How to use Callable with void return type?

Question

I am working on a project in which I have multiple interface and two Implementations classes which needs to implement these two interfaces.

Suppose my first Interface is

Answer 1

Why would you need a void for running something in Parallel? For one, if you don't need the return value, you can simply return null.

To make something parallel you need to use threading/scheduling. I would personally recommend avoiding Callables, and using Runnables instead (and hey, no return value).