// sorting
  @Override
  public String getOnlineUsersInAlphabeticalOrder() {
    String users = "";

    for (UserModel user : this.users.values()) {
      if (user.isOnline()) {
        users += String.format("* %s", user.getUserName());
      }
    }

    return users;
  }
  @Override
  public synchronized void readUsers() {
    for (String userPropertyKey : config.listKeys()) {
      UserProperty property = new UserProperty(userPropertyKey, config.getString(userPropertyKey));

      UserModel user = getUser(property.getUsername());

      if (user == null) user = new UserModel(property.getUsername());

      PropertyMapper mapper = new PropertyMapper<UserModel>(user);

      mapper.map(property.getPropertyName(), property.getPropertyValue());

      users.put(user.getUserName(), user);
    }
  }