Showing Error :Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
<p>I'm creating an app that display contact list in a listview.The problem is I'm getting the following error when I run my app and I'm struggling to fix it:</p>

<pre><code>04-05 13:41:48.868 2488-2488/? E/AndroidRuntime: FATAL EXCEPTION: main
                                             Process: com.kksworld.jsonparsing, PID: 2488
                                             java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                 at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:337)
                                                 at android.widget.ListView.setAdapter(ListView.java:491)
                                                 at com.kksworld.jsonparsing.MainActivity$JSONTask.onPostExecute(MainActivity.java:120)
                                                 at com.kksworld.jsonparsing.MainActivity$JSONTask.onPostExecute(MainActivity.java:48)
                                                 at android.os.AsyncTask.finish(AsyncTask.java:651)
                                                 at android.os.AsyncTask.-wrap1(AsyncTask.java)
                                                 at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                 at android.os.Looper.loop(Looper.java:148)
                                                 at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
</code></pre>

<p>Here is my code :</p>

<p><strong>MainActivity.java</strong></p>

<pre><code>package com.kksworld.jsonparsing;
import android.content.Context;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import com.kksworld.jsonparsing.models.ContactModel;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private ListView listView;

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

    listView = (ListView)findViewById(R.id.lvContacts)
    }


public class JSONTask extends AsyncTask&lt;String,String,List&lt;ContactModel&gt;&gt;{

    @Override
    protected List&lt;ContactModel&gt; doInBackground(String... params) {

        HttpURLConnection httpURLConnection = null;
        BufferedReader bufferedReader = null;

        try {
            URL url = new URL(params[0]);
            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.connect();
            InputStream inputStream = httpURLConnection.getInputStream();
            // Through this we can read the data line by line
            bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
            StringBuffer stringBuffer = new StringBuffer();
            String line = "";
            while ((line = bufferedReader.readLine()) != null) {
                stringBuffer.append(line);
            }

            String finalJSON = stringBuffer.toString();
            JSONArray mainArray = new JSONArray(finalJSON);
            JSONObject parentObject = mainArray.getJSONObject(0);
            JSONArray parentArray = parentObject.getJSONArray("contacts");

            List&lt;ContactModel&gt; contactModelList = new ArrayList&lt;&gt;();

            // For parsing a list of items you have to use for loop
            for(int i=0;i&lt;11;i++) {
                JSONObject finalObject = parentArray.getJSONObject(i);
                ContactModel contactModel = new ContactModel();
                contactModel.setName(finalObject.getString("name"));
                contactModel.setPhone(finalObject.getString("phone"));
                contactModel.setEmail(finalObject.getString(("email")));
                contactModel.setOfficePhone(finalObject.getString(("officePhone")));
                contactModel.setLatitude((float) finalObject.getDouble("latitude"));
                contactModel.setLongitude((float) finalObject.getDouble("longitude"));
                // Adding the finalObject in the List
                contactModelList.add(contactModel);

            }
            return contactModelList;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (httpURLConnection != null) {
                httpURLConnection.disconnect();
                try {
                    if (bufferedReader != null) {
                        bufferedReader.close();
                    }
                }catch(IOException e){
                    e.printStackTrace();
                }
            }
        }
        return null;
    }

    // Value returned by doInBackGround will be passed here...

    @Override
    protected void onPostExecute(List&lt;ContactModel&gt; result) {
        super.onPostExecute(result);

        ContactAdapter contactAdapter = new ContactAdapter(getApplicationContext(),R.layout.row,result);
        listView.setAdapter(contactAdapter);
    }
}

public class ContactAdapter extends ArrayAdapter{

    private List&lt;ContactModel&gt; contactModelList;
    private int resource;
    private LayoutInflater inflater;

    public ContactAdapter(Context context, int resource, List&lt;ContactModel&gt; objects) {
        super(context, resource, objects);
        contactModelList=objects;
        this.resource=resource;
        inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        if(convertView == null)
        {
            convertView = inflater.inflate(resource,null);
        }
        TextView textViewName;
        TextView textViewEmail;
        TextView textViewPhone;
        TextView textViewOfficePhone;

        textViewName = (TextView)convertView.findViewById(R.id.textViewInsertName);
        textViewEmail= (TextView)convertView.findViewById(R.id.textViewInsertEmail);
        textViewPhone= (TextView)convertView.findViewById(R.id.textViewInsertPhone);
        textViewOfficePhone = (TextView)convertView.findViewById(R.id.textViewInsertOfficeNumber);

        textViewName.setText(contactModelList.get(position).getName());
        textViewEmail.setText(contactModelList.get(position).getEmail());
        textViewPhone.setText(contactModelList.get(position).getPhone());
        textViewOfficePhone.setText(contactModelList.get(position).getOfficePhone());

        return convertView;
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_refresh) {
        new JSONTask().execute("some_url");
        return true;
    }

    return super.onOptionsItemSelected(item);
}
</code></pre>

<p>}</p>

<p>Could someone please help.</p>