code: public static String fullName(User user) {
      StringJoiner name = new StringJoiner(" ");
  
      if (!isEmpty(user.getFirstName()))
        name.add(user.getFirstName());
      if (!isEmpty(user.getLastName()))
        name.add(user.getLastName());
  
      return name.toString();
    }
docs: The full name is identified as the concatenation of the first and last name, separated by a space.
  This method can return an empty name if both first and last name are empty.
  
  @return the full name of the user
  @param user