Comment by Traubenfuchs

> Curious how others tackle such problems.

What do you think about the order preserving simplicity of Java?

  List<Input> inputs = ...;

  List<Output> results = inputs.parallelStream()
                             .map(this::processTask)
                             .collect(toList());

If you want more control or have more complex use cases, you can use an ExecutorService of your choice, handle the futures yourself or get creative with Javas new structured concurrency.