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

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 return their "combined" future (using CompletableFuture.allOf()), or does nothing and returns an already-completed future.

However, allOf returns a CompletableFuture<Void>:

public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs)

And the only way to create an already-completed future that know is using completedFuture(), which expects a value:

public static <U> CompletableFuture<U> completedFuture(U value)

Returns a new CompletableFuture that is already completed with the given value.

and Void is uninstantiable, so I need another way to create an already-completed future of type CompletableFuture<Void>.

What is the best way to do this?

Solution 1

Since Void can not be instantiated, you can only complete a CompletableFuture<Void> 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<Void> cf = CompletableFuture.completedFuture(null);

to get such an already completed future.

But you can also use

CompletableFuture<Void> cf = CompletableFuture.allOf();

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

Solution 2

pass a null I guess:

CompletableFuture<Void> done = CompletableFuture.completedFuture(null);