Calculating a total price by using switch statement and sentinel controlled loop

Question

My homework is to make a program that calculates the total value of products sold. It works, but I use the if statement because it was asking for the quantity on the output before ending the loop. And for the same reason, I couldn't make the program ask the user again to enter a number from 1 to 5.

import java.util.Scanner;
public class Sales {
 public static void main(String[] args) {
 Scanner input = new Scanner(System.in);
 double sum = 0;
 int n = -1 ;
 while(n !=0 ) {
 System.out.println("Enter product number 1-5 (Enter to stop)");
 n = input.nextInt();
 if (n==0)
 break;
 System.out.println("Enter quantity of product");
 int q = input.nextInt();
 switch (n) {
 case 1:
 sum += 2.98*q;
 break;
 case 2:
 sum += 4.50*q;
 break;
 case 3:
 sum += 9.98*q;
 break;
 case 4:
 sum += 4.49*q;
 break;
 case 5:
 sum += 6.87*q;
 break;
 default:
 sum += 0;
 break;
 }
 } System.out.printf("Total cost is $%.2f",sum);
 }
}

Answer 1

• Prefer not to use single characters as variable names (except for simple loop counters). Saving a few keystrokes is not worth it if your readers cannot understand your code.

  Based on the context, it looks like n and q should respectively be named productNumber and productQuantity.

• The sum += 0 in the default looks unnecessary. Just remove it if so, even if it's only there to have the default do something. Keep things simple.

• Mathieu Guindon: Saving a few keystrokes is not worth it if your readers cannot understand the code - well said!

Answer 2

A slighty longer answer:

import java.util.InputMismatchException;
import java.util.Scanner;
/**
 * Calculates the sum of given products.
 */
public class ProductSumCalculator {
 /**
 * Represents a product with a price.
 */
 protected enum Product {
 APPLE(1, "Apple", 2.98d),
 BANANA(2, "Banana", 4.50d),
 MANGO(3, "Mango", 9.98d),
 PINEAPPLE(4, "Pineapple", 4.49),
 CHERRY(5, "Cherry", 6.87);
 protected double price;
 protected int productNumber;
 protected String productTitle;
 private Product(int productNumber, String productTitle, double price) {
 this.price = price;
 this.productNumber = productNumber;
 this.productTitle = productTitle;
 }
 public static Product getByNumber(int number) {
 for (Product product : values()) {
 if (product.productNumber == number) {
 return product;
 }
 }
 return null;
 }
 @Override
 public String toString() {
 return productNumber + " - " + productTitle + " - " + String.format("$%.2f", price);
 }
 }
 /**
 * @param args
 */
 public static void main(String[] args) {
 Scanner scanner = new Scanner(System.in);
 try {
 double sum = 0.0d;
 int input = -1;
 while (input != 0) {
 System.out.println("Please make your choice: ");
 for (Product product : Product.values()) {
 System.out.println(product);
 }
 System.out.println("0 - Quit and show sum");
 input = readNumberInput(scanner);
 if (input == 0) {
 break;
 }
 if (input != -1) {
 Product product = Product.getByNumber(input);
 if (product == null) {
 System.out.println("The entered product number was not correct. Please try again.");
 } else {
 System.out.println("Enter quantity of product [" + product.productTitle + "]:");
 input = readNumberInput(scanner);
 if (input > 0) {
 sum += product.price * input;
 }
 }
 }
 }
 System.out.println("Total sum of all the chosen products is: " + String.format("$%.2f", sum));
 } finally {
 scanner.close();
 }
 }
 /**
 * Reads a number from a scanner instance and catches erroneous input.
 * 
 * @param scanner
 * @return
 */
 private static int readNumberInput(Scanner scanner) {
 try {
 return scanner.nextInt();
 } catch (InputMismatchException ex) {
 String inputString = scanner.next();
 System.out.println("Input [" + inputString + "] was not correct. Please choose a number.");
 return -1;
 }
 }
}

Let's run over this code, shall we:

• I created an enum of 'Product' so that you can easily add/remove products and change their number, title or price and so that your main code doesn't have to change when adding/removing products
• Overridden the toString method of a Product so you can print them out in the program
• Validate the input from the user (ALWAYS validate input from the user)
• Closed the scanner instance (ALWAYS close your resources when possible)
• Added more clear instructions for the user with better guidance through the 'program'

I know it's a stupid little program, but it's an easy example of how your code will be able to change more easily later on, by adding/removing products, validating user input, etc... The one thing I could have added is more documentation, but in this case it's very trivial :-)