An aggregation is a special form of association between classes that represents the concept of "WHOLE -PART". Each object of one of the classes that belong to the aggregation (the composed class) is composed of objects of the other class of the aggregation (the component class). The composed class is often called "whole" andthe component classes are often called "parts".
A simple aggregation is an aggregation where each part can be part of more than one whole.
Composition is a special kind of aggregation in which the parts are physically linked to the whole. So, a composition defines restrictions with regard the aggregation concept:
- A part can not simultaneously belong to more than one whole.
- Once a part has been created it lives and dies with its whole.