/** A person has a first and last name. */
public class Person {
  private String firstName;
  private String lastName;
  
  /** create a person with the given first and last names.
    * precondition: neither first nor last contain the ' ' character.
    */
  public Person(String firstName, String lastName) {
    assert !firstName.contains(" ");
    assert !lastName.contains(" ");
    
    this.firstName= firstName;
    this.lastName= lastName;
  }
  
  public String getName() {
    return firstName + ' ' + lastName;
  }
}

/** 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 
    * pre: names are not null and don't contain ' ',
    * gradYear is not in the future. */
//  public PhD(String firstName, String lastName, int gradYear) {
//    // TODO
//  }

}