How to calculate money in Java

Short answer

double stores decimal numbers approximately, so money is handled with BigDecimal:

void main() {
    IO.println(0.1 + 0.2);
    IO.println(new BigDecimal("0.10").add(new BigDecimal("0.20")));
}

Output:

0.30000000000000004
0.30

Arithmetic

The + and * operators do not work with BigDecimal — you need methods. Each method returns a new number:

void main() {
    var price = new BigDecimal("19.99");
    var quantity = BigDecimal.valueOf(3);
    var discount = new BigDecimal("5.00");
 
    var total = price.multiply(quantity).subtract(discount);
    IO.println(total);
}

Output:

54.97

Tax with rounding

After multiplying by a percentage you get more digits — round to cents:

void main() {
    var net = new BigDecimal("49.99");
    var vat = net.multiply(new BigDecimal("0.19"));
 
    IO.println(vat);
    IO.println(vat.setScale(2, RoundingMode.HALF_UP));
}

Output:

9.4981
9.50

Division

When dividing, specify the scale and the rounding rule, otherwise an endless fraction throws an exception:

void main() {
    var bill = new BigDecimal("100.00");
    var share = bill.divide(BigDecimal.valueOf(3), 2, RoundingMode.HALF_UP);
 
    IO.println(share);
}

Output:

33.33

Comparison

equals() takes the number of decimal places into account, while compareTo() looks only at the value:

void main() {
    var a = new BigDecimal("2.0");
    var b = new BigDecimal("2.00");
 
    IO.println(a.equals(b));
    IO.println(a.compareTo(b) == 0);
}

Output:

false
true

Common mistake

Creating a BigDecimal from a double. The inaccuracy of the double is carried over completely:

void main() {
    IO.println(new BigDecimal(0.1));
    IO.println(new BigDecimal("0.1"));
    IO.println(BigDecimal.valueOf(0.1));
}

Output:

0.1000000000000000055511151231257827021181583404541015625
0.1
0.1

Create it from a string or with BigDecimal.valueOf().

Thanks for reading!

Short answer

Arithmetic

Tax with rounding

Division

Comparison

Common mistake

Functions used in this recipe

Thanks for reading!