import java.awt.*;

/**
 * <strong>OffscreenImageExample</strong> -- creating and drawing
 * to an offscreen image and graphics object.
 *
 * <p>
 * The image is created once off screen and then rendered repeatedly
 * onto random locations in the applet.
 */
public class OffscreenImageExample extends java.applet.Applet
				   implements Runnable{

    Image offscreenImage;
    Thread paintThread;

    /**
     * Creates and draws to an offscreen image (a series of
     * green squares on top of a black background.
     */
    public void init() {
	Dimension dim = size();
	int offscreenSize = Math.min(dim.width / 2,
				     dim.height / 2);
	offscreenImage = createImage(offscreenSize,
				     offscreenSize);
	Graphics offscreenGraphics = offscreenImage.getGraphics();

	offscreenGraphics.setColor(Color.black);
	offscreenGraphics.fillRect(0, 0,
				   offscreenSize, offscreenSize);
	offscreenGraphics.setColor(Color.green);
	for (int i = 1; i < offscreenSize; i += 6) {
	    int x = (offscreenSize - i) / 3;
	    int y = (offscreenSize - i) / 3;
	    offscreenGraphics.drawRect(x, y, i, i);
	}
    }

    /** Starts the animation thread when the applet starts. */
    public void start() {
	paintThread = new Thread(this);
	paintThread.start();
    }

    /** Stops the animation thread when the applet stops. */
    public void stop() {
	paintThread.stop();
	paintThread = null;
    }

    /**
     * Runs the repaint() loop with some sleeping time between
     * updates.
     */
    public void run() {
	while (true) {
	    try {
		Thread.sleep(500);
	    } catch (InterruptedException e) {
		System.out.println("Interrupted during sleep.");
	    }
	    repaint();
	}
    }

    /**
     * Renders the offscreen image onto the applet at random
     * points in the upper left corner of the applet.
     */
    public void paint(Graphics g) {
	Dimension dim = size();
	g.setColor(Color.blue);
	g.drawRect(0, 0, dim.width - 1, dim.height - 1);
	int x = (int) (Math.random() * dim.width / 2);
	int y = (int) (Math.random() * dim.height / 2);
	g.drawImage(offscreenImage, x, y, this);
    }

}