class GuestService {

    public List<Guest> getAllGuests() {
        CriteriaQuery<Guest> cq = this.em.getCriteriaBuilder().createQuery(Guest.class);
        cq.select(cq.from(Guest.class));
        return this.em.createQuery(cq).getResultList();
    }

}

class GuestServiceTest  {

    private GuestService guestService;
    private static final Long WELL_KNOWN_ID;

    @Test
    public void finding_all_guests_does_not_return_null() {

        assertThat(guestService.getAllGuests()).isNotNull();
    }

}