Last week, you saw the Hello World example application :

    class HelloWorld
    {
        public static void main(String args[])
        {
            System.out.println("Hello World!");
        }
    }

    import java.awt.Graphics;

    public class HelloWorldApplet extends java.applet.Applet
    {
        public void paint(Graphics g)
        {
            g.drawString("Hello World!", 5, 10);
        }
    }

The import statement on the first line enables you to refer to the java.awt.Graphics class as just Graphics. Without it, the definition of the paint method would look like:

    public void paint(java.awt.Graphics g)

There are three important differences between the application and the applet versions:

1. The HelloWorldApplet class is a public class. All applets must be public otherwise the browsers will not be able to execute them!

2. The extends keyword after the name of the class. It indicates to Java that the class preceeding the extends keyword is a subclass of the class following it. Ie, class HelloWorldApplet is a subclass of class java.applet.Applet. This is how subclassing is done in Java. If a class definition doesn't include the extends clause, its superclass is assumed to be java.lang.Object

3. Instead of the method definition for main(), HelloWorldApplet overrides the default paint() method. Recall that the order of execution of methods in applets is different to that of applications. Applets are first initialised with the init() method, then started and stopped with the start() and stop() methods. Any screen activity is done via the paint() method. All the applets that you create will be subclasses of the java.applet.Applet class. Hence, they will inherit these main activity methods as discussed in the previous section.

   Since all the HelloWorldApplet applet is doing is writing (painting) the words "Hello World!" to the screen, we only need to override the paint() method. Without any instance variables to initialise or objects to create, there is no need to override the default init() method. Similarly with the other activity methods.

The paint() method takes 1 argument, a Graphics object. When the browser loads and executes the applet, it creates a Graphics object which it passes to the paint() method whenever the paint() method is called. Ie, each time the applet is painted or repainted.

paint() contains only one statement, a method call to drawString which is a method defined in the Graphics class. It takes 3 arguments. The first argument is the string to be drawn, the second and third are the x and y coordinates respectively. drawString draws the string specified in the first argument at the x and y coordinates specified by the second and third arguments. In this case, it draws the string "Hello World!" starting from point (5, 10). Note that in Java (and most other programming languages), the screen coordinates start from the top-left corner of the drawing area.

The red square is point (5, 10) in Java's screen coordinate system. This is where the drawString method will start drawing the "Hello World!" string.

When you have finished typing the program as shown above, compile it by typing javac HelloWorldApplet.java. Just like in the previous example, a HelloWorldApplet.class file will be generated. However, you can't run the bytecode file with the Java interpreter. If you try, you will get the following error message:

    In class HelloWorldApplet: void main(String argv[]) is undefined

In order to run the applet, it must be embedded in a World Wide Web document, and then executed with a Java enabled browser or the appletviewer that comes with the Java Developer's kit. Create the following web page:

Now, you can test the applet with the appletviewer or a Java enabled browser. To test it with a browser, open the web page you just created and the browser will execute the applet. To test it with the appletviewer, type:

appletviewer Hello.html

You need a Java enabled Browser to view this applet.

That's it. You have just created and executed your very first applet.