Refreshing Activity after every 20 seconds and load data from url

Question

This is a Q&A activity where i want to load new question after every 20 seconds, I am loading my data from server using JSON and want to display it in my activity.

After going through lots of SO threads, I found that this can be done using handler. so i implemented the follow code, but now it is giving me NPE on JSONObject while i try to update the UI. How can i achieve my task.

public class Quiz extends ActionBarActivity {
    private SessionManager session;
    Intent intent;
    String categoryid,userid,uri="http://demopurpose.com/Quiz/API/";
    JSONArray jArray;
    InputStream is;
    JSONObject json_data;
    int len,countdown=20,lastquestion=0;
    private Handler mHandler,handler;
    protected static final long TIME_DELAY = 20000;
    TextView txtcountdown,txtquestion,txtanswer;
    Button btn1,btn2,btn3,btn4;
    String opt1,opt2,opt3,opt4,question,answer ;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz);
        this.session = new SessionManager(this);
        mHandler = new Handler();
        handler = new Handler();
        txtcountdown =(TextView)findViewById(R.id.txtcountdown);
        txtquestion = (TextView)findViewById(R.id.txtquestion);
        txtanswer = (TextView)findViewById(R.id.txtanswer);

        userid= session.getuserid();
        intent =getIntent();
        categoryid = intent.getExtras().getString("categoryid");

        mHandler.post(refreshActivity);
        handler.post(refreshCounter);
    }

    Runnable refreshActivity=new Runnable(){  
        Handler handler =new Handler();  
        public void run() {  
              countdown =20;
              getQuestionurl(userid,categoryid,lastquestion);
              lastquestion++;
              mHandler.postDelayed(this, TIME_DELAY);  
             }  
         };  

    Runnable refreshCounter = new Runnable() {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            countdown--;
            txtcountdown.setText(""+countdown);
            handler.postDelayed(this, 1000);
        }
    }; 

    public void getQuestionurl(final String uid, final String cid,final int lastindex){
        Thread t = new Thread() {
            public void run() {
                getquiz(uid,cid,lastindex);
            }
            };
        t.start();

    }

    public void getquiz(String uid, String cid,int lastindex) {

        String result = "";

        try{
                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost(uri+"question.php");
                List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
                nameValuePairs.add(new BasicNameValuePair("userId", uid));
                nameValuePairs.add(new BasicNameValuePair("categoryId", cid));

                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                is = entity.getContent();
        }
        catch(Exception e){
                Log.e("log_tag", "Error in http connection "+e.toString());
        }
        //convert response to string
        try{
                BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
                StringBuilder sb = new StringBuilder();
                String line = null;
                while ((line = reader.readLine()) != null) {
                        sb.append(line + "\n");
                }
                is.close();

                result=sb.toString();
                Log.e("result...",result);
                jArray = new JSONArray(result);
                loadjson(jArray,lastindex);
        }catch(Exception e){
                Log.e("log_tag", "Error converting result "+e.toString());
        }
    }

    private void loadjson(JSONArray arr,int lastindex) {
        try{
            json_data = arr.getJSONObject(lastindex);
            String flag="";
            flag =json_data.getString("success");

            if(flag.equalsIgnoreCase("0"))
            {   
                runOnUiThread(new Runnable() {
                    public void run() {
                        Toast.makeText(Quiz.this, "Unable to load Quiz...Try again", Toast.LENGTH_SHORT).show();
                    }
                });

            }
            else
            {   
                runOnUiThread(new Runnable() {
                    public void run() {
                        Toast.makeText(Quiz.this, "Loading Quiz...", Toast.LENGTH_SHORT).show();
                        try{
                            txtquestion.setText(json_data.getString("title"));
                            btn1.setText(json_data.getString("option1")); //NPE OCCOURS HERE
                            btn2.setText(json_data.getString("option2"));
                            btn3.setText(json_data.getString("option3"));
                            btn4.setText(json_data.getString("option4"));
                        }
                        catch(JSONException je){
                            je.printStackTrace();
                        }
                    }
                });
            }
        }
        catch(JSONException je){
            je.printStackTrace();
        }
    }
}

Below is my logcat response. As you can see i am getting the data correctly, but still it gives me NPE.

06-22 17:58:21.417: E/result...(27307): [{"success":"1","questionId":"1","title":"First chief minister","categoryId":"1","option1":"jivraj maheta","option2":"babubhai patel","option3":"narendra modi","option4":"kesubhai patel","answer":"jivraj maheta"},{"success":"1","questionId":"2","title":"test","categoryId":"1","option1":"test","option2":"test","option3":"test","option4":"test","answer":"test"},{"success":"1","questionId":"4","title":"First Chief Minister","categoryId":"1","option1":"jivraj maheta","option2":"babubhai patel","option3":"narendra modi","option4":"kesubhai patel","answer":"jivaraj maheta"},{"success":"1","questionId":"9","title":"Affordable","categoryId":"1","option1":"testt","option2":"fdgfdgf","option3":"fgfdsgdf","option4":"dfhdfh","answer":"3"}]
06-22 17:58:21.427: E/AndroidRuntime(27307): FATAL EXCEPTION: main
06-22 17:58:21.427: E/AndroidRuntime(27307): Process: com.example.knowledgeup, PID: 27307
06-22 17:58:21.427: E/AndroidRuntime(27307): java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference
06-22 17:58:21.427: E/AndroidRuntime(27307):    at com.example.knowledgeup.Quiz$5.run(Quiz.java:163)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at android.os.Handler.handleCallback(Handler.java:739)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at android.os.Handler.dispatchMessage(Handler.java:95)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at android.os.Looper.loop(Looper.java:155)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at android.app.ActivityThread.main(ActivityThread.java:5696)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at java.lang.reflect.Method.invoke(Native Method)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at java.lang.reflect.Method.invoke(Method.java:372)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1029)
06-22 17:58:21.427: E/AndroidRuntime(27307):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:824)

Answer 1

You have to set the variable of btn1 to a concrete view object by using something like btn1 = findViewById(R.id.my_button_1)

Answer 2

Null Pointer Exception is coming on Button objects.

Button btn1,btn2,btn3,btn4;

You nee to assign ids to these button elements.