If the input is UTF8 bytes, use the

String s = new string (utf8_bytes, "utf-8"); //utf8_bytes is of type byte[].

If the input is a string, directly

String s= original return.

When the output is converted to gb 18030, it is not necessary to convert when it is not output.

OS . write(s . getbytes(" GB 18030 "))； //os is the output stream.