code: public void addUser(String username, Role role) {
          if (StringUtils.isNullOrEmpty(username)) {
              throw new IllegalArgumentException("A username must be specified.");
          }
  
          if (role == null) {
              throw new IllegalArgumentException("A role must be specified.");
          }
  
          users.add(new User(username, role));
      }
docs: Adds a user, with the specified username and role.
  
  @param username      the username (e.g. an e-mail address)
  @param role          the user's role