问题
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?
回答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.
回答2:
pass a null I guess:
CompletableFuture<Void> done = CompletableFuture.completedFuture(null);
来源：https://stackoverflow.com/questions/46641700/what-is-the-correct-way-to-create-an-already-completed-completablefuturevoid