BigDecimal setScale method working exactly in the opposite way I am expecting

Question

Consider this code:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class RoundingTests {

    public static void main(String[] args) {
        BigDecimal bd1 = new BigDecimal(265.345d);
        BigDecimal bd2 = new BigDecimal(265.335d);

        System.out.println("Setting scale 265.345: " + bd1.setScale(2, RoundingMode.HALF_EVEN));
        System.out.println("Setting scale 265.335: " + bd2.setScale(2, RoundingMode.HALF_EVEN));
    }
}

The output is:

Setting scale 265.345: 265.35
Setting scale 265.335: 265.33

Now this is exactly the opposite of what I was expecting. With RoundingMode.HALF_EVEN (also called Bankers rounding) I was expecting both these values to become the value 265.34

Note that I am not using the BigDecimal.round method on purpose because it also does not what I need. If I add this code (and import java.math.MathContext) :

System.out.println("Rounding 265.345: " + bd1.round(new MathContext(2, RoundingMode.HALF_EVEN)));
System.out.println("Rounding 265.335: " + bd2.round(new MathContext(2, RoundingMode.HALF_EVEN)));

Then the output is:

Rounding 265.345: 2.7E+2
Rounding 265.335: 2.7E+2

This is expected and also explained in BigDecimal setScale and round, but means it's basically useless for my purpose.

Can someone explain the issue with setScale here?

Update: So it was just another floating point problem, with no easy way to fix it instead of using BigDecimals from the start.

Answer 1

You might want to change your code to use BigDecimal.valueOf() instead of new BigDecimal().

The code

  Double dValue = 265.345d;
  Double dValue2 = 265.335d;
  BigDecimal value = BigDecimal.valueOf(dValue);
  BigDecimal bd1 = new BigDecimal(265.345d);
  BigDecimal bd2 = new BigDecimal(265.335d);
  BigDecimal value2 = BigDecimal.valueOf(dValue2);
  System.out.println("BigDecimal.valueOf(dValue);");
  System.out.println(value.toPlainString());
  System.out.println(String.valueOf(dValue));
  value = value.setScale(2, BigDecimal.ROUND_HALF_EVEN);
  System.out.println(value);
  System.out.println("BigDecimal.valueOf(dValue2);");
  System.out.println(value2.toPlainString());
  System.out.println(String.valueOf(dValue2));
  value2 = value2.setScale(2, BigDecimal.ROUND_HALF_EVEN);
  System.out.println(value2);
  System.out.println("BigDecimal bd1 = new BigDecimal(265.345d);");
  System.out.println(bd1.setScale(2, BigDecimal.ROUND_HALF_EVEN));
  System.out.println("BigDecimal bd2 = new BigDecimal(265.335d);");
  System.out.println(bd2.setScale(2, BigDecimal.ROUND_HALF_EVEN));

outputs :

BigDecimal.valueOf(dValue);
265.345
265.345
265.34
BigDecimal.valueOf(dValue2);
265.335
265.335
265.34
BigDecimal bd1 = new BigDecimal(265.345d);
265.35
BigDecimal value2 = BigDecimal.valueOf(dValue2);
265.33