Inheritance

No multiple class inheritance (diamond problem: ambiguous method resolution). Yes, multiple interface implementation. If two interfaces have the same default method, the implementing class must override to resolve the conflict explicitly.

Inheritance creates tight coupling (IS-A, fragile base class, single only). Composition creates loose coupling (HAS-A, swappable, multiple). Prefer composition. Use inheritance only for true IS-A relationships with shared state.

Fragile base class means parent methods leak to the child, breaking the child's contract. Stack extends Vector exposes add(0, x) which breaks LIFO. Fix: composition with a private field and controlled API.