How to use method that returns CompletionStage inside thenCombineAsync

Question

I have 3 functions that return CompletionStage. Let's say that they looks like:

CompletionStage<A> funcA();
CompletionStage<B> funcB();
CompletionStage<C> funcC(A a, B b);

Now I would like to write fuction funcD that returns CompletionStage<C>. Result is computed by funcC and params cames from both funcA and funcB. Now the question is how to do it properly?

My attempts after reading documentation looks like this, but I'm not sure if it is proper usage. The problem is that after thenCombineAsync I receive CompletionStage<CompletionStage<C>> and the last line looks like ugly workaround to extract proper result. Can it be accomplished any better?

CompletionStage<C> funcD() {
    CompletionStage<B> completionStageB = funcB();
    return funcA()
        .thenCombineAsync(completionStageB, (a,b) -> funcC(a,b))
        .thenComposeAsync(result -> result);
}

Let's assume that method's declarations cannot be changed.

Answer 1

There is no thenComposeWithBoth. If you cannot modify method signatures, I would just leave it as is (but remove Async -- see below). The only way to make this shorter is to join() inside the Combine stage:

funcA()
    .thenCombineAsync(completionStageB, (a,b) -> funcC(a,b).join());

On a separate note, you are using ...Async methods needlessly. Since your funcC returns a CompletableFuture, it is probably not long-running and there is no need to schedule it asynchronously. And result -> result certainly does not need to run in a separate thread.