Principles That Earn Their Keep

Principles That Earn Their Keep — Tech Epitome

Encapsulate what varies

This is the most actionable principle in the set, because it is a procedure rather than an aspiration. Walk the requirements, and for each one ask: has this changed before, or is it on the roadmap? Those are your variation points. Everything else is stable, and stable things do not need seams.

Pricing, scoring, ranking, matching — almost always varies.
Which external provider is called — varies, eventually.
The set of subtypes of a domain concept — usually grows.
The order of two lines in a constructor — does not vary. Leave it alone.

Program to interfaces

Depend on the narrowest type that does the job. A parameter typed List<Order> when the method only iterates should be Iterable<Order>: it accepts more callers and promises less, which is the right direction on both counts.

Dependency injection, without ceremony

Dependency injection means a class is given its collaborators rather than constructing them. That is the whole idea; a container is an optional convenience for wiring, not the principle.

java

1// Not injected: impossible to test without a real clock and gateway.2final class Booking {3    private final Clock clock = Clock.systemUTC();4    private final PaymentGateway gateway = new StripeGateway();5}6 7// Injected: every collaborator is visible in the signature, and8// substitutable in a test.9final class Booking {10    private final Clock clock;11    private final PaymentGateway gateway;12 13    Booking(Clock clock, PaymentGateway gateway) {14        this.clock = clock;15        this.gateway = gateway;16    }17}

The second version's constructor is also its documentation.

Immutability where it pays

An immutable object cannot be in an invalid state after construction, cannot be changed by a method you passed it to, and is safe to share between threads. Those three properties remove entire categories of bug.

The cost is allocation on every change, which matters for large objects mutated in a tight loop and essentially never elsewhere. Value-like things — money, dates, identifiers, coordinates, events — should be immutable by default. Long-lived entities with genuine identity and lifecycle are the reasonable exception.