As a summary of everything covered across this notebook, here's a practical checklist experienced OOP developers run through when designing or reviewing a class or system:
On a single class:
- Does this class have one clear, nameable responsibility? (SRP)
- Is only what genuinely needs to be public, public? Everything else
private/protected? (Encapsulation, least privilege) - Are properties always kept in a valid, consistent state by the class's own methods, never by external code reaching in directly?
- Would a new person on the team understand what this class does just from its public method names, without reading the implementation? (Abstraction)
On relationships between classes:
- For every
extends, is the 'is-a' relationship genuinely true and likely to remain true? Or would composition ('has-a') be more honest and flexible? - Are dependencies received from outside (constructor injection) rather than hardcoded with
newinside the class? (DIP) - Could a subclass be substituted anywhere its parent is expected, without surprising a caller? (LSP)
- Are interfaces small and focused, rather than one giant interface every implementer must fully satisfy? (ISP)
On extensibility:
- Can new functionality typically be added by writing new classes, rather than editing and re-testing existing, working code? (OCP)
- Is a recognized design pattern (Strategy, Factory, Observer, Decorator, etc.) a natural fit here, or would forcing one in add unnecessary complexity for a problem that doesn't need it yet?
On overall system health:
- Are there any 'God classes' that know or do far too much?
- Are inheritance chains shallow and easy to trace?
- Is the domain model's data and behavior meaningfully coupled together, or has logic drained out into separate 'service' classes, leaving anemic data objects behind?
The meta-principle underlying all of it: good OOP design is fundamentally about managing change — organizing code so that the parts likely to change together are grouped together, and the parts likely to change independently are decoupled from each other. Every principle in this notebook — encapsulation, SOLID, composition over inheritance, the classic design patterns — is, at its core, a specific, well-tested technique for achieving exactly that goal. Internalizing this underlying purpose, rather than memorizing rules in isolation, is what separates a beginner who follows OOP conventions from a pro who understands why they exist.