Add source.completionStage example #25468 (#28338)

akka-docs/src/main/paradox/stream/operators/Source/completionStage.md

@@ -30,3 +30,9 @@ For the corresponding operator for the Scala standard library `Future` see @ref:
 
 @@@
 
+## Example
+
+Java
+:  @@snip [SourceFromCompletionStage.java](/akka-docs/src/test/java/jdocs/stream/operators/source/FromCompletionStage.java) { #sourceFromCompletionStage }
+
+For the corresponding operator for the Scala standard library `Future` see @ref:[future](future.md).

akka-docs/src/main/paradox/stream/operators/Source/future.md

@@ -1,4 +1,4 @@
-# fromFuture
+# future
 
 Send the single value of the `Future` when it completes and there is demand.
 
@@ -20,6 +20,7 @@ If the future fails the stream is failed with that exception.
 For the corresponding operator for the Java standard library `CompletionStage` see @ref:[completionStage](completionStage.md).
 
 ## Example
+
 Scala
 :  @@snip [SourceFromFuture.scala](/akka-docs/src/test/scala/docs/stream/operators/SourceOperators.scala) { #sourceFromFuture }
 

akka-docs/src/test/java/jdocs/stream/operators/source/FromCompletionStage.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2018-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+// #sourceFromCompletionStage
+import java.util.concurrent.CompletionStage;
+import java.util.concurrent.CompletableFuture;
+
+import akka.NotUsed;
+import akka.Done;
+import akka.actor.typed.ActorSystem;
+import akka.stream.javadsl.*;
+
+// #sourceFromCompletionStage
+
+class FromCompletionStage {
+
+  public static void sourceFromCompletionStage() {
+    // Use one ActorSystem per application
+    ActorSystem system = null;
+
+    // #sourceFromCompletionStage
+    CompletionStage<Integer> stage = CompletableFuture.completedFuture(10);
+
+    Source<Integer, NotUsed> source = Source.completionStage(stage);
+
+    Sink<Integer, CompletionStage<Done>> sink = Sink.foreach(i -> System.out.println(i.toString()));
+
+    source.runWith(sink, system); // 10
+    // #sourceFromCompletionStage
+  }
+}