ExampleDifficulty: Intermediate1 min

Polymorphism Example: One Loop Refunding Three Payment Methods

A card, a bank transfer and store credit, each reversing itself, and a refund loop that names none of them.

Polymorphism works the same way through an interface as through a parent class. A refund run over yesterday's failed orders: each order was paid in a different way, and each way of paying knows how to reverse itself.

java

interface PaymentMethod { Receipt refund(BigDecimal amount); }

class Card implements PaymentMethod {
    public Receipt refund(BigDecimal amount) { return gateway.reverse(token, amount); }
}
class BankTransfer implements PaymentMethod {
    public Receipt refund(BigDecimal amount) { return bank.credit(iban, amount); }
}
class StoreCredit implements PaymentMethod {
    public Receipt refund(BigDecimal amount) { return wallet.topUp(userId, amount); }
}

java

for (Order order : failedYesterday) {
    Receipt receipt = order.paymentMethod().refund(order.total());
    audit.record(receipt);
}

Three completely different things happen on line 2, a call to a card gateway, a bank credit and a wallet top-up, and the loop names none of them. Adding a fourth way to pay adds a class and changes nothing here.

Put this beside a loop that calls draw() on a list of circles, squares and lines, each drawing itself, and the shared structure comes out, which is worth stating rather than leaving to be noticed: in both, one variable's declared type names a capability, the objects behind it are several different classes, and neither loop contains a single if. The domains have nothing in common; the mechanism is identical.

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