Defensive programming

addfriend.java

public void addFriendToList(List<Friend> friends, Friend newFriend) {
  if (friends != null && newFriend != null) {
    friends.add(newFriend);
  }   
}

find.java

public List<Friend> findFavoriteFriends(Person person) {
  List<Friend> favoriteFriends = new ArrayList<Friend>();

  if (person != null) {
    List<Friend> friends = person.getFriends();

    if (friends != null) {
      for (Friend friend : friends) {
        if (friend != null) {
          if (friend.isFavorite()) {
              favoriteFriends.add(friend);
          }
        }
      }
    }
  }

  return favoriteFriends;
}

unuse.java

public Person newPerson(String name) {
  Person person = new Person();
  if( name != null ) {
    person.setName(name);
  }
  return person;
}