+str Add Flow#flattenOptional (#31285)

akka-docs/src/main/paradox/stream/operators/Flow/flattenOptional.md

@@ -0,0 +1,29 @@
+# Flow.flattenOptional
+
+Collect the value of `Optional` from all the elements passing through this flow , empty `Optional` is filtered out.
+
+@ref[Simple operators](../index.md#simple-operators)
+
+## Signature
+
+@apidoc[Flow.flattenOptional](Flow$) { java="#flattenOptional(akka.stream.javadsl.Flow)" }
+
+
+## Description
+
+Streams the elements through the given future flow once it successfully completes. 
+If the future fails the stream is failed.
+
+## Reactive Streams semantics
+
+@@@div { .callout }
+
+**Emits when** the current @javadoc[Optional](java.util.Optional)'s value is present.
+
+**Backpressures when** the value of the current @javadoc[Optional](java.util.Optional) is present and downstream backpressures.
+
+**Completes when** upstream completes.
+
+**Cancels when** downstream cancels.
+@@@
+

akka-docs/src/main/paradox/stream/operators/index.md

@@ -150,6 +150,7 @@ depending on being backpressured by downstream or not.
 |Source/Flow|<a name="dropwhile"></a>@ref[dropWhile](Source-or-Flow/dropWhile.md)|Drop elements as long as a predicate function return true for the element|
 |Source/Flow|<a name="filter"></a>@ref[filter](Source-or-Flow/filter.md)|Filter the incoming elements using a predicate.|
 |Source/Flow|<a name="filternot"></a>@ref[filterNot](Source-or-Flow/filterNot.md)|Filter the incoming elements using a predicate.|
+|Flow|<a name="flattenoptional"></a>@ref[flattenOptional](Flow/flattenOptional.md)|Collect the value of `Optional` from all the elements passing through this flow , empty `Optional` is filtered out.|
 |Source/Flow|<a name="fold"></a>@ref[fold](Source-or-Flow/fold.md)|Start with current value `zero` and then apply the current and next value to the given function. When upstream completes, the current value is emitted downstream.|
 |Source/Flow|<a name="foldasync"></a>@ref[foldAsync](Source-or-Flow/foldAsync.md)|Just like `fold` but receives a function that results in a @scala[`Future`] @java[`CompletionStage`] to the next value.|
 |Source/Flow|<a name="frommaterializer"></a>@ref[fromMaterializer](Source-or-Flow/fromMaterializer.md)|Defer the creation of a `Source/Flow` until materialization and access `Materializer` and `Attributes`|
@@ -423,6 +424,7 @@ For more background see the @ref[Error Handling in Streams](../stream-error.md)
 * [flatMapConcat](Source-or-Flow/flatMapConcat.md)
 * [flatMapMerge](Source-or-Flow/flatMapMerge.md)
 * [flatMapPrefix](Source-or-Flow/flatMapPrefix.md)
+* [flattenOptional](Flow/flattenOptional.md)
 * [fold](Source-or-Flow/fold.md)
 * [fold](Sink/fold.md)
 * [foldAsync](Source-or-Flow/foldAsync.md)

akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java

@@ -1179,16 +1179,21 @@ public class SourceTest extends StreamTest {
   }
 
   @Test
-  public void mustRunSourceAndIgnoreElementsItOutputsAndOnlySignalTheCompletion() {
+  public void mustRunSourceAndIgnoreElementsItOutputsAndOnlySignalTheCompletion() throws Exception {
     final Iterator<Integer> iterator = IntStream.range(1, 10).iterator();
     final Creator<Iterator<Integer>> input = () -> iterator;
     final Done completion =
-        Source.fromIterator(input).map(it -> it * 10).run(system).toCompletableFuture().join();
+        Source.fromIterator(input)
+            .map(it -> it * 10)
+            .run(system)
+            .toCompletableFuture()
+            .get(1, TimeUnit.SECONDS);
     assertEquals(Done.getInstance(), completion);
   }
 
   @Test
-  public void mustRunSourceAndIgnoreElementsItOutputsAndOnlySignalTheCompletionWithMaterializer() {
+  public void mustRunSourceAndIgnoreElementsItOutputsAndOnlySignalTheCompletionWithMaterializer()
+      throws Exception {
     final Materializer materializer = Materializer.createMaterializer(system);
     final Iterator<Integer> iterator = IntStream.range(1, 10).iterator();
     final Creator<Iterator<Integer>> input = () -> iterator;
@@ -1197,7 +1202,7 @@ public class SourceTest extends StreamTest {
             .map(it -> it * 10)
             .run(materializer)
             .toCompletableFuture()
-            .join();
+            .get(3, TimeUnit.SECONDS);
     assertEquals(Done.getInstance(), completion);
   }
 
@@ -1235,4 +1240,31 @@ public class SourceTest extends StreamTest {
             1346269, 2178309, 3524578, 5702887, 9227465),
         resultList);
   }
+
+  @Test
+  public void flattenOptional() throws Exception {
+    // #flattenOptional
+    final CompletionStage<List<Integer>> resultList =
+        Source.range(1, 10)
+            .map(x -> Optional.of(x).filter(n -> n % 2 == 0))
+            .via(Flow.flattenOptional())
+            .runWith(Sink.seq(), system);
+    // #flattenOptional
+    Assert.assertEquals(
+        Arrays.asList(2, 4, 6, 8, 10), resultList.toCompletableFuture().get(3, TimeUnit.SECONDS));
+  }
+
+  @Test
+  public void flattenOptionalOptional() throws Exception {
+    final List<Integer> resultList =
+        Source.range(1, 10)
+            .map(x -> Optional.of(x).filter(n -> n % 2 == 0))
+            .map(Optional::ofNullable)
+            .via(Flow.flattenOptional())
+            .via(Flow.flattenOptional())
+            .runWith(Sink.seq(), system)
+            .toCompletableFuture()
+            .get(3, TimeUnit.SECONDS);
+    Assert.assertEquals(Arrays.asList(2, 4, 6, 8, 10), resultList);
+  }
 }

akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala

@@ -371,6 +371,23 @@ object Flow {
   def upcast[In, SuperOut, Out <: SuperOut, M](flow: Flow[In, Out, M]): Flow[In, SuperOut, M] =
     flow.asInstanceOf[Flow[In, SuperOut, M]]
 
+  /**
+   * Collect the value of [[Optional]] from the elements passing through this flow, empty [[Optional]] is filtered out.
+   *
+   * Adheres to the [[ActorAttributes.SupervisionStrategy]] attribute.
+   *
+   * '''Emits when''' the current [[Optional]]'s value is present.
+   *
+   * '''Backpressures when''' the value of the current [[Optional]] is present and downstream backpressures
+   *
+   * '''Completes when''' upstream completes
+   *
+   * '''Cancels when''' downstream cancels
+   * * */
+  def flattenOptional[Out, In <: Optional[Out]](): Flow[In, Out, NotUsed] =
+    new Flow(scaladsl.Flow[In].collect {
+      case optional: Optional[Out @unchecked] if optional.isPresent => optional.get()
+    })
 }
 
 /**