Teach Yourself Java in 21 Days tyj01x.htm

- Day 1

- An Introduction to Java Programming

- What Is Java?
- Java's Past, Present, and Future
- Why Learn Java?

- Java Is Platform-Independent
- Java Is Object-Oriented
- Java Is Easy to Learn

- Getting Started with Programming in Java

- Getting the Software
- Applets and Applications
- Creating a Java Application
- Creating a Java Applet

- Summary
- Q&A

#
Day 1

## An Introduction to Java Programming

by Laura Lemay

Hello and welcome to Teach Yourself Java in 21 Days! Starting today and for the next three weeks you'll learn all about the Java language and how to use it to create applets, as well as how to create stand-alone Java applications that you can use for just about anything.

An applet is a dynamic and interactive program that can run inside a Web page displayed by a Java-capable browser such as HotJava or Netscape 2.0.

The HotJava browser is a World Wide Web browser used to view Web pages, follow links, and submit forms. It can also download and play applets on the reader's system.

That's the overall goal for the next three weeks. Today, the goals are somewhat more modest, and you'll learn about the following:

- What exactly Java and HotJava are, and their current status
- Why you should learn Java—its various features and advantages over other programming languages
- Getting started programming in Java—what you'll need in terms of software and background, as well as some basic terminology
- How to create your first Java programs—to close this day, you'll create both a simple Java application and a simple Java applet!

### What Is Java?

Java is an object-oriented programming language developed by Sun Microsystems, a company best known for its high-end Unix workstations. Modeled after C++, the Java language was designed to be small, simple, and portable across platforms and operating systems, both at the source and at the binary level (more about this later).

Java is often mentioned in the same breath as HotJava, a World Wide Web browser from Sun like Netscape or Mosaic (see Figure 1.1). What makes HotJava different from most other browsers is that, in addition to all its basic Web features, it can also download and play applets on the reader's system. Applets appear in a Web page much in the same way as images do, but unlike images, applets are dynamic and interactive. Applets can be used to create animations, figures, or areas that can respond to input from the reader, games, or other interactive effects on the same Web pages among the text and graphics.

Figure 1.1. The HotJava browser.

Although HotJava was the first World Wide Web browser to be able to play Java applets, Java support is rapidly becoming available in other browsers. Netscape 2.0 provides support for Java applets, and other browser developers have also announced support for Java in forthcoming products.

To create an applet, you write it in the Java language, compile it using a Java compiler, and refer to that applet in your HTML Web pages. You put the resulting HTML and Java files on a Web site much in the same way that you make ordinary HTML and image files available. Then, when someone using the HotJava browser (or other Java-enabled browser) views your page with the embedded applet, that browser downloads the applet to the local system and executes it, and then the reader can view and interact with your applet in all its glory (readers using other browsers may see text, a static graphic, or nothing). You'll learn more about how applets, browsers, and the World Wide Web work together further on in this book.

The important thing to understand about Java is that you can do so much more with it besides create applets. Java was written as a full-fledged programming language in which you can accomplish the same sorts of tasks and solve the same sorts of problems that you can in other programming languages, such as C or C++. HotJava itself, including all the networking, display, and user interface elements, is written in Java.

### Java's Past, Present, and Future

The Java language was developed at Sun Microsystems in 1991 as part of a research project to develop software for consumer electronics devices—television sets, VCRs, toasters, and the other sorts of machines you can buy at any department store. Java's goals at that time were to be small, fast, efficient, and easily portable to a wide range of hardware devices. It is those same goals that made Java an ideal language for distributing executable programs via the World Wide Web, and also a general-purpose programming language for developing programs that are easily usable and portable across different platforms.

The Java language was used in several projects within Sun, but did not get very much commercial attention until it was paired with HotJava. HotJava was written in 1994 in a matter of months, both as a vehicle for downloading and running applets and also as an example of the sort of complex application that can be written in Java.

This book is current as of the 1.0 Java Developer's Kit (JDK). The 1.0 JDK includes tools for developing Java applets and applications on Sun systems running Solaris 2.3 or higher and for Windows NT and Windows 95. By the time you read this, new releases of the JDK may be available, or there may be support for the JDK on other systems and platforms. All the examples and code in this book should work with any Java development environment that supports the 1.0 API.

To run and view Java applets, you'll need a Java-capable browser or other tool. Netscape 2.0 provides Java capabilities for most platforms, and other browsers may also be Java-enabled. As of this writing, however, Sun's HotJava does not support applets written under the 1.0 JDK. By the time you read this a more current version of HotJava may exist.

Sun's 1.0 JDK does include an application called appletviewer that allows you to test your Java applets as you write them. If an applet works in the appletviewer, it should work with any Java-capable browser. You'll learn more about applet viewer later today.

What's in store for the future? In addition to the next Java release from Sun, other companies have announced support for Java in their own World Wide Web browsers. Netscape Communications Corporation has already incorporated Java capabilities into the 2.0 version of their very popular Netscape Navigator Web browser—pages with embedded Java applets can be viewed and played with Netscape. With support for Java available in as popular a browser as Netscape, tools to help develop Java applications (debuggers, development environments, and so on) most likely will be rapidly available as well.

### Why Learn Java?

At the moment, probably the most compelling reason to learn Java—and probably the reason you bought this book—is that HotJava applets are written in Java. Even if that were not the case, Java as a language has significant advantages over other languages and other programming environments that make it suitable for just about any programming task. This section describes some of those advantages.

#### Java Is Platform-Independent

Platform independence is one of the most significant advantages that Java has over other programming languages, particularly for systems that need to work on many different platforms. Java is platform-independent at both the source and the binary level.

Platform-independence is a program's capability of moving easily from one computer system to another.

At the source level, Java's primitive data types have consistent sizes across all development platforms. Java's foundation class libraries make it easy to write code that can be moved from platform to platform without the need to rewrite it to work with that platform.

Platform-independence doesn't stop at the source level, however. Java binary files are also platform-independent and can run on multiple platforms without the need to recompile the source. How does this work? Java binary files are actually in a form called bytecodes.

Bytecodes are a set of instructions that look a lot like machine code, but are not specific to any one processor.

Normally, when you compile a program written in C or in most other languages, the compiler translates your program into machine codes or processor instructions. Those instructions are specific to the processor your computer is running—so, for example, if you compile your code on a Pentium system, the resulting program will run only on other Pentium systems. If you want to use the same program on another system, you have to go back to your original source, get a compiler for that system, and recompile your code. Figure 1.2 shows the result of this system: multiple executable programs for multiple systems.

Figure 1.2. Traditional compiled programs.

Things are different when you write code in Java. The Java development environment has two parts: a Java compiler and a Java interpreter. The Java compiler takes your Java program and instead of generating machine codes from your source files, it generates bytecodes.

To run a Java program, you run a program called a bytecode interpreter, which in turn executes your Java program (see Figure 1.3). You can either run the interpreter by itself, or—for applets—there is a bytecode interpreter built into HotJava and other Java-capable browsers that runs the applet for you.

Figure 1.3. Java programs.

Why go through all the trouble of adding this extra layer of the bytecode interpreter? Having your Java programs in bytecode form means that instead of being specific to any one system, your programs can be run on any platform and any operating or window system as long as the Java interpreter is available. This capability of a single binary file to be executable across platforms is crucial to what enables applets to work, because the World Wide Web itself is also platform-independent. Just as HTML files can be read on any platform, so applets can be executed on any platform that is a Java-capable browser.

The disadvantage of using bytecodes is in execution speed. Because system-specific programs run directly on the hardware for which they are compiled, they run significantly faster than Java bytecodes, which must be processed by the interpreter. For many Java programs, the speed may not be an issue. If you write programs that require more execution speed than the Java interpreter can provide, you have several solutions available to you, including being able to link native code into your Java program or using tools to convert your Java bytecodes into native code. Note that by using any of these solutions, you lose the portability that Java bytecodes provide. You'll learn about each of these mechanisms on Day 20.

#### Java Is Object-Oriented

To some, object-oriented programming (OOP) technique is merely a way of organizing programs, and it can be accomplished using any language. Working with a real object-oriented language and programming environment, however, enables you to take full advantage of object-oriented methodology and its capabilities for creating flexible, modular programs and reusing code.

Many of Java's object-oriented concepts are inherited from C++, the language on which it is based, but it borrows many concepts from other object-oriented languages as well. Like most object-oriented programming languages, Java includes a set of class libraries that provide basic data types, system input and output capabilities, and other utility functions. These basic classes are part of the Java development kit, which also has classes to support networking, common Internet protocols, and user interface toolkit functions. Because these class libraries are written in Java, they are portable across platforms as all Java applications are.

You'll learn more about object-oriented programming and Java tomorrow.

#### Java Is Easy to Learn

In addition to its portability and object-orientation, one of Java's initial design goals was to be small and simple, and therefore easier to write, easier to compile, easier to debug, and, best of all, easy to learn. Keeping the language small also makes it more robust because there are fewer chances for programmers to make difficult-to-find mistakes. Despite its size and simple design, however, Java still has a great deal of power and flexibility.

Java is modeled after C and C++, and much of the syntax and object-oriented structure is borrowed from the latter. If you are familiar with C++, learning Java will be particularly easy for you, because you have most of the foundation already.

Although Java looks similar to C and C++, most of the more complex parts of those languages have been excluded from Java, making the language simpler without sacrificing much of its power. There are no pointers in Java, nor is there pointer arithmetic. Strings and arrays are real objects in Java. Memory management is automatic. To an experienced programmer, these omissions may be difficult to get used to, but to beginners or programmers who have worked in other languages, they make the Java language far easier to learn.

### Getting Started with Programming in Java

Enough background! Let's finish off this day by creating two real Java programs: a stand-alone Java application and an applet that you can view in either in the appletviewer (part of the JDK) or in a Java-capable browser. Although both these programs are extremely simple, they will give you an idea of what a Java program looks like and how to compile and run it.

#### Getting the Software

In order to write Java programs, you will, of course, need a Java development environment. At the time this book is being written, Sun's Java Development Kit provides everything you need to start writing Java programs. The JDK is available for Sun SPARC systems running Solaris 2.3 or higher and for Windows NT and Windows 95. You can get the JDK from several places:

- The CD-ROM that came with this book contains the full 1.0 JDK distribution. See the CD information for installation instructions.
- The latest JDK can be downloaded from Sun's Java FTP site at ftp://java.sun.com/pub/ or from a mirror site (ftp://www.blackdown.org/pub/Java/pub/ is one).

Note: The Java Development Kit is currently in release 1.0. By the time you read this, the JDK may be available for other platforms, and other organizations may be selling Java development tools as well.

Although Netscape and other Java-enabled browsers provide an environment for running Java applets, they do not provide a mechanism for developing Java applications. For that, you need separate tools—merely having a browser is not enough.

#### Applets and Applications

Java programs fall into two main groups: applets and applications.

Applets, as you have learned, are Java programs that are downloaded over the World Wide Web and executed by a Web browser on the reader's machine. Applets depend on a Java-capable browser in order to run (although they can also be viewed using a tool called the appletviewer, which you'll learn about later today).

Java applications are more general programs written in the Java language. Java applications don't require a browser to run, and in fact, Java can be used to create all the kinds of applications that you would normally use a more conventional programming language to create. HotJava itself is a Java application.

A single Java program can be an applet or an application or both, depending on how you write that program and the capabilities that program uses. Throughout this first week, you'll be writing mostly HotJava applications; then you'll apply what you've learned to write applets in Week 2. If you're eager to get started with applets, be patient. Everything that you learn while you're creating simple Java applications will apply to creating applets, and it's easier to start with the basics before moving onto the hard stuff. You'll be creating plenty of applets in Week 2.

#### Creating a Java Application

Let's start by creating a simple Java application: the classic Hello World example that all language books use to begin.

As with all programming languages, your Java source files are created in a plain text editor, or in an editor that can save files in plain ASCII without any formatting characters. On Unix, emacs, pico, or vi will work; on Windows, Notepad or DOS Edit are both text editors.

Fire up your editor of choice, and enter the Java program shown in Listing 1.1. Type this program, as shown, in your text editor. Be careful that all the parentheses, braces, and quotes are there.

Listing 1.1. Your first Java application.

    1: class HelloWorld {

    2:     public static void main (String args[]) {

    3:         System.out.println("Hello World!");

    4:     }

    5: }

Warning: The numbers before each line are part of the listing and not part of the program; they're there so I can refer to specific line numbers when I explain what's going on in the program. Do not include them in your own file.

This program has two main parts:

- All the program is enclosed in a class definition—here, a class called HelloWorld.
- The body of the program (here, just the one line) is contained in a method (function) called main(). In Java applications, as in a C or C++ program, main() is the first method (function) that is run when the program is executed.

You'll learn more about both these parts of a Java application as the book progresses.

Once you finish typing the program, save the file. Most of the time, Java source files are named the same name as the class they define, with an extension of .java. This file should therefore be called HelloWorld.java.

Now, let's compile the source file using the Java compiler. In Sun's JDK, the Java compiler is called javac.

To compile your Java program, Make sure the javac program is in your execution path and type javac followed by the name of your source file:

    javac HelloWorld.java

Note: In these examples, and in all the examples throughout this book, we'll be using Sun's Java compiler, part of the JDK. If you have a third-party development environment, check with the documentation for that program to see how to compile your Java programs.

The compiler should compile the file without any errors. If you get errors, go back and make sure that you've typed the program exactly as it appears in Listing 1.1.

When the program compiles without errors, you end up with a file called HelloWorld.class, in the same directory as your source file. This is your Java bytecode file. You can then run that bytecode file using the Java interpreter. In the JDK, the Java interpreter is called simply java. Make sure the java program is in your path and type java followed by the name of the file without the .class extension:

    java HelloWorld

If your program was typed and compiled correctly, you should get the string "Hello World!" printed to your screen as a response.

Note: Remember, the Java compiler and the Java interpreter are different things. You use the Java compiler (javac) for your Java source files to create .class files, and you use the Java interpreter (java) to actually run your class files.

#### Creating a Java Applet

Creating applets is different from creating a simple application, because Java applets run and are displayed inside a Web page with other page elements and as such have special rules for how they behave. Because of these special rules for applets in many cases (particularly the simple ones), creating an applet may be more complex than creating an application.

For example, to do a simple Hello World applet, instead of merely being able to print a message, you have to create an applet to make space for your message and then use graphics operations to paint the message to the screen.

Note: If you run the Hello World application as an applet, the Hello World message prints to a special window or to a log file, depending on how the browser has screen messages set up. It will not appear on the screen unless you write your applet to put it there.

In the next example, you create that simple Hello World applet, place it inside a Web page, and view the result.

First, you set up an environment so that your Java-capable browser can find your HTML files and your applets. Much of the time, you'll keep your HTML files and your applet code in the same directory. Although this isn't required, it makes it easier to keep track of each element. In this example, you use a directory called HTML that contains all the files you'll need.

    mkdir HTML

Now, open up that text editor and enter Listing 1.2.

Listing 1.2. The Hello World applet.

    1: import java.awt.Graphics;

    2:

    3: public class HelloWorldApplet extends java.applet.Applet {

    4:

    5:     public void paint(Graphics g) {

    6:        g.drawString("Hello world!", 5, 25);

    7:    }

    8:}

Save that file inside your HTML directory. Just like with Java applications, give your file a name that has the same name as the class. In this case, the filename would be HelloWorldApplet.java.

Features to note about applets? There are a couple I'd like to point out:

- The import line at the top of the file is somewhat analogous to an #include statement in C; it enables this applet to get access to the JDK classes for creating applets and for drawing graphics on the screen.
- The paint() method displays the content of the applet onto the screen. Here, the string Hello World gets drawn. Applets use several standard methods to take the place of main(), which include init() to initialize the applet, start() to start it running, and paint() to display it to the screen. You'll learn about all of these in Week 2.

Now, compile the applet just as you did the application, using javac, the Java compiler.

    javac HelloWorldApplet.java

Again, just as for applications, you should now have a file called HelloWorldApplet.class in your HTML directory.

To include an applet in a Web page, you refer to that applet in the HTML code for that Web page. Here, you create a very simple HTML file in the HTML directory (see Listing 1.3).

Listing 1.3. The HTML with the applet in it.

    1: <HTML>

    2: <HEAD>

    3: <TITLE>Hello to Everyone!</TITLE>

    4: </HEAD><BODY>

    5: <P>My Java applet says:

    6: <APPLET CODE="HelloWorldApplet.class" WIDTH=150 HEIGHT=25><APPLET>

    7: </BODY>

    8: </HTML>

You refer to an applet in your HTML files with the <APPLET> tag. You'll learn more about <APPLET> later on, but here are two things to note:

- Use the CODE attribute to indicate the name of the class that contains your applet.
- Use the WIDTH and HEIGHT attributes to indicate the size of the applet. The browser uses these values to know how big a chunk of space to leave for the applet on the page. Here, a box 150 pixels wide and 25 pixels high is created.

Save the HTML file in your HTML directory, with a descriptive name (for example, you might name your HTML file the same name as your applet—HelloWorldApplet.html).

And now, you're ready for the final test—actually viewing the result of your applet. To view the applet, you need one of the following:

- A browser that supports Java applets, such as Netscape 2.0.
- The appletviewer application, which is part of the JDK. The appletviewer is not a Web browser and won't enable you to see the entire Web page, but it's acceptable for testing to see how an applet will look and behave if there is nothing else available.

Note: Do not use the alpha version of HotJava to view your applets; applets developed with the beta JDK and onward cannot be viewed by the alpha HotJava. If, by the time you read this, there is a more recent version of HotJava, you can use that one instead.

If you're using a Java-capable browser such as Netscape to view your applet files, you can use the Open Local... item under the File menu to navigate to the HTML file containing the applet (make sure you open the HTML file and not the class file). You don't need to install anything on a Web server yet; all this works on your local system.

If you don't have a Web browser with Java capabilities built into it, you can use the appletviewer program to view your Java applet. To run appletviewer, just indicate the path to the HTML file on the command line:

    appletviewer HTML/HelloWorldApplet.html

Tip: Although you can start appletviewer from the same directory as your HTML and class files, you may not be able to reload that applet without quitting appletviewer first. If you start appletviewer from some other directory (as in the previous command line), you can modify and recompile your Java applets and then just use the Reload menu item to view the newer version.

Now, if you use the browser to view the applet, you see something similar to the image shown in Figure 1.4. If you're using appletviewer, you won't see the text around the applet (My Java applet says...), but you will see the Hello World itself.

Figure 1.4. The Hello World applet.

### Summary

Today, you got a basic introduction to the Java language and its goals and features. Java is a programming language, similar to C or C++, in which you can develop a wide range of programs. The most common use of Java at the moment is in creating applets for HotJava, an advanced World Wide Web browser also written in Java. Applets are Java programs that are downloaded and run as part of a Web page. Applets can create animations, games, interactive programs, and other multimedia effects on Web pages.

Java's strengths lie in its portability—both at the source and at the binary level, in its object-oriented design—and in its simplicity. Each of these features help make applets possible, but they also make Java an excellent language for writing more general-purpose programs that do not require HotJava or other Java-capable browser to run. These general-purpose Java programs are called applications. HotJava itself is a Java application.

To end this day, you experimented with an example applet and an example application, getting a feel for the differences between the two and how to create, compile, and run Java programs—or, in the case of applets, how to include them in Web pages. From here, you now have the foundation to create more complex applications and applets.

### Q&A

Q: I'd like to use HotJava as my regular Web browser. You haven't mentioned much about HotJava today.

A: The focus of this book is primarily on programming in Java and its library of standard classes, rather than on using HotJava itself. Documentation for using the HotJava browser comes with HotJava (see http://java.sun.com/).

Q: I know a lot about HTML, but not much about computer programming. Can I still write Java programs?

A: If you have no programming experience whatsoever, you most likely will find programming Java significantly more difficult. However, Java is an excellent language to learn programming with, and if you patiently work through the examples and the exercises in this book, you should be able to learn enough to get started with Java.

Q: According to today's lesson, Java applets are downloaded via a Java-enabled browser such as HotJava or Netscape 2.0 and run on the reader's system. Isn't that an enormous security hole? What stops someone from writing an applet that compromises the security of my system—or worse, that damages my system?

A: Sun's Java team has thought a great deal about the security of applets within Java-capable browsers and has implemented several checks to make sure applets cannot do nasty things:

1. Java applets cannot read or write to the disk on the local system.
2. Java applets cannot execute any programs on the local system.
3. Java applets cannot connect to any machines on the Web except for the server from which they are originally downloaded.
   Note that some of these restrictions may be allowed in some browsers or may be turned on in the browser configuration. However, you cannot expect any of these capabilities to be available.
   In addition, the Java compiler and interpreter check both the Java source code and the Java bytecodes to make sure that the Java programmer has not tried any sneaky tricks (for example, overrunning buffers or stack frames).
   These checks obviously cannot stop every potential security hole (no system can promise that!), but they can significantly reduce the potential for hostile applets. You'll learn more about security issues later on in this book.

Q: I followed all the directions you gave for creating a Java applet. I loaded it into HotJava, but Hello World didn't show up. What did I do wrong?

A: I'll bet you're using the alpha version of HotJava to view the applet. Unfortunately, between alpha and 1.0, significant changes were made as to how applets are written. The result is that you can't view 1.0 applets (as this one was) in the alpha version of HotJava, nor can you view alpha applets in browsers that expect 1.0 applets. To view the applet, either use a different browser, or use the appletviewer application that comes with the JDK.