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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);
    }
}
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  • 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.

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    \$\begingroup\$ Saving a few keystrokes is not worth it if your readers cannot understand the code - well said! \$\endgroup\$ Nov 19 '13 at 2:33
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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 :-)

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