Aggregation vs Composition vs Association vs Direct Association

Question

I am reviewing my knowledge in object-oriented programming. Under the relationship between classes topic, I have encountered some relationships which are a bit ambiguous to

Answer 1

Direct association has nothing in common with the other three. It does not belong to UML at all, it is the IBM requirements modelling term.

As for others,

Association A->B is a child of Dependency. Association means, that A (or its instance) has some easy way to get to instance of B. For example, a.x.y.b. Or by function, or by some local variable. Or by a direct reference or pointer, or something else (there are many languages in the world). As you see, there is no strict border between dependency and association.

One of attributes of Association is Aggregation, it can have values: None, shared (often incorrectly called aggregation), and composition.

If A (or instance) has some (or one) instances of B so, that destroying of association means the destroying of B instances, it is the composition.

If you or a tool author had decided, that some has-a relationship, that is weaker that composition, needs to be specially shown, you can use shared aggregation. Usually it is some collections of references to B in A.

There are some more interesting attributes of associations. Look here if you are interested.