public void add(Product prod, double unitValue){
		if(unitValue < 0) throw new IllegalArgumentException("unitValue cannot be negative.");
		if(prod == null) throw new IllegalArgumentException("Product must not be null.");
		
		if(products.containsKey(prod)){	// Check if the product exists in the HashMap
			products.put(prod, unitValue + products.get(prod));
		}else{
			products.put(prod, unitValue);
			productIndex.add(prod);
		}
	}