+str #19020 reduce combinator

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

@@ -396,6 +396,22 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def fold[T](zero: T)(f: function.Function2[T, Out, T]): SubSource[T, Mat] =
     new SubSource(delegate.fold(zero)(f.apply))
 
+  /**
+   * Similar to `fold` but uses first element as zero element.
+   * Applies the given function towards its current and next value,
+   * yielding the next current value.
+   *
+   * '''Emits when''' upstream completes
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes
+   *
+   * '''Cancels when''' downstream cancels
+   */
+  def reduce(f: function.Function2[Out, Out, Out @uncheckedVariance]): SubSource[Out, Mat] =
+    new SubSource(delegate.reduce(f.apply))
+
   /**
    * Intersperses stream with provided element, similar to how [[scala.collection.immutable.List.mkString]]
    * injects a separator between a List's elements.