Concrete implementation to load tasks from data source to the cache

ChallengesRepo.java

import android.database.Cursor;
import android.support.annotation.NonNull

/**
* Concrete implementation to load tasks from data source to the cache
*/
public class ChallengesRepo implements ChallengesDataStore {

    private static ChallengesRepo instance = null;
    private final ChallengesRemoteDataSource mChallengesRemoteDataSource;
    private final ChallengesLocalDataStore mChallengesLocalDataStore;
    
    // prevent direct instantiation
    private ChallengesRepo(@NonNull ChallengesRemoteDataSource challengesRemoteDataSource,
                           ChallengesLocalDataStore challengesLocalDataStore) {
        this.mChallengesRemoteDataSource = challengesRemoteDataSource;
        this.mChallengesLocalDataStore = challengesLocalDataStore;
    }

    public static ChallengesRepo getInstance(ChallengesRemoteDataSource challengesDataStore,
                                             ChallengesLocalDataStore challengesLocalDataStore) {
        if (instance == null) {
            instance = new ChallengesRepo(challengesDataStore, challengesLocalDataStore);
        }
        return instance;
    }
    
    @Override
    public void getChallenges(@NonNull final GetChallengesCallback getChallengesCallback) {

        mChallengesRemoteDataSource.getChallenges(new GetChallengesCallback() {
            @Override
            public void onChallengesLoaded(List<Challenge> challenges) {
                getChallengesCallback.onChallengesLoaded(challenges);
            }

            @Override
            public void onError(Exception exception) {
                getChallengesCallback.onError(exception);
            }

            @Override
            public void onDataNotAvailable() {
                getChallengesCallback.onDataNotAvailable();
            }
        });
    }
    
    @Override
    public void saveChallenges(@NonNull Challenge challenge) {
        mChallengesRemoteDataSource.saveChallenges(challenge);
    }
    
    @Override
    public void deleteAllChallenges() {
        mChallengesLocalDataStore.deleteAllChallenges();
    }

    public interface LoadDataCallback {
        void onDataLoaded(Cursor cursor);

        void onDataNotAvailable();

        void onDataReset();
    }
}