求Java大神帮助啊！

package test;
import java.awt.*;
import java.applet.*;
public class UseDoubleBuffering extends Applet implements Runnable
{
int X,Y,moveX,moveY,width,height;
Thread newThread;
Image OffScreen;
Graphics drawOffScreen;
int ballRadius=20;
public void init()
{
X = 0;
Y = 0;
moveX = 2;
moveY = 3;
width = getSize().width;
height = getSize().height;
OffScreen = createImage(width,height);
drawOffScreen = OffScreen.getGraphics();
}
public void start()
{
newThread = new Thread(this);
newThread.start();
}
public void stop()
{
newThread = null;
}
public void paint(Graphics g)
{
drawOffScreen.setColor(Color.white);
drawOffScreen.fillRect(0,0,width,height);
drawOffScreen.setColor(Color.black);
drawOffScreen.fillOval(X,Y,ballRadius,ballRadius);
g.drawImage(OffScreen,0,0,this);
}
public void update(Graphics g)
{
paint(g);
}
public void run()
{
while(newThread != null)
{
repaint();
try
{
Thread.sleep(50);
}
catch(InterruptedException E){ }

X = X + moveX;
Y = Y + moveY;

if(X >= (width - ballRadius))
{
X = width - ballRadius;
moveX = -moveX;
}
if(X <= 0)
{
X = 0;
moveX = -moveX;
}
if(Y >= (height - ballRadius))
{
Y = height - ballRadius;
moveY = -moveY;
}
if(Y <= 0)
{
Y = 0;
moveY = -moveY;
}
}
}
}

抢个沙发，修改中，最近等offer，多做好事。。
这个本来就完成了啊,要修改成什么样的？