Obscurity - "Redefining the Service Layer with Groovy Categories"

Service.groovy

class LocalPaymentService implements PaymentService {
  
	// Dependency injected from container
	CreditCardPaymentGateway creditCardPaymentGateway
	GiftCardPaymentGateway giftCardPaymentGateway
	CheckPaymentGateway checkPaymentGateway
	EmployeePaymentGateway employeePaymentGateway

	public String process(PaymentType paymentType) throws IllegalArgumentException {
		if (paymentType instanceof CreditCardPaymentType) {
			return doProcess(paymentType)
		} else if (paymentType instanceof GiftCardPaymentType) {
			return doProcess(paymentType)
		} else if (paymentType instanceof CheckPaymentType) {
			return doProcess(paymentType)
		} else if (paymentType instanceof EmployeePaymentType) {
			return doProcess(paymentType)
		}
		throw new IllegalArgumentException("Supplied payment type is not supported.");
	}

	private String doProcess(CreditCardPaymentType paymentType) {
		if ("CA".equalsIgnoreCase(paymentType.getPayee().getBillingAddress().getState())) {
			return doProcessCalifornia(paymentType)
		}
		return doProcessOther(paymentType)
	}

	private String doProcessCalifornia(CreditCardPaymentType paymentType) {
		// ...
	}
	private String doProcessOther(CreditCardPaymentType paymentType) {
		// …
	}

}