Association vs Aggregation vs Composition

If two classes in a model need to communicate with each other, there must be a link between them, and that can be represented by an association (connector).

Association can be represented by a line between these classes with an arrow indicating the navigation direction. In case an arrow is on both sides, the association is known as a bidirectional association.

Association = link between two classes. Can be unidirectional or bidirectional. Can be one to many or one to one

Aggregation and Composition are subsets of association meaning they are specific cases of association. In both aggregation and composition object of one class “owns” object of another class. But there is a subtle difference:

• Aggregation implies a relationship where the child can exist independently of the parent. Example: Class (parent) and Student (child). Delete the Class and the Students still exist.

  

• Composition implies a relationship where the child cannot exist independent of the parent. Example: House (parent) and Room (child). Rooms don’t exist separate to a House.