Optional parametrization in Java with Generics

There is never really a good reason to do this. Typically you specify generic type parameters because some method arguments take or return a parameter of that type. If unspecified were valid, that would seem to suggest you intended not to perform any meaningful implementation of those methods.

Anyway, to solve your perceived problem, just specify "Object" for any type parameter that you don't care to specify. Or extend the abstract class with another abstract class which has only one type parameter (specifying Object as the second type parameter in your extends call).

Simple answer: no, java does not support default parametrizations like that.