code: public String substring(int start, int end) {
  
      return new String(this.buffer, this.initialOffset + start, end - start);
    }
docs: @see String#substring(int, int)
  @see #appendSubstring(StringBuffer, int, int)
  
  @param start the start index, inclusive.
  @param end the end index, exclusive.
  @return the specified substring.