Aggregation is a UML whole-part relationship in which a container refers to contained objects that can exist independently. The container uses an open diamond at the relationship tail. section 4.3.2
Use aggregation when:
- A Catalogue contains zero or more Book objects, while each Book can exist outside the Catalogue. section 4.3.2
- A class should control access to another object without exposing invalid operations. section 7.2
- Aggregation better expresses a has-a relationship than inheritance when substitution would permit inappropriate operations. section 7.2 section 7.3
Refactoring CircularBuffer
A CircularBuffer implemented as a list exposes indexing and pop(), allowing callers to corrupt its invariants. Refactoring it as a standalone class that aggregates a private list permits only add() and remove(), preventing the demonstrated crash. section 7.2
Figure 4.8 Class Catalogue aggregates class Book. Each Catalogue object contains zero or more Book objects in a list instance variable named booklist. The open diamond at the tail of the dependency arrow is next to the diagram for the container class Catalogue.
Shows the open-diamond notation and the Catalogue-to-Book relationship, including the container's list of contained objects. section 4.3.2
