read data firestore java

1DocumentReference docRef = db.collection("cities").document("SF");
2docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
3    @Override
4    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
5        if (task.isSuccessful()) {
6            DocumentSnapshot document = task.getResult();
7            if (document.exists()) {
8                Log.d(TAG, "DocumentSnapshot data: " + document.getData());
9            } else {
10                Log.d(TAG, "No such document");
11            }
12        } else {
13            Log.d(TAG, "get failed with ", task.getException());
14        }
15    }
16});