SOLID: The First 5 Principles of Object-Oriented Design

SOLID is a set of five design principles that make object-oriented code easier to extend, test and maintain. I lean on them constantly — not as dogma, but as a checklist when a class starts doing too much. Here's how I think about each one.

S — Single Responsibility

A class should have one reason to change. When a module owns both business rules and, say, its own persistence, a change to either drags the other along. Split responsibilities so each unit has a single, well-named job.

O — Open/Closed

Software should be open for extension but closed for modification. New behaviour ought to arrive as new code — a new strategy, a new handler — rather than edits to code that already works and is already tested.

L — Liskov Substitution

Subtypes must be usable anywhere their base type is expected, without surprises. If a subclass weakens a guarantee or throws where the parent wouldn't, the abstraction is leaking and callers pay for it.

I — Interface Segregation

Prefer many small, focused interfaces over one fat one. Clients shouldn't be forced to depend on methods they never call — narrow contracts keep implementations honest and decoupled.

D — Dependency Inversion

Depend on abstractions, not concretions. High-level policy shouldn't know about low-level detail; both should meet at an interface. This is what makes code testable and swappable — the backbone of every service I've shipped.

Applied together, SOLID keeps a codebase flexible as it grows. Treat the principles as heuristics that prompt better questions — not rules to satisfy for their own sake.