    @Test
    public void testMyFirstActor_Greeting() {

        final TestKit probe = new TestKit(actorSystem);
        final ActorRef myFirstActor = actorSystem.actorOf(MyFirstActor.props());
        myFirstActor.tell(new Greeting(HELLO_WORLD), probe.getRef());
        final Greeting greeting = probe.expectMsgClass(Greeting.class);
        assertEquals(HELLO_WORLD, greeting.getMessage());
    }