public static void groupByLastNameExample(List<Person> people) {
    //before Guava - 9 lines
    Map<String, Collection<Person>> peopleForLastNameOld = new HashMap<String, Collection<Person>>();
    for (Person p : people) {
      List<Person> peopleWithLastName = peopleForLastNameOld.get(p.getLast());
      if (peopleWithLastName == null) {
        peopleWithLastName = new ArrayList<Person>();
        peopleForLastNameOld.put(p.getLast(), peopleWithLastName);
      }
      peopleWithLastName.add(p);
    }
    //peopleForLastNameOld.get("Smith") will return Collection<Person>

    //new way - 4 lines
    Multimap<String, Person> peopleForLastName = HashMultimap.create();
    for (Person p : people) {
      peopleForLastName.put(p.getLast(), p);
    }
    //peopleForLastName.get("Smith") will return Collection<Person>
  }