Attempt to invoke direct method 'void on a null object reference

Question

Why do I get the error Attempt to invoke direct method 'void asyncFinished(com.ats.atsver2.ui.db.classes.User)' on a null object reference. On the onPostExecute, I get a result but when I try to send it through the delegate, I get the error.

Here is my code:

public class UserRepository {
    private UserDao userDao;
    private User user;
    private LiveData<List<User>> allUsers;

    public UserRepository(Application application) {
        AtsDatabase database = AtsDatabase.getInstance(application);
        userDao = database.userDao();
        allUsers = userDao.getUsers();
    }

    private MutableLiveData<User> userSearched = new MutableLiveData<>();
    private void asyncFinished(User userResult) {
        if(userResult == null) return;
        Log.i("TAG", "asyncFinished: " + userResult.getLastName());
        userSearched.setValue(userResult);
    }

    public LiveData<User> getUserByUserName(String userName) {
        new GetUserByUserNameAsyncTask(userDao).execute(userName);
    }

    public LiveData<List<User>> getAllUsers() { return allUsers; }

    private static class InsertUserAsyncTask extends AsyncTask<User, Void, Void> {
        private UserDao userDao;

        private InsertUserAsyncTask(UserDao userDao) { this.userDao = userDao; }

        @Override
        protected Void doInBackground(User... users) {
            userDao.insert(users[0]);
            return null;
        }
    }

    private static class GetUserByUserNameAsyncTask extends AsyncTask<String, Void, User> {
        private LiveData<User> user;
        private UserDao userDao;
        private UserRepository delegate = null;

        private GetUserByUserNameAsyncTask(UserDao userDao) { this.userDao = userDao; }

        @Override
        protected User doInBackground(String... strings) {
             user = userDao.getUserByUserName(strings[0]); ======> I GET RESULT HERE 
             return null;
        }
        
        @Override
        protected void onPostExecute(User user) {
            super.onPostExecute(user);
            Log.i("TAG", "onPostExecute: "+user.getLastName());
            delegate.asyncFinished(user);
        }
    }
}

This delegate is within the same class except anonymously so why is it a null object? How can I make this right?

Answer 1

You are never assigning delegate to the outer class instance when you create a new GetUserByUserNameAsyncTask. Since it's null when you invoke asyncFinished() it will throw a NPE.

private static class GetUserByUserNameAsyncTask extends AsyncTask<String, Void, User> {
    private LiveData<User> user;
    private UserDao userDao;
    private UserRepository delegate;

    private GetUserByUserNameAsyncTask(UserDao userDao, UserRepository delegate) { 
        this.userDao = userDao; 
        this.delegate = delegate;
    }

    @Override
    protected User doInBackground(String... strings) {
         user = userDao.getUserByUserName(strings[0]); ======> I GET RESULT HERE 
         return null;
    }

    @Override
    protected void onPostExecute(User user) {
        super.onPostExecute(user);
        Log.i("TAG", "onPostExecute: "+user.getLastName());
        delegate.asyncFinished(user);
    }
}

You can now create it with your UserRepository instance (this):

public LiveData<User> getUserByUserName(String userName) {
    new GetUserByUserNameAsyncTask(userDao, this).execute(userName);
}