UML - how to identify strong aggregation (composition) relationship between classes and what is difference

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how to identify strong aggregation (composition) relationship between classes and what is difference between association and aggregation(while programming implementation of them)?

Total Answers and Comments: 2

Last Update: November 02, 2007 Asked by: Pritesh Dubey

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April 20, 2007 07:09:29

rashmitambe

Member Since: April 2007 Contribution: 3

RE: how to identify strong aggregation (composition) r...

In strong aggregation or composition, a part entity cannot exist without the containing whole entity in 'whole-part' relationship.
For example. an university has multiple department. This a composition relationship between university and department entity. A department cannot exist on its own with out the university.

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November 02, 2007 12:09:26

Dusan B

RE: how to identify strong aggregation (composition) r...

Difference between association and aggregation (while programming implementation for both):

the main difference is conceptual - Aggregation says that an object is made up of other objects, and association says that they have a relationship, they know of each other, or they use each other in some way.

As far as programming, the main difference is in the multiplicities. Aggregation parent can have * and any subset of multiplicities here, but the child making up the aggregation can only have one parent.

This is different from association where you can have any number of possible relationships, going from 1-1, all the way up to *-*.

To clarify, when implementing aggregation Car and Tire, the Car has an aggregation to Tire, and the car has between 4-5 Tire (as an example, one is spare in the trunk) but in no case can a tire belong to two cars at the same time.

So when implementing the Car, you have to check that it has at least 4 and no more than 5 tires, and when implementing the Tire, you have to at all times have a reference to the Car (which means passing the Car reference in the constructor). Furthermore, when the Car reference is set to null, you have to pro grammatically make sure that the Tire references are also set to null in case of Aggregation but if u model your system as Car just having an association to Tire, this might not necessarily be true.

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