What is the difference between association, aggregation and composition?

Question

What is the difference between association, aggregation, and composition? Please explain in terms of implementation.

Answer 1

Problem with these answers is they are half the story: they explain that aggregation and composition are forms of association, but they don't say if it is possible for an association to be neither of those.

I gather based on some brief readings of many posts on SO and some UML docs that there are 4 main concrete forms of class association:

1. composition: A is-composed-of-a B; B doesn't exist without A, like a room in a home
2. aggregation: A has-a B; B can exist without A, like a student in a classroom
3. dependency: A uses-a B; no lifecycle dependency between A and B, like a method call parameter, return value, or a temporary created during a method call
4. generalization: A is-a B

When a relationship between two entities isn't one of these, it can just be called "an association" in the generic sense of the term, and further described other ways (note, stereotype, etc).

My guess is that the "generic association" is intended to be used primarily in two circumstances:

• when the specifics of a relationship are still being worked out; such relationship in a diagram should be converted as soon as possible to what it actually is/will be (one of the other 4).
• when a relationship doesn't match any of those 4 predetermined by UML; the "generic" association still gives you a way of representing a relationship that is "not one of the other ones", so that you aren't stuck using an incorrect relationship with a note "this is not actually aggregation, it's just that UML doesn't have any other symbol we could use"