public void RemoveStock(ProductType productType)
        {
            switch (productType)
            {
            case ProductType.Cola:
            {
                cola.RemoveStock();
                break;
            }

            case ProductType.Chips:
            {
                chips.RemoveStock();
                break;
            }

            case ProductType.Candy:
            {
                candy.RemoveStock();
                break;
            }
            }
        }