What does allOf return, and how do you collect the results?

allOf cannot know the types are the same, so it gives you CompletableFuture<Void> as a completion signal only. The idiom:

List<CompletableFuture<Price>> calls = ids.stream().map(this::fetchAsync).toList();

CompletableFuture<List<Price>> all = CompletableFuture
        .allOf(calls.toArray(CompletableFuture[]::new))
        .thenApply(ignored -> calls.stream().map(CompletableFuture::join).toList());

The join inside thenApply never blocks, because every future is already complete by then.

Two things to mention: allOf completes exceptionally if any input fails, but it does not cancel the others, so add a timeout with orTimeout or completeOnTimeout per call. And anyOf returns CompletableFuture<Object>, which is awkward enough that people usually write their own.