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res\password.txt中

好的，您需要在工程中创建一个名为 "res" 的文件夹，并将 "password.txt" 文件放入该文件夹中。然后，您可以使用以下代码实现用户注册和登录功能：

import java.io.*;

public class UserAuthentication {
    private static final String FILE_PATH = "res/password.txt";

    public static void main(String[] args) {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Welcome to User Authentication System.");

        // Register
        System.out.println("\n-----Registration-----");
        System.out.print("Enter your username: ");
        String username = null;
        try {
            username = reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }

        System.out.print("Enter your password: ");
        String password = null;
        try {
            password = reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }

        if (registerUser(username, password)) {
            System.out.println("Registration successful.");
        } else {
            System.out.println("Registration failed. User already exists.");
        }

        // Login
        System.out.println("\n-----Login-----");
        System.out.print("Enter your username: ");
        username = null;
        try {
            username = reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }

        System.out.print("Enter your password: ");
        password = null;
        try {
            password = reader.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }

        if (loginUser(username, password)) {
            System.out.println("Login successful.");
        } else {
            System.out.println("Login failed. Invalid username or password.");
        }
    }

    private static boolean registerUser(String username, String password) {
        try {
            // Check if user already exists
            if (userExists(username)) {
                return false;
            }

            // Add new user
            BufferedWriter writer = new BufferedWriter(new FileWriter(FILE_PATH, true));
            writer.write(username + ":" + password);
            writer.newLine();
            writer.close();
            return true;
        } catch (IOException e) {
            e.printStackTrace();
            return false;
        }
    }

    private static boolean loginUser(String username, String password) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader(FILE_PATH));
            String line = null;
            while ((line = reader.readLine()) != null) {
                String[] parts = line.split(":");
                if (parts[0].equals(username) && parts[1].equals(password)) {
                    reader.close();
                    return true;
                }
            }
            reader.close();
            return false;
        } catch (IOException e) {
            e.printStackTrace();
            return false;
        }
    }

    private static boolean userExists(String username) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader(FILE_PATH));
            String line = null;
            while ((line = reader.readLine()) != null) {
                String[] parts = line.split(":");
                if (parts[0].equals(username)) {
                    reader.close();
                    return true;
                }
            }
            reader.close();
            return false;
        } catch (IOException e) {
            e.printStackTrace();
            return false;
        }
    }
}

您可以运行该代码，按照提示输入用户名和密码进行注册和登录。该程序会将用户信息保存在 "res/password.txt" 文件中，并在登录时读取该文件进行验证。