What is the correct way to create an already-completed CompletableFuture

Question

I am using Completable futures in java 8 and I want to write a method that, based on a received parameter, either runs multiple tasks with side effects in parallel and then

Answer 1

Since Void can not be instantiated, you can only complete a CompletableFuture with a null result, which is exactly what you also will get when calling join() on the future returned by allOf() once it has been successfully completed.

So you can use

CompletableFuture cf = CompletableFuture.completedFuture(null);

to get such an already completed future.

But you can also use

CompletableFuture cf = CompletableFuture.allOf();

to denote that there are no jobs the result depends on. The result will be exactly the same.