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");
        }
    }

Do not use `double` to represent money. Use a `BigDecimal` or some other exact data type. — Keppil, Jan 27 '14 at 20:19
because floating point numbers can not represent arbitrary real numbers. it is always just an approximation — kmera, Jan 27 '14 at 20:23
Yep, in general you should never use float/double numbers to represent money (except perhaps if you're doing stock market analysis and precise to-the-penny results are not needed). As to why, you've demonstrated that above. — Hot Licks, Jan 27 '14 at 20:24
possible duplicate of [Why not use Double or Float to represent currency?](http://stackoverflow.com/questions/3730019/why-not-use-double-or-float-to-represent-currency) — ThanksForAllTheFish, Jan 27 '14 at 20:24
@HotLicks, well I could vote to close for lack of research. I just think to find a duplicate question is easier to prove than the lack of research — ThanksForAllTheFish, Jan 27 '14 at 20:27
@HotLicks, my fault, I misunderstood you. I cannot vote to close (not enough reputation), instead I flagged the question — ThanksForAllTheFish, Jan 27 '14 at 20:40
@DavidWallace I did not claim they do (admittedly it does look so based on the previous comment). I was merely pointing out the reason for the issue/behavior posted by OP. — kmera, Jan 27 '14 at 21:24
Hah, yeah, true. Maybe I should post an answer, instead of a series of snide comments. — Dawood ibn Kareem, Jan 27 '14 at 21:26

Keppil · Accepted Answer · 2014-01-27T20:30:25.937

4

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.

edited Jan 27 '14 at 20:30

answered Jan 27 '14 at 20:22

Keppil

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Thanks for your answer. Is there any other way? My professor hasn't been over BigDecimal yet. This project should not cover BigDecimal, any other suggestion. – user3241911 Jan 27 '14 at 20:27
@user3241911: I added an alternative solution using `int`. – Keppil Jan 27 '14 at 20:30

How do I properly return money double variable in Java

1 Answers1