code: public static StringBuilder appendZeros(StringBuilder buf, int zeros) {
      for(int i = zeros; i > 0; i -= ZEROPADDING.length) {
        buf.append(ZEROPADDING, 0, i < ZEROPADDING.length ? i : ZEROPADDING.length);
      }
      return buf;
    }
docs: Append zeros to a buffer.
  
  @param buf Buffer to append to
  @param zeros Number of zeros to append.
  @return Buffer