How to sum a Stream of CompleteableFuture<BigDecimal> conveniently?

Question

I'm looking for a non-blocking way to sum a Stream of CompleteableFuture<BigDecimal>.

I already found topics closely related to this problem, like this. But unfortunately in my case I do have the BigDecimal packed into a CompleteableFuture and therefore I need to wait for completion first. In the end I would like to get another CompleteableFuture which contains the sum of all Futures within the Stream, once it's completed.

EDIT: Actually I did manage to find the following solution:

Stream<CompletableFuture<BigDecimal>> lotOfWork;
CompletableFuture.supplyAsync(() -> lotOfWork.map(CompletableFuture::join)
                                             .reduce(
                                                 BigDecimal.valueOf(0.0), 
                                                 BigDecimal::add
                                              )
);

But since this is not using any of the CompletionStage methods, I'm pretty sure there is an even better way to do this job.

Answer 1

Here is the solution with the CompletableFuture::thenCombine as suggested by you directly

I would have prefer a solution that does not enforce the reduction order but didn't find it in the javadoc.

CompletableFuture<BigDecimal> result = lotOfWork.reduce((a,b) -> a.thenCombine(b, BigDecimal::add)).orElse(CompletableFuture.completedFuture(BigDecimal.ZERO));