NullPointerException when using MediatorLiveData

Question

I would like to display a sortable list of restaurants. It worked when I was using a simple LiveData, but I get a NullPointerException since I changed it for a mediatorLiveData.

In the fragment, i use these lines to observe the mediatorLiveData

vm.getAllRestaurantsWithOrderMediatorLD().observe(getViewLifecycleOwner(), listRestaurants -> {
            datas.clear();
            datas.addAll(listRestaurants);
            adapter.notifyDataSetChanged();
        });

And here the viewmodel class :

public class ListRestaurantsViewModel extends ViewModel {

    private final RestaurantRepository restaurantRepository;
    private final SortRepository sortRepository;
    private final MutableLiveData<List<RestaurantViewState>> allRestaurantsViewStateLD = new MutableLiveData<>();
    private final MutableLiveData<SortRepository.OrderBy> orderLiveData = new MutableLiveData<>();
    private final MediatorLiveData<List<RestaurantViewState>> allRestaurantsWithOrderMediatorLD;

    Context ctx;

    public ListRestaurantsViewModel(RestaurantRepository restaurantRepository, SortRepository sortRepository, Context ctx) {
        this.ctx = ctx;
        this.restaurantRepository = restaurantRepository;
        this.sortRepository = sortRepository;

        allRestaurantsWithOrderMediatorLD = new MediatorLiveData<>();
        allRestaurantsWithOrderMediatorLD.addSource(getAllRestaurantsViewStateLD(), value -> allRestaurantsWithOrderMediatorLD.setValue(value));

        allRestaurantsWithOrderMediatorLD.addSource(getOrderLiveData(), order -> {
                                                       
                      List<RestaurantViewState> restaurants = getAllRestaurantsViewStateLD().getValue();
                      if (restaurants != null && !restaurants.isEmpty()) {
                      List<RestaurantViewState> newList = new ArrayList<>();
                      if (order == SortRepository.OrderBy.DISTANCE)
                                                                newList =
                                                                        Stream.of(restaurants).sorted((a, b) -> a.getDistance() - b.getDistance()).toList();

                     else if (order == SortRepository.OrderBy.RATING)
                                                                newList = Stream.of(restaurants).sorted((a, b) -> Double.compare(a.getStarsCount(),
                                                                                                                                 b.getStarsCount())).toList();

         allRestaurantsWithOrderMediatorLD.setValue(newList);
        }
    }
);
    }

    public MediatorLiveData<List<RestaurantViewState>> getAllRestaurantsWithOrderMediatorLD() {
        return allRestaurantsWithOrderMediatorLD;
    }

    public LiveData<SortRepository.OrderBy> getOrderLiveData() {
        return sortRepository.getOrderLiveData();
    }

    public LiveData<List<RestaurantViewState>> getAllRestaurantsViewStateLD() {
        return Transformations.map(restaurantRepository.getRestaurantsLiveData(), restaurantsList -> {
            List<RestaurantViewState> restaurantViewStates = new ArrayList<>();
            for (Restaurant r : restaurantsList) {

                restaurantViewStates.add(new RestaurantViewState(
                                                 r.getId(),
                                                 r.getName(),
                                                 r.getType(),
                                                 r.getAdress(),
                                                 r.getImage()
                                         )
                                        );
            }
            return restaurantViewStates;
        });
    }

This is the error, I get java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observeForever(androidx.lifecycle.Observer)' on a null object reference

I have no error if I observe the livedata getAllRestaurantsViewStateLD() in the fragment, so I think I did not get something in the use of mediatorLiveData Do you see what it is?

Answer 1

The crash is on the call to observeForever, but your code snippet doesn't show you are using it. So the problem is somewhere else.