Introduction - "Redefining the Service Layer with Groovy Categories"

DataModel.groovy

class Payee {
  	String name
	String phoneNumber
	Address billingAddress
}

public interface PaymentType {
	Payee getPayee();
}

class CreditCardPaymentType implements PaymentType {
	enum CreditCardType {
		VISA, MASTERCARD, AMEX, DISCOVER
	}

	CreditCardType cardType

	String cardNumber
	String securityCode
	Date expirationDate

	Payee payee
}

class GiftCardPaymentType implements PaymentType {
	String cardNumber
	String securityCode

	Payee payee
}

Service.groovy

public interface PaymentService {
  	/**
	 * Returns an authorization code
	 */
	public String process(PaymentType paymentType);
}

class LocalPaymentService implements PaymentService {
	
	// Dependency injected from container
	CreditCardPaymentGateway creditCardPaymentGateway
	GiftCardPaymentGateway giftCardPaymentGateway

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

	private String doProcess(CreditCardPaymentType paymentType) {
		// ...
	}
	private String doProcess(GiftCardPaymentType paymentType) {
		// ...
	}

}