Double-Buffered Bouncing Ball Program

Source Code

import java.awt.Graphics;
import java.awt.Color;
import java.awt.Image;
import java.applet.Applet;

public class DBall extends Applet implements Runnable {
  Thread thr;
  Image buffer;
  Graphics GC;
  int x = 20, y = 300, r = 7;
  int vx = 5, vy = 0, a = 3;
  double d = 0.88;

  public void init() {
    buffer = createImage(400, 400);
    GC = buffer.getGraphics();
    GC.setColor(Color.white);
    GC.fillRect(0, 0, 400, 400);
    GC.setColor(Color.blue);
    GC.fillOval(50+x-r, 380-y+r, 2*r, 2*r);
  }
  public void start() {
    if (thr == null) {
      thr = new Thread(this);
      thr.start();
    }
  }
  public void run() {
    while(true) {
      repaint();
      try{
	Thread.sleep(50);
      }
      catch(InterruptedException e) { }
    }
  }
  void calc() {
    x += vx; y -= vy;
    vy += a;
    if (x > 300) {
      x = 600-x; vx *= -1;
    }
    else if (x < 0) {
      x *= -1; vx *= -1;
    }
    if (y < 0) {
      y = (int)(-(double)y * d); vy = (int) (-(double) vy * d);
    }
    else if (y == 0 && vy == 0) {
      x = 20; y = 300;
    }
  }
  public void paint(Graphics g) {
    update(g);
  }
  public void update(Graphics g) {
    g.drawImage(buffer, 0, 0, this);
    calc();
    GC.setColor(Color.white);
    GC.fillRect(0, 0, 400, 400);
    GC.setColor(Color.blue);
    GC.fillOval(50+x-r, 380-y+r, 2*r, 2*r);
  }
  public void stop() {
    if (thr != null) {
      thr.stop();
      thr = null;
    }
  }
}