System.java

import java.util.Collections;

public class System {

  private Set<User> users;

  public Set<User> getUsers() {
      return Collections.unmodifiableSet(this.users);
  }

  public boolean addUser(String login, String password) {
      return this.users.add(new User(login, password));
  }

  public boolean removeUser(User user) {
      return this.users.remove(user);
  }

  ...

}