How do I properly return money double variable in Java

Question

I'm currently working on an assignment for school where I need to write a program that it needs to return your change in this mode

Input: purchaseAmount: $12.45 givenAmount: $15.00

Output: Change: $2.55 2 one dollar bills 2 quarters 1 nickel

But, if I work with this case scenario it works fine. Now, If I want this scenario to work this what I get

Input: purchaseAmount: $19.51 givenAmount: $50.01

Output:

Twenty dollar bill 1 Ten dollar bill: 1 Quarters: 1 Dimes: 2 Pennies: 4

Which is incorrect since I'm returning to the costumer 30.49 instead of 30.5

Can't figure out whats the issue here.

Thanks

 boolean isFiveBillOn = false, isTwentybillOn = false, isTenbillOn = false, isDollarOn = false, isQuartersOn = false, isDimesOn = false, isNicklesOn = false, isPenniesOn = false;

    //retrieve amount due
    String moneyd = JOptionPane.showInputDialog("Enter the amount due");
    double amountd = Double.parseDouble(moneyd);

    String moneyr = JOptionPane.showInputDialog("Enter amount you would like to pay");
    double amountr = Double.parseDouble(moneyr);

    //calculate difference
    double difference = (amountr - amountd);

    //convert to pennies
    int balance = (int)(difference * 100);

    //twenty dollar bill
    int twentydollars = balance / 2000;
    balance = (balance % 2000);
    if (twentydollars > 0) {
        isTwentybillOn = true;
    }

    //ten dollar bill
    int tendollars = balance / 1000;
    balance = (balance % 1000);
    if (tendollars > 0) {
        isTenbillOn = true;
    }

    //five dollar bill
    int fivedollars = balance / 500;
    balance = (balance % 500);
    if (fivedollars > 0) {
        isFiveBillOn = true;
    }

    //dollar bill
    int dollars = balance / 100;
    balance = (balance % 100);
    if (dollars > 0) {
        isDollarOn = true;
    }

    //quartes
    int quarters = balance / 25;
    balance = (balance % 25);
    if (quarters > 0) {
        isQuartersOn = true;
    }

    //dimes
    int dimes = balance / 10;
    balance = (balance % 10);
    if (dimes > 0) {
        isDimesOn = true;
    }

    //nickles
    int nickles = balance / 5;
    balance = (balance % 5);
    if (nickles > 0){
        isNicklesOn = true;
    }

    //pennies
    int pennies = balance / 1;
    balance = (balance % 1);
    if (pennies > 0){
        isPenniesOn = true;
    }

    //checking for difference over 0
    if (difference < 0) {
        JOptionPane.showMessageDialog(null, "The amount received can't be less than the amount owned");
    } else {
        // Returning Message
        } if (isTwentybillOn) {
                    System.out.println("Twenty dollar bill " + twentydollars);
        } if (isTenbillOn) {
                    System.out.println("Ten dollar bill: " + tendollars);
        } if (isDollarOn) {
                    System.out.println("One Dollar Bill: " + dollars);
        } if (isQuartersOn) {
                    System.out.println("Quarters: " + quarters);
        } if (isDimesOn) {
                    System.out.println("Dimes: " + dimes);
        } if (isNicklesOn) {
                    System.out.println("Nickles: " + nickles);
        } if (isPenniesOn) {
                    System.out.println("Pennies: " + pennies);
        } else {
            System.out.println("Thanks! have a good day");
        }
    }

Answer 1

You should never use a double or any other floating-point data type to represent money. The reason is that they can't represent all values exactly. For example, the following line:

System.out.println(new BigDecimal(0.1));

prints

0.1000000000000000055511151231257827021181583404541015625

For a longer explanation why, see What Every Computer Scientist Should Know About Floating-Point Arithmetic.

I suggest using BigDecimal instead.

An alternative if you only want to use primitives is to use an int to store the currency value in cents instead, and divide by 100 when displaying the value.