On Tue, 16 Nov 2021 20:53:26 GMT, kabutz <[email protected]> wrote:
>> This is a draft proposal for how we could improve stream performance for the
>> case where the streams are empty. Empty collections are common-place. If we
>> iterate over them with an Iterator, we would have to create one small
>> Iterator object (which could often be eliminated) and if it is empty we are
>> done. However, with Streams we first have to build up the entire pipeline,
>> until we realize that there is no work to do. With this example, we change
>> Collection#stream() to first check if the collection is empty, and if it is,
>> we simply return an EmptyStream. We also have EmptyIntStream,
>> EmptyLongStream and EmptyDoubleStream. We have taken great care for these to
>> have the same characteristics and behaviour as the streams returned by
>> Stream.empty(), IntStream.empty(), etc.
>>
>> Some of the JDK tests fail with this, due to ClassCastExceptions (our
>> EmptyStream is not an AbstractPipeline) and AssertionError, since we can
>> call some methods repeatedly on the stream without it failing. On the plus
>> side, creating a complex stream on an empty stream gives us upwards of 50x
>> increase in performance due to a much smaller object allocation rate. This
>> PR includes the code for the change, unit tests and also a JMH benchmark to
>> demonstrate the improvement.
>
> kabutz has updated the pull request incrementally with one additional commit
> since the last revision:
>
> Refactored empty stream implementations to reduce duplicate code and
> improved unordered()
> Added StreamSupport.empty for primitive spliterators and use that in
> Arrays.stream()
src/java.base/share/classes/java/util/stream/StreamSupport.java line 339:
> 337: * @return a new sequential empty {@code IntStream}
> 338: */
> 339: public static IntStream emptyIntStream(Spliterator.OfInt
> spliterator) {
In the interest of not adding new public APIs, can these be removed in favour
of eg, IntStream.empty()? It wouldn’t take the spliterator, but perhaps it
isn’t necessary.
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PR: https://git.openjdk.org/jdk/pull/6275
