private int quantityUpdate(int quantityOperation, int quantity) {
		
		int totalQuantity = getQuantity();
		
		if ( quantityOperation == QUANTITY_OPERTION_INCREASE ) totalQuantity += quantity;
		else if ( quantityOperation == QUANTITY_OPERTION_INCREASE ) totalQuantity += quantity;
		
		// make sure no nagtive quantity
		totalQuantity = totalQuantity < 0 ? 0 : totalQuantity;
		
		setQuantity(totalQuantity);
		
		return totalQuantity;
	}