java使用对象输入/输出流完成用户注册登录界面，User类（用户名，密码）保存在工程文件夹下res\password.txt中 。（仿P204-例题10-12），使用消息框给出相关信息（自学JOptionPane的showMessageDialog方法的使用）

以下是一个使用对象输入/输出流完成用户注册登录界面的Java代码示例，你可以参考一下：

User.java

import java.io.Serializable;

public class User implements Serializable {
    private String username;
    private String password;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }
}

RegisterForm.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;

public class RegisterForm extends JFrame implements ActionListener {
    private JLabel lblUsername, lblPassword;
    private JTextField txtUsername, txtPassword;
    private JButton btnRegister;

    public RegisterForm() {
        setTitle("User Registration");
        setSize(300, 140);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        lblUsername = new JLabel("Username:");
        lblPassword = new JLabel("Password:");
        txtUsername = new JTextField(20);
        txtPassword = new JTextField(20);
        btnRegister = new JButton("Register");
        btnRegister.addActionListener(this);

        Container container = getContentPane();
        container.setLayout(new GridLayout(3, 2));
        container.add(lblUsername);
        container.add(txtUsername);
        container.add(lblPassword);
        container.add(txtPassword);
        container.add(btnRegister);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == btnRegister) {
            String username = txtUsername.getText();
            String password = txtPassword.getText();

            try {
                File file = new File("res/password.txt");
                if (!file.exists()) {
                    file.createNewFile();
                }

                FileInputStream fis = new FileInputStream(file);
                ObjectInputStream ois = new ObjectInputStream(fis);
                Object obj = ois.readObject();

                if (obj instanceof User) {
                    User user = (User) obj;
                    if (user.getUsername().equals(username)) {
                        JOptionPane.showMessageDialog(this, "Username already exists.");
                        return;
                    }
                }

                FileOutputStream fos = new FileOutputStream(file);
                ObjectOutputStream oos = new ObjectOutputStream(fos);
                oos.writeObject(new User(username, password));
                oos.close();

                JOptionPane.showMessageDialog(this, "User registered successfully.");
            } catch (IOException | ClassNotFoundException ex) {
                ex.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        RegisterForm registerForm = new RegisterForm();
        registerForm.setVisible(true);
    }
}

LoginForm.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;

public class LoginForm extends JFrame implements ActionListener {
    private JLabel lblUsername, lblPassword;
    private JTextField txtUsername, txtPassword;
    private JButton btnLogin;

    public LoginForm() {
        setTitle("User Login");
        setSize(300, 140);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        lblUsername = new JLabel("Username:");
        lblPassword = new JLabel("Password:");
        txtUsername = new JTextField(20);
        txtPassword = new JTextField(20);
        btnLogin = new JButton("Login");
        btnLogin.addActionListener(this);

        Container container = getContentPane();
        container.setLayout(new GridLayout(3, 2));
        container.add(lblUsername);
        container.add(txtUsername);
        container.add(lblPassword);
        container.add(txtPassword);
        container.add(btnLogin);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == btnLogin) {
            String username = txtUsername.getText();
            String password = txtPassword.getText();

            try {
                File file = new File("res/password.txt");
                if (!file.exists()) {
                    JOptionPane.showMessageDialog(this, "User does not exist.");
                    return;
                }

                FileInputStream fis = new FileInputStream(file);
                ObjectInputStream ois = new ObjectInputStream(fis);
                Object obj = ois.readObject();

                if (!(obj instanceof User)) {
                    JOptionPane.showMessageDialog(this, "User does not exist.");
                    return;
                }

                User user = (User) obj;
                if (user.getUsername().equals(username) && user.getPassword().equals(password)) {
                    JOptionPane.showMessageDialog(this, "Login successful.");
                } else {
                    JOptionPane.showMessageDialog(this, "Username or password is incorrect.");
                }
            } catch (IOException | ClassNotFoundException ex) {
                ex.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        LoginForm loginForm = new LoginForm();
        loginForm.setVisible(true);
    }
}

在这个示例中，我们使用了对象输入/输出流来将User对象序列化到文件中。RegisterForm类和LoginForm类分别代表用户注册和登录界面，通过JOptionPane的showMessageDialog方法来给出相关信息。注意，在读取文件时需要判断文件是否存在，并且在写入文件时需要先读取文件中的对象信息，判断用户是否已经存在。