StreamConverters provides extension methods and other functionality to ease interoperability of Scala collections with java.util.stream classes.

Scala collections gain extension methods seqStream and parStream that allow them to be used as the source of a Stream. Some collections either intrinsically cannot be paralellized, or could be but an efficient implementation is missing. It this case, only seqStream is provided. If a collection cannot be stepped over at all (e.g. Traversable), then it gains neither method.

Array also gains seqStream and parStream methods, and calling those on Array[Double], Array[Int], or Array[Long] will produce the corresponding primitive stream.

Streams gain accumulate and toScala[_] methods, which collect the stream into a custom high-performance scala.collection.mutable.java8.Accumulator, which is not part of the standard collections hierarchy, or into a named Scala collection, respectively.

Generic streams also gain an unboxed method that will convert to the corresponding unboxed primitive stream, if appropriate. Unboxed streams have custom accumulators with improved performance.

Accumulators have toArray, toList, iterator, and to[_] methods to convert to standard Scala collections. Note that if you wish to create an array from a Stream, going through an Accumulator is not the most efficient option: just create the Array directly.

Internally, Scala collections implement a hybrid of Iterator and java.util.Spliterator to implement Stream compatibility; these are called Steppers. In particular, they can test for the presence of a next element using hasStep, can retrieve the next value with nextStep, or can optionally retrieve and operate on a value if present with tryStep, which works like tryAdvance in java.util.Spliterator.

Every Scala collection that can be stepped through has a stepper method implicitly provided. In addition, maps have keyStepper and valueStepper methods. A limited number of collections operations are defined on Steppers, including conversion to Scala collections with to or accumulation via accumulate. Steppers also implement seqStream and parStream to generate Streams. These are provided regardless of whether a Stepper can efficiently subdivide itself for parallel processing (though one can check for the presence of the EfficientSubstep trait to know that parallel execution will not be limited by long sequential searching steps, and one can call anticipateParallelism to warn a Stepper that it will be used in a parallel context and thus may wish to make different tradeoffs).

Examples:

import scala.compat.java8.StreamConverters._

val s = Vector(1,2,3,4).parStream    // Stream[Int]
val si = s.unboxed                   // Stream.OfInt
val ai = si.accumulate               // IntAccumulator
val v = ai.to[Vector]                // Vector[Int] again

val t = Array(2.0, 3.0, 4.0).parStream               // DoubleStream
val q = t.toScala[scala.collection.immutable.Queue]  // Queue[Double]

val x = List(1L, 2L, 3L, 4L).stepper.parStream.sum   // 10, potentially computed in parallel