Association
Association is a relationship between two software components in which one component uses, depends on, or otherwise has knowledge of the other. It is the most general of the relationships between classes in object-oriented programming, and the one from which the more specific composition and aggregation relationships are derived.
An association is typically realized as one object holding a reference to another, either to a concrete class or through an interface. The holder can invoke the collaborator’s operations, and the collaborator may in turn hold a reference back, forming a bidirectional association. Directionality and multiplicity — one-to-one, one-to-many, many-to-many — are part of what distinguishes one association from another.
In UML
The Unified Modeling Language formalizes association as a
solid line drawn between two classes in a class diagram. The line may carry
multiplicities at each end (eg. 1, 0..1, *), role names describing each
end’s part, and an open arrowhead indicating navigability. A line with no
arrowheads denotes a bidirectional association.
Two refined forms sit under association.
- Aggregation. A "has a" relationship in which the part can exist independently of the whole. It is drawn with a hollow diamond at the whole end.
- Composition. A stronger "owns a" relationship in which the part’s lifetime is bound to the whole. When the whole is destroyed, so are its parts. It is drawn with a filled diamond at the whole end.
Association versus inheritance
Where inheritance expresses an "is a" relationship between classes, association expresses a "uses a" or "has a" relationship. The two are often contrasted. Inheritance ties a subclass to its parent’s implementation at compile time, while an association lets a class reach a collaborator through a reference that can be swapped — at runtime when the reference is held behind an interface. This is why composition, the strongest form of association, is the recommended alternative to inheritance for code reuse. It keeps coupling lower and behaviors replaceable.