Composition over Inheritance — Notes

Why it usually wins

• Behaviours can be swapped at runtime.
• Easier to test (mock the collaborator).
• Avoids deep hierarchies (which are hard to reason about).
• Sidesteps multiple-inheritance issues.

When inheritance is fine

• Closed, stable hierarchies (Shape → Circle/Rect/Triangle).
• Templates and frameworks that explicitly require it (Spring's HandlerAdapter, Android Activity).
• Trivial extension where overriding is the natural mechanism.

"Favor composition" ≠ "never inherit"

• Single-level inheritance is often perfectly fine.
• Inheritance chains > 2 levels are a smell.
• "Marker interfaces" or sealed hierarchies model algebraic data types cleanly.

Refs

• Effective Java — Items 18 (favor composition over inheritance), 19 (design for inheritance or prohibit it).
• Design Patterns (GoF) — many patterns are composition-based alternatives to inheritance.
• Go: design and evolution talks — Go intentionally drops inheritance.