Java Swing Gui第一个程序！

package hello;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;

class SimpleFrame extends JFrame implements ActionListener{
    JButton b1;
    JLabel l1;
    JLabel l2;
    public SimpleFrame(String title){
        setTitle(title);
        setSize(300,500);
        l1 = new JLabel("一个新的GUI应用程序的例子！",JLabel.CENTER);
        l2 = new JLabel();
        b1 = new JButton("时间");
        b1.setActionCommand("time");
        b1.addActionListener(this);
        this.getContentPane().add(l1,BorderLayout.NORTH);
        this.getContentPane().add(l2,BorderLayout.CENTER);
        this.getContentPane().add(b1,BorderLayout.SOUTH);
    }
    public void actionPerformed(ActionEvent e){
        Calendar c1 = Calendar.getInstance();
        if(e.getActionCommand().equals("time")){
            l2.setText(Calendar.HOUR_OF_DAY+"时"+c1.get(Calendar.MONTH)+"分");
        }
        else
            System.exit(0);
    }
}
public class Hello{
    public static void main(String args[])throws Exception{
        String title = new String("测试程序");
        SimpleFrame frame = new SimpleFrame(title);
        frame.setVisible(true);
    }
}