设计实现一个“用户注册登录”系统，实现用户登录、用户注册和查看用户信息等功能，其中用户登录需包含动态验证码功能，怎么写代码

好的，下面是一个简单的代码实现，仅供参考：

1. 用户类的实现：

public class User {
    private String username;
    private String password;
    private String email;
    private String phone;
    private Date registerTime;
    private Date lastLoginTime;

    public User(String username, String password, String email, String phone, Date registerTime, Date lastLoginTime) {
        this.username = username;
        this.password = password;
        this.email = email;
        this.phone = phone;
        this.registerTime = registerTime;
        this.lastLoginTime = lastLoginTime;
    }

    // getter and setter methods
}

2. 用户管理类的实现：

public class UserManager {
    private List<User> userList;
    private Map<String, String> captchaMap;

    public UserManager() {
        userList = new ArrayList<>();
        captchaMap = new HashMap<>();
    }

    public void register(String username, String password, String email, String phone) {
        // check if username already exists
        for (User user : userList) {
            if (user.getUsername().equals(username)) {
                throw new RuntimeException("Username already exists.");
            }
        }

        // generate register time
        Date registerTime = new Date();

        // create new user
        User newUser = new User(username, password, email, phone, registerTime, null);
        userList.add(newUser);
    }

    public void login(String username, String password, String captcha) {
        // check if username and password are correct
        User user = null;
        for (User u : userList) {
            if (u.getUsername().equals(username) && u.getPassword().equals(password)) {
                user = u;
                break;
            }
        }
        if (user == null) {
            throw new RuntimeException("Invalid username or password.");
        }

        // check captcha
        String savedCaptcha = captchaMap.get(username);
        if (savedCaptcha == null || !savedCaptcha.equals(captcha)) {
            throw new RuntimeException("Invalid captcha.");
        }

        // update last login time
        user.setLastLoginTime(new Date());
    }

    public User getUser(String username) {
        for (User user : userList) {
            if (user.getUsername().equals(username)) {
                return user;
            }
        }
        return null;
    }

    public void sendCaptcha(String username, String email) {
        // generate captcha
        Random random = new Random();
        String captcha = String.format("%06d", random.nextInt(1000000));
        captchaMap.put(username, captcha);

        // send email
        // ...

        System.out.println("Captcha sent to email: " + email);
    }
}

3. 用户界面的实现：

public class UserInterface {
    private UserManager userManager;
    private Scanner scanner;

    public UserInterface(UserManager userManager) {
        this.userManager = userManager;
        scanner = new Scanner(System.in);
    }

    public void run() {
        while (true) {
            System.out.println("1. Register");
            System.out.println("2. Login");
            System.out.println("3. View user info");
            System.out.println("4. Exit");
            System.out.print("Enter your choice: ");
            int choice = scanner.nextInt();
            scanner.nextLine();

            switch (choice) {
                case 1:
                    register();
                    break;
                case 2:
                    login();
                    break;
                case 3:
                    viewUserInfo();
                    break;
                case 4:
                    return;
                default:
                    System.out.println("Invalid choice.");
            }
        }
    }

    private void register() {
        System.out.print("Enter username: ");
        String username = scanner.nextLine();

        System.out.print("Enter password: ");
        String password = scanner.nextLine();

        System.out.print("Enter email: ");
        String email = scanner.nextLine();

        System.out.print("Enter phone: ");
        String phone = scanner.nextLine();

        try {
            userManager.register(username, password, email, phone);
            System.out.println("Registration successful.");
        } catch (Exception e) {
            System.out.println("Registration failed: " + e.getMessage());
        }
    }

    private void login() {
        System.out.print("Enter username: ");
        String username = scanner.nextLine();

        System.out.print("Enter password: ");
        String password = scanner.nextLine();

        System.out.print("Enter captcha: ");
        String captcha = scanner.nextLine();

        try {
            userManager.login(username, password, captcha);
            System.out.println("Login successful.");
        } catch (Exception e) {
            System.out.println("Login failed: " + e.getMessage());
        }
    }

    private void viewUserInfo() {
        System.out.print("Enter username: ");
        String username = scanner.nextLine();

        User user = userManager.getUser(username);
        if (user == null) {
            System.out.println("User not found.");
        } else {
            System.out.println("Username: " + user.getUsername());
            System.out.println("Email: " + user.getEmail());
            System.out.println("Phone: " + user.getPhone());
            System.out.println("Register time: " + user.getRegisterTime());
            System.out.println("Last login time: " + user.getLastLoginTime());
        }
    }

    private void sendCaptcha() {
        System.out.print("Enter username: ");
        String username = scanner.nextLine();

        User user = userManager.getUser(username);
        if (user == null) {
            System.out.println("User not found.");
            return;
        }

        userManager.sendCaptcha(username, user.getEmail());
    }
}

注意，在上述代码中，动态验证码的实现部分没有完整实现，需要根据实际情况进行修改。同时，还需要实现数据库的设计和连接，这部分代码也没有包含在内。