Note. I'm not sure if it's a better fit for Software Recommendations, but I'll give it a try.
Suppose there's a long IO operation that may result in some return value — but doesn't have to. Completion with no return value is different from the operation being pending.
Do I have better options other than returning CompletableFuture<Optional<MyType>> from my asynchronous method? It's a bit of a mouthful, especially if MyType is generic itself. If I need to return a list of some entities, it can get as ugly as CompletableFuture<Optional<List<MyType<MyOtherType>>>> (remeber, an empty list is different from no list, I can't skip the Optional layer).
In my case it's a bit different, though, Optional<CompletableFuture<List<MyType<MyOtherType>>>>. A cache class may have a list of entities that you may fetch by a key, but you may also need to wait for the loading to finish. Thus, cacheable, cached, not cacheable are three distinct states.
Java 8.