PrivacyInvander.java

public class PrivacyInvader {

  private System system;
  
  public boolean addUser(String login, String password) {
      return system.getUsers().add(new User(login, password));
  }
  
  public boolean removeUser(User user) {
      return system.getUsers().remove(user);
  }
  
  ...
  
}