public void addCashOffice(CashOffice cashOffice) {
    if (cashOffice == null) {
      throw new NullObjectInsertionException("Null CashOffice cannot be added");
    }

    if ( !existCashOffice(cashOffice) && !hasCashOfficeWithSameNumber(cashOffice)) {
      cashOffices.add(cashOffice);
      cashOffice.setCinema(this);
    }
  }