diff --git a/akka-stream-tests/src/test/java/akka/stream/actor/ActorSubscriberTest.java b/akka-stream-tests/src/test/java/akka/stream/actor/ActorSubscriberTest.java
index 738acf9046..5787bb28a1 100644
--- a/akka-stream-tests/src/test/java/akka/stream/actor/ActorSubscriberTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/actor/ActorSubscriberTest.java
@@ -28,7 +28,7 @@ public class ActorSubscriberTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("FlowTest", AkkaSpec.testConf());
+      new AkkaJUnitActorSystemResource("ActorSubscriberTest", AkkaSpec.testConf());
 
   public static class TestSubscriber extends UntypedActorSubscriber {
 
diff --git a/akka-stream-tests/src/test/java/akka/stream/io/OutputStreamSourceTest.java b/akka-stream-tests/src/test/java/akka/stream/io/OutputStreamSourceTest.java
index 94aabc2786..df6e0bce74 100644
--- a/akka-stream-tests/src/test/java/akka/stream/io/OutputStreamSourceTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/io/OutputStreamSourceTest.java
@@ -30,7 +30,7 @@ public class OutputStreamSourceTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("OutputStreamSourceTest2", Utils.UnboundedMailboxConfig());
+      new AkkaJUnitActorSystemResource("OutputStreamSourceTest", Utils.UnboundedMailboxConfig());
 
   @Test
   public void mustSendEventsViaOutputStream() throws Exception {
diff --git a/akka-stream-tests/src/test/java/akka/stream/io/SinkAsJavaSourceTest.java b/akka-stream-tests/src/test/java/akka/stream/io/SinkAsJavaSourceTest.java
index 8cd3c0641c..d812404279 100644
--- a/akka-stream-tests/src/test/java/akka/stream/io/SinkAsJavaSourceTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/io/SinkAsJavaSourceTest.java
@@ -27,7 +27,7 @@ public class SinkAsJavaSourceTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("OutputStreamSource", Utils.UnboundedMailboxConfig());
+      new AkkaJUnitActorSystemResource("SinkAsJavaSourceTest", Utils.UnboundedMailboxConfig());
 
   @Test
   public void mustBeAbleToUseAsJavaStream() throws Exception {
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
index e2cd937305..78bda44fe5 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
@@ -1301,4 +1301,11 @@ public class FlowTest extends StreamTest {
 
     assertEquals((Object) 1, result);
   }
+
+  @Test
+  public void mustBeAbleToConvertToJavaInJava() {
+    final akka.stream.scaladsl.Flow<Integer, Integer, NotUsed> scalaFlow =
+        akka.stream.scaladsl.Flow.apply();
+    Flow<Integer, Integer, NotUsed> javaFlow = scalaFlow.asJava();
+  }
 }
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/KillSwitchTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/KillSwitchTest.java
index f4b2b76c40..ae2c72fe1f 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/KillSwitchTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/KillSwitchTest.java
@@ -27,7 +27,7 @@ public class KillSwitchTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("FlowTest", AkkaSpec.testConf());
+      new AkkaJUnitActorSystemResource("KillSwitchTest", AkkaSpec.testConf());
 
   @Test
   public void beAbleToUseKillSwitch() throws Exception {
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/RunnableGraphTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/RunnableGraphTest.java
new file mode 100644
index 0000000000..eccc6057eb
--- /dev/null
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/RunnableGraphTest.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.stream.javadsl;
+
+import akka.NotUsed;
+import akka.stream.StreamTest;
+import akka.testkit.AkkaJUnitActorSystemResource;
+import akka.testkit.AkkaSpec;
+import org.junit.ClassRule;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class RunnableGraphTest extends StreamTest {
+  public RunnableGraphTest() {
+    super(actorSystemResource);
+  }
+
+  @ClassRule
+  public static AkkaJUnitActorSystemResource actorSystemResource =
+      new AkkaJUnitActorSystemResource("RunnableGraphTest", AkkaSpec.testConf());
+
+  @Test
+  public void beAbleToConvertFromJavaToScala() {
+    final RunnableGraph<NotUsed> javaRunnable = Source.empty().to(Sink.ignore());
+    final akka.stream.scaladsl.RunnableGraph<NotUsed> scalaRunnable = javaRunnable.asScala();
+    assertEquals(NotUsed.getInstance(), scalaRunnable.run(materializer));
+  }
+
+  @Test
+  public void beAbleToConvertFromScalaToJava() {
+    final akka.stream.scaladsl.RunnableGraph<NotUsed> scalaRunnable =
+        akka.stream.scaladsl.Source.empty().to(akka.stream.scaladsl.Sink.ignore());
+    final RunnableGraph<NotUsed> javaRunnable = scalaRunnable.asJava();
+    assertEquals(NotUsed.getInstance(), javaRunnable.run(materializer));
+  }
+}
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/SinkTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/SinkTest.java
index 3502420adf..145bef5180 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/SinkTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/SinkTest.java
@@ -34,7 +34,7 @@ public class SinkTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("FlowTest", AkkaSpec.testConf());
+      new AkkaJUnitActorSystemResource("SinkTest", AkkaSpec.testConf());
 
   @Test
   public void mustBeAbleToUseFanoutPublisher() throws Exception {
@@ -146,4 +146,11 @@ public class SinkTest extends StreamTest {
             .addAttributes(Attributes.asyncBoundary())
             .named("");
   }
+
+  @Test
+  public void mustBeAbleToConvertToJavaInJava() {
+    final akka.stream.scaladsl.Sink<Integer, NotUsed> scalaSink =
+        akka.stream.scaladsl.Sink.cancelled();
+    Sink<Integer, NotUsed> javaSink = scalaSink.asJava();
+  }
 }
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
index 94d43dd8ec..485be034bb 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
@@ -1112,4 +1112,11 @@ public class SourceTest extends StreamTest {
     final Pair<NotUsed, Source<Integer, NotUsed>> p =
         Source.<Integer>empty().preMaterialize(materializer);
   }
+
+  @Test
+  public void mustBeAbleToConvertToJavaInJava() {
+    final akka.stream.scaladsl.Source<Integer, NotUsed> scalaSource =
+        akka.stream.scaladsl.Source.empty();
+    Source<Integer, NotUsed> javaSource = scalaSource.asJava();
+  }
 }
diff --git a/akka-stream-tests/src/test/java/akka/stream/stage/StageTest.java b/akka-stream-tests/src/test/java/akka/stream/stage/StageTest.java
index 8a35fbf812..3dd4f44cae 100644
--- a/akka-stream-tests/src/test/java/akka/stream/stage/StageTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/stage/StageTest.java
@@ -27,7 +27,7 @@ public class StageTest extends StreamTest {
 
   @ClassRule
   public static AkkaJUnitActorSystemResource actorSystemResource =
-      new AkkaJUnitActorSystemResource("FlowTest", AkkaSpec.testConf());
+      new AkkaJUnitActorSystemResource("StageTest", AkkaSpec.testConf());
 
   @Test
   public void javaStageUsage() throws Exception {
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/RunnableGraphSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/RunnableGraphSpec.scala
index cea94c8237..4b11a17d35 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/RunnableGraphSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/RunnableGraphSpec.scala
@@ -7,6 +7,7 @@ package akka.stream.scaladsl
 import akka.NotUsed
 import akka.stream.{ ActorMaterializer, Attributes }
 import akka.stream.testkit.StreamSpec
+import akka.stream.javadsl
 
 class RunnableGraphSpec extends StreamSpec {
 
@@ -18,8 +19,18 @@ class RunnableGraphSpec extends StreamSpec {
       import Attributes._
       val r: RunnableGraph[NotUsed] = RunnableGraph.fromGraph(Source.empty.to(Sink.ignore)).async.addAttributes(none).named("useless")
 
-      r.traversalBuilder.attributes.getFirst[Name] shouldEqual Some(Name("useless"))
-      r.traversalBuilder.attributes.getFirst[AsyncBoundary.type] shouldEqual (Some(AsyncBoundary))
+      r.traversalBuilder.attributes.get[Name] shouldEqual Some(Name("useless"))
+      r.traversalBuilder.attributes.get[AsyncBoundary.type] shouldEqual (Some(AsyncBoundary))
+    }
+
+    "allow conversion from scala to java" in {
+      val runnable: javadsl.RunnableGraph[NotUsed] = Source.empty.to(Sink.ignore).asJava
+      runnable.run(materializer) shouldBe NotUsed
+    }
+
+    "allow conversion from java to scala" in {
+      val runnable: RunnableGraph[NotUsed] = javadsl.Source.empty.to(javadsl.Sink.ignore).asScala
+      runnable.run() shouldBe NotUsed
     }
 
   }
diff --git a/akka-stream/src/main/mima-filters/2.5.20.backward.excludes b/akka-stream/src/main/mima-filters/2.5.20.backward.excludes
new file mode 100644
index 0000000000..7312f7b3af
--- /dev/null
+++ b/akka-stream/src/main/mima-filters/2.5.20.backward.excludes
@@ -0,0 +1,2 @@
+# #26246 Add asJava/asScala to RunnableGraph, not to be extended by users
+ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.javadsl.RunnableGraph.asScala")
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index b0aca138fc..3e4e7d6091 100755
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -3313,6 +3313,8 @@ object RunnableGraph {
       if (newRunnable eq runnable) this
       else new RunnableGraphAdapter(newRunnable)
     }
+
+    override def asScala: scaladsl.RunnableGraph[Mat] = runnable
   }
 }
 /**
@@ -3338,4 +3340,9 @@ abstract class RunnableGraph[+Mat] extends Graph[ClosedShape, Mat] {
 
   override def named(name: String): RunnableGraph[Mat] =
     withAttributes(Attributes.name(name))
+
+  /**
+   * Converts this Java DSL element to its Scala DSL counterpart.
+   */
+  def asScala: scaladsl.RunnableGraph[Mat]
 }
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Hub.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Hub.scala
index 12816d0d8f..ec71ccc6c4 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Hub.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Hub.scala
@@ -35,7 +35,7 @@ object MergeHub {
    */
   def of[T](clazz: Class[T], perProducerBufferSize: Int): Source[T, Sink[T, NotUsed]] = {
     akka.stream.scaladsl.MergeHub.source[T](perProducerBufferSize)
-      .mapMaterializedValue(_.asJava[T, NotUsed])
+      .mapMaterializedValue(_.asJava[T])
       .asJava
   }
 
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
index 2c5a13088e..a1d76ccfc3 100755
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
@@ -309,7 +309,7 @@ final class Flow[-In, +Out, +Mat](
       }
 
   /** Converts this Scala DSL element to it's Java DSL counterpart. */
-  def asJava[JIn <: In, JOut >: Out, JMat >: Mat]: javadsl.Flow[JIn, JOut, JMat] =
+  def asJava[JIn <: In]: javadsl.Flow[JIn, Out @uncheckedVariance, Mat @uncheckedVariance] =
     new javadsl.Flow(this)
 }
 
@@ -613,6 +613,9 @@ final case class RunnableGraph[+Mat](override val traversalBuilder: TraversalBui
    */
   override def async(dispatcher: String, inputBufferSize: Int): RunnableGraph[Mat] =
     super.async(dispatcher, inputBufferSize).asInstanceOf[RunnableGraph[Mat]]
+
+  /** Converts this Scala DSL element to it's Java DSL counterpart. */
+  def asJava: javadsl.RunnableGraph[Mat] = javadsl.RunnableGraph.fromGraph(this)
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
index 9d0a5778d6..2a91306354 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
@@ -23,6 +23,8 @@ import scala.util.{ Failure, Success, Try }
 import scala.collection.immutable
 import akka.util.ccompat._
 
+import scala.annotation.unchecked.uncheckedVariance
+
 /**
  * A `Sink` is a set of stream processing steps that has one open input.
  * Can be used as a `Subscriber`
@@ -120,7 +122,7 @@ final class Sink[-In, +Mat](
   /**
    * Converts this Scala DSL element to it's Java DSL counterpart.
    */
-  def asJava[JIn <: In, JMat >: Mat]: javadsl.Sink[JIn, JMat] = new javadsl.Sink(this)
+  def asJava[JIn <: In]: javadsl.Sink[JIn, Mat @uncheckedVariance] = new javadsl.Sink(this)
 }
 
 object Sink {
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
index 0861849ef1..0676cd1c00 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -194,7 +194,7 @@ final class Source[+Out, +Mat](
   /**
    * Converts this Scala DSL element to it's Java DSL counterpart.
    */
-  def asJava[JOut >: Out, JMat >: Mat]: javadsl.Source[JOut, JMat] = new javadsl.Source(this)
+  def asJava: javadsl.Source[Out @uncheckedVariance, Mat @uncheckedVariance] = new javadsl.Source(this)
 
   /**
    * Combines several sources with fan-in strategy like `Merge` or `Concat` and returns `Source`.