### Question:
ProfileSubject { public void addObserver(ProfileObserver profileObserver){ observers.add(profileObserver); } ProfileSubject(); void addObserver(ProfileObserver profileObserver); void removeObserver(ProfileObserver profileObserver);  }### Answer:
@Test void testAddObserver() { ProfileObserver mockObserver = Mockito.mock(ProfileObserver.class); sut.addObserver(mockObserver); }