public static void addDisplay()
	{

		boolean isValid = false;

		while (!isValid)
		{
			Scanner sc = new Scanner(System.in);
			System.out.print("Enter a UPC code: ");
			String upc = sc.nextLine();

			if (upc == null || upc.isEmpty())
			{
				if (lastUpc == null)
				{
					System.out.println("Error: No UPC provided.");

				}
				else
				{
					upc = lastUpc;
					System.out.println("Your UPC has defaulted to your last " +
							                   "used UPC " + lastUpc);
				}
			}

			for (Product p : ExtractProductsandInventory.prod)
			{
				if (p.getUpc().equals(upc))
				{
					lastUpc = upc;
					askForQuantity();

					Integer quantity;
					String s = sc.nextLine();
					if (s != null && InvoiceApp.isInteger(s))
					{
						quantity = Integer.valueOf(s);
					}
					else
					{
						quantity = 1;
					}

					Integer invoiceQuantity = invoice.get(p);
					if (invoiceQuantity != null)
					{

						quantity += invoice.get(p);

						invoice.put(p, quantity);
					}
					invoice.put(p, quantity);
					InvoiceDisplay.printInvoice(invoice);
					InitialSalesDisplay.initialDisplay();
				}

			}
			System.out.println("Error! The UPC code was not found in add display.");
			addDisplay();
		}

	}