@Test public void mustWorkFromRange() throws Exception { CompletionStage<List<Integer>> f = Source.range(0, 10).grouped(20).runWith(Sink.<List<Integer>>head(), materializer); final List<Integer> result = f.toCompletableFuture().get(3, TimeUnit.SECONDS); assertEquals(11, result.size()); Integer counter = 0; for (Integer i : result) assertEquals(i, counter++); }