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Why Line by line execution does not happen in Android Studio?

I have created a Place[] type array p to store the place obtained through fetchPlace() method. I have just shown the some lines code of a function which I have to call. But when I call this function the 1st logd line is executed first which shows value of p[0] as null. As fetching a place takes some time, so I delayed 2nd logd line. Here, p[0] is not null. Please have a look at the code below:

final Place[] p = new Place[1];

final String placeId = place.get("place_id");

        final List<Place.Field> placeFields = Arrays.asList(
                Place.Field.PHONE_NUMBER,
                Place.Field.LAT_LNG
        );

        final FetchPlaceRequest request = FetchPlaceRequest.newInstance(placeId, placeFields);

        placesClient.fetchPlace(request).addOnSuccessListener((response) -> {

            p[0] = response.getPlace();
            
        }).addOnFailureListener((exception) -> {
            Log.i(TAG, "Place not found: " + exception.getMessage());
        });

        Log.d(TAG, "Place: " + p[0]);
        
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                Log.d(TAG, "Place Delayed: " + p[0]);
            }
        }, 1000);

Here is the two logd outputs:
1st : D/MapsFragment: Place: null
2nd : D/MapsFragment: Place Delayed: Place{address=null, addressComponents=null, businessStatus=null, attributions=[], id=null, latLng=lat/lng: (19.0255944,73.0935353), name=null, openingHours=null, phoneNumber=+91 1800 208 1234, photoMetadatas=null, plusCode=null, priceLevel=null, rating=null, types=null, userRatingsTotal=null, utcOffsetMinutes=null, viewport=null, websiteUri=null}

I want to ask why does the code inside addOnSuccessListener doesn’t run before logd which is below it ?
Thanks in advance !

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2

Answers


  1. 0

    You are assigning the p[0] in addOnSuccessListener which is a callback which will be executed not instantly but after when the response is received. So, the value to p[0] is never assigned before the Log statement.

    Refer to this stack overflow answer for more info regarding callbacks.

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  2. 0

    The comment of nanofarad actually sums it up pretty nicely.

    […] the code passed to addOnSuccessListener is specifically set aside to be run only once success occurs. You add the listener, and immediately print the value of p before the listener is called

    Your OnSuccessListener is only getting executed if the lookup of the place was actually successful. Your code however continues to execute and results in p[0] to be null.

    I am not familiar with the Google Places API nor Android Development, but judging from the docs / 2, you shouldn’t need the listener if you don’t make use of it.

    Just submit your Task normally and wait until an result is available.

    Task<FetchPlaceResponse> fetchPlaceRequest = placesClient.fetchPlace(request);

while (true) {
  Thread.sleep(100);
  try {
    TResult result = fetchPlaceRequest.getResult();
    p[0] = result.getPlace();
    break;
  } catch (IllegalStateException ise) {} catch (InterruptedException ie) {
    break;
  }
}

    The sleep function is added to avoid high CPU use. However, this is my generic Java approach since I am not familiar with the API / environment.

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