Android - ช่วยผมด้วยครับ ผมจะลองเขียน Chat Application ขึ้นมา แต่ติดปัญหา

public class MainActivity extends ActionBarActivity {
    private static String url = "http://www.fbcredibility.com/cloudobject/usg02/find/student2";

    //JSON Node Names
    private static final String TAG_DATA = "data";

    private static final String TAG_MSG = "msg";
    private static final String TAG_USER = "name";

    JSONArray user = null;
    Random  ran =new Random();
    int  n = ran.nextInt(50)+1;
    String n2=String.valueOf(n);





        @Override
        protected void onCreate(Bundle savedInstanceState) {

            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
            StrictMode.setThreadPolicy(policy);


            JSONParser jParser = new JSONParser();

            JSONObject json = jParser.getJSONFromUrl(url);

            try {

                user = json.getJSONArray(TAG_DATA);

                for (int i = 0; i < user.length(); i++) {
                    JSONObject c = user.getJSONObject(i);


                    String ms = c.getString(TAG_MSG);
                    String na = c.getString(TAG_USER);


                    TextView text = (TextView) findViewById(R.id.singleMessage);

                    if (!n2.equals(na)) {

                        text.setTextColor(Color.RED);
                        text.append("ผู้ใช้ที่ " + na + " : " + ms + "\n\n");

                    } else {
                        text.setTextColor(Color.BLUE);
                        text.append("ผู้ใช้ที่ "+ na + " : " + ms + "\n\n");
                    }


                }

                findViewById(R.id.sendButton).setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        sendMessage();
                    }
                });


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

        }