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

A shorter version:

ExecutorService executor = ... // e.g. Executors.newFixedThreadPool(4);
Future future = executor.submit(() -> testA.abc());
testB.xyz();
future.get(); // wait for completion of testA.abc()

To be noted that having to run something in parallel with nothing to be returned could be a sign of a bad pattern :)

Also, if you are in a Spring environment, you could use: https://spring.io/guides/gs/async-method/