Jpanel 上画图的问题

Question

我想在一JPanel上画出一实时显示当前时间的字符串，用一线程画字符串，
代码如下：
   class SimplePanel1 extends JPanel
{
SimpleDateFormat formatter;
java.util.Date currentDate;
String lastdate;
SimplePanel1()
{
    super();
    formatter=new SimpleDateFormat("EEE MMM dd hh:mm:ss yyyy",Locale.getDefault());       
}

class DrawRunner implements Runnable
{
Graphics g;
DrawRunner(Graphics g)
{
this.g=g;
}
public void run()
{
 while(true)
{
          currentDate =new java.util.Date();
 lastdate=formatter.format(currentDate);
          g.drawString(lastdate,5,20);          
          currentDate=null;
          try
          {
           Thread.sleep(300);
          } 
          catch(InterruptedException e){}
}
}
}public void paintComponent(Graphics g)
  {
    super.paintComponent(g);
     DrawRunner drawrunner=new DrawRunner(g);
        Thread DrawThread=new Thread(drawrunner);
        DrawThread.start();  
  }
}为什么Jpanel上的字符不会自动刷新，而且有时切换界面后字符串会消失呢？
请各位帮忙看看，是否还有其他方法实现？谢谢

Answer 1

import java.awt.Graphics;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;import javax.swing.JPanel;class SimplePanel1 extends JPanel {
SimpleDateFormat formatter; SimplePanel1() {
super();
formatter = new SimpleDateFormat("EEE MMM dd hh:mm:ss yyyy", Locale.getDefault()); DrawRunner drawrunner = new DrawRunner();
Thread DrawThread = new Thread(drawrunner);
DrawThread.start();
} class DrawRunner implements Runnable {
public void run() {
while (true) {
try {
Thread.sleep(300);
repaint();
} catch (InterruptedException e) {
}
}
}
} public void paintComponent(Graphics g) {
super.paintComponent(g);
Date currentDate = new java.util.Date();
String dateString = formatter.format(currentDate);
g.drawString(dateString, 5, 20);
}
}