MovingImage.java


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

/* 
 * This applet moves one image in front of a background image.  It
 * eliminates flashing by double buffering.
 */

public class MovingImage extends Applet implements Runnable {
    int frameNumber;
    int delay;
    Thread animatorThread;

    Dimension offDimension;
    Image offImage;
    Graphics offGraphics;

    Image stars;
    Image pizza;

    public void init() {
        String str;

        // How many milliseconds between frames?
        str = getParameter("fps");
        int fps = (str != null) ? Integer.parseInt(str) : 10;
        delay = (fps > 0) ? (1000 / fps) : 100;

        stars = getImage(getDocumentBase(), "images/starfield.gif");
        pizza = getImage(getDocumentBase(), "images/pizza.gif");
    }

    public void start() {
        if (animatorThread == null) {
            animatorThread = new Thread(this);
            animatorThread.start();
        }
    }

    public void stop() {
        animatorThread = null;
        offImage = null;
        offGraphics = null;
    }

    public boolean mouseDown(Event e, int x, int y) {
        if (animatorThread == null) {
            start();
        }
        else {
            animatorThread = null;
        }
        return false;
    }

    public void run() {
        // Remember the starting time
        long startTime = System.currentTimeMillis();

        while (Thread.currentThread() == animatorThread) {
            // Display the next frame of animation.
            repaint();

            // Delay depending on how far we are behind.
            try {
                startTime += delay;
                Thread.sleep(Math.max(0, 
                                      startTime-System.currentTimeMillis()));
            } catch (InterruptedException e) {
                break;
            }

            frameNumber++;
        }
    }

    // Paint the previous frame (if any).
    public void paint(Graphics g) {
        if (offImage != null) {
            g.drawImage(offImage, 0, 0, this);
        }
    }

    public void update(Graphics g) {
        Dimension d = size();

        // Create the offscreen graphics context, if no good one exists.
        if ( (offGraphics == null)
          || (d.width != offDimension.width)
          || (d.height != offDimension.height) ) {
            offDimension = d;
            offImage = createImage(d.width, d.height);
            offGraphics = offImage.getGraphics();
        }

        // Erase the previous image.
        offGraphics.setColor(getBackground());
        offGraphics.fillRect(0, 0, d.width, d.height);
        offGraphics.setColor(Color.black);

        //Paint the frame into the image.
        paintFrame(offGraphics);

        // Paint the image onto the screen.
        g.drawImage(offImage, 0, 0, this);
    }

    void paintFrame(Graphics g) {
        Dimension d = size();
        int w = stars.getWidth(this);
        int h = stars.getHeight(this);

        // If we've loaded any of the background image, draw it.
        if ((w > 0) && (h > 0)) { 
            g.drawImage(stars, (d.width - w)/2, (d.height - h)/2, this);
        }

        w = pizza.getWidth(this);
        h = pizza.getHeight(this);

        // If we've loaded any of the moving image, draw it.
        if ((w > 0) && (h > 0)) { 
            g.drawImage(pizza, ((frameNumber*5) % (w + d.width)) - w,
                        (d.height - h)/2, this);
        }
    }
}