FireStore v7

private void addRealtimeUpdate() {
    DocumentReference contactListener = db.collection("PhoneBook").document("Contacts");
    contactListener.addSnapshotListener(new EventListener<DocumentSnapshot>() {
        @Override
        public void onEvent(DocumentSnapshot documentSnapshot, FirebaseFirestoreException e) {
            if (e != null ){
                Log.d("ERROR", e.getMessage());
                return;
            }
            if (documentSnapshot != null && documentSnapshot.exists()){
                message.setText(documentSnapshot.getData().toString());
            }
        }
    });