+str #19020 reduce combinator

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

@@ -29,6 +29,16 @@ object Sink {
   def fold[U, In](zero: U, f: function.Function2[U, In, U]): javadsl.Sink[In, Future[U]] =
     new Sink(scaladsl.Sink.fold[U, In](zero)(f.apply))
 
+  /**
+   * A `Sink` that will invoke the given function for every received element, giving it its previous
+   * output (from the second element) and the element as input.
+   * The returned [[scala.concurrent.Future]] will be completed with value of the final
+   * function evaluation when the input stream ends, or completed with `Failure`
+   * if there is a failure signaled in the stream.
+   */
+  def reduce[In](f: function.Function2[In, In, In]): Sink[In, Future[In]] =
+    new Sink(scaladsl.Sink.reduce[In](f.apply))
+
   /**
    * Helper to create [[Sink]] from `Subscriber`.
    */