Java tutorial
/* * Copyright (c) 2000 David Flanagan. All rights reserved. * This code is from the book Java Examples in a Nutshell, 2nd Edition. * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied. * You may study, use, and modify it for any non-commercial purpose. * You may distribute it non-commercially as long as you retain this notice. * For a commercial use license, or to purchase the book (recommended), * visit http://www.davidflanagan.com/javaexamples2. */ import java.applet.Applet; import java.awt.Color; import java.awt.Graphics; import java.awt.Rectangle; /** An applet that displays a simple animation */ public class BouncingCircle extends Applet implements Runnable { int x = 150, y = 50, r = 50; // Position and radius of the circle int dx = 11, dy = 7; // Trajectory of circle Thread animator; // The thread that performs the animation volatile boolean pleaseStop; // A flag to ask the thread to stop /** This method simply draws the circle at its current position */ public void paint(Graphics g) { g.setColor(Color.red); g.fillOval(x - r, y - r, r * 2, r * 2); } /** * This method moves (and bounces) the circle and then requests a redraw. * The animator thread calls this method periodically. */ public void animate() { // Bounce if we've hit an edge. Rectangle bounds = getBounds(); if ((x - r + dx < 0) || (x + r + dx > bounds.width)) dx = -dx; if ((y - r + dy < 0) || (y + r + dy > bounds.height)) dy = -dy; // Move the circle. x += dx; y += dy; // Ask the browser to call our paint() method to draw the circle // at its new position. repaint(); } /** * This method is from the Runnable interface. It is the body of the thread * that performs the animation. The thread itself is created and started in * the start() method. */ public void run() { while (!pleaseStop) { // Loop until we're asked to stop animate(); // Update and request redraw try { Thread.sleep(100); } // Wait 100 milliseconds catch (InterruptedException e) { } // Ignore interruptions } } /** Start animating when the browser starts the applet */ public void start() { animator = new Thread(this); // Create a thread pleaseStop = false; // Don't ask it to stop now animator.start(); // Start the thread. // The thread that called start now returns to its caller. // Meanwhile, the new animator thread has called the run() method } /** Stop animating when the browser stops the applet */ public void stop() { // Set the flag that causes the run() method to end pleaseStop = true; } }