Java BigDecimal Round Down issue

Question

I have some simple code to round up and round down, but it is producing some unexpected results.

    public static void main(String[] args) throws Exception {
    // TODO Auto-generated method stub

    double a = 46.66;
    System.out.println("Roundup: " + roundUp(a,2) + "\nRound Down: " + roundDown(a,2));

}

public static double roundUp(double a, int scale)
{
    BigDecimal value = new BigDecimal(a);
    value = value.setScale(scale, RoundingMode.UP);
    return value.doubleValue();
}

public static double roundDown(double a, int scale)
{
    BigDecimal value = new BigDecimal(a);
    value = value.setScale(scale, RoundingMode.DOWN);
    return value.doubleValue();
}

When I use 3 digits after decimal, it is working as expected. If a = 44.661, the output is as expected as below.

Roundup: 46.67 Round Down: 46.66

When a=44.66, the round down value reduces by 1 which is unexpected as below.

Roundup: 46.66 Round Down: 46.65

How do I get 44.66 for round down and 44.67 for roundUp while still retaining the above results.

Thanks in advance for your help

Answer 1

That's because you're not really using BigDecimals, you're using doubles. When you type 44.66 it gets converted to a double, which results in a value slightly less than 44.66. BigDecimal can't fix rounding errors that have already occurred before you give it the input.

Try this instead.

public static void main(String[] args) throws Exception {
    BigDecimal a = new BigDecimal("46.66");
    System.out.println("Roundup: " + roundUp(a,2) + "\nRound Down: " + roundDown(a,2));
}

public static BigDecimal roundUp(BigDecimal a, int scale)
{
    return a.setScale(scale, RoundingMode.UP);
}

public static BigDecimal roundDown(BigDecimal a, int scale)
{
    return a.setScale(scale, RoundingMode.DOWN);
}