Android studio – Java Null-pointer on Firebase invocation

The actual error I receive when running the program is:

java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)' on a null object reference

This is my firebase database class:

import android.location.Location;
import android.util.Log;

import androidx.annotation.NonNull;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;

public class FirebaseDatabaseHelper {
    public static FirebaseDatabase mDatabase ;
    public static DatabaseReference mReferenceCrossings;
   // public static DataSnapshot dataSnapshot;

    public FirebaseDatabaseHelper(FirebaseDatabase mDatabase) {
       mDatabase = FirebaseDatabase.getInstance();
        mReferenceCrossings = mDatabase.getReference();
    }




    public static ArrayList<Location> sortedQuery(String reference, String orderKey) {
        ArrayList<Location> results = new ArrayList<Location>();
        DatabaseReference targetRef = mReferenceCrossings.child(reference);
        targetRef.orderByChild(orderKey).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot datasnapshot) {
                for (DataSnapshot child : datasnapshot.getChildren()) {
                    double latitude = Double.parseDouble(child.child("Latitude").getValue().toString());
                    double longitude = Double.parseDouble(child.child("Longitude").getValue().toString());
                    Location entry = new Location("coordinates");
                    entry.setLatitude(latitude);
                    entry.setLongitude(longitude);
                    results.add(entry);
                    Log.d("coordinates", results.toString());
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                Log.d("Firebase error", error.getDetails());
            }
        });
        return results;

    }
    
    }

This is my main class:

package com.example.locationpushing;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.fragment.app.FragmentActivity;

import android.Manifest;
import android.content.Intent;
import android.content.IntentSender;
import android.content.pm.PackageManager;
import android.graphics.Color;
import android.location.Address;
import android.location.Geocoder;
import android.location.Location;
import android.os.Bundle;
import android.os.Looper;
import android.util.Log;

import com.google.android.gms.common.api.ResolvableApiException;
import com.google.android.gms.location.FusedLocationProviderClient;
import com.google.android.gms.location.LocationCallback;
import com.google.android.gms.location.LocationRequest;
import com.google.android.gms.location.LocationResult;
import com.google.android.gms.location.LocationServices;
import com.google.android.gms.location.LocationSettingsRequest;
import com.google.android.gms.location.LocationSettingsResponse;
import com.google.android.gms.location.SettingsClient;
import com.google.android.gms.maps.CameraUpdate;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.LocationSource;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.Circle;
import com.google.android.gms.maps.model.CircleOptions;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.gms.tasks.Task;

import androidx.annotation.NonNull;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.fragment.app.FragmentActivity;

import android.Manifest;
import android.content.pm.PackageManager;
import android.location.Address;
import android.location.Geocoder;
import android.location.Location;
import android.os.Bundle;
import android.util.Log;

import com.google.android.gms.location.FusedLocationProviderClient;
import com.google.android.gms.location.LocationServices;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.messaging.FirebaseMessaging;
import com.google.firebase.messaging.RemoteMessage;

import java.io.IOException;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    private static final String TAG = "MapsActivity";
    private GoogleMap mMap;
    private Geocoder geocoder;
    private int ACCESS_LOCATION_REQUEST_CODE = 10001;
    FusedLocationProviderClient fusedLocationProviderClient;
    LocationRequest locationRequest;

    Marker userLocationMarker;
    Circle userLocationAccuracyCircle;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_maps);
        // Obtain the SupportMapFragment and get notified when the map is ready to be used.
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
        geocoder = new Geocoder(this);
        fusedLocationProviderClient = LocationServices.getFusedLocationProviderClient(this);
        locationRequest = LocationRequest.create();
        locationRequest.setInterval(330);
        locationRequest.setFastestInterval(330);
        locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);

        Intent intentBackgroundService = new Intent(this, FirebasePushNotification.class);
        startService(intentBackgroundService);
       FirebaseDatabaseHelper.sortedQuery("mReferenceCrossings","Latitude");
}

I’ve attempted different solutions but I still receive nullPointerException for the database. How could I properly initialize the database for access?

I would really appreciate the help on this. Thank you for any advice given.

Answer

I found problem in your code. Your error is occurred when call child method of mReferenceCrossings.

public class FirebaseDatabaseHelper {
    public static FirebaseDatabase mDatabase ;
    public static DatabaseReference mReferenceCrossings;

   ..................

    public static ArrayList<Location> sortedQuery(String reference, String orderKey) {
        ArrayList<Location> results = new ArrayList<Location>();
        
        // Your error happend by this line. because mReferenceCrossings is null.
        DatabaseReference targetRef = mReferenceCrossings.child(reference);
        
        .....................
    }
}

Because mReferenceCrossings is null. mReferenceCrossings will be assigned in following method of your code.

public FirebaseDatabaseHelper(FirebaseDatabase mDatabase) {
   mDatabase = FirebaseDatabase.getInstance();
    mReferenceCrossings = mDatabase.getReference();
}

But I don’t find any part this method calling in your main class. You just called sort method without creating FirebaseDatabaseHelper. So your code have error.

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    ................................   

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

        .................................................

        // This method called without init firebase instance        
        FirebaseDatabaseHelper.sortedQuery("mReferenceCrossings","Latitude");
   }

}

EDIT: update for solution.

Solution is several ways.
Here is one of those.
So you need to update FirebaseDatabaseHelper like this.

public class FirebaseDatabaseHelper {
    public static FirebaseDatabase mDatabase = FirebaseDatabase.getInstance();
    public static DatabaseReference mReferenceCrossings = mDatabase.getReference();

    ...........
}