请教一个乱码问题

Question

public static String JptoISO (String str) {
         if(str!=null) {
            byte[] b=null;
            try {
                b = str.getBytes("ISO-8859-1");
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }
            String newStr=null;
            newStr=new String(b,"GBK");
             return newStr;
         } else{
             return str;
         }
     }