Firestore Android查询第一次返回null

Question

public boolean cemail(final String email){
    CollectionReference users = db.collection("Users");
    Query query = users.whereEqualTo("email", email);
    query.get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()){
                        for (DocumentSnapshot ds : task.getResult()){
                            if (ds.getString("email").equals(email)){
                                res = "asd";
                                x = false;
                            }
                        }
                        if (task.getResult().size() == 0){
                            res = "asd2";
                            x = true;
                        }
                        if (task.getResult().isEmpty()){
                            res = "asd3";
                            x = true;
                        }
                    }else {
                        if (task.getResult().size() == 0){
                            res = "asd2";
                            x = true;
                        }
                        if (task.getResult().isEmpty()){
                            res = "asd3";
                            x = true;
                        }
                    }
                    if (task.isCanceled()){
                        Toast.makeText(RegisterActivity.this, "Error:"+task.getException(), Toast.LENGTH_SHORT).show();
                    }

                }

            });
    Toast.makeText(this, "->"+res, Toast.LENGTH_SHORT).show();
    return x;
}

我有这段代码，当我第一次调用该方法时，如果再次调用该方法可以正常返回正确的值，那么“ res”值为null，我的问题是为什么第一次返回null以及如何修复它。