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From the May 2010 Issue of Prestwood eMag   Coder Object Orientation (OO): OO/UML: Aggregation versus Composition By Mike Prestwood 10/24/2002, Last updated 5/21/2010   Take Away: Explains the "is a", "has a", "uses a", and "looks like" relationships (updated May 2007). "Is a" is inheritance, "looks like" is interfaces, "has a" is aggregation, and "uses a" is composition.  Tags: OO , UML , aggregation , composition , inheritance , uses a , has a , , Role-Based Tech Talk, Coding & OO, Object Orientation (OO)

I've been getting a lot of questions about the distinction between aggregation and composition lately. Both deal with part-of relationships and are extremely important concepts to understand.

Let's start with a defition of aggregation from the UML 1.4 glossary:

• Aggregation - A special form of an association that specifies a whole-part relationship between the aggregate (whole) and a component part.

Types and Characteristics
There are various types and characteristics of aggregation relationsips (whole-part relationships) that describe the relationship between the whole and part in more detail. Lets continue with a few more definitions related to aggregation sub-types and characteristics (the Composition definition is from the UML 1.4 glossary):

• Composition / Composition Aggregation - A form of aggregation which requires that a part instance be included in at most one composite at a time, and that the composite object is responsible for the creation and destruction of the parts.
• Shareable Aggregation - A form of aggregation in which the part can exist on it's own and is therefore shareable among multiple owner classes.

More On Aggregations ("has a" relationship)
In an aggregation relationship, the part may be independent of the whole but the whole requires the part. An aggregation relationship is indicated in the UML with an unfilled diamond and a line as follows:

An Aggregation is an Association which denotes an "is part of" relationship. Unfortunately, the definition of this relationship is quite lax, so basically everyone is using his own interpretation. The only definitive (?) property is that in an instance graph, aggregations are not allowed to be circular - that is, an object can not be "a part of itself".

Composition (a strong type of "uses a" relationship)
A composition relationship, also known as a composite aggregation, is a stronger form of aggregation where the part is created and destroyed with the whole. A composition relationship is indicated...

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More Info

Article:	An Introduction to Object Orientation
Definition:	Composition
Link:	DotNetCoders.com UML Home Page
Definition:	Interface
Article:	Introduction to OO for the Paradox Community
Article:	Introduction to the Unified Modeling Language
Definition:	Polymorphism
KB Post:	PSDP Phases compared to UML Workflows
KB Post:	PSDP: Step 2 Planning Overview (Gen & Design Phases)

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First Comment
Author... Prestwood IT Solutions Staff Moderator Mike Prestwood Rank: Fleet Admiral Joined: Oct 2000 Location: Citrus Heights, CA USA Mike Prestwood Email Approved! E CA USA Member Points: 56899 Visits: 7856 MB Posts: 1333 KB Articles: 1484	Comment 1 of 6 Thursday, August 06, 2009 Thanks Soniya. It's always nice to get positive feedback.

Reader... inghamc Rank: Cadet 3rd Year Joined: Apr 2005 Location: CO USA inghamc Email Approved! E CO USA Member Points: 26 Visits: 2 MB Posts: 0 KB Articles: 0

Comment 2 of 6 Monday, April 25, 2005 I don't understand the composition example - how is an engine responsible for the creation of gas?

Reader... call_krmohan Rank: Cadet 1st Year Joined: Oct 2007 Location: bangalore IN call_krmohan Email Approved! E IN Member Points: 6 Visits: 2 MB Posts: 0 KB Articles: 0

Comment 3 of 6 Wednesday, October 31, 2007 fine

Reader... mohitsi Rank: Cadet 1st Year Joined: Feb 2008 Location: USA mohitsi Email Inactive E USA  (Inactive) Member Points: 6 Visits: 1 MB Posts: 0 KB Articles: 0

Comment 4 of 6 Tuesday, February 05, 2008 Hi Mike,  Just would like add some comment on the examples given above: -         In aggregation relationship, the ‘part’ object reference can be re-used & creation of ‘part’ object is not the responsibility of the ‘whole‘object usully. It would have been created somewhere else, and passed to the ‘whole’ object as a method argument. -         In composition, the life cycle of the ‘part’ is controlled by the ‘whole’. So, if I am right then your pseudo code examples would need to get swapped within each other. ie. SparkPlug/Car example shows composition & Room/building example shows aggregation. Waiting for your response.... Thanks in advance, Mohit

Reader... vletter1-1 Rank: Cadet 2nd Year Joined: Apr 2009 vletter1-1 Email Approved! E Unk Member Points: 22 Visits: 2 MB Posts: 0 KB Articles: 0

Comment 5 of 6 Saturday, April 25, 2009 Hi Mike, with the pseudo code you provided from aggregate and composition, I totally understand what it means. Can you also provide the pseudo code from inheritance and interface? thanks Andrew

Latest Comment
Reader... soniya_ahuja Rank: Cadet 1st Year Joined: Aug 2009 Location: Raigad, UNK IN soniya_ahuja Email Approved! E Unk Member Points: 10 Visits: 1 MB Posts: 0 KB Articles: 0	Comment 6 of 6 Thursday, August 06, 2009 The article is indeed impressive. I tried reading on this topic at a few other places, but none were as comprehensible as this. A concise but effective way of explaining aggregation and its types.

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