Round decimals up to multiples of 5 with BigDecimal

I want to round BigDecimal decimal part according to the following rules:

1710.10 becomes 1710.10
1710.11 becomes 1710.15
1710.15 becomes 1710.15
1710.16 becomes 1710.20

I tried this way new BigDecimal("1710.11").setScale(2, RoundingMode.HALF_UP)), expecting to get 1710.15 but I get 1710.11. I also tried with Apache Math Utils and Decimal Format but no way to achieve this.

Solution

Your misunderstanding

First of all setting the scale on a BigDecimal only returns a BigDecimal with a different value if your current BigDecimal has a higher scale than you specified.

For example

new BigDecimal("1710.113").setScale(2, RoundingMode.HALF_UP)

returns a new BigDecimal equal to new BigDecimal("1710.11")

However, if you already have a scale of 2 or lower in this example, the new BigDecimal stays the same (read: equal).

HALF_UP simply means that a BigDecimal ending on a 5 will result in the higher value, e.g.

new BigDecimal("1710.115").setScale(2, RoundingMode.HALF_UP)

returns a result equal to new BigDecimal("1710.12")

Now to your question

Since your way of "rounding" doesn't actually change the scale, I doubt there is an already existing function you could use. However, doing it by hand is actually quite simple:

public class QuickMathz {

    public static void main(String[] args) {
        System.out.println(roundToNext5(new BigDecimal("1710.10"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.11"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.15"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.16"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.1135"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.1635"), 2));
        System.out.println(roundToNext5(new BigDecimal("1710.1675"), 2));
        System.out.println();
        System.out.println(roundToNext5(new BigDecimal("1710.10"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.11"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.15"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.16"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.1135"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.1635"), 3));
        System.out.println(roundToNext5(new BigDecimal("1710.1675"), 3));
    }

    public static BigDecimal roundToNext5(BigDecimal bigDecimal, int scale) {
        // Get the last digit we need to decide if we have to round to 0, 5 or 10
        int lastDigit = bigDecimal
                .movePointRight(scale)
                .remainder(BigDecimal.TEN).intValue();

        // Setting the Scale to scale - 1 to remove one more digit than we need
        // and then increase the scale to what we want
        BigDecimal result = bigDecimal
                .setScale(scale - 1, RoundingMode.DOWN)
                .setScale(scale, RoundingMode.UNNECESSARY);

        if (lastDigit == 0) {
            // Last digit is a 0 upscaling adds a 0
            return result;
        } else if (lastDigit <= 5) {
            // rounding up to 5
            return result.add(new BigDecimal("5").movePointLeft(scale));
        } else {
            // rounding up to 10
            return result.add(new BigDecimal("1").movePointLeft(scale - 1));
        }
    }
}

This class yields the output of

(It's neither optimized nor checked for negative values, so don't use in critical environments)