Java Applets and Applications

Applets and Applications

Java applications 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.

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 most other 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 lecture, you’ll be writing mostly HotJava applications; then you’ll apply what you’ve learned to write applets in next lecture. 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 next lectures.

        

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, ped, 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 routine called main(). In Java applications, as in a C or C++ program, main() is the first routine that is run when the program is executed. You’ll learn more about both these parts of a Java application as the class progresses.

Once you finish typing the program, save the file. Conventionally, 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 these lectures, 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

Analysis

1

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: Actually, 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