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Aggregation
Aggregation is stronger form of association. It represents the "has-a" or "part-of" relationship. An aggregation association depicts a complex object which is composed of other objects. You can characterize a house as a combination of its roof, floors, walls, foundation, windows, and rooms and so on. A room may in turn be, composed of walls, floor, ceiling, doors and windows, as shown in Figure.
In UML, a link is placed among the "whole" and "parts" classes, with a diamond head (Figure) attached to the complete class to indicate which association is an aggregation. Multiplicity can be precise at the end of the association for each of "part-of" classes to indicate constituent parts. The method of decomposing a complex object into its component objects can be extended until desired level of detail is reached.
Figure: A house and some of its component
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