private void addStudent(StudentDirectory dir2, int i) {
   String id = "" + i;
   Student student = new Student(id);
   student.setId(id);
   student.addCredits(i);
   dir.add(student);
 }
  private void verifyStudentLookup(StudentDirectory dir, int i) {
    String id = "" + i;
    Student student = dir.findById(id);

    assertEquals(id, student.getId());
    assertEquals(id, student.getLastName());
    assertEquals(i, student.getCredits());
  }