为什么我的这个程序运行时会出错？

Question

import java.awt.*;
import java.awt.event.*;
import java.applet.*;public class doublebuffer extends Applet implements Runnable
{
        Image memoryimage;
        Graphics memorygraphics;
        Thread doublebufferthread;
        boolean animateFlag = true;
        int i;
        //Initialize the applet
        public void init()
        {
                memoryimage = createImage(100,100);
                memorygraphics = memoryimage.getGraphics();
        }        public void start()
        {
                doublebufferthread = new Thread(this);
                doublebufferthread.start();
        }        public void stop()
        {
                animateFlag = false;
        }        public void run()
        {
                try
                {
                  while(true) {
                    Thread.sleep(200);
                    repaint();
                  }
                }
                catch(InterruptedException e)
                {                }
        }        public void paint(Graphics g)
        {
                i += 5;
                if(i>=100) i=5;                memorygraphics.setColor(new Color(255,255,255));
                memorygraphics.fillRect(0,0,100,100);
                memorygraphics.setColor(new Color(0,0,0));
                memorygraphics.drawRect(0,0,i,i);
                g.drawImage(memoryimage,10,10,this);
        }
}

Answer 1

怎么一样，我的代码里有while循环和sleep，我这里运行是正常的
改成如下：
import java.awt.*;
import java.awt.event.*;
import java.applet.*;public class doublebuffer extends Applet implements Runnable
{
        Image memoryimage=null;
        Graphics memorygraphics;
        Thread doublebufferthread;
        boolean animateFlag = true;
        int i;
        //Initialize the applet
        public void init()
        {
                memoryimage=createImage(100,100);
                memorygraphics = memoryimage.getGraphics();
        }        public void start()
        {
                doublebufferthread = new Thread(this);
                doublebufferthread.start();
        }        public void stop()
        {
                animateFlag = false;
        }        public void run()
        {
                try
                {
                  while(true) {
                    Thread.sleep(200);
                    repaint();
                  }
                }
                catch(InterruptedException e)
                {                }
        }        public void paint(Graphics g)
        {
                i += 5;
                if(i>=100) i=5;
                if(memorygraphics==null)
                  memorygraphics = memoryimage.getGraphics();
                memorygraphics.setColor(new Color(255,255,255));
                memorygraphics.fillRect(0,0,100,100);
                memorygraphics.setColor(new Color(0,0,0));
                memorygraphics.drawRect(0,0,i,i);
                g.drawImage(memoryimage,10,10,this);
        }
}