
restrictions, 1
RuntimeException, 1
specification, 1
termination vs. resumption, 1
Throwable, 1
throwing an exception, 1, 2
try, 1
try block, 1
typical uses of exceptions, 1
exceptional condition, 1
exceptions
and JNI , 1
executeQuery( ), 1, 2
Exponential notation, 1
extending a class during inheritance, 1
extends, 1, 2, 3
and interface, 1
keyword, 1
extensible
program, 1
extension
pure inheritance vs. extension, 1
extension, sign, 1
extension, zero, 1
Externalizable, 1
alternative approach to using, 1
Extreme Programming (XP), 1, 2
F

fail fast containers, 1
false, 1
FeatureDescriptor, 1
Field, for reflection, 1
fields, initializing fields in interfaces, 1
FIFO, 1
File, 1, 2, 3
characteristics of files, 1
class, 1
data file output shorthand, 1
File.list( ), 1
formatted file output shorthand, 1
input shorthand, 1
JAR file, 1
File Transfer Protocol (FTP), 1
FileDescriptor, 1
FileDialog, 1
FileInputStream, 1, 2
FilenameFilter, 1, 2
FileNotFoundException, 1
FileOutputStream, 1, 2
FileReader, 1, 2
FileWriter, 1
fillInStackTrace( ), 1
FilterInputStream, 1
FilterOutputStream, 1
FilterReader, 1
FilterWriter, 1
final, 1
and efficiency, 1
and private, 1
and static, 1
argument, 1, 2
blank finals, 1
classes, 1
data, 1
keyword, 1
method, 1
methods, 1, 2
static primitives, 1
with object references, 1
finalize( ), 1, 2
and inheritance, 1
and super, 1
calling directly, 1
order of finalization of objects, 1
finally, 1, 2
and constructors, 1
keyword, 1
pitfall, 1
finding .class files during loading, 1
flat-file database, 1
flavor, clipboard, 1
float, literal value marker(F), 1
floating point
true and false, 1
FlowLayout, 1
Focus traversal, 1
FocusAdapter, 1
FocusEvent, 1, 2
focusGained( ), 1
FocusListener, 1
focusLost( ), 1
folding, constant, 1
for keyword, 1
forName( ), 1, 2
FORTRAN, 1
forward referencing, 1
Fowler, Martin, 1, 2, 3
Frame, 1, 2
framework
application framework, 1
control framework and inner classes, 1
friendly, 1, 2
and interface, 1
and protected, 1
less accessible than protected, 1
FTP
File Transfer Protocol (FTP), 1
function
member function, 1
overriding, 1
functor, 1
G

garbage collection, 1, 2, 3
and cleanup, 1
and native method execution, 1
forcing finalization, 1
how the collector works, 1
order of object reclamation, 1
reachable objects, 1
setting references to null to allow cleanup, 1
generator, 1
generator object, to fill arrays and containers, 1
get( ), ArrayList, 1, 2
get( ), HashMap, 1
getAppletContext( ), 1
getBeanInfo( ), 1
getClass( ), 1, 2
getConstructor( ), 1
getConstructors( ), 1
getContents( ), 1
getDirectory( ), 1
getEventSetDescriptors( ), 1
getFile( ), 1
getFloat( ), 1, 2
getInputStream( ), 1
getInt( ), 1, 2
getInterfaces( ), 1
getMethodDescriptors( ), 1
getMethods( ), 1
getModel( ), 1
getName( ), 1, 2
getOutputStream( ), 1
getPriority( ), 1
getPropertyDescriptors( ), 1
getPropertyType( ), 1
getReadMethod( ), 1
getSelectedItems( ), 1
getState( ), 1
getString( ), 1, 2
getSuperclass( ), 1
getText( ), 1
getTransferData( ), 1
getTransferDataFlavors( ), 1
getWriteMethod( ), 1
Glass, Robert, 1
gotFocus( ), 1
goto
lack of goto in Java, 1
graphical user interface (GUI), 1, 2
graphics, 1
greater than (>), 1
greater than or equal to (>=), 1
GridBagLayout, 1
GridLayout, 1, 2
guarded region, in exception handling, 1
GUI
builders, 1
graphical user interface, 1, 2
guidelines
object development, 1
guidelines, coding standards, 1
GZIPInputStream, 1
GZIPOutputStream, 1
H

handleEvent( ), 1, 2
handler, exception, 1
hardware devices, interfacing with, 1
has-a, 1
has-a relationship, composition, 1
hash code, 1, 2
hash function, 1
hashCode( )
and hashed data structures, 1
issues when writing, 1
hashCode( ), 1, 2
overriding for HashMap, 1
hashing, 1
external chaining, 1
perfect hashing function, 1
HashMap, 1, 2
HashSet, 1, 2
Hashtable, 1, 2, 3, 4
used with Vector, 1
hasNext( ), Iterator, 1
Hexadecimal, 1
hiding
implementation, 1
hiding, implementation, 1
high concept, 1
HTML, 1
name, 1
param, 1
value, 1
I

Icon, 1
idltojava, 1
if-else statement, 1, 2
IllegalMonitorStateException, 1
ImageIcon, 1
immutable objects, 1
implementation, 1
and interface, 1, 2
and interface, separating, 1
and interface, separation, 1
hiding, 1, 2, 3
implements keyword, 1
import keyword, 1
increment operator, 1
incremental development, 1
indexed property, 1
indexing operator [ ], 1
indexOf( )
String, 1, 2
InflaterInputStream, 1
inheritance, 1, 2, 3, 4, 5
and cloning, 1
and final, 1
and finalize( ), 1
and synchronized, 1
choosing composition vs. inheritance, 1
class inheritance diagrams, 1
combining composition & inheritance, 1
designing with inheritance, 1
diagram, 1
extending a class during, 1
extending interfaces with inheritance, 1
from an abstract class, 1
from inner classes, 1
inheritance and method overloading vs. overriding, 1
initialization with inheritance, 1
multiple inheritance in C++ and Java, 1
pure inheritance vs. extension, 1
specialization, 1
vs composition, 1
vs. composition, 1
init( ), 1, 2
initial capacity, of a HashMap or HashSet, 1
initialization
and class loading, 1
array initialization, 1
base class, 1
class member, 1
constructor initialization during inheritance and composition, 1
initializing class members at point of definition, 1
initializing with the constructor, 1
instance initialization, 1, 2
member initializers, 1
non-static instance initialization, 1
of class data members, 1
of method variables, 1
order of initialization, 1, 2
static, 1
with inheritance, 1
inizialization
lazy, 1
inline method calls, 1
inner class, 1, 2, 3
inner class
access rights, 1
and overriding, 1
and super, 1
and control frameworks, 1
and the AWT, 1
and upcasting, 1
anonymous inner class, 1, 2
anonymous inner class and constructors, 1
anonymous, and table-driven code, 1
callback, 1
closure, 1
hidden reference to the object of the enclosing class, 1
identifiers and .class files, 1
in methods & scopes, 1
inheriting from inner classes, 1
nesting within any arbitrary scope, 1
private, 1
private inner classes, 1
referring to the outer class object, 1
static inner classes, 1
input
console input, 1
file input shorthand, 1
InputStream, 1, 2
InputStreamReader, 1, 2, 3
insertNodeInto( ), 1
instance
instance initialization , 1
non-static instance initialization, 1
instance of a class, 1
instanceof
dynamic instanceof, 1
keyword, 1
Integer
parseInt( ), 1
Integer wrapper class, 1
interface
and implementation, separation, 1
and inheritance, 1
base-class interface, 1
Cloneable interface used as a flag, 1
common interface, 1
defining the class, 1
for an object, 1
graphical user interface (GUI), 1, 2
implementation, separation of, 1
initializing fields in interfaces, 1
keyword, 1
Runnable, 1
upcasting to an interface, 1
user, 1
vs. abstract, 1
vs. implemenation, 1
interfaces
name collisions when combining interfaces, 1
interfacing with hardware devices, 1
internationalization, in IO library, 1
Internet
Internet Protocol, 1
Internet Service Provider (ISP), 1
interrupt( ), 1
InterruptedException, 1
intranet, 1
Introspector, 1
IO
and threads, blocking, 1
available( ), 1
blocking on IO, 1
blocking, and available( ), 1
BufferedInputStream, 1, 2
BufferedOutputStream, 1, 2
BufferedReader, 1, 2
BufferedWriter, 1
ByteArrayInputStream, 1
ByteArrayOutputStream, 1
characteristics of files, 1
CharArrayReader, 1
CharArrayWriter, 1
CheckedInputStream, 1
CheckedOutputStream, 1
close( ), 1
console input, 1
controlling the process of serialization, 1
data file output shorthand, 1
DataInput, 1
DataInputStream, 1, 2, 3, 4, 5, 6
DataOutput, 1
DataOutputStream, 1, 2, 3
DeflaterOutputStream, 1
deprecated, 1
directory lister, 1
directory, creating directories and paths, 1
Externalizable, 1
File, 1, 2, 3
File class, 1
file input shorthand, 1
File.list( ), 1
FileDescriptor, 1
FileInputStream, 1, 2
FilenameFilter, 1, 2
FileOutputStream, 1, 2
FileReader, 1, 2
FileWriter, 1
FilterInputStream, 1
FilterOutputStream, 1
FilterReader, 1
FilterWriter, 1
formatted file output shorthand, 1
from standard input, 1
GZIPInputStream, 1
GZIPOutputStream, 1
InflaterInputStream, 1
input, 1
InputStream, 1, 2
InputStreamReader, 1, 2, 3
internationalization, 1
Java 1.1 compression library, 1
library, 1
lightweight persistence, 1
LineNumberInputStream, 1, 2
LineNumberReader, 1
mark( ), 1
mkdirs( ), 1
nextToken( ), 1
ObjectOutputStream, 1
output, 1
OutputStream, 1, 2, 3
OutputStreamWriter, 1, 2, 3
pipe, 1
piped stream, 1
PipedInputStream, 1, 2
PipedOutputStream, 1, 2, 3
PipedReader, 1
PipedWriter, 1
PrintStream, 1, 2, 3
PrintWriter, 1, 2
pushBack( ), 1
PushbackInputStream, 1
PushBackReader, 1
RandomAccessFile, 1, 2, 3
read( ), 1
readChar( ), 1
readDouble( ), 1
Reader, 1, 2, 3
readExternal( ), 1
readLine( ), 1, 2, 3, 4, 5, 6
readObject( ), 1
redirecting standard IO, 1
renameTo( ), 1
reset( ), 1
seek( ), 1, 2
SequenceInputStream, 1, 2
Serializable, 1
setErr(PrintStream), 1
setIn(InputStream), 1
setOut(PrintStream), 1
StreamTokenizer, 1, 2, 3, 4
StringBuffer, 1
StringBufferInputStream, 1, 2
StringReader, 1
StringWriter, 1
System.err, 1
System.in, 1, 2
System.out, 1
transient, 1
typical IO configurations, 1
Unicode, 1
write( ), 1
writeBytes( ), 1
writeChars( ), 1
writeDouble( ), 1
writeExternal( ), 1
writeObject( ), 1
Writer, 1, 2, 3
ZipEntry, 1
ZipInputStream, 1
ZipOutputStream, 1
IP (Internet Protocol), 1
is-a, 1
relationship, inheritance, 1
relationship, inheritance & upcasting, 1
vs. is-like-a relationships, 1
is-like-a, 1
isDaemon( ), 1
isDataFlavorSupported( ), 1
isFocusTraversable( ), 1
isInstance, 1
isInterface( ), 1
ISP (Internet Service Provider), 1
ItemEvent, 1, 2, 3
ItemListener, 1, 2
itemStateChanged( ), 1
iteration, in program development, 1
iterator, 1, 2, 3, 4
hasNext( ), 1
next( ), 1
iterator( ), 1
J

Jacobsen, Ivar, 1
JAR, 1
archive tag, for HTML and JAR files, 1
file, 1, 2
jar files and classpath, 1
Java 1.1 JAR utility, 1
packaging applets to optimize loading, 1
Java, 1, 2
and pointers, 1
and set-top boxes, 1
capitalization style source-code checking tool, 1
compiling and running a program, 1
containers library, 1, 2
public Java seminars, 1
versions, 1
Java 1.0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30
Java 1.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52
and Swing, 1
IO streams, 1
JAR utility, 1
reflection, 1, 2
Java 2, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
Swing library, 1
Java Beans
see Beans, 1
Java Foundation Classes (JFC/Swing), 1
Java operators, 1
Java Virtual Machine, 1
javah, 1
JButton, 1
JCheckbox, 1
JColorChooser, 1
JComboBox, 1
JComponent, 1
JDBC
createStatement( ), 1, 2
database URL, 1
DatabaseMetaData, 1
executeQuery( ), 1, 2
flat-file database, 1
getFloat( ), 1, 2
getInt( ), 1, 2
getString( ), 1, 2
Java DataBase Connectivity, 1
join, 1
relational database, 1
ResultSet, 1, 2
SQL stored procedures, 1
Statement, 1, 2
Structured Query Language (SQL), 1
JDK
downloading and installing, 1
JFC
Java Foundation Classes (JFC/Swing), 1
JFileChooser, 1
JFrame, 1
JHTMLPane, 1
JInternalFrame, 1
JIT
Just-In Time compilers, 1
JLabel, 1
JLabels, 1
JLayeredPane, 1
JList, 1
JMenu, 1
JMenuItem, 1, 2
JMenuItems, 1
JNI functions, 1
JNICALL, 1
JNIEnv, 1
JNIEXPORT, 1
join, 1
JOptionPane, 1
JPasswordField, 1
JPopupMenu, 1
JProgressBar, 1
JRadioButton, 1
JScrollPane, 1, 2, 3
JSlider, 1
JTextField, 1
JTextPane, 1
JToggleButton, 1
JToolbar, 1
JTree, 1, 2
JVM (Java Virtual Machine), 1
K

KeyAdapter, 1
keyboard navigation, and Swing, 1
keyDown( ), 1
KeyEvent, 1, 2
KeyListener, 1
keyPressed( ), 1
keyReleased( ), 1
keySet( ), 1
keyTyped( ), 1
keyUp( ), 1
keywords
class, 1, 2
Koenig, Andrew, 1
L

label, 1, 2
labeled break, 1
labeled continue, 1
late binding, 1, 2, 3
layout
controlling layout, 1
layout manager, 1
lazy inizialization, 1
left-shift operator (<<), 1
length, array member, 1
length, for arrays, 1
less than (<), 1
less than or equal to (<=), 1
lexicographic vs. alphabetic sorting, 1
library
creator, vs. client programmer, 1
design, 1
use, 1
LIFO, 1
lightweight
Swing components, 1
lightweight persistence, 1
LineNumberInputStream, 1, 2
LineNumberReader, 1
linked list, 1
LinkedList, 1, 2, 3
List, 1, 2, 3, 4, 5
boxes, 1
drop-down list, 1
Java 1.1, 1
sorting and searching, 1
list box, 1
listener adapters, 1
listener classes, 1
Lister, Timothy, 1
ListIterator, 1
literal
class literal, 1, 2
double, 1
float, 1
long, 1
values, 1
load factor, of a HashMap or HashSet, 1
loading
.class files, 1
initialization & class loading, 1
loding a class, 1
local loopback IP address, 1
localhost, 1
lock, for multithreading, 1
logarithms
natural logarithms, 1
logical
AND, 1
operator and short-circuiting, 1
operators, 1
OR, 1
long, literal value marker (L), 1
lostFocus( ), 1
lvalue, 1
M

main( ), 1
maintenance, program, 1
management obstacles, 1
manifest file, for JAR files, 1, 2
Map, 1, 2, 3, 4, 5, 6
Map.Entry, 1
mark( ), 1
Math.random( ), 1
values produced by, 1
mathematical operators, 1
max( ), 1
MDI, 1
member
member function, 1
object, 1
member initializers, 1
memory exhaustion, solution via References, 1
mentoring
and training, 1, 2
Menu, 1
Java 1.1, 1
popup menu, 1
Swing, 1
menu shortcuts, 1
MenuBar, 1, 2
MenuComponent, 1
MenuItem, 1, 2
Java 1.1, 1
message box, in Swing, 1
message, sending, 1
meta-class, 1
Method, 1
adding more methods to a design, 1
aliasing during a method call, 1
aliasing during method calls, 1
behavior of polymorphic methods inside constructors, 1
distinguishing overloaded methods, 1
final, 1, 2
final methods, 1
for reflection, 1
inline method calls, 1
inner classes in methods & scopes, 1
lookup tool, 1
method call binding, 1
overloading, 1
passing a reference into a method, 1
polymorphic method call, 1
private, 1
protected methods, 1
recursive, 1
static, 1
synchronized method and blocking, 1
initialization of method variables, 1
MethodDescriptors, 1
methodology, analysis and design, 1
Meyers, Scott, 1
Microsoft, 1
Visual Basic, 1
min( ), 1
mission statement, 1
mistakes, and design, 1
mkdirs( ), 1
modulus, 1
monitor, for multithreading, 1
MouseAdapter, 1
mouseClicked( ), 1
mouseDown( ), 1, 2
mouseDrag( ), 1
mouseDragged( ), 1
mouseEnter( ), 1
mouseEntered( ), 1
MouseEvent, 1, 2
mouseExit( ), 1
mouseExited( ), 1
MouseListener, 1
MouseMotionAdapter, 1
MouseMotionListener, 1
mouseMove( ), 1
mouseMoved( ), 1
mousePressed( ), 1
mouseReleased( ), 1
mouseUp( ), 1
multi-tiered systems, 1
multicast, 1
event, and Java Beans, 1
multicast events, 1
multidimensional arrays, 1
Multimedia CD ROM for book, 1
multiparadigm programming, 1
multiple inheritance, in C++ and Java, 1
multiple-document interface, 1
multiplication, 1
multitasking, 1
multithreading, 1, 2
and containers, 1
and Java Beans, 1
blocking, 1
deciding what methods to synchronize, 1
drawbacks, 1
Runnable, 1
when to use it, 1
N

name, 1
clash, 1
collisions, 1
creating unique package names, 1
spaces, 1
name collisions when combining interfaces, 1
name, HTML keyword, 1
Naming
bind( ), 1
rebind( ), 1
unbind( ), 1
narrowing conversion, 1, 2, 3
native method interface (NMI) in Java 1.0, 1
natural logarithms, 1
network programming, 1
accept( ), 1
client, 1
Common-Gateway Interface (CGI), 1
datagrams, 1
dedicated connection, 1
displaying a Web page from within an applet, 1
DNS (Domain Name System), 1
dotted quad, 1
getInputStream( ), 1
getOutputStream( ), 1
HTML, 1
identifying machines, 1
Internet Protocol (IP), 1
Java DataBase Connectivity (JDBC), 1
local loopback IP address, 1
localhost, 1
multithreading, 1
port, 1
reliable protocol, 1
server, 1
serving multiple clients, 1
showDocument( ), 1
Socket, 1
stream-based sockets, 1
testing programs without a network, 1
Transmission Control Protocol (TCP), 1
unreliable protocol, 1
URL, 1
User Datagram Protocol (UDP), 1
new operator, 1
and primitives, array, 1
newInstance( ), 1
reflection, 1
next( ), Iterator, 1
nextToken( ), 1
NMI
Java 1.0 native method interface, 1
no-arg
constructors, 1
non-Java code, calling, 1
North, 1
NOT
logical (!), 1
not equivalent (!=), 1
notify( ), 1
notifyAll( ), 1
notifyListeners( ), 1
null, 1, 2
garbage collection, allowing cleanup, 1
NullPointerException, 1
numbers, binary, 1
O

object, 1, 2
aliasing, 1
array of Object, 1
arrays are first-class objects, 1
assigning objects by copying references, 1
assignment and reference copying, 1
business object/logic, 1
Class object, 1, 2, 3
clone( ), 1, 2
creation, 1
equals( ) method, 1
equivalence, 1
equivalence vs reference equivalence, 1
final, 1
five stages of object design, 1
getClass( ), 1
guidelines for object development, 1
hashCode( ), 1
immutable objects, 1
interface to, 1
lock, for multithreading, 1
member, 1
object-oriented programming, 1
order of finalization of objects, 1
process of creation, 1
reference equivalence vs. object equivalence, 1
serialization, 1, 2
standard root class, default inheritance from, 1
web of objects, 1, 2
object-oriented
analysis and design, 1
basic concepts of object-oriented programming (OOP), 1
ObjectOutputStream, 1
obstacles, management, 1
Octal, 1
ODBC, 1
OMG, 1
ones complement operator, 1
OOP, 1
analysis and design, 1
basic characteristics, 1
basic concepts of object-oriented programming, 1
protocol, 1
Simula programming language, 1
substitutability, 1
operator, 1
+ and += overloading for String, 1
+, for String, 1
== and !=, 1
binary, 1
bitwise, 1
casting, 1
comma, 1
comma operator, 1
common pitfalls, 1
indexing operator [ ], 1
logical, 1
logical operators and short-circuiting, 1
ones-complement, 1
operator overloading for String, 1
overloading, 1
precedence, 1
precedence mnemonic, 1
relational, 1
shift, 1
unary, 1, 2
ternary, 1
optional methods, in the Java 2 containers, 1
OR, 1
(||), 1
order
of constructor calls with inheritance, 1
of finalization of objects, 1
of initialization, 1, 2, 3
organization, code, 1
output
data file output shorthand, 1
formatted file output shorthand, 1
OutputStream, 1, 2, 3
OutputStreamWriter, 1, 2, 3
overflow
and primitive types, 1
overloading
and constructors, 1
distinguishing overloaded methods, 1
lack of name hiding during inheritance, 1
method overloading, 1
on return values, 1
operator + and += overloading for String, 1
operator overloading, 1
operator overloading for String, 1
overloading vs. overriding, 1
vs. overriding, 1
overriding
and inner classes, 1
function, 1
overloading vs. overriding, 1
vs. overloading, 1
P

package, 1
access, and friendly, 1
and applets, 1
and directory structure, 1
creating unique package names, 1
default package, 1
names, capitalization, 1
visibility, friendly, 1
paint( ), 1, 2
pair programming, 1
paralysis, analysis, 1
param, HTML keyword, 1
parameter, applet, 1
parameterized type, 1
parseInt( ), 1
pass
pass by value, 1
passing a reference into a method, 1
patterns, design, 1, 2
patterns, design patterns, 1
perfect hashing function, 1
performance
and final, 1
performance issues, 1
persistence, 1
lightweight persistence, 1
PhantomReference, 1
pipe, 1
piped stream, 1
PipedInputStream, 1, 2
PipedOutputStream, 1, 2, 3
PipedReader, 1
PipedWriter, 1
planning, software development, 1
Plauger, P.J., 1
pointer
Java exclusion of pointers, 1
pointers, and Java, 1
polymorphism, 1, 2, 3, 4, 5
and constructors, 1
behavior of polymorphic methods inside constructors, 1
popup menu, 1
port, 1
portability in C, C++ and Java, 1
precedence
operator precedence mnemonic, 1
prerequisites, for this book, 1
primitive
comparison, 1
containers of primitives, 1
data types, and use with operators, 1
dealing with the immutability of primitive wrapper classes, 1
final, 1
final static primitives, 1
initialization of class data members, 1
wrappers, 1
printInfo( ), 1
printing arrays, 1
println( ), 1
printStackTrace( ), 1, 2
PrintStream, 1, 2, 3
PrintWriter, 1, 2
priority
default priority for a Thread group, 1
thread, 1
private, 1, 2, 3, 4, 5, 6
illusion of overriding private methods, 1
inner class, 1
inner classes, 1
methods, 1
problem space, 1, 2
process, and threading, 1
program
maintenance, 1
programmer, client, 1
programming
basic concepts of object-oriented programming (OOP), 1
coding standards, 1
event-driven programming, 1
Extreme Programming (XP), 1, 2
in the large, 1
multiparadigm, 1
object-oriented, 1
pair, 1
progress bar, 1
promotion
of primitive types, 1
type promotion, 1
Properties, 1
property, 1
bound properties, 1
constrained properties, 1
custom property editor, 1
custom property sheet, 1
indexed property, 1
PropertyChangeEvent, 1
PropertyDescriptors, 1
ProptertyVetoException, 1
protected, 1, 2, 3, 4, 5
and friendly, 1
is also friendly, 1
more accessible than friendly, 1
use in clone( ), 1
protocol, 1
unreliable protocol, 1
prototyping
rapid, 1
public, 1, 2, 3, 4
and interface, 1
class, and compilation units, 1
pure
substitution, 1
pure inheritance, vs. extension, 1
pure substitution, 1
pushBack( ), 1
PushbackInputStream, 1
PushBackReader, 1
put( ), HashMap, 1
Python, 1, 2
Q

queue, 1, 2
R

RAD (Rapid Application Development), 1
radio button, 1
random number generator, values produced by, 1
random( ), 1
RandomAccessFile, 1, 2, 3
rapid prototyping, 1
re-throwing an exception, 1
reachable objects and garbage collection, 1
read( ), 1
readChar( ), 1
readDouble( ), 1
Reader, 1, 2, 3, 4
readExternal( ), 1
reading from standard input, 1
readLine( ), 1, 2, 3, 4, 5, 6
readObject( ), 1
with Serializable , 1
rebind( ), 1
recursion, unintended via toString(), 1
redirecting standard IO, 1
refactoring, 1
reference
assigning objects by copying references, 1
equivalence vs object equivalence, 1
final, 1
finding exact type of a base reference, 1
null, 1
reference equivalence vs. object equivalence, 1
Reference, from java.lang.ref, 1
referencing, forward referencing, 1
reflection, 1, 2, 3
and Beans, 1
difference between RTTI and reflection, 1
registry
remote object registry, 1
relational
database, 1
operators, 1
reliable protocol, 1
Remote Method Invocation (RMI), 1
RemoteException, 1
removeActionListener( ), 1, 2, 3
removeAdjustmentListener( ), 1
removeComponentListener( ), 1
removeContainerListener( ), 1
removeFocusListener( ), 1
removeItemListener( ), 1
removeKeyListener( ), 1
removeMouseListener( ), 1
removeMouseMotionListener( ), 1
removeTextListener( ), 1
removeWindowListener( ), 1
renameTo( ), 1
reporting errors in book, 1
request, in OOP, 1
requestFocus( ), 1
requirements analysis, 1
reset( ), 1
ResultSet, 1, 2
resume( ), 1, 2
and deadlocks, 1
deprecation in Java 2, 1
resumption, termination vs. resumption, exception handling, 1
return
an array, 1
constructor return value, 1
overloading on return value, 1
reusability, 1
reuse, 1
code reuse, 1
existing class libraries, 1
reusable code, 1
right-shift operator (>>), 1
RMI
AlreadyBoundException, 1
and CORBA, 1
bind( ), 1
localhost, 1
rebind( ), 1
Remote, 1
remote interface, 1
Remote Method Invocation, 1
remote object registry, 1
RemoteException, 1, 2
rmic, 1
rmic and classpath, 1
rmiregistry, 1
RMISecurityManager, 1
Serializable arguments, 1
skeleton, 1
stub, 1
TCP/IP, 1
unbind( ), 1
UnicastRemoteObject, 1
rmic, 1
rmiregistry, 1
RMISecurityManager, 1
rollover, 1
RTI
Class, 1
RTTI
and cloning, 1
cast, 1
Class object, 1
ClassCastException, 1
Constructor, 1, 2
difference between RTTI and reflection, 1
downcast, 1
Field, 1
getConstructor( ), 1
instanceof keyword, 1
isInstance, 1
meta-class, 1
Method, 1
newInstance( ), 1
reflection, 1
run-time type identification (RTTI), 1
type-safe downcast, 1
using the Class object, 1
Rumbaugh, James, 1
run-time binding, 1
polymorphism, 1
run-time type identification
(RTTI), 1
misuse, 1
shape example, 1
when to use it, 1
runFinalizersOnExit( ), 1, 2
Runnable, 1
interface, 1
Thread, 1
running a Java program, 1
RuntimeException, 1, 2
rvalue, 1
S

safety, and applet restrictions, 1
scenario, 1
scheduling, 1
scope
inner class nesting within any arbitrary scope , 1
inner classes in methods & scopes, 1
use case, 1
searching
sorting and searching Lists, 1
searching an array, 1
section, critical section and synchronized block, 1
seek( ), 1, 2
seminars
public Java seminars, 1
training, provided by Bruce Eckel, 1
sending a message, 1
separation of interface and implementation, 1, 2
SequenceInputStream, 1, 2
Serializable, 1, 2, 3, 4, 5
readObject( ), 1
writeObject( ), 1
serialization
and object storage, 1
and transient, 1
controlling the process of serialization, 1
defaultReadObject( ), 1
defaultWriteObject( ), 1
RMI arguments, 1
to perform deep copying, 1
Versioning, 1
server, 1
Set, 1, 2, 3, 4, 5
setActionCommand( ), 1
setAlignment( ), 1
setBorder( ), 1
setCheckboxGroup( ), 1
setContents( ), 1
setDaemon( ), 1
setDirectory( ), 1
setEditable( ), 1
setErr(PrintStream), 1
setFile( ), 1
setIcon( ), 1
setIn(InputStream), 1
setLayout( ), 1
setOut(PrintStream), 1
setPriority( ), 1
setText( ), 1
setToolTipText( ), 1
shallow copy, 1, 2
shape
example, 1, 2
example, and run-time type identification, 1
shift operators, 1
short-circuit, and logical operators, 1
shortcut, menu shortcuts, 1
show( ), 1
showConfirmDialog( ), 1
showDocument( ), 1
showMessageDialog( ), 1
showStatus( ), 1
shuffle( ), 1
side effect, 1, 2, 3, 4
sign extension, 1
signed two’s complement, 1
Simula programming language, 1
Simula-67, 1
singleton
design pattern, 1
size( ), ArrayList, 1
Size, of a HashMap or HashSet, 1
sizeof( )
lack of in Java, 1
skeleton, RMI, 1
sleep( ), 1, 2, 3, 4
slider, 1
Smalltalk, 1, 2
Socket, 1
sockets, stream-based, 1
SoftReference, 1
software
development methodology, 1
Software Development Conference, 1
solution space, 1
sorting, 1
and searching Lists, 1
source code copyright notice, 1
South, 1
space
problem, 1
solution, 1
specialization, 1
specification
system specification, 1
specification, exception, 1
specifier
access specifiers, 1, 2, 3
splitter control, 1
SQL
stored procedures, 1
Structured Query Language, 1
Stack, 1, 2
standard input
Reading from standard input, 1
standards
coding standards, 1, 2
start( ), 1, 2
startup costs, 1
Statement, 1, 2
mission, 1
static, 1
and final, 1
block, 1
clause, 1
construction clause, 1
data initialization, 1
final static primitives, 1
initialization, 1
inner classes, 1
keyword, 1
method, 1
synchronized static, 1
status line, Web browser , 1
STL
C++, 1
stop( ), 1, 2
and deadlocks, 1
deprecation in Java 2, 1
stored procedures in SQL, 1
stream-based sockets, 1
StreamTokenizer, 1, 2, 3, 4
String
automatic type conversion, 1
class methods, 1
concatenation with operator +, 1
immutability, 1
indexOf( ), 1, 2
lexicographic vs. alphabetic sorting, 1
methods, 1
Operator +, 1, 2
operator + and += overloading, 1
toString( ), 1, 2
StringBuffer, 1, 2
methods, 1
StringBufferInputStream, 1, 2
StringReader, 1
StringSelection, 1
StringTokenizer, 1
StringWriter, 1
stub, RMI, 1
style of creating classes, 1
subobject, 1, 2
substitutability, in OOP, 1
substitution
principle, 1
subtraction, 1
super, 1
and finalize( ), 1
and inner classes, 1
super keyword, 1
super.action( ), 1
super.clone( ), 1, 2, 3
superclass, 1
suspend( ), 1, 2
and deadlocks, 1
deprecation in Java 2, 1
Swing
AbstractButton, 1
BasicArrowButton, 1
BoxLayout, 1
ButtonGroup, 1
combo box, 1
DefaultMutableTreeNode, 1
DefaultTreeModel, 1
getModel( ), 1
Icon, 1
ImageIcon, 1
insertNodeInto( ), 1
Java Foundation Classes (JFC), 1
JButton, 1
JCheckbox, 1
JColorChooser, 1
JComboBox, 1
JComponent, 1
JFileChooser, 1
JFrame, 1
JHTMLPane, 1
JInternalFrame, 1
JLabel, 1
JLabels, 1
JLayeredPane, 1
JList, 1
JMenu, 1
JMenuItem, 1, 2
JMenuItems, 1
JOptionPane, 1
JPasswordField, 1
JPopupMenu, 1
JProgressBar, 1
JRadioButton, 1
JScrollPane, 1, 2, 3
JSlider, 1
JTextField, 1
JTextPane, 1
JToggleButton, 1
JToolbar, 1
JTree, 1, 2
keyboard navigation, 1
list box, 1
MDI, 1
Menus, 1
message box, 1
multiple-document interface, 1
popup menu, 1
progress bar, 1
rollover, 1
setBorder( ), 1
setIcon( ), 1
setToolTipText( ), 1
showConfirmDialog( ), 1
showMessageDialog( ), 1
slider, 1
splitter control, 1
table, 1
TitledBorder, 1
tool tip, 1
toolbar, 1
tree, 1
UI Component library, 1
undo, 1
switch keyword, 1
synchronized, 1, 2
and inheritance, 1
and wait( ) & notify( ), 1
containers, 1
deciding what methods to synchronize, 1
efficiency, 1
method, and blocking, 1
static, 1
synchronized block, 1
system clipboard, 1
system specification, 1
System.arraycopy( ), 1
System.err, 1
System.gc( ), 1
System.in, 1, 2
System.out, 1
System.out.println( ), 1
System.runFinalization( ), 1
T

table, 1
table-driven code, and anonymous inner classes, 1
target, of an event, 1
TCP, Transmission Control Protocol, 1
TCP/IP, and RMI, 1
template
in C++, 1
termination vs. resumption, exception handling, 1
ternary operator, 1
testing
automated, 1
Extreme Programming (XP), 1
unit testing, 1
testing techniques, 1
TextArea, 1, 2
Java 1.1, 1
TextComponent, 1
Java 1.1, 1
TextEvent, 1, 2
TextField, 1
Java 1.1, 1
TextListener, 1
textValueChanged( ), 1
this keyword, 1
Thread, 1, 2
and Java Beans, 1
and Runnable, 1
blocked, 1
combined with main class, 1
daemon threads, 1
dead, 1
deadlock, 1
deciding what methods to synchronize, 1
destroy( ), 1
drawbacks, 1
getPriority( ), 1
interrupt( ), 1
IO and threads, blocking, 1
isDaemon( ), 1
new Thread, 1
notify( ), 1
notifyAll( ), 1
order of execution of threads, 1
priority, 1
properly suspending & resuming, 1
resume( ), 1, 2
resume( ) , deprecation in Java 2, 1
resume( ), and deadlocks, 1
run( ), 1
Runnable, 1
Runnable interface, 1
setDaemon( ), 1
setPriority( ), 1
sharing limited resources, 1
sleep( ), 1, 2, 3
start( ), 1
states, 1
stop( ) , deprecation in Java 2, 1
stop( ), and deadlocks, 1
stopping, 1
suspend( ), 1, 2
suspend( ) , deprecation in Java 2, 1
suspend( ), and deadlocks, 1
synchronized method and blocking, 1
thread group, 1
thread group, default priority, 1
threads and efficiency, 1
wait( ), 1, 2
when they can be suspended, 1
when to use threads, 1
yield( ), 1
throw keyword, 1
Throwable, 1
base class for Exception, 1
throwing an exception, 1
time-critical code sections, 1
TitledBorder, 1
toArray( ), 1
token, 1
tool tip, 1
toolbar, 1
TooManyListenersException, 1, 2
toString( ), 1, 2, 3, 4
training, 1
and mentoring, 1, 2
training seminars provided by Bruce Eckel, 1
Transferable, 1
transient, 1
translation unit, 1
Transmission Control Protocol (TCP), 1
tree, 1
TreeMap, 1, 2
TreeSet, 1, 2
true, 1
try, 1, 2
try block in exceptions, 1
two’s complement, signed, 1
type
base, 1
data type equivalence to class, 1
derived, 1
finding exact type of a base reference, 1
parameterized type, 1
primitive data types and use with operators, 1
type checking and arrays, 1
type safety in Java, 1
type-safe downcast in run-time type identification, 1
weak typing, 1
TYPE field, for primitive class literals, 1
type safe sets of constants, 1
type-conscious ArrayList, 1
U

UDP, User Datagram Protocol, 1
UML, 1
indicating composition, 1
Unified Modeling Language, 1, 2
unary
minus (-), 1
operator, 1
operators, 1
plus (+), 1
unbind( ), 1
undo, 1
unicast, 1
unicast events, 1
UnicastRemoteObject, 1
Unicode, 1
Unified Modeling Language (UML), 1, 2
unit testing, 1
unmodifiable, making a Collection or Map unmodifiable, 1
unsupported methods, in the Java 2 containers, 1
UnsupportedOperationException, 1
upcasting, 1, 2, 3, 4
and interface, 1
inner classes and upcasting, 1
updates of the book, 1
URL, 1
use case, 1
iteration, 1
scope, 1
User Datagram Protocol (UDP), 1
user interface, 1
and threads, for responsiveness, 1
responsive, with threading, 1
V

value
preventing change at run-time, 1
value, HTML keyword, 1
variable
defining a variable, 1
initialization of method variables, 1
variable argument lists (unknown quantity and type of arguments), 1
Vector, 1, 2, 3
and deep copying, 1
of change, 1
used with Hashtable, 1
vector of change, 1
versioning, serialization, 1
versions of Java, 1
visibility, package visibility, (friendly), 1
visual
programming, 1
programming environments, 1
Visual Basic, Microsoft, 1
W

wait( ), 1, 2
Waldrop, M. Mitchell, 1
weak
weakly typed language, 1
WeakHashMap, 1
WeakReference, 1
Web
displaying a Web page from within an applet, 1
placing an applet inside a Web page, 1
safety, and applet restrictions, 1
Web browser
status line, 1
web of objects, 1, 2
West, 1
while, 1
widening conversion, 1
wild-card, 1
window
Abstract Window Toolkit (see AWT), 1
standalone windowed application, 1
Web browser status line, 1
windowActivated( ), 1
WindowAdapter, 1
windowClosed( ), 1
windowClosing( ), 1
windowDeactivated( ), 1
windowDeiconified( ), 1
windowed applications, 1
WindowEvent, 1, 2, 3
windowIconified( ), 1
WindowListener, 1
windowOpened( ), 1
WINDOW_CLOSING, 1
WINDOW_DESTROY, 1
wrapper, dealing with the immutability of primitive wrapper classes, 1
write( ), 1
writeBytes( ), 1
writeChars( ), 1
writeDouble( ), 1
writeExternal( ), 1
writeObject( ), 1
with Serializable, 1
Writer, 1, 2, 3, 4
X

XOR, 1
XP, Extreme Programming, 1
Y

yield( ), 1
Z

zero extension, 1
ZipEntry, 1
ZipInputStream, 1
ZipOutputStream, 1
[

[ ]
indexing operator [ ], 1
^

^, 1
^=, 1
|

|, 1
|=, 1
||, 1


Thinking in Java, 2nd ed. Revision 9 - Title Page

[ Short TOC ] [ Table of Contents ] [ Index ] [ Next Chapter ]

Thinking

in

Java

Second Edition

Bruce Eckel

President, MindView, Inc.

Comments from readers:

Much better than any other Java book I’ve seen. Make that “by an order of magnitude”... very complete, with excellent right-to-the-point examples and intelligent, not dumbed-down, explanations ... In contrast to many other Java books I found it to be unusually mature, consistent, intellectually honest, well-written and precise. IMHO, an ideal book for studying Java. Anatoly Vorobey, Technion University, Haifa, Israel

One of the absolutely best programming tutorials I’ve seen for any language. Joakim Ziegler, FIX sysop

Thank you for your wonderful, wonderful book on Java. Dr. Gavin Pillay, Registrar, King Edward VIII Hospital, South Africa

Thank you again for your awesome book. I was really floundering (being a non-C programmer), but your book has brought me up to speed as fast as I could read it. It’s really cool to be able to understand the underlying principles and concepts from the start, rather than having to try to build that conceptual model through trial and error. Hopefully I will be able to attend your seminar in the not-too-distant future. Randall R. Hawley, Automation Technician, Eli Lilly & Co.

The best computer book writing I have seen. Tom Holland

This is one of the best books I’ve read about a programming language... The best book ever written on Java. Ravindra Pai, Oracle Corporation, SUNOS product line

This is the best book on Java that I have ever found! You have done a great job. Your depth is amazing. I will be purchasing the book when it is published. I have been learning Java since October 96. I have read a few books, and consider yours a “MUST READ.” These past few months we have been focused on a product written entirely in Java. Your book has helped solidify topics I was shaky on and has expanded my knowledge base. I have even used some of your explanations as information in interviewing contractors to help our team. I have found how much Java knowledge they have by asking them about things I have learned from reading your book (e.g., the difference between arrays and Vectors). Your book is great! Steve Wilkinson, Senior Staff Specialist, MCI Telecommunications

Great book. Best book on Java I have seen so far. Jeff Sinclair, Software Engineer, Kestral Computing

Thank you for Thinking in Java. It’s time someone went beyond mere language description to a thoughtful, penetrating analytic tutorial that doesn’t kowtow to The Manufacturers. I’ve read almost all the others–only yours and Patrick Winston’s have found a place in my heart. I’m already recommending it to customers. Thanks again. Richard Brooks, Java Consultant, Sun Professional Services, Dallas

Other books cover the WHAT of Java (describing the syntax and the libraries) or the HOW of Java (practical programming examples). Thinking in Java is the only book I know that explains the WHY of Java; why it was designed the way it was, why it works the way it does, why it sometimes doesn’t work, why it’s better than C++, why it’s not. Although it also does a good job of teaching the what and how of the language, Thinking in Java is definitely the thinking person’s choice in a Java book. Robert S. Stephenson

Thanks for writing a great book. The more I read it the better I like it. My students like it, too. Chuck Iverson

I just want to commend you for your work on Thinking in Java. It is people like you that dignify the future of the Internet and I just want to thank you for your effort. It is very much appreciated. Patrick Barrell, Network Officer Mamco, QAF Mfg. Inc.

Most of the Java books out there are fine for a start, and most just have beginning stuff and a lot of the same examples. Yours is by far the best advanced thinking book I’ve seen. Please publish it soon! ... I also bought Thinking in C++ just because I was so impressed with Thinking in Java. George Laframboise, LightWorx Technology Consulting, Inc.

I wrote to you earlier about my favorable impressions regarding your Thinking in C++ (a book that stands prominently on my shelf here at work). And today I’ve been able to delve into Java with your e-book in my virtual hand, and I must say (in my best Chevy Chase from Modern Problems) “I like it!” Very informative and explanatory, without reading like a dry textbook. You cover the most important yet the least covered concepts of Java development: the whys. Sean Brady

Your examples are clear and easy to understand. You took care of many important details of Java that can’t be found easily in the weak Java documentation. And you don’t waste the reader’s time with the basic facts a programmer already knows. Kai Engert, Innovative Software, Germany

I’m a great fan of your Thinking in C++ and have recommended it to associates. As I go through the electronic version of your Java book, I’m finding that you’ve retained the same high level of writing. Thank you! Peter R. Neuwald

VERY well-written Java book...I think you’ve done a GREAT job on it. As the leader of a Chicago-area Java special interest group, I’ve favorably mentioned your book and Web site several times at our recent meetings. I would like to use Thinking in Java as the basis for a part of each monthly SIG meeting, in which we review and discuss each chapter in succession. Mark Ertes

I really appreciate your work and your book is good. I recommend it here to our users and Ph.D. students. Hugues Leroy // Irisa-Inria Rennes France, Head of Scientific Computing and Industrial Tranfert

OK, I’ve only read about 40 pages of Thinking in Java, but I’ve already found it to be the most clearly written and presented programming book I’ve come across...and I’m a writer, myself, so I am probably a little critical. I have Thinking in C++ on order and can’t wait to crack it–I’m fairly new to programming and am hitting learning curves head-on everywhere. So this is just a quick note to say thanks for your excellent work. I had begun to burn a little low on enthusiasm from slogging through the mucky, murky prose of most computer books–even ones that came with glowing recommendations. I feel a whole lot better now. Glenn Becker, Educational Theatre Association

Thank you for making your wonderful book available. I have found it immensely useful in finally understanding what I experienced as confusing in Java and C++. Reading your book has been very satisfying. Felix Bizaoui, Twin Oaks Industries, Louisa, Va.

I must congratulate you on an excellent book. I decided to have a look at Thinking in Java based on my experience with Thinking in C++, and I was not disappointed. Jaco van der Merwe, Software Specialist, DataFusion Systems Ltd, Stellenbosch, South Africa

This has to be one of the best Java books I’ve seen. E.F. Pritchard, Senior Software Engineer, Cambridge Animation Systems Ltd., United Kingdom

Your book makes all the other Java books I’ve read or flipped through seem doubly useless and insulting. Brett g Porter, Senior Programmer, Art & Logic

I have been reading your book for a week or two and compared to the books I have read earlier on Java, your book seems to have given me a great start. I have recommended this book to a lot of my friends and they have rated it excellent. Please accept my congratulations for coming out with an excellent book. Rama Krishna Bhupathi, Software Engineer, TCSI Corporation, San Jose

Just wanted to say what a “brilliant” piece of work your book is. I’ve been using it as a major reference for in-house Java work. I find that the table of contents is just right for quickly locating the section that is required. It’s also nice to see a book that is not just a rehash of the API nor treats the programmer like a dummy. Grant Sayer, Java Components Group Leader, Ceedata Systems Pty Ltd, Australia

Wow! A readable, in-depth Java book. There are a lot of poor (and admittedly a couple of good) Java books out there, but from what I’ve seen yours is definitely one of the best. John Root, Web Developer, Department of Social Security, London

I’ve *just* started Thinking in Java. I expect it to be very good because I really liked Thinking in C++ (which I read as an experienced C++ programmer, trying to stay ahead of the curve). I’m somewhat less experienced in Java, but expect to be very satisfied. You are a wonderful author. Kevin K. Lewis, Technologist, ObjectSpace, Inc.

I think it’s a great book. I learned all I know about Java from this book. Thank you for making it available for free over the Internet. If you wouldn’t have I’d know nothing about Java at all. But the best thing is that your book isn’t a commercial brochure for Java. It also shows the bad sides of Java. YOU have done a great job here. Frederik Fix, Belgium

I have been hooked to your books all the time. A couple of years ago, when I wanted to start with C++, it was C++ Inside & Out which took me around the fascinating world of C++. It helped me in getting better opportunities in life. Now, in pursuit of more knowledge and when I wanted to learn Java, I bumped into Thinking in Java—no doubts in my mind as to whether I need some other book. Just fantastic. It is more like rediscovering myself as I get along with the book. It is just a month since I started with Java, and heartfelt thanks to you, I am understanding it better now. Anand Kumar S., Software Engineer, Computervision, India

Your book stands out as an excellent general introduction. Peter Robinson, University of Cambridge Computer Laboratory

It’s by far the best material I have come across to help me learn Java and I just want you to know how lucky I feel to have found it. THANKS! Chuck Peterson, Product Leader, Internet Product Line, IVIS International

The book is great. It’s the third book on Java I’ve started and I’m about two-thirds of the way through it now. I plan to finish this one. I found out about it because it is used in some internal classes at Lucent Technologies and a friend told me the book was on the Net. Good work. Jerry Nowlin, MTS, Lucent Technologies

Of the six or so Java books I’ve accumulated to date, your Thinking in Java is by far the best and clearest. Michael Van Waas, Ph.D., President, TMR Associates

I just want to say thanks for Thinking in Java. What a wonderful book you’ve made here! Not to mention downloadable for free! As a student I find your books invaluable (I have a copy of C++ Inside Out, another great book about C++), because they not only teach me the how-to, but also the whys, which are of course very important in building a strong foundation in languages such as C++ or Java. I have quite a lot of friends here who love programming just as I do, and I’ve told them about your books. They think it’s great! Thanks again! By the way, I’m Indonesian and I live in Java. Ray Frederick Djajadinata, Student at Trisakti University, Jakarta

The mere fact that you have made this work free over the Net puts me into shock. I thought I’d let you know how much I appreciate and respect what you’re doing. Shane LeBouthillier, Computer Engineering student, University of Alberta, Canada

I have to tell you how much I look forward to reading your monthly column. As a newbie to the world of object oriented programming, I appreciate the time and thoughtfulness that you give to even the most elementary topic. I have downloaded your book, but you can bet that I will purchase the hard copy when it is published. Thanks for all of your help. Dan Cashmer, B. C. Ziegler & Co.

Just want to congratulate you on a job well done. First I stumbled upon the PDF version of Thinking in Java. Even before I finished reading it, I ran to the store and found Thinking in C++. Now, I have been in the computer business for over eight years, as a consultant, software engineer, teacher/trainer, and recently as self-employed, so I’d like to think that I have seen enough (not “have seen it all,” mind you, but enough). However, these books cause my girlfriend to call me a ”geek.” Not that I have anything against the concept—it is just that I thought this phase was well beyond me. But I find myself truly enjoying both books, like no other computer book I have touched or bought so far. Excellent writing style, very nice introduction of every new topic, and lots of wisdom in the books. Well done. Simon Goland, simonsez@smartt.com, Simon Says Consulting, Inc.

I must say that your Thinking in Java is great! That is exactly the kind of documentation I was looking for. Especially the sections about good and poor software design using Java. Dirk Duehr, Lexikon Verlag, Bertelsmann AG, Germany

Thank you for writing two great books (Thinking in C++, Thinking in Java). You have helped me immensely in my progression to object oriented programming. Donald Lawson, DCL Enterprises

Thank you for taking the time to write a really helpful book on Java. If teaching makes you understand something, by now you must be pretty pleased with yourself. Dominic Turner, GEAC Support

It’s the best Java book I have ever read—and I read some. Jean-Yves MENGANT, Chief Software Architect NAT-SYSTEM, Paris, France

Thinking in Java gives the best coverage and explanation. Very easy to read, and I mean the code fragments as well. Ron Chan, Ph.D., Expert Choice, Inc., Pittsburgh PA

Your book is great. I have read lots of programming books and your book still adds insights to programming in my mind. Ningjian Wang, Information System Engineer, The Vanguard Group

Thinking in Java is an excellent and readable book. I recommend it to all my students. Dr. Paul Gorman, Department of Computer Science, University of Otago, Dunedin, New Zealand

You make it possible for the proverbial free lunch to exist, not just a soup kitchen type of lunch but a gourmet delight for those who appreciate good software and books about it. Jose Suriol, Scylax Corporation

Thanks for the opportunity of watching this book grow into a masterpiece! IT IS THE BEST book on the subject that I’ve read or browsed. Jeff Lapchinsky, Programmer, Net Results Technologies

Your book is concise, accessible and a joy to read. Keith Ritchie, Java Research & Development Team, KL Group Inc.

It truly is the best book I’ve read on Java! Daniel Eng

The best book I have seen on Java! Rich Hoffarth, Senior Architect, West Group

Thank you for a wonderful book. I’m having a lot of fun going through the chapters. Fred Trimble, Actium Corporation

You have mastered the art of slowly and successfully making us grasp the details. You make learning VERY easy and satisfying. Thank you for a truly wonderful tutorial. Rajesh Rau, Software Consultant

Thinking in Java rocks the free world! Miko O’Sullivan, President, Idocs Inc.

About Thinking in C++:

Best Book! Winner of the
1995 Software Development Magazine Jolt Award!

“This book is a tremendous achievement. You owe it to yourself to have a copy on your shelf. The chapter on iostreams is the most comprehensive and understandable treatment of that subject I’ve seen to date.”

Al Stevens
Contributing Editor, Doctor Dobbs Journal

“Eckel’s book is the only one to so clearly explain how to rethink program construction for object orientation. That the book is also an excellent tutorial on the ins and outs of C++ is an added bonus.”

Andrew Binstock
Editor, Unix Review

“Bruce continues to amaze me with his insight into C++, and Thinking in C++ is his best collection of ideas yet. If you want clear answers to difficult questions about C++, buy this outstanding book.”

Gary Entsminger
Author, The Tao of Objects

“Thinking in C++ patiently and methodically explores the issues of when and how to use inlines, references, operator overloading, inheritance, and dynamic objects, as well as advanced topics such as the proper use of templates, exceptions and multiple inheritance. The entire effort is woven in a fabric that includes Eckel’s own philosophy of object and program design. A must for every C++ developer’s bookshelf, Thinking in C++ is the one C++ book you must have if you’re doing serious development with C++.”

Richard Hale Shaw
Contributing Editor, PC Magazine

Thinking

in

Java

Second Edition

Bruce Eckel

President, MindView Inc.

Prentice Hall
Upper Saddle River, New Jersey 07458
http://www.phptr.comLibrary of Congress Cataloging-in-Publication Data

Eckel, Bruce.

Thinking in Java / Bruce Eckel.

p. cm.

Includes index.

ISBN 0-13-027363-5

1. Java (Computer program language) I. Title.

QA76.73.J38E25 1998

005.13'3--dc21 97-52713

CIP

Editorial/Production Supervision: Craig Little
Acquisitions Editor: Jeffrey Pepper
Manufacturing Manager: Alexis R. Heydt
Marketing Manager: Miles Williams
Cover Design: Daniel Will-Harris
Interior Design: Daniel Will-Harris, www.will-harris.com

© 2000 by Bruce Eckel, President, MindView, Inc.
Published by Prentice-Hall Inc.
Pearson Higher Education
Upper Saddle River, NJ 07458

The information in this book is distributed on an “as is” basis, without warranty. While every precaution has been taken in the preparation of this book, neither the author nor the publisher shall have any liability to any person or entitle with respect to any liability, loss or damage caused or alleged to be caused directly or indirectly by instructions contained in this book or by the computer software or hardware products described herein.

All rights reserved. No part of this book may be
reproduced, in any form or by any means, without
permission in writing from the publisher.

Prentice Hall books are widely used by corporations and government agencies for training, marketing, and resale. The publisher offers discounts on this book when ordered in bulk quantities. For more information, contact the Corporate Sales Department at 800-382-3419, fax: 201-236-7141, email: corpsales@prenhall.com or write: Corporate Sales Department, Prentice Hall PTR, One Lake Street, Upper Saddle River, New Jersey 07458.

Java is a registered trademark of Sun Microsystems, Inc. Windows 95 and Windows NT are trademarks of Microsoft Corporation. All other product names and company names mentioned herein are the property of their respective owners.

Printed in the United States of America
10 9 8 7 6 5 4 3 2 1

ISBN 0-13-659723-8

Prentice-Hall International (UK) Limited, London

Prentice-Hall of Australia Pty. Limited, Sydney

Prentice-Hall Canada, Inc., Toronto

Prentice-Hall Hispanoamericana, S.A., Mexico

Prentice-Hall of India Private Limited, New Delhi

Prentice-Hall of Japan, Inc., Tokyo

Pearson Education Asia Ltd., Singapore

Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro

Dedication

To the person who, even now,
is creating the next great computer language

Overview

What’s Inside

[ Short TOC ] [ Table of Contents ] [ Index ] [ Next Chapter ]


Thinking in Java, 2nd ed. - Contents

 Expanded Table of Contents

Title Pages
Preface
 Introduction
 1: Introduction to Objects
 2: Everything is an Object
 3: Controlling Program Flow
 4: Initialization & Cleanup
 5: Hiding the Implementation
 6: Reusing Classes
 7: Polymorphism
 8: Interfaces & Inner Classes
 9: Holding Your Objects
 10: Error Handling with Exceptions
 11: The Java IO System
 12: Run-time Type Identification
 13: Creating Windows & Applets
 14: Multiple Threads
 15: Distributed Computing
 A: Passing & Returning Objects
 B: The Java Native Interface (JNI)
 C: Java Programming Guidelines
 D: Resources
 Index