Hello! Current implementation of Collections.nCopies().stream() is as follows:
http://hg.openjdk.java.net/jdk9/dev/jdk/file/f160dec9a350/src/java.base/share/classes/java/util/Collections.java#l5066 public Stream<E> stream() { return IntStream.range(0, n).mapToObj(i -> element); } @Override public Stream<E> parallelStream() { return IntStream.range(0, n).parallel().mapToObj(i -> element); } The problem is that it adds a lambda expression to the RangeIntSpliterator type profile which can be polluted by some other code and vice versa: using nCopies().stream() may pollute the type profile for other code making it slower. Another thing which is missing in current implementation is unordered mode. This collection is unordered by nature, its stream is similar to Stream.generate(), so to my opinion it should be unordered which may improve the parallel reduction in some cases. This can be improved by introducing the custom spliterator class which is quite simple: https://gist.github.com/amaembo/62f3efee9923b1468e86 On pre-polluted type profile with simple mapping and reduction using custom spliterator is about 25-30% faster in both parallel and sequential cases as benchmarking shows (performed on 4-core cpu). What do you think? With best regards, Tagir Valeev.
