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   ► KBRole-Based T...Coding & OOObject Orien...   Print This     
  From the November 2015 Issue of Prestwood eMag
 
Coder Object Orientation (OO):
Aggregation
 
Posted 11 years ago on 6/30/2008 and updated 1/5/2009
Take Away:

Aggregations indicate a whole-part relationship, and are known as "has-a" or "is part of" relationships. An Aggregation relationship is indicated by a line with a hollow diamond.

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KB101263



Aggregation -

Aggregations indicate a whole-part relationship, and are known as "has-a" or "is part of" relationships. An Aggregation relationship is indicated by a line with a hollow diamond.


Aggregation is a type of class or object association.  Aggregations indicate a whole-part relationship, and are known as "is part of" or "has-a" relationships. An Aggregation relationship is indicated by a line with a hollow diamond.

UML Symbol:

UML Aggregation Arrow

 

Aggregation vs. Composition

It's interesting to contrast aggregation with composition, which is also known as composite aggregation. General aggregation does not indicate whether the class can exist by itself or not. It simply designates a whole-part relationship. Composition indicates the class cannot exist by itself such as a button class control must exist as part of a container form class.

Summary of Relationship types

An aggregation relationship represents an "is-part-of" or "has-a" relationship. Contrast this with composition which represents a "uses-a" relationship. You can also contrast this with inheritance which represents an "is-a" relationship and with interfaces which represents a "behaves-as" or "looks-like" relationship.

More Info

Definition:  Association
Definition:  Composition
Definition:  Dependency

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Definition Contributed By Mike Prestwood:

Mike Prestwood is a drummer, an author, and creator of the PrestwoodBoards online community. He is the President & CEO of Prestwood IT Solutions. Prestwood IT provides Coding, Website, and Computer Tech services. Mike has authored 6 computer books and over 1,200 articles. As a drummer, he maintains play-drums.com and has authored 3 drum books. If you have a project you wish to discuss with Mike, you can send him a private message through his PrestwoodBoards home page or call him 9AM to 4PM PST at 916-726-5675 x205.

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