Membuat Obyek Bola Bergerak (Materi Applet Java)

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

public class BolaBergerak extends Applet implements Runnable{
 private Thread runner ;
 private int x=0, y=0 ;

 public void init() {
 }

 public void start(){
  if (runner==null){
   runner = new Thread(this);
   runner.start();
  }
 }

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

 public void run(){
 
  while(true){
   try{
    runner.sleep(250);
   }catch(Exception e){}
   if (x<=400)
    x = x+10 ;
   else
    x = 0 ;
   if (y<=400) {
    y = y + 10 ;
   }
   else
    y = 0 ;
   repaint();
  }
 }

 public void paint(Graphics g){
  g.setColor(Color.white);
  g.fillRect(0, 0, 400, 400) ;
  g.setColor(Color.red);
  g.fillOval(x,y,50,50);
 }
}