import java.util.List;
public interface FirebaseCallback {
    void onCallback (List<Sentinel> list);
}

public class SentinelStorage { // Singleton class
    public static SentinelStorage sentinelStorage;
    private List<Sentinel> sentinelsList;

    ...
    public List<Sentinel> readSentinelsListFromDB(DatabaseReference dbRef, final FirebaseCallback firebaseCallback){
        final Sentinel sentinel = new Sentinel();
        ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                GenericTypeIndicator<Map<String, Object>> t = new GenericTypeIndicator<Map<String, Object>>(){};
                Map<String, Object> sentinelsMap = dataSnapshot.getValue(t);
                for (Map.Entry<String,Object> entry : sentinelsMap.entrySet()){
                    // convert every node of Map to Sentinel instance
                    sentinelsList.add(sentinel.mapToSentinel((Map)entry.getValue()));
                }
                firebaseCallback.onCallback(sentinelsList);
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.d(TAG,databaseError.getMessage());
            }
        };
        dbRef.addListenerForSingleValueEvent(valueEventListener);
        return sentinelsList; // here sentinelsList still is not empty
    }
    ...
}

public class ActivitySentinel extends BaseActivity {
    RecyclerView mRecyclerView;
    SentinelViewAdapter adapter;
    private final String DBSentintelName = "db_sentinel";
    Sentinel sentinel = new Sentinel();
    FirebaseDatabase mFirebaseDatabase = FirebaseDatabase.getInstance();
    DatabaseReference dbRef = mFirebaseDatabase.getReference(DBSentintelName);
    List<Sentinel> list = new ArrayList<>(); // must be passed to SentinelViewAdapter
    SentinelStorage sentinelStorage; // singletone that contain a List<Sentinel>
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
    ...
        sentinelStorage = SentinelStorage.get(); // get the only instance of SentinelStorage
    // exactly this list below makes me nervous
    list = sentinelStorage.readSentinelsListFromDB(dbRef, new FirebaseCallback() {
        @Override
        public void onCallback(List<Sentinel> lst) {
            list = lst;
            Log.d(TAG, "onCallback.Is list empty "+list.isEmpty());// not EMPTY
        }
    });

    Log.d(TAG, " Is list empty :: "+list.isEmpty()); // TRUE - list is EMPTY
    adapter = new SentinelViewAdapter(list);
    mRecyclerView.setAdapter(adapter);
    ...
    }
}