java UTF-8转换GB2312

private String utf8Togb2312(String str){
      StringBuffer sb = new StringBuffer();
      for(int i=0; i<str.length(); i++) {
          char c = str.charAt(i);
          switch (c) {
             case '+':
                 sb.append(' ');
             break;
             case '%':
                 try {
                      sb.append((char)Integer.parseInt(
                      str.substring(i+1,i+3),16));
                 }
                 catch (NumberFormatException e) {
                     throw new IllegalArgumentException();
                }
                i += 2;
                break;
             default:
                sb.append(c);
                break;
           }
      }
      // Undo conversion to external encoding
      String result = sb.toString();
      String res=null;
      try{
          byte[] inputBytes = result.getBytes("8859_1");
          res= new String(inputBytes,"UTF-8");
      }
      catch(Exception e){}
      return res;
}