Getting null object reference using MVVM & Firestore

I'm trying to use MVVM in my project for so many days. But unable to achieve it.

My code:

FeedRepository.java

public class FeedsRepository {
private static FeedsRepository instance;
private List<Feed> feedsToAppend = new ArrayList<>();
public static FeedsRepository getInstance() {
if (instance == null) {
instance = new FeedsRepository();
}
return instance;
}
public MutableLiveData<List<Feed>> getFeeds() {
MutableLiveData<List<Feed>> mFeeds = new MutableLiveData<>();
Query query;
query = feedsRef.whereEqualTo("userID", currentUID).whereEqualTo("not_interested", false)
.orderBy("created_at", Query.Direction.DESCENDING).limit(10);
query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
@Override
public void onComplete(@NonNull Task<QuerySnapshot> task) {
if (task.isSuccessful()) {
QuerySnapshot snapshot = task.getResult();
if (snapshot.isEmpty()) {
} else {
List<DocumentSnapshot> documents = snapshot.getDocuments();
for (DocumentSnapshot doc: documents) {
final Feed feed = doc.toObject(Feed.class);
feed.setFeedID(doc.getId());
feed.setFeedAvailable(true);
feedsToAppend.add(feed);
System.out.println("Feed: " + feed.getFeedID());
}
System.out.println("Total feeds to append: " + feedsToAppend.size());
mFeeds.setValue(feedsToAppend);
}
}
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
}
});
return mFeeds;
}

HomeViewModel.java

public class HomeViewModel extends ViewModel {
private MutableLiveData<List<Feed>> mFeeds;
private FeedsRepository repo;
public void init() {
if (mFeeds != null) {
return;
}
repo = FeedsRepository.getInstance();
mFeeds = repo.getFeeds();
}
public LiveData<List<Feed>> getFeeds() {
return mFeeds;
}
}

HomeFragment.java

    public View onCreateView(@NonNull LayoutInflater inflater,
ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.home_fragment, container, false);
feedsRecycleView = view.findViewById(R.id.feeds_recycler_view);
homeViewModel = ViewModelProviders.of(this).get(HomeViewModel.class);
homeViewModel.init();
homeViewModel.getFeeds().observe(getViewLifecycleOwner(), new Observer<List<Feed>>() {
@Override
public void onChanged(List<Feed> feeds) {
System.out.println("Feed adapter feeds: "+feeds.size());
mFeeds = feeds;
feedAdapter.notifyDataSetChanged();
}
});
List<Feed> testFeeds = homeViewModel.getFeeds().getValue();
System.out.println("Test feeds are : " + testFeeds.size());
feedAdapter = new FeedAdapter(getContext(), homeViewModel.getFeeds().getValue());
feedsRecycleView.setAdapter(feedAdapter);
feedsRecycleView.setHasFixedSize(true);
LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getContext());
linearLayoutManager.setStackFromEnd(true); //newer posts will be shown on top
linearLayoutManager.setReverseLayout(true);
feedsRecycleView.setLayoutManager(linearLayoutManager);
return view;
}

I get error

> "Attempt to invoke interface method 'int java.util.List.size()' on a
> null object reference"

on this line

List<Feed> testFeeds = homeViewModel.getFeeds().getValue();

and

feedAdapter = new FeedAdapter(getContext(), homeViewModel.getFeeds().getValue());

Please anyone tell me where am I making mistake.

You are getting the following error:

> "Attempt to invoke interface method 'int java.util.List.size()' on a null object reference"

Most likely at this particular line of code:

System.out.println("Test feeds are : " + testFeeds.size());

And this is because your testFeeds object is null, meaning that:

homeViewModel.getFeeds().getValue();

Return null. This also means that getValue() method returns a LiveData object that is null, which is indeed the case as in your HomeViewModel class the getFeeds() method returns a LiveData object which is never initialized. To solve this, please change the following line of code:

private MutableLiveData<List<Feed>> mFeeds

to

private MutableLiveData<List<Feed>> mFeeds = new MutableLiveData<>();

Okay. I have figured it out.

I had to add this line in FeedsRepository,

 public MutableLiveData<ArrayList<Feed>> getFeeds() {
MutableLiveData<ArrayList<Feed>> mFeeds = new MutableLiveData<>();
mFeeds.setValue(feedsToAppend);
......

and after fetching records from backend, again

mFeeds.setValue(feedsToAppend);

As the backend process is asynchronous, and would take time to fetch
it would return with null object.