Design to Contracts

Design to contracts

An interface is a promise callers can rely on and implementations can swap. The intermediate skill is choosing what belongs in the promise:

public interface PaymentGateway {
    PaymentResult charge(Money amount, Card card);
}

High-level code (checkout) should depend on PaymentGateway, not on StripeGateway. That is the Dependency Inversion Principle in one line: details depend on abstractions, never the reverse.

public final class CheckoutService {
    private final PaymentGateway gateway;
    public CheckoutService(PaymentGateway gateway) {   // injected
        this.gateway = gateway;
    }
    public PaymentResult checkout(Order order) {
        return gateway.charge(order.total(), order.card());
    }
}

Why this pays off:

• tests swap in a fake gateway — no network, no secrets
• adding PayPal means writing a new class, not editing checkout
• the interface documents the only methods checkout actually needs (interface segregation: small promises beat fat ones)