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javaroundingbigdecimal

BigDecimal wrong rounding

I have a problem with rounding a number using BigDecimal's scale and RoundingMode.

This is my piece of code:

BigDecimal taxAmount = new BigDecimal("0.8445");
taxAmount = taxAmountPrecision.setScale(2, RoundingMode.HALF_UP);

If I put 0.845 it is rounding well, but if there is 0.8445 the taxAmount is 0.84.

It should be 0.8445 -> 0.845 -> 0.85.

Solution

  • That's the exact expected behavior of that rounding method.

    Rounding mode to round towards "nearest neighbor" unless both neighbors are equidistant, in which case round up.

    The nearest neighbor of 0.8445 to 2 decimal places is 0.84. HALF_UP will round to 0.85 if the value is 0.845 or greater (halfway between the values), and will round to 0.844 if it's less than that. 0.8445 is less than 0.845, so it rounds down.

    There are no intermediate rounding steps to propagate the 5 like you want. If you want that sort of rounding, you'll have to write a loop which rounds to steadily decreasing precisions.