RecyclerView not showing JSON sent from localhost just blank screen

Issue

Hi guys I dont have much experience with AsyncTask and this is my first time using RecyclerView besides a couple of tutorials I done to learn about it.

If I use dummy data in EventActivity everything works fine and a list shown on the screen. But when I create an ArrayList of EventItems and pass that to the adapter it's just a blank screen. The JSON from localhost is being parsed and sent to the EventItem class. I have tried several things to get it to work but as my experience with AsyncTask and RecyclerView are limited I end up just crashing the app and getting a null pointer exception.

I think that the RecyclerView is being created before the JSON has been retrieved from localhost and this is what's causing the blank screen or null pointer exception but I'm not 100% sure and dont know how to fix the issue if I am correct.

Any help is appreciated.

EventActivity

public class EventActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private RecyclerView.Adapter mAdapter;
    private RecyclerView.LayoutManager mLayoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_event);
        fetchEvents();//call EventBackgroundWorker

        //ArrayList<EventItem> eventList = new EventItem().getRecyclerList();//causes java null pointer exception
        ArrayList<EventItem> eventList = new ArrayList<>();


        //Dummie data
       // eventList.add(new EventItem (1, "Title1", "Date", "endDate", "Location1", 5, 1111));
       // eventList.add(new EventItem (R.drawable.ic_favourite, "Title2", "11/07/2018", "11/07/2018", "Wales", 10, 5));
       // eventList.add(new EventItem (R.drawable.ramspeed_custom, "Title3", "11/01/2018", "11/09/2018", "Paris, France", 0, 90));
       // eventList.add(new EventItem (R.drawable.ramspeed_custom, "Title4", "12/01/2018", "11/09/2018", "New York", 20, 500));
       // eventList.add(new EventItem (R.drawable.ic_favourite, "Title5", "Mon 11/05/2015", "11/09/2018", "London, England", 5, 500));
       // eventList.add(new EventItem (R.drawable.biker, "Title6", "Mon 11/05/2018", "20/07/2018", "Swords Dublin", 0, 500));

        mRecyclerView = (RecyclerView) findViewById(R.id.event_recycler_view);
        mRecyclerView.setHasFixedSize(true);//increase performance of app if recyclerView does not increase in size
        mLayoutManager = new LinearLayoutManager(this);
        // mAdapter = new EventAdapter(eventList);
        mAdapter = new EventAdapter(eventList);//context added for testing //////////////////////////////
        // mAdapter = new EventAdapter(new ArrayList<EventItem>(0));
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setAdapter(mAdapter);


       // buildRecyclerView();
    }


//run  background thread
    private void fetchEvents(){
       // String username = "user";
       // String userPassword = "password";
        String startConnectionCondition = "event";
        //this passes the context
        EventBackgroundWorker backgroundWorker = new EventBackgroundWorker(this);
        backgroundWorker.execute(startConnectionCondition, null);
    }

 /*   private void buildRecyclerView(){
        ArrayList<EventItem> eventList = new EventItem().getRecyclerList();

        mRecyclerView = (RecyclerView) findViewById(R.id.event_recycler_view);
        mRecyclerView.setHasFixedSize(true);//increase performance of app if recyclerView does not increase in size
        mLayoutManager = new LinearLayoutManager(this);
        // mAdapter = new EventAdapter(eventList);
        mAdapter = new EventAdapter(eventList);//context added for testing 
        // mAdapter = new EventAdapter(new ArrayList<EventItem>(0));
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setAdapter(mAdapter);
    }
*/
   /* public void showEventList(boolean b){
        ArrayList<EventItem> eventList = new ArrayList<>();
        for(int i =0; i< eventList.size(); i++){
            eventList.get(i);
            Log.d("tag","eventList from main " + eventList);
        }

        mAdapter = new EventAdapter(eventList);
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setAdapter(mAdapter);
    }*/
}//MainActivity

EventAdapter

public class EventAdapter extends RecyclerView.Adapter<EventAdapter.EventViewHolder> {
    private ArrayList<EventItem> mEventList;
    Context context;

    //constructor for EventAdapter
    public EventAdapter(ArrayList<EventItem> eventList){
        // context =c;
        mEventList = eventList;
        Log.d("tag","EventAdapter eventlist variable called from constructor " + eventList);
    }


    public static class EventViewHolder extends RecyclerView.ViewHolder{
        public ImageView mPromoImage;
        public TextView mTitle;
        public TextView mStartDate;
        public TextView mEndDate;
        public TextView mLocation;
        public TextView mFee;
        public ImageView mFavourite;

        //constructor for EventViewHolder class
        public EventViewHolder(View itemView) {
            super(itemView);
          //  mPromoImage = (ImageView) itemView.findViewById(R.id.event_promotional_image);
            mTitle = (TextView) itemView.findViewById(R.id.event_title_textView);
            mStartDate = (TextView) itemView.findViewById(R.id.event_start_textView);
            mEndDate = (TextView) itemView.findViewById(R.id.event_end_textView);
            mLocation = (TextView) itemView.findViewById(R.id.event_location_textView);
            mFee = (TextView) itemView.findViewById(R.id.event_fee_textView);
         //   mFavourite = (ImageView) itemView.findViewById(R.id.event_favourite_image);
            Log.d("tag", "EventViewHolder being called");
        }
    }//EventViewHolder class



    @Override
    public EventViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_event, parent, false);
        //create view holder
        EventViewHolder viewHolder = new EventViewHolder(v);
        return viewHolder;
    }

    //pass values to inflated XML Views in item_event.xml
    @Override
    public void onBindViewHolder(EventViewHolder holder, int position) {
        EventItem currentEvent = mEventList.get(position);
      //  holder.mPromoImage.setImageResource(currentEvent.getEventPromoImage());
        holder.mTitle.setText(currentEvent.getEventTitle());
        holder.mStartDate.setText(currentEvent.getEventStartDate());
        holder.mEndDate.setText(currentEvent.getEventEndDate());
        holder.mLocation.setText(currentEvent.getEventLocation());
        holder.mFee.setText(String.valueOf(currentEvent.getEventFee()));//int value passed
      //  holder.mFavourite.setImageResource(currentEvent.getEventFavourite());
        //Log.d("tag", "position" + position );
        Log.d("tag","eventAdapter " + currentEvent.getEventTitle());
    }

    //how many items there will be
    @Override
    public int getItemCount() {
        return mEventList.size();
    }
}

EventItem

public class EventItem {
    private int mEventPromoImage;
    private int mId;
    private String mEventTitle;
    private String mEventStartDate;
    private String mEventEndDate;
    private String mEventLocation;
    private int mEventFee;
    private int mEventViews;


   // public EventItem(){}

    public EventItem(int id, String eventTitle, String eventStartDate,
                     String eventEndDate, String eventLocation, int eventFee, int eventViews){
       // mEventPromoImage = eventPromoImage;
        mId = id;
        mEventTitle = eventTitle;
        mEventStartDate = eventStartDate;
        mEventEndDate = eventEndDate;
        mEventLocation = eventLocation;
        mEventFee = eventFee;
        mEventViews = eventViews;
        Log.d("tag","EVENTITEM title EventItem" + mEventTitle);
      //  Log.d("tag","EVENTITEM startDate EventItem" + mEventStartDate );
      //  Log.d("tag","EVENTITEM endDate EventItem" + mEventEndDate );
      //  Log.d("tag","EVENTITEM address EventItem" + mEventLocation);
      //  Log.d("tag","EVENTITEM fee EventItem" + mEventFee);
    }



    public int getEventPromoImage() {
        return mEventPromoImage;
    }

    public void setEventPromoImage(int mEventPromoImage) {
        this.mEventPromoImage = mEventPromoImage;
    }

    public int getEventId() {
        return mId;
    }

    public void setEventId(int mId){
        this.mId = mId;
    }

    public String getEventTitle() {
        Log.d("tag","getEventTitle() " + mEventTitle);
        return mEventTitle;
    }

    public void setEventTitle(String mEventTitle) {
        this.mEventTitle = mEventTitle;
    }

    public String getEventStartDate() {
        return mEventStartDate;
    }

    public void setEventStartDate(String mEventStartDate) {
        this.mEventStartDate = mEventStartDate;
    }

    public String getEventEndDate(){
        return mEventEndDate;
    }

    public void setEventEndDate(String mEventEndDate){
        this.mEventEndDate = mEventEndDate;
    }

    public String getEventLocation() {
        return mEventLocation;
    }

    public void setEventLocation(String mEventLocation) {
        this.mEventLocation = mEventLocation;
    }

    public int getEventFee() {
        return mEventFee;
    }

    public void setEventFee(int mEventFee) {
        this.mEventFee = mEventFee;
    }

    public int getEventViews(){
        return mEventViews;
    }

    public void getEventViews(int mEventViews) {
        this.mEventViews = mEventViews;
    }

    public ArrayList<EventItem> getRecyclerList(){
        ArrayList event = new ArrayList();
        event.add(mEventTitle);
        event.add(mEventStartDate);
        event.add(mEventEndDate);
        event.add(mEventLocation);
        event.add(mEventFee);
        event.add(mEventViews);
        return event;
    }
}

EventBackgroundWorker

public class EventBackgroundWorker extends AsyncTask<String, Void, String> {
    Context context;
    AlertDialog alert;
    public static final String REQUEST_URL = "http://10.0.2.2/m/event/";

    public EventBackgroundWorker(Context ctxt) {
        context = ctxt;
    }

    //Invoked on UI thread before doInBackground is called
    @Override
    protected void onPreExecute() {
        alert = new AlertDialog.Builder(context).create();
        alert.setTitle("Result from server");

    }

    @Override
    protected String doInBackground(String... params) {
        String type = params[0];//eventList
        //String login_url = "http://10.0.2.2/m/event/";
        if(type.equals("event")){
            try {
                String user_name = params[1];
                URL url = new URL(REQUEST_URL);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                httpURLConnection.setDoInput(true);
                httpURLConnection.connect();//works without this connect Find out why

                OutputStream outputStream = httpURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("getEvents", "UTF-8");
                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                //read response
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "";
                String line;
                while((line = bufferedReader.readLine()) != null){
                    result += line;
                }

                //parse JSON event Array

                // Create an empty ArrayList that we can start adding events to
                ArrayList<EventItem> eventArray = new ArrayList<>();

                JSONArray baseJsonResponse = new JSONArray(result);
                for(int i =0; i < baseJsonResponse.length(); i++){
                    JSONObject event = baseJsonResponse.getJSONObject(i);
                    int id = event.getInt("id");
                    String title = event.getString("title");
                    String address = event.getString("address");
                    String startDate = event.getString("startDate");
                    String endDate = event.getString("endDate");
                    int fee = event.getInt("fee");
                    int views = event.getInt("views");

                    //Send data to eventItem Object
                    EventItem eventObject = new EventItem(id,title,address,startDate,endDate,fee,views);
                    eventArray.add(eventObject);
                  /*  Log.d("tag", "JSON id " + id);
                    Log.d("tag", "JSON title " + title);
                    Log.d("tag", "JSON address " + address);
                    Log.d("tag", "JSON startDate " + startDate);
                    Log.d("tag", "JSON endDate " + endDate);
                    Log.d("tag", "JSON fee " + fee);
                    Log.d("tag", "JSON views " + views);*/
                }

                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();

                return result;

            } catch (MalformedURLException e) {//added for URL object
                e.printStackTrace();
            } catch (IOException e) {
                //HTPURLConnection
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    //results from doInBackground are passed here
    @Override
    protected void onPostExecute(String result) {
        Log.d("tag", "onPostExecute called" + result);
        alert.setMessage(result);
        alert.show();

    }

}

Solution

The AsyncTask creates eventArray but does nothing with it. doInBackground() should return the eventArray and onPostExecute() should set it as adapter content (by a Callback or by setting the Adapter instance at the AsyncTask object). Your Adapter should have a setter for the content which should call notifyDataSetChanged().

Answered By - user9906968