A Java 8 compatibility kit for Scala.
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README.md

scala-java8-compat

A Java 8 compatibility kit for Scala.

The API is currently still experimental: we do not yet guarantee source or binary compatibility with future releases.

Functional Interfaces for Scala functions

A set of Functional Interfaces for scala.FunctionN. These are designed for convenient construction of Scala functions using Java 8 lambda syntax.

Usage

import scala.concurrent.*;
import static scala.compat.java8.JFunction.*;

class Test {
    private static Future<Integer> futureExample(Future<String> future, ExecutionContext ec) {
        return future.map(func(s -> s.toUpperCase()), ec).map(func(s -> s.length()), ec);
    }
}

More Examples / Documentation

Converters between scala.FunctionN and java.util.function

A set of converters that enable interconversion between Java's standard Functional Interfaces defined in java.util.function and Scala's Function0, Function1, and Function2 traits. These are intended for use when you already have an instance of a java.util.function and need a Scala function, or have a Scala function and need an instance of a java.util.function.

The .asScala extension method will convert a java.util.function to the corresponding Scala function. The .asJava extension method will convert a Scala function to the most specific corresponding Java functional interface. If you wish to obtain a less specific functional interface, there are named methods that start with asJava and continue with the name of the Java functional interface. For instance, the most specific interface corresponding to the Scala function val rev = (s: String) => s.reverse is UnaryOperator[String], and that is what rev.asJava will produce. However, asJavaFunction(rev) will return a java.util.function.Function[String, String] instead.

The asJava methods can also be called conveniently from Java. There are additional asScalaFrom methods (e.g. asScalaFromUnaryOperator) that will perform the functional-interface-to-Scala-function conversion; this is primarily of use when calling from Java since the .asScala extension method is more convenient in Scala.

Usage examples

In Scala:

import java.util.function._
import scala.compat.java8.FunctionConverters._

val foo: Int => Boolean = i => i > 7
def testBig(ip: IntPredicate) = ip.test(9)
println(testBig(foo.asJava))  // Prints true

val bar = new UnaryOperator[String]{ def apply(s: String) = s.reverse }
List("cod", "herring").map(bar.asScala)    // List("doc", "gnirrih")

def testA[A](p: Predicate[A])(a: A) = p.test(a)
println(testA(asJavaPredicate(foo))(4))  // Prints false

// println(testA(foo.asJava)(4))  <-- doesn't work
//                                    IntPredicate does not extend Predicate!

In Java:

import java.util.function.*;
import scala.compat.java8.FunctionConverters;

class Example {
  String foo(UnaryOperator<String> f) {
    return f.apply("halibut");
  }
  String bar(scala.Function1<String, String> f) {
    return foo(functionConverters.asJavaUnaryOperator(f));
  }
  String baz(Function<String, String> f) {
    return bar(functionConverters.asScalaFromFunction(f));
  }
}

Converters between scala.concurrent and java.util.concurrent

Converters between scala.Option and java.util classes Optional, OptionalDouble, OptionalInt, and OptionalLong.

A set of extension methods to enable explicit conversion between Scala Option and the Java 8 optional types, Optional, OptionalDouble, OptionalInt, and OptionalLong.

Note that the four Java classes have no inheritance relationship despite all encoding optional types.

Usage example

import scala.compat.java8.OptionConverters._

class Test {
  val o = Option(2.7)
  val oj = o.asJava        // Optional[Double]
  val ojd = o.asPrimitive  // OptionalDouble
  val ojds = ojd.asScala   // Option(2.7) again
}

Converters from Scala collections to Java 8 Streams

Scala collections gain seqStream and parStream as extension methods that produce a Java 8 Stream running sequentially or in parallel, respectively. These are automatically specialized to a primitive type if possible, including automatically applied widening conversions. For instance, List(1,2).seqStream produces an IntStream, and so does List(1.toShort, 2.toShort).parStream. Maps additionally have seqKeyStream, seqValueStream, parKeyStream, and parValueStream methods.

Scala collections also gain accumulate and stepper methods that produce utility collections that can be useful when working with Java 8 Streams. accumulate produces an Accumulator or its primitive counterpart (DoubleAccumulator, etc.), which is a low-level collection designed for efficient collection and dispatching of results to and from Streams. Unlike most collections, it can contain more than Int.MaxValue elements.

stepper produces a Stepper which is a fusion of Spliterator and Iterator. Steppers underlie the Scala collections' instances of Java 8 Streams. Steppers are intended as low-level building blocks for streams. Usually you would not create them directly or call their methods but you can implement them alongside custom collections to get better performance when streaming from these collections.

Java 8 Streams gain toScala[Coll] and accumulate methods, to make it easy to produce Scala collections or Accumulators, respectively, from Java 8 Streams. For instance, myStream.to[Vector] will collect the contents of a Stream into a scala.collection.immutable.Vector. Note that standard sequential builders are used for collections, so this is best done to gather the results of an expensive computation.

Finally, there is a Java class, ScalaStreamSupport, that has a series of stream methods that can be used to obtain Java 8 Streams from Scala collections from within Java.

Performance Considerations

For sequential operations, Scala's iterator almost always equals or exceeds the performance of a Java 8 stream. Thus, one should favor iterator (and its richer set of operations) over seqStream for general use. However, long chains of processing of primitive types can sometimes benefit from the manually specialized methods in DoubleStream, IntStream, and LongStream.

Note that although iterator typically has superior performance in a sequential context, the advantage is modest (usually less than 50% higher throughput for iterator).

For parallel operations, parStream and even seqStream.parallel meets or exceeds the performance of Scala parallel collections methods (invoked with .par). Especially for small collections, the difference can be substantial. In some cases, when a Scala (parallel) collection is the ultimate result, Scala parallel collections can have an advantage as the collection can (in some cases) be built in parallel.

Because the wrappers are invoked based on the static type of the collection, there are also cases where parallelization is inefficient when interfacing with Java 8 Streams (e.g. when a collection is typed as Seq[String] so might have linear access like List, but actually is a WrappedArray[String] that can be efficiently parallelized) but can be efficient with Scala parallel collections. The parStream method is only available when the static type is known to be compatible with rapid parallel operation; seqStream can be parallelized by using .parallel, but may or may not be efficient.

If the operations available on Java 8 Streams are sufficient, the collection type is known statically with enough precision to enable parStream, and an Accumulator or non-collection type is an acceptable result, Java 8 Streams will essentially always outperform the Scala parallel collections.

Scala Usage Example

import scala.compat.java8.StreamConverters._

object Test {
  val m = collection.immutable.HashMap("fish" -> 2, "bird" -> 4)
  val s = m.parValueStream.sum          // 6, potientially computed in parallel
  val t = m.seqKeyStream.toScala[List]  // List("fish", "bird")
  val a = m.accumulate                  // Accumulator[(String, Int)]

  val n = a.stepper.fold(0)(_ + _._1.length) +
          a.parStream.count             // 8 + 2 = 10

  val b = java.util.Arrays.stream(Array(2L, 3L, 4L)).
          accumulate                    // LongAccumulator
    val l = b.to[List]                    // List(2L, 3L, 4L)
}

Using Java 8 Streams with Scala Function Converters

Scala can emit Java SAMs for lambda expressions that are arguments to methods that take a Java SAM rather than a Scala Function. However, it can be convenient to restrict the SAM interface to interactions with Java code (including Java 8 Streams) rather than having it propagate throughout Scala code.

Using Java 8 Stream converters together with function converters allows one to accomplish this with only a modest amount of fuss.

Example:

import scala.compat.java8.FunctionConverters._
import scala.compat.java8.StreamConverters._

def mapToSortedString[A](xs: Vector[A], f: A => String, sep: String) =
  xs.parStream.                     // Creates java.util.stream.Stream[String]
    map[String](f.asJava).sorted.   // Maps A to String and sorts (in parallel)
    toArray.mkString(sep)           // Back to an Array to use Scala's mkString

Note that explicit creation of a new lambda will tend to lead to improved type inference and at least equal performance:

def mapToSortedString[A](xs: Vector[A], f: A => String, sep: String) =
  xs.parStream.
    map[String](a => f(a)).sorted.  // Explicit lambda creates a SAM wrapper for f
    toArray.mkString(sep)

Java Usage Example

To convert a Scala collection to a Java 8 Stream from within Java, it usually suffices to call ScalaStreamSupport.stream(xs) on your collection xs. If xs is a map, you may wish to get the keys or values alone by using fromKeys or fromValues. If the collection has an underlying representation that is not efficiently parallelized (e.g. scala.collection.immutable.List), then fromAccumulated (and fromAccumulatedKeys and fromAccumulatedValues) will first gather the collection into an Accumulator and then return a stream over that accumulator. If not running in parallel, from is preferable (faster and less memory usage).

Note that a Scala Iterator cannot fulfill the contract of a Java 8 Stream (because it cannot support trySplit if it is called). Presently, one must call fromAccumulated on the Iterator to cache it, even if the Stream will be evaluated sequentially, or wrap it as a Java Iterator and use static methods in Spliterator to wrap that as a Spliterator and then a Stream.

Here is an example of conversion of a Scala collection within Java 8:

import scala.collection.mutable.ArrayBuffer;
import scala.compat.java8.ScalaStreamSupport;

public class StreamConvertersExample {
  public int MakeAndUseArrayBuffer() {
    ArrayBuffer<String> ab = new ArrayBuffer<String>();
    ab.$plus$eq("salmon");
    ab.$plus$eq("herring");
    return ScalaStreamSupport.stream(ab).mapToInt(x -> x.length()).sum();  // 6+7 = 13
  }
}