How to use OKHTTP to make a post request

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How to use OKHTTP to make a post request?

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public JSONObject getJSONFromUrl(String str_url, List<NameValuePair> params) {      
        String reply_str = null;
        BufferedReader reader = null;

        try {
            URL url = new URL(str_url);
            OkHttpClient client = new OkHttpClient();
            HttpURLConnection con = client.open(url);                           
            con.setDoOutput(true);
            OutputStreamWriter writer = new OutputStreamWriter(con.getOutputStream());
            writer.write(getEncodedParams(params));
            writer.flush();     
            StringBuilder sb = new StringBuilder();
            reader = new BufferedReader(new InputStreamReader(con.getInputStream()));           
            String line;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }           
            reply_str = sb.toString();              
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                    return null;
                }
            }
        }

        // try parse the string to a JSON object. There are better ways to parse data.
        try {
            jObj = new JSONObject(reply_str);            
        } catch (JSONException e) {   
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }     
      return jObj;
    }

    //in this case it‘s NameValuePair, but you can use any container
    public String getEncodedParams(List<NameValuePair> params) {
        StringBuilder sb = new StringBuilder();
        for (NameValuePair nvp : params) {
            String key = nvp.getName();
            String param_value = nvp.getValue();
            String value = null;
            try {
                value = URLEncoder.encode(param_value, "UTF-8");
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }

            if (sb.length() > 0) {
                sb.append("&");
            }
            sb.append(key + "=" + value);
        }
        return sb.toString();
    }

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