* implementation, it is <i>possible</i> that this method will return a different number of chars
   * than would be returned by reading all of the chars.
   *
   * <p>In either case, for mutable sources such as files, a subsequent read may return a different
   * number of chars if the contents are changed.
   *
   * @throws IOException if an I/O error occurs while reading the length of this source
   * @since 19.0
   */