Update on JEP-461: Stream Gatherers (Preview)

[Viktor Klang]

Greetings,

As you may have already seen, Stream Gatherers is now a Preview JEP with Candidate status<https://openjdk.org/jeps/461>

Work-in-progress (interfaces, implementation, tests, benches, and documentation) for JEP-461 is currently available here<https://github.com/viktorklang-ora/jdk/tree/gatherer>.

While the design<https://cr.openjdk.org/~vklang/Gatherers.html> has held up well, there are some important improvements made since the original design document was written.

Notable changes (without any particular order):

  *   Stream::gather() now has a default implementation.

  *   Gatherer::supplier() was renamed to Gatherer::initializer() to better reflect intent.

  *   Gatherer.Sink<R> was renamed to Gatherer.Downstream<R> to better signal what it represents.

  *   Gatherer::collect(Collector) and its companion type Gatherer.ThenCollector was dropped due to compatibility concerns with existing code which operates on Collector.

  *
Gatherer.Characteristics have been eliminated and superseded by having default values that are used as sentinels.
 (discussed further down the list)
This is important because with the Characteristics-model keeping alignment between Characteristics and actual implementation proved brittle, and under composition of Gatherers computing the union was inefficient and mostly lead to an empty set in the end.

  *   Gatherer.defaultInitializer(), Gatherer.defaultCombiner(), and Gatherer.defaultFinisher() were added as static methods—these are the sentinels used to elide calling the initializer, to elide calling the combiner (avoid parallelization), and to elide calling the finisher, respectively.

  *   Gatherer::initializer(), Gatherer::combiner(), and Gatherer::finisher() default implementations return the respective sentinels.

  *   A subtype of Integrator named Greedy was added, together with a factory method to guide lambda-to-nominal-type conversion. This allows creators of Gatherers to signal that an Integrator will never initiate a short-circuit (but may relay one from downstream), and that is available during evaluation to determine if the operation can short-circuit or not.

  *   Factories for creating anonymous Gatherers were expanded upon to include Gatherer.of() and Gatherer.ofSequential() with different sets of parameters, primarily to make it more ergonomical and easier to read and write the code using those factories.

  *   A curated set of initial built-in Gatherers is located in java.util.stream.Gatherers

I recently presented Gatherers at Devoxx<https://www.youtube.com/watch?v=8fMFa6OqlY8>, which I'd recommend watching for an introduction to the feature.
[cid:d048f613-b144-4ff9-a2d3-98e0765aab57]<https://www.youtube.com/watch?v=8fMFa6OqlY8>
Teaching old Streams new tricks By Viktor Klang<https://www.youtube.com/watch?v=8fMFa6OqlY8>
Have you ever wanted to perform an operation on a java.util.stream.Stream only to find that the existing set of operations didn't provide what you needed—forcing you to break out early from the Stream and perform the logic outside of it? As a matter of fact, java.util.stream was the first JDK API designed with lambdas in mind and was ...
www.youtube.com
      

Cheers,
√


Viktor Klang
Software Architect, Java Platform Group
Oracle
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