BouncingCircle.java Source code

Introduction

Here is the source code for BouncingCircle.java

Source

/*
 * 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;
    }
}