This object provides extension methods to createJava Streams that operate on Scala collections (sequentially or in parallel). For more information on Java streams, consult the documentation (https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html).

When writing Java code, use the explicit conversion methods defined injavaapi.StreamConverters instead.

The methodsasJavaSeqStream andasJavaParStream convert a collection to a Java Stream:

scala> import scala.jdk.StreamConverters._scala> val s = (1 to 10).toList.asJavaSeqStreams: java.util.stream.IntStream = java.util.stream.IntPipeline$Head@7b1e5e55scala> s.map(_ * 2).filter(_ > 5).toScala(List)res1: List[Int] = List(6, 8, 10, 12, 14, 16, 18, 20)

Note: using parallel streams in the Scala REPL causes deadlocks, seehttps://github.com/scala/bug/issues/9076. As a workaround, usescala -Yrepl-class-based.

scala> def isPrime(n: Int): Boolean = !(2 +: (3 to Math.sqrt(n).toInt by 2) exists (n % _ == 0))isPrime: (n: Int)Booleanscala> (10000 to 1000000).asJavaParStream.filter(isPrime).toScala(Vector)res6: scala.collection.immutable.Vector[Int] = Vector(10007, 10009, 10037, 10039, ...

A JavaStream provides operations on a sequence of elements. Streams are created fromSpliterators, which are similar to Iterators with the additional capability to partition off some of their elements. This partitioning, if supported by the Spliterator, is used for parallelizing Stream operations.

Scala collections have a methodstepper that returns ascala.collection.Stepper for the collection, which in turn can be converted to a Spliterator for creating a Java Stream.

TheasJavaSeqStream extension method is available on any Scala collection. TheasJavaParStream extension method can only be invoked on collections where the return type of thestepper method is marked with thescala.collection.Stepper.EfficientSplit marker trait. This trait is added to steppers that support partitioning, and therefore efficient parallel processing.

The following extension methods are available:

TheasJavaPrimitiveStream method converts aStream[Int] to anIntStream. It is the dual of theboxed method defined on primitive streams (e.g.,IntStream.boxed is aStream[Integer]).

ThetoScala extension methods on Java streams collects the result of a stream pipeline into a Scala collection, for examplestream.toScala(List),stream.toScala(Vector). Note that transformation operations on streams are lazy (also called "intermediate"), terminal operations such asforEach,count ortoScala trigger the evaluation.

Collecting a parallel stream to a collection can be performed in parallel. This is beneficial if the target collection supports efficient merging of the segments that are built in parallel. To support this use case, the Scala standard library provides theAccumulator collection. This collection supports efficient parallel construction, and it has specialized subtypes forInt,Long andDouble so that primitive Java streams can be collected to a Scala collection without boxing the elements.