What is the difference between Composition and Association relationship?

Question

In OOP, what is the difference between composition (denoted by filled diamond in UML) and association (denoted by empty diamond in UML) relationship between classes. I\'m a

Answer 1

Owning and using.

Composition: the object with the reference owns the object referred to, and is responsible for its "lifetime", its destruction (and often creation, though it may be passed in). Also known as a has-a relationship.

Association: the object with the reference uses the object referred to, may not be an exclusive user, and isn't responsible for he referred-to object's lifetime. Also known as a uses-a relationship.

The OP comments:

Can you provide a real world example. Also, what is aggregation? – Marc

Aggregation: an Association that is from whole to part, and that can't be cyclic.

Examples:

Composition: a Car has-an Engine, a Person has-an Address. Basically, must have, controls lifetime.

Association: A Car has-a Driver, some class instance has-an ErrorLogger. Lifetime not controlled, may be shared.

Aggregation: A DOM (Document Object Model, that is the objects that make up a tree of HTML elements) Node has-a (an array of) child Nodes. The Node is top (well, higher) level; it "contains" its children, they don't contain it.