Change string to a double assign value update and assign back to a string

Question

I am trying to convert a string that the user inputs (price) to a double (price1) and assign it to a total price variable. Then I will convert it back to a string and store it in a map. I have copied the code I have written so far.

public class SalesTax {

    public static void main(String[] args) {
        // Input items for shopping cart
        HashMap<String, String> cart = new HashMap<String, String>();
        HashMap<String, String> shoppingcart = new HashMap<String, String>();

        // Create a Scanner
        Scanner input = new Scanner(System.in);

        // variables
        char done;
        boolean goods;
        double totalprice = 0;
        double totalwtax = 0;
        double totaltax;


        // Pick items for list.
        do { 
            System.out.print("Please enter an item.");
            String item = input.nextLine();

            System.out.print("Please enter the price for "+ item + ": ");
            String price = input.nextLine();    

            double price1 = Double.parseDouble(price);
            totalprice += price1;

            String price2 = String.valueOf(price1);
            shoppingcart.put(item, price2);

            cart.put(item, price);

            //determine if item will have additional tax
            if (item.contains("music") == true)
            {
                double newprice = Double.parseDouble(price);
                newprice = newprice *1.10;

                totalwtax +=newprice;

                String newprice2 = String.valueOf(newprice);
                shoppingcart.put(item, newprice2);
            }
            else if (item.contains("imported") == true)
            {
                double newprice = Double.parseDouble(price);
                newprice = newprice *1.05;

                totalwtax +=newprice;

                String newprice2 = String.valueOf(newprice);
                shoppingcart.put(item, newprice2);
            }
            else if(item.contains("bottle") == true)
            {
                double newprice = Double.parseDouble(price);
                newprice = newprice *1.10;

                totalwtax +=newprice;

                String newprice2 = String.valueOf(newprice);
                shoppingcart.put(item, newprice2);
            }
            else 
            {
                shoppingcart.put(item, price);
            }


            System.out.print("Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.");
            done = input.nextLine().charAt(0);
            totaltax = totalprice - totalwtax;

        } while(Character.toUpperCase(done) == 'Y');
       System.out.println("Your cart contains the following at items with tax" + shoppingcart);
       System.out.println("Total Tax: " + totalwtax);

    }
}

Test Case

The problem is that I'm not getting the total with tax if I input 12.49, 14.99, 0.85 and a 10% tax is on the 14.99 the total of all three should come out to be 29.83:

Please enter an item.standard
Please enter the price for standard: 12.49
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.y
Please enter an item.music
Please enter the price for music: 14.99
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.y
Please enter an item.discount
Please enter the price for discount: 0.85
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.n
Your cart contains the following at items with tax{music=16.489, standard=12.49, discount=0.85}
Total Tax: 16.489

Why does this code produce the wrong answer?

Side note: why change it back to String? If you can, store it in the Map as a Double. — splungebob, Aug 14 '14 at 19:34
The problem is that I'm not getting the total with tax if I input 12.49, 14.99, 0.85 and a 10% tax is on the 14.99 the total of all three should come out to be 29.83 — pgray10, Aug 14 '14 at 22:19

score 2 · Answer 1 · answered Aug 14 '14 at 19:36

In order to minimize the number of conversions you have to make, you could read in the price directly as a double instead of as a string using Scanner's nextDouble() method instead of nextLine(). This will read in the price as a double so you don't have to convert it. Also, it seems like it would make more sense to store the price as a double in the map (i.e. have a Map<String, Double> instead of a Map<String, String>, so that you don't have to do any conversion.

score 1 · Answer 2 · answered Aug 15 '14 at 09:19

One problem is that totalwtax is not being calculated correctly because any items that are zero rated for tax are not included in the total. Changing the else clause to this:

        else
        {
            double newprice = Double.parseDouble(price);
            totalwtax +=newprice;

            shoppingcart.put(item, price);
        }

Produces this output for your test case:

Please enter an item.standard
Please enter the price for standard: 12.49
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.y
Please enter an item.music
Please enter the price for music: 14.99
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.y
Please enter an item.discount
Please enter the price for discount: 0.85
Would you like to continue to add items? (Type Y) for Yes and (Type N) for No.n
Your cart contains the following at items with tax{music=16.489, standard=12.49, discount=0.85}
Total Tax: 29.829

This still isn't exactly the same as the expected answer because the expected answer assumes some rounding in the calculation. You would either need to round the values yourself or it might be appropriate to use a Money class so that all the fractions of cents are correctly accounted for.

+1 for showing that floating point artihmetics and variables are not suitable to calculate and store money values. See e.g. this answer: http://stackoverflow.com/questions/8148684/what-is-the-best-data-type-to-use-for-money-in-java-app — david a., Aug 15 '14 at 09:32

Change string to a double assign value update and assign back to a string

2 Answers2