public interface UserRepository {
    UserEntity save(UserEntity car);
    void update(UserEntity user);
    List<UserEntity> getAll();
    List<UserEntity> getAllByFirstName();
    List<UserEntity> getAllByLastName();
    List<UserEntity> getAllByGender();
    UserEntity getById(Long id);
    void remove(Long id);
}

@Repository
@Transactional()
public class UserRepositoryImpl  implements UserRepository {

    @PersistenceContext
    private EntityManager entityManager;

    @Override
    @Transactional
    public UserEntity save(UserEntity user) {
        if(user.getId() != null){
            return entityManager.merge(user);
        } else {
            entityManager.persist(user);
            return user;
        }
    }

    @Override
    public void update(UserEntity user) {

    }

    @Override
    public List<UserEntity> getAll() {
        CriteriaQuery<UserEntity> criteria = entityManager.getCriteriaBuilder().createQuery(UserEntity.class);
        criteria.select(criteria.from(UserEntity.class));
//        criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

        return entityManager.createQuery(criteria).getResultList();
    }

    @Override
    public List<UserEntity> getAllByFirstName() {
        return null;
    }

    @Override
    public List<UserEntity> getAllByLastName() {
        return null;
    }

    @Override
    public List<UserEntity> getAllByGender() {
        return null;
    }

    @Override
    public UserEntity getById(Long id) {
        return entityManager.find(UserEntity.class, id);
    }

    @Override
    public void remove(Long id) {
        UserEntity user = getById(id);
        if(user != null) {
            entityManager.remove(user);
        }
    }
}