@Override
public void unregister(User user, T appointment) throws UnregisterPeriodOverException, NotRegisteredException, AppointmentNotFoundException {
List<User> users = getRegisteredUsers(appointment, user);
if (!users.contains(user)) {
throw new NotRegisteredException();
}
appointment = getRealAppointment1(appointment);
if (appointment.getRegisterStart().after(new Date()) || appointment.getUnregisterEnd().before(new Date())) { //nach dem heutigen Tag -> Anmeldung noch nicht begonnen