URL Encode and Decode Special character in Java

In Java, I need to use HTTP Post to send request to server, but if in the parameter of the URL contains some special character it throws below Exception

java.lang.IllegalArgumentException: URLDecoder: Illegal hex characters in escape (%) pattern - For input string: "&'"

The code to send data

DefaultHttpClient httpclient = new DefaultHttpClient(); 
   HttpPost httpPost = new HttpPost(URL); 

   String sessionId = RequestUtil.getRequest().getSession().getId();
   String data = arg.getData().toString();

   List<NameValuePair> params = new ArrayList<NameValuePair>();   
   params.add(new BasicNameValuePair(param1, data));
   params.add(new BasicNameValuePair(param2, sessionId));
         httpPost.setEntity(new UrlEncodedFormEntity(params, "UTF-8"));           

   HttpResponse response = (HttpResponse) httpclient.execute(httpPost);

And at the server side, i use the below code to read information

 String data = request.getParameter(param1);
   if (data != null) {
    actionArg = new ChannelArg(URLDecoder.decode(data, "UTF-8"));
   }

The code works correctly but if i input some special character like [aああ#$%&'(<>?/.,あああああ], it will throw exception. I wonder if someone could help me some hint to be able to encode and decode special characters?

Thank you very much in advance.