// 程序：重載update()的可能問題
// 范例文件：OnlyChangeUpdate.java

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

public class OnlyChangeUpdate extends Applet implements Runnable
{
   int    X,Y,moveX,moveY,width,height;  
   Thread newThread;       

   public void init()
   {
      X      = 0;         
      Y      = 0;           
      moveX  = 40;           
      moveY  = 40;              
      width  = getSize().width;    
      height = getSize().height;   

      setBackground(Color.black);    
   }

   public void start()             
   {
      newThread = new Thread(this);  
      newThread.start();
   }

   public void stop()               
   {
      newThread = null;               
   }

   public void paint(Graphics g)
   {
      g.setColor(Color.white);       
      g.fillOval(X,Y,30,30);         
   }

   public void update(Graphics g)   
   {
      paint(g);                    
   }

   public void run()
   {
      while(newThread != null)     
      {
         repaint();               

         try
         {
            Thread.sleep(80);         
         }
         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;
         } 
      }
   }
}