void viewExistingCustomers() {
        boolean quit = false;
        while (!quit) {
            System.out.println("------------");
            System.out
                .println("1.) Print All Registered Customer Information");
            System.out.println("2.) Print Customer by ID");
            System.out.println("3.) Print All Customers by Last Name");
            System.out.println("4.) Go Back");
            System.out.print("Select [1/2/3/4]: ");
            String userInput = inputScanner.nextLine();

            if (userInput.equals("1")) {
                System.out.println(getCustomerRoll().toString());
                quit = true;
            }
            else if (userInput.equals("2")) {
                System.out.println("------------");
                System.out.print("User ID: ");
                String idInput = inputScanner.nextLine();
                Integer customerID = 0;

                try {
                    customerID = Integer.valueOf(idInput);
                }
                catch (Exception e) {
                    System.out.println("[ERROR] Not a valid input");
                    continue;
                }

                if (isRegisteredCustomer(customerID)) {
                    System.out.println(getCustomerRoll().getCustomer(
                        customerID));
                    System.out.println(getOrderList().getOrdersByCustomerID(
                        customerID));
                    quit = true;
                }
                else {
                    System.out
                        .println("[ERROR] No customer exists with that ID");
                }

            }
            else if (userInput.equals("3")) {
                System.out.println("------------");
                System.out.print("Last Name: ");
                String lastName = inputScanner.nextLine();
                System.out.println(getCustomerRoll().getCustomersByLastName(
                    lastName).toString());
            }
            else if (userInput.equals("4")) {
                quit = true;
            }

            else {
                System.out.println("[ERROR] Invalid input.");
            }
        }
    }