public List<Achievement> getCompletedAchievements() {
    if (achievements.size() == 0) {
      return Collections.emptyList();
    }

    List<Achievement> completed = new LinkedList<Achievement>();

    for (Achievement achievement : achievements.values()) {
      if (achievement.isCompleted()) {
        completed.add(achievement);
      }
    }

    return completed;
  }