JAVA为什么暂停后运行不了啊QAQ

Question

import java.awt.FlowLayout; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.text.DateFormat; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; import javax.swing.*; public class Demo5 extends JFrame implements Runnable, ActionListener { JButton jb1, jb2, jb3; JLabel jl; Thread thread = null; boolean flag;     Calendar i=Calendar.getInstance(); int nian=i.get(Calendar.YEAR); int yue=i.get(Calendar.MONTH); int tian=i.get(Calendar.DAY_OF_MONTH); int shi=i.get(Calendar.HOUR_OF_DAY); int feng=i.get(Calendar.MINUTE); int miao=i.get(Calendar.SECOND);    Demo5() { jb1 = new JButton("开始"); jb2 = new JButton("停止"); jb3 = new JButton("切换"); jl = new JLabel(""); } public void init() { this.setTitle("计时器"); this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); this.setVisible(true); this.setLocation(500, 400); this.setSize(500, 300); this.setLayout(new FlowLayout()); this.add(jl); this.add(jb1); this.add(jb2); this.add(jb3); jb1.addActionListener(this); jb2.addActionListener(this); jb3.addActionListener(this); } public static void main(String[] args) { new Demo5().init(); } @Override public void run() { int i=miao; int k=feng; int y=shi; while (flag) { jl.setText(nian+"-"+(yue+1)+"-"+tian+" "+y+":"+k+":"+i); if(i>=59){ k+=1; i=-1; } if(k>=59){ y+=1; k=0; } try { Thread.currentThread().sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } if(flag) { i++; } } } @Override public void actionPerformed(ActionEvent e) { if (e.getSource().equals(jb1)) { if (thread == null) { thread = new Thread(this); thread.start(); } flag=true; } else if (e.getSource().equals(jb2)) { flag=false; } } }