Correctly divide a double to be able to get back the correct amount

Question

I have a input to a method that represents an amount of money, a total price of X items.
This can be an amount under a number of currencies and is represented by a double.
Question: I want to break this amount into the price for each of the X items. Considering that the amount itself is a double my concern is that if I simply do: amount/X I could get a number that number*X does not give me the amount exactly due to e.g. rounding. How can I do this correctly?
Note: Please give me help taking for granted that I can not change the amount to be something other than a double

Answer 1

As you already mentioned, this is not possible in every case. Usually, when the total price is really the product of the single item price and a quantity, the resulting double will have enough precision to do this calculation. But generally you have to store both prices.

I implemented a whole ERP, and we also have a feature where the user can specify the total line amount directly, maybe to round the sum after discussing with a customer. And this makes it possible to sell 7 items for 1000$. Then there is no way to create a precise representation of a price for a single item. And therefore we also store the item price rounded (cut after 4 digits).

To come back to your answer: Depending on context you will need the total and single item amount later anyway, so just store both.

Answer 2

You can use BigDecimal class:

    double a=456.6556756;
    BigDecimal amount=BigDecimal.valueOf(a);
    BigDecimal priceOfEachItem=amount.divide(BigDecimal.valueOf(8));   
    BigDecimal amountAgain=priceOfEachItem.multiply(BigDecimal.valueOf(8));
    System.out.println(amountAgain.doubleValue());

Output:456.6556756

As you can see you can get the exact original amount

Answer 3

You can do this by taking the single item price as double value which won't round off the value(Yes,you may limit the number of digits after decimal).

double totalPrice = 500.33;
int totalItems = 6;
double singleItemPrice = totalPrice/totalItems;
System.out.println("Single Item Price = " + singleItemPrice);
System.out.println("Total Price = " + (singleItemPrice * totalItems));
DecimalFormat df = new DecimalFormat("#.##"); 
Double formatted = Double.parseDouble(df.format(singleItemPrice * totalItems)); 
System.out.println("Formatted Value = "formatted);

Output:

Single Item Price = 83.38833333333334
Total Price = 500.33000000000004
Formatted Value = 500.33

Or use BigDecimal as mentioned by someone else(but maybe the above will have more precision, let us know about that):

import java.math.BigDecimal;

BigDecimal premium = BigDecimal.valueOf(1586.6d);
BigDecimal netToCompany = BigDecimal.valueOf(708.75d);
BigDecimal commission = premium.subtract(netToCompany);
System.out.println(commission + " = " + premium + " - " + netToCompany);

Output:

877.85 = 1586.6 - 708.75

Hope this was what you were looking for.

Answer 4

You can use Math.round for rounding. The problem you have is that you don't know what rounding for you amount/X you need. This is a problem whether you use double or BigDecimal

You can do this

public static double round2(double d) {
    if (d < -Long.MAX_VALUE / 100 || d > Long.MAX_VALUE / 100)
        return d;
    return Math.round(d * 100) / 100.0;
}

public static void main(String[] args) throws IOException {
    double amount = 1000.00;
    int units = 9;
    System.out.println("amount: " + amount );
    System.out.println("units: " + units);
    double perUnit = amount / units;
    double amount2 = round2(perUnit * units);
    System.out.println("perUnit: " + perUnit);
    System.out.println("amount2: " + amount2);
}

prints

amount: 1000.0
units: 9
perUnit: 111.11111111111111
amount2: 1000.0