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

// This appears in Core Web Programming from
// Prentice Hall Publishers, and may be freely used
// or adapted. 1997 Marty Hall, hall@apl.jhu.edu.

/** Draw random circles in a background thread.
 *  Needs an external event to start/stop drawing.
 */

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() {
    drawCircles = true;
    Color[] colors = { Color.lightGray, Color.gray,
                       Color.darkGray, Color.black };
    int colorIndex = 0;
    Graphics g = getGraphics();
    while(drawCircles) {
      int x =
        (int)Math.round(size().width * Math.random());
      int y =
        (int)Math.round(size().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) {}
  }
}