Seams & Test Doubles

Seams and test doubles

A seam is a replaceable point in code — usually a constructor-injected interface. Doubles plug into seams:

• Stub — returns canned data (FakeGateway from Module 2)
• Fake — a working lightweight implementation (in-memory repo)
• Spy — records calls for later verification
• Mock — pre-programmed expectations (use sparingly; mocks that know too much make refactoring painful)

interface Clock { Instant now(); }

class SubscriptionService {
    private final Clock clock;
    SubscriptionService(Clock clock) { this.clock = clock; }
    boolean isExpired(Instant until) { return clock.now().isAfter(until); }
}

// test: freeze time instead of sleeping/_waiting
Clock frozen = () -> Instant.parse("2026-09-14T00:00:00Z");

The rule: if code calls Instant.now() or Math.random() deep inside, it has no seam — extract the decision behind an interface. Deterministic tests are the payoff.