Is this the correct behavior for rounding with BigDecimals?

Question

I am trying to round BigDecimals like this: 5.46597 -> 5.46, I thought the code below does this for me, but not.

I tried it with BigDecimal.round and BigDecimal.setScale.

BigDecimal bD = new BigDecimal(5.46597); // whole number: 5.4659700000000004393996277940459549427032470703125 
bD.setScale(2, RoundingMode.HALF_DOWN); // 5.47
bD.round(new MathContext(3, RoundingMode.HALF_DOWN)); // 5.47

Isn't this should be 5.46, or what do I misunderstood?

Answer 1

HALF_DOWN only rounds down when the actual value is exactly half-way between the two possible rounded values. I.e. 5.46500 -> 5.46. On the other hand 5.4650000001 -> 5.47 because that's nearer to 5.47 than 5.46.

Perhaps what you're looking for is RoundingMode.DOWN, which always rounds down.

Answer 2

Rounding with BigDecimal using ROUND_HALF_DOWN is defined in JavaDoc as "ROUND_HALF_DOWN - Rounding mode to round towards "nearest neighbor" unless both neighbors are equidistant, in which case round down." Documentation ROUND_HALF_DOWN Documentation RoundingMode.HALF_DOWN

For BigDecimal.valueOf("5.46597").setScale(2, BigDecimal.ROUND_HALF_DOWN) the nearest neighbors are 5.46 and 5.47 with distances of 0.00597 and 0.00403. So the distance to 5.47 is smaller (nearer) than to 5.46 which results in rounding to 5.47.