Working on JSON Async Task

ConcertConcierge Main Activity

package com.toneloc.concertconcierge;

import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {

    private ArrayList<Concert> mConcertList = new ArrayList<>();
    private TextView mResult;

    private class FetchConcertsTask extends AsyncTask<String, Void, JSONObject> {
        private OkHttpClient http = new OkHttpClient();

        // while the task runs....
        @Override
        protected JSONObject doInBackground(String... strings) {
            //allows for debugging in async task
            android.os.Debug.waitForDebugger();

            // async call the API
            String url = "http://api.songkick.com/api/3.0/metro_areas/24426/calendar.json?apikey=";
            Request req = new Request.Builder().url(url).build();
            try {
                Response res = http.newCall(req).execute();
                String data = res.body().string();

                //format JSON response
                JSONObject rootObject = new JSONObject(data);
                JSONArray events = rootObject.getJSONArray("events");
                String string = rootObject.optString("displayName");

                // purge prior results
                mConcertList.clear();
                Integer length = events.length();

                //loop through the array of events to get relevant concert data
                for (int i = 0; i < length; i++) {

                    //get a single event JSON object
                    JSONObject singleEvent = events.getJSONObject(i);

                    //event is JSON object
                    int id = singleEvent.getInt("id");
                    String displayName = singleEvent.getString("displayName");
                    double popularity = singleEvent.getDouble("popularity");

                    //start is JSON object
                    JSONObject start = singleEvent.getJSONObject("start");
                    double startTime = start.getDouble("time");
                    double startDate = start.getDouble("date");

                    //location is also JSON object
                    JSONObject location = singleEvent.getJSONObject("location");
                    String city = location.getString("city");
                    double latitude = location.getDouble("latitude");
                    double longitude = location.getDouble("longitude");

                    Concert tempConcert = new Concert(id, displayName, popularity, city, latitude, longitude, startTime, startDate);
                    mConcertList.add(tempConcert);
                }

            } catch (IOException ex) {
                ex.printStackTrace();
            } catch (JSONException ex) {
                ex.printStackTrace();
            }

            return null;
        }

        // when the task is done
        @Override
        protected void onPostExecute(JSONObject location) {
            // when done, we need to populate page with concerts, somehow

        }

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mResult = (TextView) findViewById(R.id.result);

        // find our asynctask
        FetchConcertsTask fetchConcertsTask = new FetchConcertsTask();
        // start our task
        fetchConcertsTask.execute();

        //upcoming events for a metro area
        //hard coded london as the metro area
        //can make get lat/lng, then make separate API call to
        //get closest songkick metro area
    }

}