org.json.JSONObject cannot be converted to JSONArray

I am getting a exception while working in json . My JSONPresr class is as follow

 import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.UnsupportedEncodingException;

 import org.apache.http.HttpEntity;
 import org.apache.http.HttpResponse;
 import org.apache.http.client.ClientProtocolException;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.impl.client.DefaultHttpClient;
 import org.json.JSONException;
 import org.json.JSONObject;

 import android.os.AsyncTask;
 import android.util.Log;

public class JSONParser {
 static InputStream is = null;
    static JSONObject jObj = null;
    static String json = "";

    // constructor
    public JSONParser() {

    }

    public JSONObject getJSONFromUrl(String url) {

        // Making HTTP request
        try {
            // defaultHttpClient
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url);

            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();          

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "UTF-8"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            json = sb.toString();
            System.out.println(" value in json sting"+json);
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }

        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
            Log.e("JSON Parser", "Error parsing data [" + e.getMessage()+"] "+json);
            System.out.println(" error is here");

        }

        // return JSON String
        return jObj;

    }

    }

I am getting my data by these code

          String TAG_user_detail = "user_details";
              String TAG_user_id = "user_id";
             String TAG_user_name = "user_name";
             String TAG_user_phone = "user_phone";
             String TAG_ref_id = "ref_id";
             JSONArray user_detail_jsonarray = null;
             JSONParser jParser = new JSONParser();

             // getting JSON string from URL
             JSONObject json = jParser.getJSONFromUrl(url);
             try {
                 // Getting Array of Contacts
                 user_detail_jsonarray = json.getJSONArray(TAG_user_detail);

                 // looping through All Contacts
                 for(int i = 0; i < user_detail_jsonarray.length(); i++){
                     JSONObject c = user_detail_jsonarray.getJSONObject(i);

                     // Storing each json item in variable
                     int id = c.getInt(TAG_user_id);
                     System.out.println("user id from json"+id);

                     String name_fromjson = c.getString(TAG_user_name);
                     System.out.println("user name from json"+name_fromjson);

                     int phone_no_fromjson = c.getInt(TAG_user_phone);
                     System.out.println("user phone from json"+phone_no_fromjson);

                     int ref_id_fromjson = c.getInt(TAG_ref_id);
                     System.out.println("user ref id from json"+ref_id_fromjson);





                 }
             } catch (JSONException e) {
                 e.printStackTrace();
             }

But i am countiniously getting Exception in this line

    user_detail_jsonarray = json.getJSONArray(TAG_user_detail);

My Stake Trace message is as

       {"ref_id":1295,"user_name":"chand","user_phone":"9620085675","user_id":"1"} at user_details of type org.json.JSONObject cannot be converted to JSONArray

I tried many link but unfortunately i am not able to get my result. Please any body just help me to solve this issue. Thanks in advance to all