How to read new child from firebase in an android app?

Question

I am creating a app that will read data from firebase and display it on a map. The app keeps crashing because there are hell lot of records to retrieve from the database. I need a way with which I get only the latest node data and not the entire data snapshot.

MainActivity.java:

public class MainActivity extends AppCompatActivity implements OnMapReadyCallback{
    FirebaseDatabase db;
    DatabaseReference dbReference;
    TextView speed_disp;
    public List<LatLng> polygon;

    public double getLatitude() {
        return latitude;
    }

    public double getLongitude() {
        return longitude;
    }

    private GoogleMap googleMap;

    private double latitude=19.10815888216128;
    private double longitude=72.83706388922495;

    public void setCord(String lat,String lon){
        this.latitude=Double.parseDouble(lat);
        this.longitude=Double.parseDouble(lon);
    }




    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        polygon = new PolyList<>(10);
        setContentView(R.layout.activity_main);

        db = FirebaseDatabase.getInstance();
        speed_disp=findViewById(R.id.speed);

        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);


//        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M && checkSelfPermission(android.Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
//            requestPermissions(new String[]{android.Manifest.permission.ACCESS_FINE_LOCATION}, 1000);
//        }

        try{
            dbReference = db.getReference("Data");
            dbReference.addValueEventListener(new ValueEventListener() {



                @Override
                public void onDataChange(@NonNull DataSnapshot datasnapshot) {
                    Log.d("Lat",datasnapshot.getValue().toString());
                    for (DataSnapshot snapshot: datasnapshot.getChildren()) {

                        String latitude = snapshot.child("lat").getValue(String.class);
                        String longitude = snapshot.child("lon").getValue(String.class);
                        String speed = snapshot.child("speed").getValue(String.class);

                        speed_disp.setText(speed);
                        setCord(latitude,longitude);
                        googleMap.clear();


                        double lat = getLatitude();
                        double lon = getLongitude();

                        LatLng current=new LatLng(lat,lon);


                        // Add marker position to polygon list
                        polygon.add(current);

                        // Draw polyline
                        if (polygon.size() >= 2) {
                            PolylineOptions polylineOptions = new PolylineOptions()
                                    .addAll(polygon)
                                    .width(5)
                                    .color(Color.RED);
                            googleMap.addPolyline(polylineOptions);
                        }

                        googleMap.addMarker(new MarkerOptions().position(current).title("Current Location"));

                        // Move camera to marker
                        googleMap.moveCamera(CameraUpdateFactory.newLatLngZoom(current,20 ));

                    }


                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                    Toast.makeText(MainActivity.this,"Read error!!!",Toast.LENGTH_SHORT).show();
                }


            });
        }catch (Error e) {
            Toast.makeText(MainActivity.this, "Read module error", Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        this.googleMap=googleMap;
        googleMap.setMapType(GoogleMap.MAP_TYPE_SATELLITE);

//        LatLng current=new LatLng(19.10815888216128,72.83706388922495);
//        this.googleMap.addMarker(new MarkerOptions().position(current).title("Dub maro pls!!"));
//        this.googleMap.moveCamera(CameraUpdateFactory.newLatLng(current));



    }


}

Firebase:

I want to read only the newest node from the real-time database. Also, I would appreciate if you guys could improve my code.

Answer 1

It looks like you use push to add children to Data, which means you can use this approach to get only the most recently added child node:

dbReference = db.getReference("Data");
Query query = dbReference.orderByKey().limitToLast(1);
query.addValueEventListener(new ValueEventListener() {
    ...

Also see the Firebase documentation on:

• Adding an item to a list
• Sorting and filtering data