Mutable* increment decrement, add and subtract methods are available on all numbers.
It would be nice to have a MutableNumber interface (or abstract class extending Number) specifying those methods. This can be useful when some object takes a "MutableNumber" as input. It would be nice as well to have a copy() method, that creates a new MutableNumber initialized with the value of the current MutableNumber. This is different from the getValue(), as getValue returns a non mutable object. I can send a patch if you are interested. Fabien.
