@Test
public void TestNotifyObservers() {

    Observer<String> a = Callback("a");
    Observer<String> b = Callback("b");
    Observer<String> c = Callback("c");
    ObserverList<String> list = concurrentListOf(a, b, c);
    assertEquals(3, list.size());
    assertFalse(list.isEmpty());
    list.notifyObservers("1");
    assertResults("a:1", "b:1", "c:1");
    list.notifyObservers("2");
    assertResults("a:2", "b:2", "c:2");
}