+str #18807 add Source.range for Java DSL

akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java

@@ -463,6 +463,16 @@ public class SourceTest extends StreamTest {
     assertEquals("A", result);
   }
 
+  @Test
+  public void mustWorkFromRange() throws Exception {
+    Future<List<Integer>> f = Source.range(0, 10).grouped(20).runWith(Sink.<List<Integer>> head(), materializer);
+    final List<Integer> result = Await.result(f, FiniteDuration.create(3, TimeUnit.SECONDS));
+    assertEquals(result.size(), 11);
+    Integer counter = 0;
+    for (Integer i: result)
+      assertEquals(i, counter++);
+  }
+
   @Test
   public void mustRepeat() throws Exception {
     final Future<List<Integer>> f = Source.repeat(42).grouped(10000).runWith(Sink.<List<Integer>> head(), materializer);

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

@@ -4,6 +4,7 @@
 package akka.stream.javadsl
 
 import java.io.{ OutputStream, InputStream, File }
+import java.util
 
 import akka.actor.{ ActorRef, Cancellable, Props }
 import akka.event.LoggingAdapter
@@ -110,6 +111,18 @@ object Source {
     new Source(scaladsl.Source(scalaIterable))
   }
 
+  /**
+   * Creates [[Source]] with `start` as the first element and each next element as `previous + 1` until
+   * it reaches `end`. It allows to create `Source` out of range as simply as on scala `Source(1 to N)`
+   */
+  def range(start: Int, end: Int): javadsl.Source[Integer, Unit] = {
+    require(start <= end, "start must be less or equal than end")
+    from(new util.AbstractList[Integer]() {
+      override def get(index: Int) = start + index
+      override def size() = end - start + 1
+    })
+  }
+
   /**
    * Start a new `Source` from the given `Future`. The stream will consist of
    * one element when the `Future` is completed with a successful value, which