Composition

- A more specific form of aggregation…is something called composition.…Like aggregation, composition is based around…a has-a relationship between objects…but it specifically implies ownership.…I might say that a spaceship has an engine,…but more accurately, I should that…the spaceship owns the engine.…Composition implies ownership and an engine has…no meaning or purpose in the system without a spaceship.…- It's like attaching the object to…one of these model ships.…The spaceship has a shield and,…since it's a star fighter, it has a weapon.…

Or I should say, it owns a weapon.…- Right.…The space ship is composed of several different objects…that each have their own attributes and behaviors.…But a key difference between composition and aggregation…is that in a composition situation,…if the owning object is destroyed by, say,…an asteroid ...…- Or an Olivia.…- The contained objects are destroyed, too.…The ships engine, its shield, its weapons,…those are all gone.…Now, although aggregation may not always…be worth showing in a UML diagram,…