public void demonstrateACustomMaterializedValue() throws Exception { // tests: RunnableGraph<CompletionStage<Integer>> flow = Source.from(Arrays.asList(1, 2, 3)) .viaMat(new FirstValue(), Keep.right()) .to(Sink.ignore()); CompletionStage<Integer> result = flow.run(mat); assertEquals(new Integer(1), result.toCompletableFuture().get(3, TimeUnit.SECONDS)); }