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

/** Draw random circles in a background thread.
 *  Needs an external event to start/stop drawing.
 *
 *  Taken from Core Web Programming from 
 *  Prentice Hall and Sun Microsystems Press,
 *  http://www.corewebprogramming.com/.
 *  © 2001 Marty Hall and Larry Brown;
 *  may be freely used or adapte 
 */

public class RandomCircles extends Applet
                           implements Runnable {
  private boolean drawCircles = false;

  public void init() {
    setBackground(Color.white);
  }

  public void startCircles() {
    Thread t = new Thread(this);
    t.start();
  }

  public void run() {
    Color[] colors = { Color.lightGray, Color.gray,
                       Color.darkGray, Color.black };
    int colorIndex = 0;
    int x, y;
    int width = getSize().width;
    int height = getSize().height;

    Graphics g = getGraphics();
    drawCircles = true;

    while(drawCircles) {
      x = (int)Math.round(width * Math.random());
      y = (int)Math.round(height * Math.random());
      g.setColor(colors[colorIndex]);
      colorIndex = (colorIndex + 1) % colors.length;
      g.fillOval(x, y, 10, 10);
      pause(0.1);
    }
  }

  public void stopCircles() {
    drawCircles = false;
  }

  private void pause(double seconds) {
    try {
      Thread.sleep((int)(Math.round(seconds * 1000.0)));
    } catch(InterruptedException ie) {}
  }
}