public TeacherDTO[] findTeachersByFirstName(String firstName) {
    String query = "SELECT e FROM TeacherEntity e WHERE e.teacherName = ?1";
    List<TeacherEntity> result;

    try {
        result = entityManager.createQuery(query, TeacherEntity.class).setParameter(1, param).getResultList();
    } catch(NoResultException e) {
        return null;
    }

    TeacherDTO[] teachers = new TeacherDTO[result.size()];

    for( int i = 0; i < result.size(); i++ )
        teachers[i] = createTeacherDTO(result.get(i));

    return teachers;

} // public TeacherDTO[] findTeachersByFirstName(String firstName)