PaymentMethod.java

public interface PaymentMethod {
    enum PaymentMethodType {
        GIFT_CARD, CREDIT_CARD
    }

    PaymentMethodType getType();
}

public class GiftCard implements PaymentMethod {
    String number;
    BigDecimal balance;

    @Override
    public PaymentMethodType getType() {
        return PaymentMethod.PaymentMethodType.GIFT_CARD;
    }
}

public class CreditCard implements PaymentMethod {
    String number;
    Date expiration;
    Address billingAddress;

    @Override
    public PaymentMethodType getType() {
        return PaymentMethod.PaymentMethodType.CREDIT_CARD;
    }
}