title: how to add google map marker from firebase database
body: <p>I want to retrieve location markers from firebase database but I can't see any markers after running this code. I tried to make <code>EventListener</code> but the map runs without any markers. This my code:</p>
  
  <pre><code>public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {
  
      private GoogleMap mMap;
  
      @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);
  
      }
  
  
      /**
       * Manipulates the map once available.
       * This callback is triggered when the map is ready to be used.
       * This is where we can add markers or lines, add listeners or move the camera. In this case,
       * we just add a marker near Sydney, Australia.
       * If Google Play services is not installed on the device, the user will be prompted to install
       * it inside the SupportMapFragment. This method will only be triggered once the user has
       * installed Google Play services and returned to the app.
       */
      @Override
      public void onMapReady(GoogleMap googleMap) {
          mMap = googleMap;
          DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
  
  
  
  
                      String rightLocation = ref.child("location_right").toString();
                      String leftLocation = ref.child("location_left").toString();
  
                      double location_left = Double.valueOf(leftLocation);
                      double location_right = Double.valueOf(rightLocation);
                      LatLng sydney = new LatLng(location_left, location_right);
                      mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney"));
                      mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(sydney,18));
  
  
  
  
  
  
  
  
  
  
  
          // Add a marker in Sydney and move the camera
  
      }
  }
  </code></pre>
  
  <p>What might be wrong here?</p>