Authentication system

import java.util.HashMap;
import java.util.Map;

public class AuthenticationSystem {

    private Map<String, User> users;

    public AuthenticationSystem() {
        this.users = new HashMap<>();
    }

    public void registerUser(User user) {
        this.users.put(user.getUsername(), user);
    }

    public User login(String username, String password) {
        User user = this.users.get(username);
        if (user != null && user.isAuthenticated(password)) {
            return user;
        } else {
            return null;
        }
    }

    public static class User {

        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;
        }

        public boolean isAuthenticated(String password) {
            return this.password.equals(password);
        }
    }
}