simple LoginValidate, of no use

LoginValidate.java

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

public class LoginValidate extends JFrame {

	//db存放用户名和喜闻乐见的明文密码
	private HashMap<String, String> db;

	//GUI各种界面元素
	private JFrame mainFrame;
	private Container mainContentPane;
	private JTextArea resultTextArea;
	private JLabel usernameLabel;
	private JTextField usernameField;
	private JLabel passwordLabel;
	private JPasswordField passwordField;
	private JButton submitButton;

	//构造函数会建立用户数据库和GUI，并显示
	LoginValidate(){
		buildDB();
		buildUI();
	}

	//建立用户数据库
	private void buildDB(){
		//用HashMap存放用户名和密码的键值对
		db = new HashMap<String, String>();
		//添加两个用户做为测试
		db.put("admin", "nimda");
		db.put("user",  "password");
	}

	private void buildUI(){
		//主窗口
		mainFrame = new JFrame("Login");
		mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		//设置窗口大小
		mainFrame.setPreferredSize(new Dimension(400, 300));

		//使用FlowLayout
		mainContentPane = mainFrame.getContentPane();
		mainContentPane.setLayout(new FlowLayout());

		//显示结果的文本框
		resultTextArea = new JTextArea();
		resultTextArea.setPreferredSize(new Dimension(300, 60));
		resultTextArea.setEditable(false);
		mainContentPane.add(resultTextArea);

		//用户名标签
		usernameLabel = new JLabel("Username:");
		usernameLabel.setPreferredSize(new Dimension(300, 30));
		mainContentPane.add(usernameLabel);

		//输入用户名
		usernameField = new JTextField();
		usernameField.setPreferredSize(new Dimension(300, 30));
		mainContentPane.add(usernameField);

		//密码标签
		passwordLabel = new JLabel("Password:");
		passwordLabel.setPreferredSize(new Dimension(300, 30));
		mainContentPane.add(passwordLabel);

		//输入密码
		passwordField = new JPasswordField();
		passwordField.setPreferredSize(new Dimension(300, 30));
		mainContentPane.add(passwordField);

		//登录按钮
		//点击调用loginAction，loginAction验证登录
		submitButton = new JButton(loginAction);
		submitButton.setPreferredSize(new Dimension(300, 30));
		mainContentPane.add(submitButton);

		mainFrame.pack();
		//显示
		mainFrame.setVisible(true);
	}

	//登录动作
	Action loginAction = new AbstractAction("Login") {
		public void actionPerformed(ActionEvent e) {
			//获得各自输入
			String usernameInput = usernameField.getText();
			String passwordInput = passwordField.getText();
			//在db中查找
			String realPassword =  db.get(usernameInput);

			//设置结果的文本框
			String outputString = "Username: " + usernameInput + "\n";
			if (realPassword != null && realPassword.equals(passwordInput)) {
				outputString += "Success";
			} else {
				outputString += "Fail";
			}
			resultTextArea.setText(outputString);
		}
	};

	public static void main(String[] args) {
	 	LoginValidate app = new LoginValidate();
	}
}