saving data on arraylist

  myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                userList.clear();
                Iterator<DataSnapshot> userIterator = dataSnapshot.getChildren().iterator();

                while (userIterator.hasNext()){
                    User temp = userIterator.next().getValue(User.class);
                    userList.add(temp);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

 public void addToFavorite(){
        for(User u : userList){
            myRef.child(u.getId()).child("check").setValue(true);
        }
    }