Java 1.2 How-To/JavaHowTo.iso/javafile/ch09/Bounce.java

Java Source | 1998-12-14 | 4.7 KB | 245 lines

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.awt.image.ImageObserver;

/*
 * a class describing a single ball
 */
class Ball {

/*
 * the image for this ball
 */
Image img;

/*
 * x position and velocity
 */
double x, dx;    // x position and velocity

/*
 * y position and velocity
 */
double y, dy;    // y position and velocity

/*
 * initialize the position and velocity
 * to random values
 */
void random () {
    x = 10 + 380*Math.random ();
    y = 10 + 200*Math.random ();
    dx = 5 - 10*Math.random ();
    dy = 5 - 10*Math.random ();
}

/**
 * calculate the next position of this ball
 * and make sure it bounces off the edge of the panel
 * @param d - dimension of the bounding panel
 */
void compute (Dimension d) {
    if (x <= 0 || x > d.width) dx = -dx;    // bounce horizontal
    if (y <= 0 || y > d.height) dy = -dy;    // bounce vertical
    x += dx;
    y += dy;
}

/**
 * draw the ball image
 * @param g - destination graphics object
 * @param obs - parent image observer
 */
public void paint (Graphics g, ImageObserver obs) {

    g.drawImage (img, (int) x-10, (int) y-10, obs);
}
}

/*
 * the panel containing the bouncing balls
 */
class BouncePanel extends Panel implements Runnable {

/*
 * the number of balls
 */
final int nballs = 4;

/*
 * the array holding all the balls
 */
Ball balls[] = new Ball[10];

/*
 * offscreen image
 */
Image offimg;

/*
 * size of offscreen image
 */
Dimension offsize;

/*
 * graphics object associated with offscreen image
 */
Graphics offg;

/*
 * thread for periodic updating
 */
Thread thread;

/*
 * The thread recalculates each ball position and
 * redraws them
 */
public void run() {

    offsize = getSize();
    offimg = createImage (offsize.width, offsize.height);
    offg = offimg.getGraphics();
    while (true) {
        for (int i=0; i<nballs; i+=1) {
            balls[i].compute (offsize);
        }
        repaint ();
        try {
            Thread.sleep (25);
        } catch (InterruptedException e) {
            break;
        }
    }
}

/**
 * Override update to avoid erase flicker
 * @param g - destination graphics object
 */
public synchronized void update (Graphics g) {

    offg.setColor (Color.lightGray);
    offg.fillRect (0, 0, offsize.width, offsize.height);

    for (int i = 0 ; i < nballs ; i++)
        balls[i].paint (offg, this);
    offg.setColor (Color.black);
    offg.drawRect (0, 0, offsize.width-1, offsize.height-1);
    g.drawImage(offimg, 0, 0, this);
}

/*
 * Start the update thread
 */
public void start() {

    thread = new Thread(this);
    thread.start();
}

/*
 * Stop the update thread
 */
public void stop()
{

   if (thread != null)
     thread = null;
}

}

/*
 * the applet proper
 */
public class Bounce extends Applet implements ActionListener
{

/*
 * instance of BouncePanel
 */
BouncePanel panel;

/*
 * an array containing the images for the balls
 */
Image img[] = new Image[4];

/*
 * the audio clip to be played in a loop
 */
AudioClip sound;
Button btnStart;
Button btnStop;
/*
 * Called when the applet is loaded
 * Create an instance of bounce panel and add the start button
 * and load images
 */
public void init() {

    setLayout(new BorderLayout());

    panel = new BouncePanel ();
    add ("Center", panel);
    Panel p = new Panel ();
    add ("South", p);
    btnStart = new Button("Start");
    p.add (btnStart);
    btnStop = new Button("Stop");
    p.add (btnStop);
    btnStart.addActionListener(this);
    btnStop.addActionListener(this);

    sound = getAudioClip(getCodeBase(),  "sound.au");

    img[0] = getImage (getDocumentBase(), "whiteball.gif");
    img[1] = getImage (getDocumentBase(), "redball.gif");
    img[2] = getImage (getDocumentBase(), "blueball.gif");
    img[3] = getImage (getDocumentBase(), "greenball.gif");
    for (int i=0; i<panel.nballs; i+=1) {
        panel.balls[i] = new Ball ();
        panel.balls[i].img = img[i & 3];
    }
}

/*
 * Called when the applet is started
 * Don't do anything
 */
public void start () {

}

/*
 * Called when the applet is stopped
 */
public void stop() {

    panel.stop();
    sound.stop ();
}

/*
 * Handle start button press by randomizing balls
 */
public void actionPerformed(ActionEvent ev)
{
    Object object1 = ev.getSource();
    if (object1 == btnStart)
    {
        for (int i=0; i<panel.nballs; i+=1)
            panel.balls[i].random ();
        panel.start ();
        sound.loop ();
    }
    if (object1 == btnStop)
    {
        sound.stop ();
    }
}

}