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

public class SingleThreadApplet extends Applet {

  Button createButton = new Button("new thread");
  Button startButton = new Button("start thread");
  Button stopButton = new Button("stop thread");
  Box box = new Box(40, 40, 300, 300, Color.black);
  Ball ball = null;

  public void init () {
    add(createButton);
    add(startButton);
    add(stopButton);
  }

  public void stop() {
    if (ball != null) {
      ball.stop();
      ball = null;
    }
  }

  public boolean action (Event evt, Object arg) {
    if (evt.target == createButton) {
      if (ball == null ) // if there is NO thread
	ball = new Ball(this, 100, 100, 4, 7, box, Color.red); // create the thread
    }
    else if (evt.target == startButton) {
      if (ball != null && !ball.isAlive()) // if there is a thread and it is NOT running
	ball.start(); // start the thread
    }
    else if (evt.target == stopButton) {
      if (ball != null ) { // if there is a thread
	ball.stop(); // stop the thread
      ball = null;
      }
    }
    repaint();
    return true;
  }	

  // double buffering of the graphics

  Dimension offDimension;
  Image offImage;
  Graphics offGraphics;
  
  public void update(Graphics g) {
    Dimension d = size();
    
    // Create the offscreen graphics context
    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);
    
    // draw the shapes into the image
    if (ball != null)
      ball.draw(offGraphics);
    box.draw(offGraphics);
    // Paint the image onto the screen
    g.drawImage(offImage, 0, 0, null);
  }
 
  public void paint(Graphics g) {
    update(g);
  }
}