Foo.java

        class Order {
            Item[] items;
            String orderId;
            Customer customer;
            Double totalPrice;

            void addItem(Item item) {
                //...
                totalPrice = null;
            }

            double getTotalPrice() {
                if (totalPrice != null) {
                    return totalPrice;
                }
                //...
                for (Item item : items) {
                    //...
                }

                totalPrice = null;
                return num;
            }
        }