Firebase firestore data retrieval throws null pointer exception?

Question

I am trying to get Realtime data from firestore and update a class level object in my android app but when execute the code the app crashes and when I examined it displays the following error:

2021-06-08 16:47:37.569 13632-13632/com.example.railtracker E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.railtracker, PID: 13632
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference
    at com.example.railtracker.Acitivity.TrainLiveTrackViewActivity$1.onEvent(TrainLiveTrackViewActivity.java:80)
    at com.example.railtracker.Acitivity.TrainLiveTrackViewActivity$1.onEvent(TrainLiveTrackViewActivity.java:69)
    at com.google.firebase.firestore.DocumentReference.lambda$addSnapshotListenerInternal$2$DocumentReference(DocumentReference.java:504)
    at com.google.firebase.firestore.-$$Lambda$DocumentReference$GF131yLOtm0PQYErAZvV1mYKmvw.onEvent(Unknown Source:6)
    at com.google.firebase.firestore.core.AsyncEventListener.lambda$onEvent$0$AsyncEventListener(AsyncEventListener.java:42)
    at com.google.firebase.firestore.core.-$$Lambda$AsyncEventListener$DNkggu2LY54oguDvcp-QtRg6Sfg.run(Unknown Source:6)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

I checked printing the data to the terminal and it displays correctly but when I try to assign it to a variable the app crashes. Please help me out with this issue. Thanks in advance.

My code:

package com.example.railtracker.Acitivity;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.app.IntentService;
import android.content.Intent;
import android.location.Location;
import android.os.Bundle;
import android.os.Handler;
import android.os.ResultReceiver;
import android.util.Log;
import android.view.MenuItem;
import android.view.View;


import com.example.railtracker.Adapter.StopViewAdapter;
import com.example.railtracker.DTO.TrainDTO;
import com.example.railtracker.DTO.TrainInfoDTO;
import com.example.railtracker.R;
import com.example.railtracker.Util.FetchLocationIntentService;
import com.github.vipulasri.timelineview.TimelineView;
import com.google.android.gms.common.logging.Logger;
import com.google.android.material.snackbar.BaseTransientBottomBar;
import com.google.android.material.snackbar.Snackbar;
import com.google.api.Logging;
import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;

public class TrainLiveTrackViewActivity extends AppCompatActivity {
    private static final String TAG= TrainLiveTrackViewActivity.class.getSimpleName();
    private RecyclerView stopsView;
    private Toolbar toolbar;
    private TrainDTO train;
    private LocationReceiver locationReceiver;
    private Location lastLocation;
    private String zipCode;
    private TrainInfoDTO trainInfoDTO=new TrainInfoDTO();
    private final String RESULT_DATA="RESULT_DATA";
    private final String COLUMN="trains";
    double sample;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_train_live_track_view);
        setUpView();
        locationReceiver=new LocationReceiver(new Handler());
        loadRealTimeData();

    }


    private void loadRealTimeData()
    {

        try{
            DocumentReference db= FirebaseFirestore.getInstance().collection(COLUMN).document( String.valueOf(train.getTrainId()));
            db.addSnapshotListener(new EventListener<DocumentSnapshot>() {
                @Override
                public void onEvent(@Nullable DocumentSnapshot value, @Nullable FirebaseFirestoreException error) {
                    if(error != null){
                        Log.e(TAG,error.getMessage());
                        error.printStackTrace();
                    }


                    if(value != null && value.exists()){
                        trainInfoDTO.setTrainId((int) value.get(getResources().getString(R.string.train_id)));
                        trainInfoDTO.setLat((double)value.get(getResources().getString(R.string.latitude)));
                        trainInfoDTO.setLon((double)value.get(getResources().getString(R.string.longitude)));
                        trainInfoDTO.setPassengerCount((int)value.get(getResources().getString(R.string.passenger_count)));
                        trainInfoDTO.setSeatingCapacity(train.getSeatingCapacity());
                        System.out.println(value.getData());

                    }
                    else{
                        showSnackbar(getResources().getString(R.string.no_train_data));
                    }
                }
            });


        }catch (Exception e){
            Log.e(TAG,e.getMessage());
            e.printStackTrace();
        }

    }

    private void showSnackbar(String text){
        View container=findViewById(android.R.id.content);
        if(container != null){
            Snackbar.make(container,text,Snackbar.LENGTH_LONG).show();
        }
    }

    private void setUpView() {
        toolbar=findViewById(R.id.ToolBarMain);
        stopsView=findViewById(R.id.stops_rv);
        Intent intent=getIntent();
        train=(TrainDTO)intent.getSerializableExtra("train");

        StopViewAdapter adapter=new StopViewAdapter(train.getCurrentShift().getStops(),this);
        stopsView.setAdapter(adapter);
        stopsView.setLayoutManager(new LinearLayoutManager(this));

        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setTitle(train.getName());

    }


    private void startIntentService()
    {
        Intent intent= new Intent(this, FetchLocationIntentService.class);
        intent.putExtra("LocationReceiver",locationReceiver);
        intent.putExtra("LastLocation",lastLocation);
        startService(intent);
    }


    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()){
            case android.R.id.home:
                finish();
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private class LocationReceiver extends ResultReceiver {
        /**
         * Create a new ResultReceive to receive results.  Your
         * {@link #onReceiveResult} method will be called from the thread running
         * <var>handler</var> if given, or from an arbitrary thread if null.
         *
         * @param handler
         */
        public LocationReceiver(Handler handler) {
            super(handler);
        }


        @Override
        protected void onReceiveResult(int resultCode, Bundle resultData) {
            zipCode= resultData.getString(RESULT_DATA);


        }
    }
}

Answer 1

I was able to figure out the problem in the above scenario. It was the document which I accessed in the firestore had a inner Map and within it was its attributes so the null pointer exception was thrown because I directly tried to access the attributes from the document. Work around I did:

Map<String,Object> map=new HashMap<>();
map=(Map)value.get(String.valueOf(train.getTrainId()));
trainInfoDTO.setLat((double) map.get(getResources().getString(R.string.latitude)));