FireStore v4

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()){
                    Toast.makeText(MainActivity.this, "Current data:" + documentSnapshot.getData(), Toast.LENGTH_SHORT).show();
                }
            }
        });
    }