+ stream add collectType operator to Source,SubSource,Flow and SubFlow for javadsl.

add docs for collectType, do mirror it in scaladsl

mima

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

@@ -19,6 +19,7 @@ import java.util.concurrent.CompletionStage
 import akka.stream.impl.fusing.MapError
 
 import scala.compat.java8.FutureConverters._
+import scala.reflect.ClassTag
 
 /**
  * A “stream of streams” sub-flow of data elements, e.g. produced by `groupBy`.
@@ -294,6 +295,24 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def collect[T](pf: PartialFunction[Out, T]): SubSource[T, Mat] =
     new SubSource(delegate.collect(pf))
 
+  /**
+   * Transform this stream by testing the type of each of the elements
+   * on which the element is an instance of the provided type as they pass through this processing step.
+   * Non-matching elements are filtered out.
+   *
+   * Adheres to the [[ActorAttributes.SupervisionStrategy]] attribute.
+   *
+   * '''Emits when''' the element is an instance of the provided type
+   *
+   * '''Backpressures when''' the element is an instance of the provided type and downstream backpressures
+   *
+   * '''Completes when''' upstream completes
+   *
+   * '''Cancels when''' downstream cancels
+   */
+  def collectType[T](clazz: Class[T]): javadsl.SubSource[T, Mat] =
+    new SubSource(delegate.collectType[T](ClassTag[T](clazz)))
+
   /**
    * Chunk up this stream into groups of the given size, with the last group
    * possibly smaller than requested due to end-of-stream.