public void setSlotNumber(Tray tray) {
		int slotNumber = -1;
		List<Integer> numbers = new ArrayList<Integer>();
		for (Tray t : this.trays) {
			numbers.add(t.getSlotNumber());
		}
		Collections.sort(numbers);
		for (Integer num : numbers) {
			if (num > slotNumber + 1) {
				tray.setSlotNumber(slotNumber + 1);
				return;
			}
			slotNumber = num;
		}
		tray.setSlotNumber(slotNumber + 1);
	}