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()