code: private void appendUserInfoFields(final StringBuilder builder,
              final User user) {
          append(builder, user.getUsername(), ",");
          append(builder, user.getFirstname(), ",");
          append(builder, user.getLastname(), ",");
          append(builder, user.getEmail(), ",");
          append(builder, user.getPassword(), "");
      }
docs: Append the core fields of the user to the string.
  
  @param builder the string builder
  @param user the user whose core fields we'll add to the string