Explain Aggregation with the help of an example.

Ans. Aggregation

 Aggregation is a stronger form of association. It represents the has-a or part-of relationship. An aggregation association depicts a complex object that is composed of other objects. You may characterize a house in terms of its roof, floors, foundation, walls, rooms, windows, and so on.  

In UML, a link is placed between the “whole “and “parts” classes, with a diamond head attached to the whole class to indicate that this association is an aggregation. Multiplicity can be specified at the end of the association for each of the part-of classes to indicate the constituent parts. The process of decomposing a complex object into its component objects can be extended until the desired level of detail is reached.

Aggregation helps us describe models of the real world that are composed of other models, as well as those that are composed of still other models. Analysts, at the time of describing a complex system of aggregates, need to describe them in enough detail for the system at hand. In the case of a customer order and services, a customer order is composed not only of header information, but also the detail lines as well.