单一职责原则

登录模块在实际项目开发中很常见，请按照教材28页（PPT49页）利用单一职责原则重构后的类图实现这一模块。

import java.sql.Connection；
import java.sql.DriverManager；
import java.sql.PreparedStatement；
import java.sql.ResultSet；
import javax.swing.JFrame；
import javax.swing.JOptionPane；
// MainClass is responsible for starting the system.public class MainClass {    

public static void main(String[] args) {        

JFrame frame = new JFrame("Login Form")；
        LoginForm loginForm = new LoginForm(frame)；
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)；
        frame.setSize(400, 200)；
        frame.setVisible(true)；
    }

}

// LoginForm is responsible for displaying the login form and handling events.

public class LoginForm extends JFrame {    

private UserDAO userDAO；
    public LoginForm(JFrame frame) {        

super("Login Form")；
        userDAO = new UserDAO()；
        // Add UI components and event handlers here

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)；
        setSize(400, 200)；
        setVisible(true)；
    }

    // Event handler methods go here}

// UserDAO encapsulates all operations related to the users table.

public class UserDAO {    

private DBUtil dbUtil；
    public UserDAO() {        

dbUtil = new DBUtil()；
    }

    public boolean findUser(String userName, String userPassword) throws Exception {        Connection connection = dbUtil.getConnection()；

        PreparedStatement statement = connection.prepareStatement(            "SELECT * FROM Users WHERE username = ? AND password = ?")；
        statement.setString(1, userName)；
        statement.setString(2, userPassword)；

        ResultSet resultSet = statement.executeQuery()；
        return resultSet.next()；
    }

}

// DBUtil provides a connection to the database.

public class DBUtil {    

private final String URL = "jdbc:mysql://localhost:3306/mydatabase"；
    private final String USER = "root"；
    private final String PASSWORD = "20223959"；
    public Connection getConnection() throws Exception {        Class.forName("com.mysql.jdbc.Driver")；
        return DriverManager.getConnection(URL, USER, PASSWORD)；
    }

}