code: private void reportInvalidInitial(int mask, int offset)
          throws IOException
      {
          // input (byte) ptr has been advanced by one, by now:
          int bytePos = _byteCount + _inputPtr - 1;
          int charPos = _charCount + offset + 1;
  
          throw new CharConversionException("Invalid UTF-8 start byte 0x"+Integer.toHexString(mask)
                  +" (at char #"+charPos+", byte #"+bytePos+")");
      }
docs: /*
  ********************************************************************
  /* Internal methods
  ********************************************************************