From the Java 11
MathContextobject is supplied with a precision setting of 0 (for example,
MathContext.UNLIMITED), arithmetic operations are exact, as are the arithmetic methods which take no
MathContextobject. (This is the only behavior that was supported in releases prior to 5.)
As a corollary of computing the exact result, the rounding mode setting of a
MathContextobject with a precision setting of 0 is not used and thus irrelevant. In the case of divide, the exact quotient could have an infinitely long decimal expansion; for example, 1 divided by 3.
If the quotient has a nonterminating decimal expansion and the operation is specified to return an exact result, an
ArithmeticExceptionis thrown. Otherwise, the exact result of the division is returned, as done for other operations.
To fix, you need to do something like this:
a.divide(b, 2, RoundingMode.HALF_UP)
where 2 is the scale and RoundingMode.HALF_UP is rounding mode
For more details see this blog post.