/**
 * A PhD is a person with a graduation year.
 */
class PhD extends Person {
  private int graduationYear; // not in the future

  /**
   * Create a PhD with given first and last names and graduation year.
   * Precondition: names are not null and don't contain ' ', and
   * gradYear is not in the future.
   */
  public PhD(String firstName, String lastName, int gradYear) {
    // TODO
  }
}