How to use Callable with void return type?

You can use java.lang.Thread for parallel execution. However, in most cases it's easier to use an java.util.concurrent.ExecutorService. The latter provides a method to submit a Callable and returns a Future to get the result later (or wait for completion).

If testA.abc() and testB.xyz() should be executed in parallel, you use the ExecutorService to execute the former in a separate thread whereas the latter is executed in the original thread. Then you wait for the completion of the former for synchronization.

ExecutorService executor = ... // e.g. Executors.newFixedThreadPool(4);Future<Void> future = executor.submit(new Callable<Void>() {    public Void call() throws Exception {        testA.abc();        return null;    }});testB.xyz();future.get(); // wait for completion of testA.abc()

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).

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/