System.java

public class System {

  private Set<User> users;
  
  public void setUsers(Set<Usuario> users) {
      this.users = users;
  }
  
  public Set<Usuario> getUsers() {
      return 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);
  }
  
  ...
  
}