Add examples for zipWithIndex #25468 (#25661)

* Add examples for zipWithIndex #25468

* better examples

akka-docs/src/main/paradox/stream/operators/Source-or-Flow/zipWithIndex.md

@@ -27,3 +27,10 @@ Zips elements of current flow with its indices.
 
 @@@
 
+## Example
+
+Scala
+:   @@snip [FlowZipWithIndexSpec.scala](/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowZipWithIndexSpec.scala) { #zip-with-index }
+
+Java
+:   @@snip [SourceOrFlow.java](/akka-docs/src/test/java/jdocs/stream/operators/SourceOrFlow.java) { #zip-with-index }

akka-docs/src/test/java/jdocs/stream/operators/SourceOrFlow.java

@@ -4,8 +4,15 @@
 
 package jdocs.stream.operators;
 
+import akka.stream.Materializer;
 import akka.stream.javadsl.Flow;
 
+//#zip-with-index
+import akka.stream.javadsl.Sink;
+import akka.stream.javadsl.Source;
+import java.util.Arrays;
+
+//#zip-with-index
 //#log
 import akka.stream.Attributes;
 import akka.stream.javadsl.Source;
@@ -29,6 +36,16 @@ class SourceOrFlow {
     ;
   }
 
+  void zipWithIndexExample() {
+    Materializer materializer = null;
+    //#zip-with-index
+    Source.from(Arrays.asList("apple", "orange", "banana"))
+            .zipWithIndex()
+            .runWith(Sink.foreach(System.out::print), materializer);
+    // this will print ('apple', 0), ('orange', 1), ('banana', 2)
+    //#zip-with-index
+  }
+  
   void conflateExample() {
     //#conflate
     Source.cycle(() -> Arrays.asList(1, 10, 100).iterator())

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowZipWithIndexSpec.scala

@@ -4,6 +4,11 @@
 
 package akka.stream.scaladsl
 
+//#zip-with-index
+import akka.stream.scaladsl.Source
+import akka.stream.scaladsl.Sink
+
+//#zip-with-index
 import akka.stream.testkit.Utils._
 import akka.stream.testkit.scaladsl.StreamTestKit._
 import akka.stream.{ ActorMaterializer, ActorMaterializerSettings }
@@ -21,7 +26,6 @@ class FlowZipWithIndexSpec extends StreamSpec {
     "work in the happy case" in assertAllStagesStopped {
       val probe = TestSubscriber.manualProbe[(Int, Long)]()
       Source(7 to 10).zipWithIndex.runWith(Sink.fromSubscriber(probe))
-
       val subscription = probe.expectSubscription()
 
       subscription.request(2)
@@ -36,5 +40,14 @@ class FlowZipWithIndexSpec extends StreamSpec {
       probe.expectComplete()
     }
 
+    "work in fruit example" in {
+      //#zip-with-index
+      Source(List("apple", "orange", "banana"))
+        .zipWithIndex
+        .runWith(Sink.foreach(println))
+      // this will print ('apple', 0), ('orange', 1), ('banana', 2)
+      //#zip-with-index
+    }
+
   }
 }