Learning Java - Chapter 8 : Java

A popular task for a thread in Java is to control an animation. A thread process can direct the drawing of each frame while other aspects of the interface, such as responding to user input, can continue in parallel.

The SimpleAnimationApplet shown below illustrates the basics of an applet animation by modifying the colors of text in each "frame". The class implements the Runnable interface and the overridden run() method contains a continuous loop for updating the display. The sleep() method in the Thread class is used to pause between each frame. Here the framing is a slow 1 per second rate.

The applet's start() method (called when the page is loaded) creates the thread and starts it. A flag (the thread's reference variable) is checked at the start of each loop in to see if the process should jump from the loop and end the thread. The applet's stop() method (called when the browser loads another page), sets the flag so that the looping in run() finishes.

Note that the repaint() method does not cause the immediate repainting of the applet component but puts a request for repainting on the AWT's task queue. This allows the AWT to coordinate repainting with other tasks such as handling interface events.

SimpleAnimationApplet.java

import java.awt.*;

/** Demo of animation a with Runnable implementation. **/
public class SimpleAnimationApplet extends java.applet.Applet
                              implements Runnable {

  Thread fThread;
  Color [] fColorSet = {Color.RED, Color.BLUE, Color.GREEN};
  Color [] fColors   = new Color[3];
  int fColorIndex = 0;

  /** Applet's start method creates the thread and starts it. **/
  public void start () {

    // Create an instance of Thread with a
    // reference to this Runnable instance.
    fThread = new Thread (this);

    // Start the thread
    fThread.start ();
  } // start

  /** Use applet stop method to stop thread loop. **/
  public void stop () {
    if (fThread != null) {
        fThread = null;
    }
  } // stop

  /** Override the Runnable run() method. **/
  public void run() {
    while (fThread != null) {
        // Rotate colors
      for (int i=0; i < 3; i++) {
           int index = fColorIndex%3;
           fColors[i] = fColorSet[index];
           fColorIndex++;
      }
      // Shift colors in next loop.
      fColorIndex += 2;
      repaint();
      // Pause
      try {
           Thread.sleep(1000);
      }
      catch (InterruptedException e) {}
     }
  } // run

  /** Paint the text. **/
  public void paint(java.awt.Graphics g) {

    g.setColor(fColors[0]);
    g.drawString("Animation", 20, 30);

    g.setColor(fColors[1]);
    g.drawString("Demo", 80, 30);

    g.setColor(fColors[2]);
    g.drawString("Program", 120, 30);

  } // paint

} // class SimpleAnimationApplet