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Exercise 6: Threads and Animation

The only functions we have seen in applets so far are init(), paint(), and functions called in response to input events. All of these functions are supposed to do a small amount of work and return quickly. There has been no opportunity, so far, for a function to loop and do some continuous work.

This applet creates a thread, a separate stream of execution, to perform a background task. The body of the thread's code is in the run() function. In this case, the purpose of the thread is to increment the variable i once every 1000 milliseconds, and cause the applet to redraw itself. The result is a simple animation.

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

public class Threads1 extends Applet implements Runnable {

   int width, height;
   int i = 0;
   Thread t = null;
   boolean threadSuspended;

   // Executed when the applet is first created.
   public void init() {
      System.out.println("init(): begin");
      width = getSize().width;
      height = getSize().height;
      setBackground( Color.black );
      System.out.println("init(): end");

   // Executed when the applet is destroyed.
   public void destroy() {

   // Executed after the applet is created; and also whenever
   // the browser returns to the page containing the applet.
   public void start() {
      System.out.println("start(): begin");
      if ( t == null ) {
         System.out.println("start(): creating thread");
         t = new Thread( this );
         System.out.println("start(): starting thread");
         threadSuspended = false;
      else {
         if ( threadSuspended ) {
            threadSuspended = false;
            System.out.println("start(): notifying thread");
            synchronized( this ) {
      System.out.println("start(): end");

   // Executed whenever the browser leaves the page containing the applet.
   public void stop() {
      System.out.println("stop(): begin");
      threadSuspended = true;

   // Executed within the thread that this applet created.
   public void run() {
      System.out.println("run(): begin");
      try {
         while (true) {
            System.out.println("run(): awake");

            // Here's where the thread does some work
            ++i;  // this is shorthand for "i = i+1;"
            if ( i == 10 ) {
               i = 0;
            showStatus( "i is " + i );

            // Now the thread checks to see if it should suspend itself
            if ( threadSuspended ) {
               synchronized( this ) {
                  while ( threadSuspended ) {
                     System.out.println("run(): waiting");
            System.out.println("run(): requesting repaint");
            System.out.println("run(): sleeping");
            t.sleep( 1000 );  // interval given in milliseconds
      catch (InterruptedException e) { }
      System.out.println("run(): end");

   // Executed whenever the applet is asked to redraw itself.
   public void paint( Graphics g ) {
      g.setColor( Color.green );
      g.drawLine( width, height, i * width / 10, 0 );

The resulting applet:

( You need to enable Java to see this applet. )

The call to showStatus() in run() will cause the value of i to appear in the browser's status bar. If you open the Java Console of your browser (in Netscape 4.7, this is accessible under the Communicator|Tools submenu), you'll see the text printed by calls to System.out.println().

Unfortunately, the source code is complicated because the applet is supposed to suspend execution of the thread whenever the browser leaves the web page containing the applet, and resume execution upon return. This is done in the stop() and start() functions, respectively. Basically, the thread monitors the value of some variable, here called threadSuspended, that behaves like a flag. When the thread sees that it is supposed to suspend itself, it calls wait(), which blocks the thread and does not allow it to continue executing until the applet calls notify().

Strictly speaking, it is not necessary to suspend the thread at all, but failing to do so is somewhat irresponsible. The user's CPU could become bogged down with useless instructions to execute long after the browser has left the page containing the applet. Those tempted to forgo "proper" coding to make their applets simpler can use this as an example. When you run this applet, open the Java Console of your browser, and then leave the page the applet is on. Notice how messages continue to be printed out in the console, and the value of i continues to be displayed in the status bar, demonstrating that the thread is still running in the background.

For more information on threads, try here, here and here.