You could also think of it as declining to _lose_ a decimal place; a.add(b) ought to be the same as b.add(a), so there's no reason to prefer a's precision to b. You need to take the higher precision of the two for an exact result; I assume that's the rationale.

> The BigDecimal class gives its user complete control over rounding behavior. If no rounding mode is specified and the exact result cannot be represented, an exception is thrown ...

- https://docs.oracle.com/javase/8/docs/api/java/math/BigDecim...