ProductOrderRules.java

public enum ProductOrderRules implements ProductOrderRulesEngine {
    /**
     * If no shipping address was explicitly selected, then we'll ship to the billing address of the payment method.
     * If the payment method doesn't have a billing address (gift card), then this rule will fail
     */
    ShippingAddressRule {
        public boolean apply(ProductOrder order) {
            // check if the shipping address is null
            if (order.shippingAddress == null && order.paymentMethod.getType() == PaymentMethod.PaymentMethodType.CREDIT_CARD) {
                // if so, find the first credit card and use its billing address as the shipping address
                order.shippingAddress = ((CreditCard)order.paymentMethod).billingAddress;
            }
            // if there wasn't a credit card, then fail the rule, otherwise, continue
            return order.shippingAddress != null ? true : false;
        }
    },

    /**
     * A coupon can apply to 1..n product types. Each coupon should discount the applicable product types by the specified discount rate.
     * If a coupon has been added to this order and there are no products for which it qualifies, then fail this rule as a last-ditch validation.
     */
    CouponsRule {
        public boolean apply(ProductOrder order) {
            // Exit quickly if we don't have any coupons to process
            if (order.coupons == null || order.coupons.size() == 0) {
                return true;
            }

            boolean result = false;
            // Loop the coupons that have been applied to the order
            for (Coupon coupon : order.coupons) {
                    // Get the list of product types that this coupon can apply to
                    for (Product.ProductType productType : coupon.appliesTo) {
                        // Find the products in the order that this coupon can apply to, and apply the coupon's discount
                        for (Product product : order.products) {
                            if (product.type == productType) {
                                result = true;
                                product.cost = product.cost.multiply(BigDecimal.valueOf(1.00 - coupon.discountRate));
                            }
                        }
                    }
            }
            return result;
        }
    },

    /**
     * We're in NowheresVille, Utopia where we are not required to charge a tax rate, but California has some fucked up laws, so
     * we have to charge them an 8% sales tax if we're shipping to them.
     */
    TaxRule {
        public boolean apply(ProductOrder order) {
            if (order.shippingAddress.state == "CA") {
                order.taxRate = 0.08;
            }
            return true;
        }
    },

    /**
     * By default, we'll use Stripe (www.stripe.com) for our payment processing, but if the chosen payment method is a gift card, then we'll use our
     * internal processor to "apply" the payment.
     */
    PaymentVendorRule {
        public boolean apply(ProductOrder order) {
            if (order.paymentMethod == null) {
                return false;
            }
            if (order.paymentMethod.getType() == PaymentMethod.PaymentMethodType.GIFT_CARD) {
                order.paymentVendor = PaymentVendor.INTERNAL;
            }
            return true;
        }
    },

    /**
     * If this is our customer's first order, then flag this order so that after the order is completed, we will send a nice 1st order email.
     */
    FirstOrderEmailRule {
        public boolean apply(ProductOrder order) {
            if (++order.account.orderCount == 1) {
                order.firstOrderThankYou = true;
            }
            return true;
        }
    };

    public static void callRules(ProductOrder order) {
        for (ProductOrderRules rule : ProductOrderRules.values()) {
            if (!rule.apply(order)) {
                throw new RuntimeException("Failed to execute rule"+rule.name()+".");
            }
        }
    }
}