AuthenticationService.java

import java.util.*;

public class AuthenticationService {
    private List<User> users;

    public AuthenticationService() {
        users = new ArrayList<>();
    }

    public boolean isValidEmail(String email) {
        return email != null && email.matches("^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+$");
    }

    public boolean registerUser(User user) {
        if (!isValidEmail(user.getEmail())) {
            System.out.println("Invalid email: " + user.getEmail());
            return false;

        }

        for (User u : users) {
            if (u.getUsername().equals(user.getUsername())) {
                System.out.println("Username already exists: " + user.getUsername());
                return false;
            }
        }
        users.add(user);
        return true;
    }
    
    public User loginUser(String username, String password) {
        for (User u : users) {
            if (u.getUsername().equals(username) && u.getPassword().equals(password)) {
                return u;
            }
        }
        return null;
    }
}