How to do something in MainActivity after the AsyncTask onPostExecute is done - android-studio

Question

I am trying to get the LatLngs so I can find the nearest place of the search result, I am trying it this way, but the getLatLngs returns null because it hasn't yet initialized.How do I access stuff from MainActivity after the onPostExecute is done.Thank you in advance.
public class GetNearbyPlaces extends AsyncTask<Object, String, String> {
private ArrayList<LatLng> LatLngs = new ArrayList<>();
private LatLng latLngMain;
private LatLng shortestLatLng;
#Override
protected void onPostExecute(String s) {
List<HashMap<String, String>> nearbyPlacesList = null;
DataParser dataParser = new DataParser();
nearbyPlacesList = dataParser.parse(s);
shortestLatLng = DisplayNearbyPlaces(nearbyPlacesList);
}
private LatLng DisplayNearbyPlaces(List<HashMap<String, String>> nearbyPlacesList)
{
int index = 0;
LatLng shortest;
double shortestDistance = Double.MAX_VALUE;
for(int i = 0;i<nearbyPlacesList.size();i++)
{
MarkerOptions markerOptions = new MarkerOptions();
HashMap<String, String> googleNearbyPlace = nearbyPlacesList.get(i);
String nameOfPlace = googleNearbyPlace.get("place_name");
String vicinity = googleNearbyPlace.get("vicinity");
double lat = Double.parseDouble(googleNearbyPlace.get("lat"));
double lng = Double.parseDouble(googleNearbyPlace.get("lng"));
LatLng latLng = new LatLng(lat,lng);
LatLngs.add(latLng);
markerOptions.position(latLng);
markerOptions.title(nameOfPlace + " : " + vicinity);
markerOptions.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_RED));
if(mMap!=null)
mMap.addMarker(markerOptions);
}
for(int i = 0;i<LatLngs.size();i++)
{
double deltaLat = latLngMain.latitude - LatLngs.get(i).latitude;
double deltaLng = latLngMain.longitude - LatLngs.get(i).longitude;
double distanceInDegree = Math.sqrt(deltaLat * deltaLat + deltaLng * deltaLng);
double distanceInMeter = distanceInDegree * 4000;
if(distanceInMeter<shortestDistance)
{
shortestDistance=distanceInMeter;
index = i;
}
}
shortest = LatLngs.get(index);
return shortest;
}
public LatLng getShortestLatLng()
{
return shortestLatLng;
}
}
In MainActivity
getNearbyPlaces.execute(transferData).get();
nearestDistance = getNearbyPlaces.getShortestLatLng();

Answer 1

You must initialize your LatLngs field before calling methods on it.
Replace
private ArrayList<LatLng> LatLngs;
to
private ArrayList<LatLng> LatLngs = new ArrayList<>();
In your code, you initialize it only in loop of DisplayNearbyPlaces method, and if, for example,
nearbyPlacesList is empty, then body of this loop does not run and field remains uninitialized.