How can I complete data from Firebase to ArrayList?

Question

There is a User object, and from the given collection you want to output all the numbers. In order to compare the uniqueness of the number. The data can be output to the console. But it cannot be added to the ArrayList.

     public DatabaseReference databaseReference;
     ArrayList<String> codesOfUsers = new ArrayList<>();

    databaseReference = FirebaseDatabase.getInstance().getReference().child("User");
    ChildEventListener сhildEventListener = new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

                String number = snapshot.child("number").getValue(String.class);
                codesOfUsers.add(number);
                System.out.println(number);
        }

        @Override
        public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

        }

        @Override
        public void onChildRemoved(@NonNull DataSnapshot snapshot) {

        }

        @Override
        public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

        }

        @Override
        public void onCancelled(@NonNull DatabaseError  error) {
            Log.e("The read failed: " ,error.getMessage());
        }
    };

    databaseReference.addChildEventListener(сhildEventListener);
    System.out.println("Output" + codesOfUsers.toString());