Animation: Applets

Animation by Example: Applets
by David Matuszek

An applet is a small Java program that can be put in a web page. There is nothing special about using applets for animation; you can do as much (and more) in an application. These pages use mostly applets so that I can show you the running applets as we go along.

Writing Applets

When you write an applet, you don't write a complete program. Themainmethod is in the browser (Netscape or Internet Explorer), and it decides what methods to call and when to call them.

The browser calls these methods, all of them defined in Applet:

public void init(): This method is called once, at the very beginning.
public void start(): This method is called each time the page containing the applet is brought to the front.
public void paint(Graphics d): This method may be called many times. Each time it is called, it paints in the entire content of the Applet area. If you are doing animation, it probably paints something different each time.
public void stop(): This method is called when the page quits being in front.
public void destroy(): This method is called once, at the very end. Use it to kill any extra threads you have created.

None of these methods actually do anything. You create an applet by extending the class Applet,

public class myApplet extends Applet {...}

and overriding some or all of the above methods to make them do something.

Just about every reasonable applet overrides paint(Graphics g). The parameter is a "graphics context" that you can draw on. There are a very large number of methods that draw things, for example, g.drawString(string, x, y) or g.fillRect(x, y, width, height). The important thing to realize is that paint is expected to draw the entire picture every time it is called, starting from a blank screen.

The init() method is useful for initializing your variables and generally setting things up.

You can override start() and stop() if you don't want the animation to keep going when you aren't looking at the page. Remember, these methods don't yet do anything--the names suggest what you should make them do.

Here is the simplest possible applet:

import java.applet.*;
public class MyApplet1 extends Applet { }

and here it is in "action":

Note that this is an actual applet, not just a picture of an applet. You may notice that it doesn't seem to be doing very much--what did you expect?

Here is another, slightly more interesting applet:

import java.applet.*;
import java.awt.*;


public class MyApplet2 extends Applet {

    public void paint(Graphics g) {

        g.setColor(Color.red);
        g.drawRect(25, 15, 20, 20);

        g.setColor(Color.green);
        g.fillOval(40, 10, 60, 30);

        g.setColor(new Color(32, 0, 128));
        g.drawString("Is this exciting, or what?", 60, 25);
    }
}

and here it is in action:

Writing HTML

Applets must be embedded in an HTML page. Here is the page used for MyApplet2 above; notice that it's just text. The file has a .html extension.

<html>
  <head>
    <title>A Do-Little Applet</title>
  </head>
  <body>
    <applet code="MyApplet2.class" width="250" height="60">
    </applet>
  </body>
</html>

For more information, see More About HTML for Applets.

Running Applets

There are two ways to run an applet:

Open the HTML page in a browser, such as Netscape Navigator or Internet Explorer
Open the HTML page with a special viewer. In an MS-DOS window, go to the appropriate directory and type: appletviewer myPage.html. On a Macintosh, use Apple Applet Runner.

These methods may give different results, because not all browsers are equal; each uses a plug-in to execute the Java code, and the plug-in may not be up to date. You can download and install more recent plug-ins. In general,appletviewer does the best job.

The applets in these pages should work for just about any browser, because I've tried to use only the most basic Java 1.1, and I've tested them under Netscape 4.76. If they don't work in your browser, you probably need an updated plug-in. If you can't get your browser updated properly, use appletviewer instead. Remember, you have to use the HTML page; you can't use the .java or the .class file.

Reading Assignment:: Understanding Applets, pages 36-43.
If your basic Java is still weak, read all of Chapter 1, Fundamental Java.

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