Why doesn't ParallelFlowable.collect(Collector) return a Single?

[jackfirth]

From my understanding, ParallelFlowable.collect(Collector) uses the collector to accumulate values from each parallel rail and then combines each rail's final state into a single output value. It emits that value through a returned Flowable<R>, which will only ever emit one value. Why doesn't this API return a Single<T>? Does the choice to not return a Single imply that if I as a caller immediately convert the returned flowable to a single using singleOrError(), I might hit some problem? Also, the documentation seems off. It claims this:

Each parallel rail receives its own Collector.accumulator() and Collector.combiner().

But why would each rail need its own combiner? The Collector interface is designed for parallel collecting to use just the accumulator in each thread, with the combiner only used to combine final results from multiple threads. Looking at the implementation, I see that this is how ParallelCollector works too: each rail only uses the accumulator, and the final parent flow only uses the combiner.

[akarnokd]

The parallel flowable came from an experimental library. In there, no other reactive type existed thus the serialized output would become a Flowable. We simply didn't re-evaluate the result type after integration with RxJava.

As for the Collector, we looked at examples and the documentation and it was unclear if it is safe to reuse the same accumulator or combiner returned from it across the rails.

[jackfirth]

It's definitely safe, since that's exactly how parallel streams work.

[akarnokd]

Looking at some standard Collectors, it appears the methods return a constant lambda so calling them once or multiple times should have no practical negative consequence.

https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/master/src/java.base/share/classes/java/util/stream/Collectors.java#L195