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

public class JPasswordFieldDemo extends JFrame
{
    JTextField userName;
    JPasswordField passWord;
    JButton logonButton;
    JButton cancelButton;
    
    public JPasswordFieldDemo()
    {
        super("JPasswordField演示");
        Container container=getContentPane();
        container.setLayout(new GridLayout(3,2,2,2));
        
        userName=new JTextField(16);
        passWord=new JPasswordField(16);
        logonButton=new JButton("登录");
        logonButton.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent evt)
            {
                char[] pw=passWord.getPassword();
                String message="您的用户名:"+userName.getText()+"\n你的密码:"+new String(pw);
                JOptionPane.showMessageDialog(JPasswordFieldDemo.this,  message);
            }    
        });
        cancelButton=new JButton("退出");
        cancelButton.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent evt)
            {
                System.exit(0);
            }
        });
        
        container.add(new JLabel("   用户名:"));
        container.add(userName);
        container.add(new JLabel("   密码:"));
        container.add(passWord);
        container.add(logonButton);
        container.add(cancelButton);
        setResizable(false);
        setSize(300,120);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    public static void main(String[] args)
    {
        new JPasswordFieldDemo();
    }
}

中的JPasswordFieldDemo.this 指的是什么啊

指当前的容器吧

可以用一个声明一个Container，可以把JPasswordFieldDemo.this替换成container

showMessageDialog（JFrame j,String message）
一定有这个方法
JPasswordFieldDemo.this 就是当前Swing容器

详见core jave 第1卷 swing 里有说明， 具体那节记不住了。

this 指的是当前的object

JOptionPane.showMessageDialog(JPasswordFieldDemo.this,  message);

这里如果只用 this ， this 指的是 logonButton
而 用JPasswordFieldDemo.this  ， 则指的是外层的  JPasswordFieldDemo 的一个实体