import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MainPane extends JFrame implements ActionListener
{  
    public static void main(String[] args)
    {
        JFrame f=new JFrame("登入窗口");
        JLabel l1=new JLabel("用户名");
        JLabel l2=new JLabel("密   码");
        JTextField jt=new JTextField(10);
        JPasswordField fp=new JPasswordField(10);
        

        JButton b1=new JButton("确定");
         b1.addActionListener(this);
        JButton b2=new JButton("取消");
    
        JButton b3=new JButton("about");
    
        Container contentPane=f.getContentPane();
        contentPane.setLayout(new GridLayout(3,1));
        JPanel p1=new JPanel();
        JPanel p2=new JPanel();
        JPanel p3=new JPanel();
        
        f.add(p1);
        f.add(p2);
        f.add(p3);
        
        p1.add(l1);
        p1.add(jt);
        p2.add(l2);
        p2.add(fp);
        
        p3.add(b1);
        p3.add(b2);
        p3.add(b3);
        
        
        f.setBounds(100,100,250,150);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
   
   public void actionPerformed(ActionEvent e)
        {
            if(e.getSource() == b1)
            {
                JOptionPane.showMessageDialog(null,"extends err","heh",JOptionPane.INFORMATION_MESSAGE);
            }
        }
}

这种编码风格太差了,在main方法里面做这些事情
并且你的类是JFrame的子类,可是又没见你用它来做什么