/** @param people a set of Person objects
   *  @return a set containing all members of people and all the
   *          immediate contacts of all members of people
   *  Postcondition: the set people is unchanged
   */
  private static Set<Person> expandContactSet(Set<Person> people)
  {
    Set<Person> c = new TreeSet<Person>(); 1
    for (Person p : people)
    {
      c.add(p);
      for (Person p1 : p.getContacts())
        c.add(p1);
    }
    return c;
  }

1. Strictly speaking, for using a TreeSet<Person>, Person must implement Comparable<Person>.  Using a TreeSet assures that the names of Persons appear in alphabetical order when the set is printed out.

   It is also possible to use a HashSet:

      Set<Person> c = new HashSet<Person>();
   

   Then Person must have a hashCode method and an equals method; we can assume they are provided but "not shown."

  /** Creates and returns a set containing all members of this person's network of contacts
   *  who are within distance dist away
   *  @param dist the maximum distance between this person and a network contact
   *         Precondition: dist > 0
   *  @return a set of contacts who are within distance dist away from this person
   *         Postcondition: this person is not included in the returned set
   */
  public Set<Person> getNetwork(int dist)
  {
    Set<Person> people = getContacts();

    for (int d = 2; d <= dist; d++)
      people = expandContactSet(people);

    people.remove(this);
    return people;
  }