public void setCinema(Cinema cinema) {
    if (cinema == null) {
      throw new IllegalArgumentException("Cinema object should not be set NULL.");
    }
    if (this.cinema != null) {
      this.cinema.removeCashOffice(this);
    }
    if ( !cinema.existCashOffice(this)) {
      cinema.addCashOffice(this);
    }
    this.cinema = cinema;
  }