code: public static String toHexString(byte[] bytes, int start, int len) {
          StringBuilder buf = new StringBuilder();
          for (int i = 0; i < len; i++) {
              int b = bytes[start + i] & 0xFF;
              if (b < 16) buf.append('0');
              buf.append(Integer.toHexString(b));
          }
          return buf.toString();
      }
docs: Writes some bytes in Hexadecimal format
  
  @param bytes Bytes to format
  @param start Start position for the conversion
  @param len   Number of bytes to convert
  @return Hex string