Generate infinite parallel stream

Question

Problem

Hi, I have a function where i going to return infinite stream of parallel (yes, it is much faster in that case) generated results. So obviously (or not) i used

Stream<Something> stream = Stream.generate(this::myGenerator).parallel()

It works, however ... it doesn't when i want to limit the result (everything is fine when the stream is sequential). I mean, it creates results when i make something like

stream.peek(System.out::println).limit(2).collect(Collectors.toList())

but even when peek output produces more than 10 elements, collect is still not finallized (generating is slow so those 10 can took even a minute)... and that is easy example. Actually, limiting those results is a future due the main expectation is to get only better than recent results until the user will kill the process (other case is to return first what i can make with throwing exception if nothing else will help [findFirst didn't, even when i had more elements on the console and no more results for about 30 sec]).

So, the question is...

how to copy with that? My idea was also to use RxJava, and there is another question - how to achieve similar result with that tool (or other).

Code sample

public Stream<Solution> generateSolutions() {
     final Solution initialSolution = initialSolutionMaker.findSolution();
     return Stream.concat(
          Stream.of(initialSolution),
          Stream.generate(continuousSolutionMaker::findSolution)
    ).parallel();
}

new Solver(instance).generateSolutions()
    .map(Solution::getPurpose)
    .peek(System.out::println)
    .limit(5).collect(Collectors.toList());

Implementation of findSolution is not important. It has some side effect like adding to solutions repo (singleton, sych etc..), but nothing more.

Answer 1

As explained in the already linked answer, the key point to an efficient parallel stream is to use a stream source already having an intrinsic size instead of using an unsized or even infinite stream and apply a limit on it. Injecting a size doesn’t work with the current implementation at all, while ensuring that a known size doesn’t get lost is much easier. Even if the exact size can’t be retained, like when applying a filter, the size still will be carried as an estimate size.

So instead of

Stream.generate(this::myGenerator).parallel()
      .peek(System.out::println)
      .limit(2)
      .collect(Collectors.toList())

just use

IntStream.range(0, /* limit */ 2).unordered().parallel()
         .mapToObj(unused -> this.myGenerator())
         .peek(System.out::println)
         .collect(Collectors.toList())

Or, closer to your sample code

public Stream<Solution> generateSolutions(int limit) {
    final Solution initialSolution = initialSolutionMaker.findSolution();
    return Stream.concat(
         Stream.of(initialSolution),
         IntStream.range(1, limit).unordered().parallel()
               .mapToObj(unused -> continuousSolutionMaker.findSolution())
   );
}

new Solver(instance).generateSolutions(5)
    .map(Solution::getPurpose)
    .peek(System.out::println)
    .collect(Collectors.toList());