Class Diagrams

Relationships between components:

Owners, pets, tails:

  • Owners feed pets, pets please owners (association)
  • A tail is a part of both dogs and cats (aggregation/composition)
  • A cat is a kind of pet (inheritance/generalization)

Aggregation and Composition

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.

  • Generalization is implemented as inheritance.

  • Inheritance: “is-a”; composition: “has-a”.

Visibility of any class members are marked by notations of: