创建普通文本文件存储登录名、姓名、密码以及注册时间，实现用户的登录、注册功能。

public class Test {

    public static void main(String[] args) {
        UserSystem system = new UserSystem();
        while (true) {
            system.printChoice();
        }
    }

}

class UserSystem {
    Scanner scanner;

    public UserSystem() {
        scanner = new Scanner(System.in);
    }

    public void printChoice() {
        System.out.println("**********用户管理系统**********");
        System.out.println("*         1、用户登录          *");
        System.out.println("*         2、用户注册          *");
        System.out.println("*         3、退出系统          *");
        System.out.print("请选择：");
        switch (scanner.nextInt()) {
            case 1:
                if (login() == null) System.exit(0);
                break;
            case 2:
                register();
                break;
            case 3:
                System.exit(0);
                break;
            default:
                System.out.println("错误选项！");
        }
    }

    private User login() {
        int loginTimes = 0;
        String username, pwd;
        do {
            System.out.print("请输入用户名：");
            username = scanner.next();
            System.out.print("请输入密码：");
            pwd = scanner.next();
            UserManager.LoginResult loginResult = UserManager.canLogin(username, pwd);
            switch (loginResult.loginResult) {
                case UserManager.SUCCESS:
                    loginResult.user.printInfo();
                    return loginResult.user;
                case UserManager.ERROR_CODE_USERNAME:
                    System.out.println("登录失败！用户名不存在，请重写登录");
                    break;
                case UserManager.ERROR_CODE_PWD:
                    System.out.println("登录失败！密码不正确，请重写登录");
                    break;
            }
            loginTimes++;
        } while (loginTimes < 3);
        return null;
    }

    private void register() {
        String username, pwd, nickname;
        System.out.print("请输入用户名：");
        username = scanner.next();
        System.out.print("请输入密码：");
        pwd = scanner.next();
        System.out.print("请输入确认密码：");
        if (!pwd.equals(scanner.next())) {
            System.out.println("密码与确认密码不符！");
            return;
        }
        System.out.print("请输入姓名：");
        nickname = scanner.next();
        UserManager.LoginResult loginResult = UserManager.canLogin(username, pwd);
        if (loginResult.loginResult != UserManager.ERROR_CODE_USERNAME) {
            System.out.println("注册失败！该用户已存在，请重新填写！");
            return;
        }
        UserManager.saveToDb(username, pwd, nickname);
    }
}

class UserManager {
    private static final String DB_NAME = "d:/user.txt";
    public static final int SUCCESS = 1;
    public static final int ERROR_CODE_USERNAME = -1;
    public static final int ERROR_CODE_PWD = -2;

    public static LoginResult canLogin(String username, String pwd) {
        try {
            Scanner scanner = new Scanner(new File(DB_NAME));
            while (scanner.hasNextLine()) {
                String userRecordString = scanner.nextLine();
                if (userRecordString.equals("")) continue;
                User userRecord = new User(userRecordString);
                int result = userRecord.canLogin(username, pwd);
                if (result == SUCCESS) {
                    return new LoginResult(SUCCESS, userRecord);
                } else if (result == ERROR_CODE_PWD) {
                    return new LoginResult(ERROR_CODE_PWD);
                }
            }
        } catch (Exception e) {
//            System.out.println("登录失败！用户名不存在，请重新填写");
        }
        return new LoginResult(ERROR_CODE_USERNAME);
    }

    public static void saveToDb(String username, String pwd, String nickname) {
        File file = new File(DB_NAME);
        if (!file.getParentFile().exists()) {
            file.getParentFile().mkdirs();
        }
        try {
            FileWriter fileWriter = new FileWriter(file, true);
            fileWriter.append('\n');
            fileWriter.append(new User(username, pwd, nickname).getRecordString());
            fileWriter.flush();
            fileWriter.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static class LoginResult {
        public int loginResult;
        public User user;

        public LoginResult(int loginResult) {
            this.loginResult = loginResult;
        }

        public LoginResult(int loginResult, User user) {
            this.loginResult = loginResult;
            this.user = user;
        }
    }
}

class User {
    private String username;
    private String pwd;
    private String nickname;
    private String registerTime;

    public User(String username, String pwd, String nickname) {
        this.username = username;
        this.pwd = pwd;
        this.nickname = nickname;
        this.registerTime = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss").format(System.currentTimeMillis());
    }

    public User(String recordLine) {
        String[] userItem = recordLine.split(" ");
        username = userItem[0];
        pwd = userItem[1];
        nickname = userItem[2];
        registerTime = userItem[3] + " " + userItem[4];
    }

    public int canLogin(String username, String pwd) {
        if (!username.equals(this.username)) {
            return UserManager.ERROR_CODE_USERNAME;
        }
        if (!pwd.equals(this.pwd)) {
            return UserManager.ERROR_CODE_PWD;
        }
        return UserManager.SUCCESS;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }

    public void printInfo() {
        System.out.println(String.format("登录名：%s，姓名：%s，密码：%s，注册日期：%s", username, nickname, pwd, registerTime));
    }

    public String getRecordString() {
        return String.format("%s %s %s %s", username, pwd, nickname, registerTime);
    }
}

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