how do I retrieve an object from inside a void method

Question

So I have been working with firestore recently but I have had been having a hard time retrieving object from my database. I want to have a method that retrieves an object and returns it, I tried to look up up this question but it seems that the only solution is to use a class attribute however the problem is that it is a static method. Here is the code:

package com.example.test2;

import com.google.android.gms.tasks.OnSuccessListener;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.FirebaseFirestore;

public class DatabaseManager {
    private static final FirebaseFirestore db = FirebaseFirestore.getInstance();
    private static final String TAG = "error";
    private static User retrievedUser;

    public static void addUser(User user){

        // Add a new document with a generated ID
        db.collection("brawlrUsers").document(user.getId())
                .set(user);
    }
    public static void setAttribute(String attribute, String value){
//        db.collection("brawlrUsers")
    }
    //----------------------------------------
    // this is the method I need help with, I want to return the user 
    // from inside the successlistener but I can't since it has void return type
    //----------------------------------------
    public static User getUser(String username){
        db.collection("brawlrUsers").document(username).get()
                .addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
                    @Override
                    public void onSuccess(DocumentSnapshot documentSnapshot) {
                        User user = documentSnapshot.toObject(User.class);

                    }
                });
        return new User();
    }
}