public void addTicket(Ticket ticket) {
    if (ticket == null) {
      throw new NullObjectInsertionException("null ticket should not be added");
    }
    if (exists(ticket)) {
      throw new DuplicateInsertionException("This Ticket already added to CashOffice. Could not be added second time");
    }
    tickets.add(ticket);
    ticket.setCashOffice(this);

  }