SOLID Principles in Java — Practical Application and Trade-offs

SOLID gives you five rules for structuring classes so changes don't ripple unpredictably through your codebase. Single Responsibility means one axis of change per class — if your UserService handles registration, emails, and reports, changing the email provider risks breaking registration logic. Extract separate services.

Open/Closed is about extending behavior without modifying existing code. In Spring, the Strategy pattern with @Component scanning is the natural implementation — add a new DiscountStrategy class and the consuming service discovers it automatically, no edits needed.

Liskov Substitution catches subtype lies. If ReadOnlyList extends ArrayList but throws on add(), any code expecting a List breaks at runtime. The fix is composition — expose only the methods the wrapper actually supports, so the mismatch becomes a compile error.

Interface Segregation keeps interfaces small. Spring Data does this well — CrudRepository for basic CRUD, JpaRepository for JPA-specific operations. Extend only what you need so implementations don't carry dead methods.

Dependency Inversion is Spring's foundation. Depend on OrderRepository the interface, inject the concrete implementation via constructor. Testing injects a mock, production injects JPA — the service doesn't know the difference.

The nuance that separates a good answer from a great one is knowing when to break these rules. A 200-line microservice doesn't need five single-responsibility classes — the navigation cost outweighs the coupling cost. My rule: follow SOLID when the abstraction earns its weight, violate it consciously when simplicity wins. DIP through constructor injection is the one I never skip — the cost is essentially zero.