example for reduce operator #25468

akka-docs/src/main/paradox/stream/operators/Sink/reduce.md

@@ -27,3 +27,10 @@ Materializes into a @scala[`Future`] @java[`CompletionStage`] that will be compl
 
 @@@
 
+## Example
+
+Scala
+:   @@snip [SinkReduceSpec.scala](/akka-stream-tests/src/test/scala/akka/stream/scaladsl/SinkSpec.scala) { #reduce-operator-example }
+
+Java
+:   @@snip [SinkDocExamples.java](/akka-docs/src/test/java/jdocs/stream/operators/SinkDocExamples.java) { #reduce-operator-example }

akka-docs/src/test/java/jdocs/stream/operators/SinkDocExamples.java

@@ -0,0 +1,35 @@
+/**
+ * Copyright (C) 2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators;
+
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.ActorMaterializer;
+import akka.stream.Materializer;
+import akka.stream.javadsl.Sink;
+import akka.stream.javadsl.Source;
+
+import java.util.Arrays;
+import java.util.concurrent.CompletionStage;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+
+public class SinkDocExamples {
+
+    static void reduceExample() throws InterruptedException, ExecutionException, TimeoutException {
+
+        final ActorSystem system = ActorSystem.create("SourceFromExample");
+        final Materializer materializer = ActorMaterializer.create(system);
+        //#reduce-operator-example
+        Source<Integer, NotUsed> ints = Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
+        CompletionStage<Integer> sum = ints.runWith(Sink.reduce((a, b) -> a + b), materializer);
+        int result = sum.toCompletableFuture().get(3, TimeUnit.SECONDS);
+        System.out.println(result);
+        // 55
+        //#reduce-operator-example
+    }
+}
+

akka-stream-tests/src/test/scala/akka/stream/scaladsl/SinkSpec.scala

@@ -324,6 +324,18 @@ class SinkSpec extends StreamSpec with DefaultTimeout with ScalaFutures {
     }
   }
 
+  "The reduce sink" must {
+    "sum up 1 to 10 correctly" in {
+      //#reduce-operator-example
+      val source = Source(1 to 10)
+      val result = source.runWith(Sink.reduce[Int]((a, b) ⇒ a + b))
+      result.map(println)(system.dispatcher)
+      // 55
+      //#reduce-operator-example
+      assert(result.futureValue == (1 to 10 sum))
+    }
+  }
+
   "Sink pre-materialization" must {
     "materialize the sink and wrap its exposed publisher in a Source" in {
       val publisherSink: Sink[String, Publisher[String]] = Sink.asPublisher[String](false)