Relationships: Association, Aggregation, Composition

Aggregation means the child survives the parent's destruction (independent lifecycle). Composition means the child dies with the parent (bound lifecycle). JPA: aggregation has no cascade; composition uses cascade ALL with orphanRemoval.

Association means uses (arrow in UML). Aggregation means has with part surviving (hollow diamond). Composition means owns with lifecycle tied (filled diamond). Weakest to strongest: association, aggregation, composition.

Delegation holds a delegate and forwards selected calls. Inheritance inherits all parent methods. Delegation gives control (expose what you want), flexibility (swap delegate), and safety (add logic around calls).