@Test public void testNullInteractions() {
    try {
      bus.register(null);
      fail("Should have thrown an NPE on register.");
    } catch (NullPointerException e) {
    }
    try {
      bus.unregister(null);
      fail("Should have thrown an NPE on unregister.");
    } catch (NullPointerException e) {
    }
    try {
      bus.post(null);
      fail("Should have thrown an NPE on post.");
    } catch (NullPointerException e) {
    }
  }
Tests that events are distributed to any subscribers to their type or any
supertype, including interfaces and superclasses.

Also checks delivery ordering in such cases.
otto/src/test/java/com/squareup/otto/BusTest.java