Docs: Example on the as/fromJavaStream operators (#28745)

akka-docs/src/test/scala/docs/stream/operators/converters/StreamConvertersToJava.scala

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.converters
+
+// #import
+import java.util.stream
+import java.util.stream.IntStream
+
+import akka.NotUsed
+import akka.stream.scaladsl.Keep
+import akka.stream.scaladsl.Sink
+import akka.stream.scaladsl.Source
+import akka.stream.scaladsl.StreamConverters
+// #import
+import akka.testkit.AkkaSpec
+import org.scalatest.concurrent.Futures
+
+import scala.collection.immutable
+import scala.concurrent.Future
+
+class StreamConvertersToJava extends AkkaSpec with Futures {
+
+  "demonstrate materialization to Java8 streams" in {
+    //#asJavaStream
+    val source: Source[Int, NotUsed] = Source(0 to 9).filter(_ % 2 == 0)
+
+    val sink: Sink[Int, stream.Stream[Int]] = StreamConverters.asJavaStream[Int]()
+
+    val jStream: java.util.stream.Stream[Int] = source.runWith(sink)
+    //#asJavaStream
+    jStream.count should be(5)
+  }
+
+  "demonstrate conversion from Java8 streams" in {
+    //#fromJavaStream
+    def factory(): IntStream = IntStream.rangeClosed(0, 9)
+    val source: Source[Int, NotUsed] = StreamConverters.fromJavaStream(factory).map(_.intValue())
+    val sink: Sink[Int, Future[immutable.Seq[Int]]] = Sink.seq[Int]
+
+    val futureInts: Future[immutable.Seq[Int]] = source.toMat(sink)(Keep.right).run
+
+    //#fromJavaStream
+    whenReady(futureInts) { ints =>
+      ints should be((0 to 9).toSeq)
+    }
+
+  }
+
+}