Execute a method of different signature in CompletableFuture

Question

From the below code, I want to execute foo3 after foo2 is complete in foo1 . The method signature of foo1 cannot be changed. One way would be to move foo3 into foo2 , but that would be breaking separation of concerns.

@Override
CompletableFuture<Integer> foo1(String str) {
    return CompletableFuture.supplyAsync(() -> foo2(str));
}

Integer foo2(String str) {
    return 0;
}

void foo3(String str) {
    System.out.println("Done");
}

Answer 1

Assuming you just want to call the foo3 after foo2 and return foo2 output, then you can do like this:

return CompletableFuture.supplyAsync(() -> {
                Integer r = foo2(str);
                foo3("testing");
                return r;
            }
    );

Answer 2

You mean like

foo1(str).thenRun(() -> foo3(str);

since you don't seem interested in foo1 's result.

Answer 3

If foo3 call should be part of foo1 , then this is one way:

CompletableFuture<Integer> foo1(String str) {
    return CompletableFuture.supplyAsync(() -> foo2(str))
            .thenApply(foo2Result -> {
                        foo3(str);
                        return foo2Result;
                    }
            );
}

Or if you want to call it, in places where foo1 is called:

foo1(str).thenApply(result -> {
            foo3(str);
            return result;
        }
);