Can i use thenCombine/thenCompose on a CompletableFuture more than once?

Question

Suppose I have CompletableFutures A, B and C is a runnable. B depends on A and C depends on A and B, can I do A thenCompose B and B thenCombine A so C gets the value from A and B even though B depended on the value from A also?

Basically what I'm asking is - is there a way to get a CompletableFuture pipeline like this:

A -- B -- C
  -------^

CompletableFuture<String> A = CompletableFuture.supplyAsync(() -> return "Hello");
CompletableFuture<String> B = A.thenApplyAsync(stringA -> return stringA + "World");
B.thenCombine(A, (stringA, StringB) -> doStuffWithAAndB(stringA, stringB));

Hope this makes sense.

Answer 1

It is perfectly fine to do this. There is no restriction on how you combine CompletableFuture s together.

It is your responsibility to make sure they eventually complete, but it is not an issue here.

Alternatively, you can access A directly from a callback, like this:

CompletableFuture<String> a = CompletableFuture.supplyAsync(() -> return "Hello");
CompletableFuture<String> b = a.thenApplyAsync(stringA -> return stringA + "World");
b.thenApply(stringB -> doStuffWithAAndB(a.join(), stringB));

but this is really a matter of preference.

Answer 2

If you can use RxJava2 , you can do it as follows:

import io.reactivex.Observable;

public class CompletableFutureDemo {

    public static void main(String[] args) {

        CompletableFuture<String> a = CompletableFuture.supplyAsync(() -> "Hello");
        Observable<String> obsA = Observable.fromFuture(a);

        Observable<String> obsB = obsA.map(stringA -> stringA + " world");

        Observable.zip(obsA, obsB, (stringA, stringB) -> {
            return stringA + " " + stringB;
        })
        .subscribe(zipped -> {
            System.out.println(zipped);
        });
    }