自己编的时钟程序，为什么不能用按钮停止时间呢？代码如下
import java.util.*;
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
public class DDate extends Applet implements ActionListener,Runnable{
    Button b1=new Button("显示时间");
    Button b2=new Button("停止时间");
    Label l=new Label("显示当前时间");
    Thread t;
    boolean flag=true;
    public void init(){
        setLayout(new BorderLayout());
        add(b1,BorderLayout.NORTH);
        add(b2,BorderLayout.SOUTH);
        add(l,BorderLayout.CENTER);
        b1.addActionListener(this);
        b2.addActionListener(this);
        
    }
    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("显示时间")) {   
            while(flag=true){
                Date now=new Date();
                l.setText("现在是北京时间："+now.getHours()+"时"+now.getMinutes()+"分"+now.getSeconds()+"秒");
                try{Thread.sleep(1000);}catch(InterruptedException f){}
            }
    }
        if (e.getActionCommand().equals("停止时间")) {   
            t.stop();
        }
}
    public void start(){        
        t=new Thread(this);
        t.start();
    }
    public void run(){
        while(true){
        Date now=new Date();
        System.out.println("现在是北京时间："+now.getHours()+"时"+now.getMinutes()+"分"+now.getSeconds()+"秒");
        try{Thread.sleep(1000);}catch(InterruptedException e){}
        }
    }
}