Problem implementing interface in an onComplete method

Question

Following the previous thread that I have created here: How do I set a variable inside of an onComplete method?

I am still trying to call the variable that I have set inside on the onComplete method. I know there is a delay in-between accessing the method. And the one solution that was brought up to me was to create an interface. Although, I have successfully implemented the interface, I am not sure whether my implementation is correct since I am still getting nulls.

Here's my Interface:

public interface MyCallBack {
void onCallFullName(String fullName);
void onCallEmail(String Email);
}

The method to fetch the data from firebase:

private void callUserCredential(MyCallBack myCallBack) {
    fAuth = FirebaseAuth.getInstance();
    fStore = FirebaseFirestore.getInstance();
    String userID = fAuth.getCurrentUser().getUid();


    DocumentReference docRef = fStore
            .collection("users")
            .document(userID);

    docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if(task.isSuccessful()){
                DocumentSnapshot document = task.getResult();
                if(document.exists()){

                    //Log for successfully fetching data from firebase
                    Log.d(TAG,"User full name: " + document.getString("FullName"));
                    Log.d(TAG, "User email: " + document.getString("Email"));

                    String fullName = document.getString("FullName");
                    String eMail = document.getString("Email");

                    //Log for successfully setting the data into the variables
                    Log.d(TAG, "donorFullName set: " + fullName);
                    Log.d(TAG, "donorEmail set: " + eMail);

                    //Interface callback
                    myCallBack.onCallFullName(fullName);
                    myCallBack.onCallEmail(eMail);

                }
            }
        }
    });
}

and Lastly, here's me calling the Interface:

//Create booking information
    BookingInformation bookingInformation = new BookingInformation();

    callUserCredential(new MyCallBack() {
        @Override
        public void onCallFullName(String fullName) {
            donorFullName = fullName;
        }

        @Override
        public void onCallEmail(String Email) {
            donorEmail = Email;
        }
    });

    //Log for successfully calling the interface
    Log.d(TAG, "donorFullName bookingInformation: " + donorFullName);
    Log.d(TAG, "donorEmail bookingInformation: " + donorEmail);
    bookingInformation.setDonorName(donorFullName);
    bookingInformation.setDonorEmail(donorEmail);

The logcat still shows the same thing:

Answer 1

You must setDonorName() and setDonorEmail() in Callback functions like:

//Create booking information
    BookingInformation bookingInformation = new BookingInformation();

    callUserCredential(new MyCallBack() {
        @Override
        public void onCallFullName(String fullName) {
            bookingInformation.setDonorName(fullName);
            //Log for successfully calling the interface
            Log.d(TAG, "donorFullName bookingInformation: " + fullName);
        }

        @Override
        public void onCallEmail(String email) {
            bookingInformation.setDonorEmail(email);
            //Log for successfully calling the interface
            Log.d(TAG, "donorEmail bookingInformation: " + email);
        }
    });

Because when you call the log function, the request is uncompleted yet!