// 程序：使用雙緩沖區(qū)
// 范例文件：UseDoubleBuffering.java

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;   

   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.black);
      drawOffScreen.fillRect(0,0,width,height);

      drawOffScreen.setColor(Color.white);
      drawOffScreen.fillOval(X,Y,30,30);  

      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 - 30))
         {
            X     = width - 30;
            moveX = -moveX;
         }
         if(X <= 0)
         {
            X = 0;
            moveX = -moveX;
         } 
         if(Y >= (height - 30))
         {
            Y     = height - 30;
            moveY = -moveY;
         }
         if(Y <= 0)
         {
            Y = 0;
            moveY = -moveY;
         } 
      }
   }
}