Association, Aggregation, Composition, Abstraction, Generalization, Realization, Dependency

转自：http://javapapers.com/oops/association-aggregation-composition-abstraction-generalization-realization-dependency/

These terms signify the relationships between classes. These are the building blocks of object oriented programming and very basic stuff. But still for some, these terms look like Latin and Greek. Just wanted to refresh these terms and explain in simpler terms.

Association

Association is a relationship between two objects. In other words, association defines the multiplicity between objects. You may be aware of one-to-one, one-to-many, many-to-one, many-to-many all these words define an association between objects. Aggregation is a special form of association. Composition is a special form of aggregation.

Example: A Student and a Faculty are having an association.

Aggregation

Aggregation is a special case of association. A directional association between objects. When an object ‘has-a’ another object, then you have got an aggregation between them. Direction between them specified which object contains the other object. Aggregation is also called a “Has-a” relationship.

Composition

Composition is a special case of aggregation. In a more specific manner, a restricted aggregation is called composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition.

Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

Difference between aggregation and composition

Composition is more restrictive. When there is a composition between two objects, the composed object cannot exist without the other object. This restriction is not there in aggregation. Though one object can contain the other object, there is no condition that the composed object must exist. The existence of the composed object is entirely optional. In both aggregation and composition, direction is must. The direction specifies, which object contains the other object.

Example: A Library contains students and books. Relationship between library and student is aggregation. Relationship between library and book is composition. A student can exist without a library and therefore it is aggregation. A book cannot exist without a library and therefore its a composition. For easy understanding I am picking this example. Don’t go deeper into example and justify relationships!

Abstraction

Abstraction is specifying the framework and hiding the implementation level information. Concreteness will be built on top of the abstraction. It gives you a blueprint to follow to while implementing the details. Abstraction reduces the complexity by hiding low level details.

Example: A wire frame model of a car.

Generalization

Generalization uses a “is-a” relationship from a specialization to the generalization class. Common structure and behaviour are used from the specializtion to the generalized class. At a very broader level you can understand this as inheritance. Why I take the term inheritance is, you can relate this term very well. Generalization is also called a “Is-a” relationship.

Example: Consider there exists a class named Person. A student is a person. A faculty is a person. Therefore here the relationship between student and person, similarly faculty and person is generalization.

Realization

Realization is a relationship between the blueprint class and the object containing its respective implementation level details. This object is said to realize the blueprint class. In other words, you can understand this as the relationship between the interface and the implementing class.

Example: A particular model of a car ‘GTB Fiorano’ that implements the blueprint of a car realizes the abstraction.

Dependency

Change in structure or behaviour of a class affects the other related class, then there is a dependency between those two classes. It need not be the same vice-versa. When one class contains the other class it this happens.

Example: Relationship between shape and circle is dependency.

========== 下面是网友的讨论 ==========

Ali says:

27/05/2013 at 6:32 pm

Hello,

What is Difference between Association and Dependency ?

Swapnil says:

11/05/2013 at 9:58 am

U Say ‘ if the contained object cannot exist without the existence of container object, then it is called composition.’–>with example Library–>Books
With your definition ‘Books can not exists without Library’..How’s this ? ..or is it the other way round ‘Library can not exists without Books’??
I am damn confused?
Please please help.Thanks
Amitabha Roysays:

04/04/2013 at 10:17 pm

Association: uses a
Aggregation: has a : strong association
Composition: owns a : weak association
Inheritance: is a
thakkar komal says:

25/02/2013 at 6:59 pm

in ooad(Object Oriented Analysis Design and UML)

1)Is association class same as ordinary classs? Explain with example.

plz send ans
furry says:

14/02/2013 at 11:35 pm

“Suppose we have implemented a scenario and we used aggregation in it, after some time we realize that use of aggregation is not a correct choice, now we want to use composition instead of aggregation. Can we replace composition with aggregation? At which phase(s) these changes are required? Justify your answer with solid reasons. ”

sir this question is related by C++
Prakash says:

10/02/2013 at 11:46 am

Can u please explain about abstraction and encapsulation oops concepts
Nagarjuna says:

30/01/2013 at 5:28 pm

Really good article , thanks a lot.

If you can provide the UML examples using symbols(aggregation,composition and generalization) will be more useful.
Venkat says:

28/01/2013 at 4:33 pm

Hi Joe, very nice explanation.
I just got doubt about the sentence in “Generalisation” , you wrote as – Common structure and behaviour are used from the specializtion to the generalized class.

or it should be as below.

Common structure and behaviour are used from the generalized to the specializtion class.

Correct me on this please.
sarang says:

19/12/2012 at 12:00 pm

cool.. :)
pradeep says:

17/12/2012 at 4:59 pm

Really helpful to brush up the concepts.Thanks for posting such a nice article.
Girish says:

18/11/2012 at 12:18 pm

Thanks.:) Clearcut definitions. :)

For association, aggregation and composition could you please put real time exmaples with come classes?

Nice work ..
Jatinder says:

16/11/2012 at 12:49 pm

Indeed this is a very good post.
If i have understood correctly,then will it be ok to say that a child in a mother’s womb is an example of composition and after birth,a mother-child relation can be called aggregation.
Request all learned to please correct me if i have understood incorrectly.
Anonymous says:

06/11/2012 at 11:39 am

Very helpful and easy t understand
archana says:

05/11/2012 at 10:02 am

super
Hamed Mirzaei says:

03/11/2012 at 7:23 pm

Thank you

it was very useful for me
vandu says:

21/10/2012 at 7:15 pm

such a nice information .it will help everyone..
js says:

14/10/2012 at 11:21 am

Very information. thanks
Priya says:

13/10/2012 at 3:26 am

Its really very easy to understand.. Thanks a lot
Anonymous says:

12/10/2012 at 3:20 pm

simple to understand…with real life egg
Anonymous says:

11/10/2012 at 12:28 pm

Nice explaination
Gaurav says:

23/08/2012 at 3:53 pm

superbly explained.. :)
Abhijeet says:

23/08/2012 at 3:15 pm

Very Nice and easy explanation.

Thanks Joe.
Anonymous says:

23/08/2012 at 3:03 pm

good article for basics
Anonymous says:

23/08/2012 at 10:04 am

nice explanation for UML easy understanding…Please continue this short of topics
shashikant pandit says:

20/08/2012 at 1:30 pm

earlier i confused to understand about these topic .but by the God i found your site, and understood these all topics with in few moments.

nice site
dheeraj says:

17/08/2012 at 11:08 am

very good explanation thanks…………..
Yuri says:

13/08/2012 at 7:48 pm

Hi, Joe!
Thank you for your explanations!
But… you write: “A book cannot exist without a library and therefore its a composition.”
I think more correctly would be as follows:
“A library cannot exist without book and therefore it’s a composition.”
jyoti says:

12/08/2012 at 2:37 pm

Thankyou
durga says:

23/07/2012 at 10:33 pm

Thank you…..
could you please explain these association,aggregation,composition and dependancies with a sample scenario with java classes implementation…..
Ajay Shah says:

20/07/2012 at 3:22 pm

This post is very helpfull to understand the basic concept of association and understands the association type aggregation and composition type ….
Ganesh says:

06/07/2012 at 6:04 pm

Dependency explanation is need to be amended as follows.

Dependency is a weaker form of relationship which indicates that one class depends on another because it uses it at some point of time. One class depends on another if the latter is a parameter variable or local variable of a method of the former. This is different from an association, where an attribute of the former is an instance of the latter.
Anonymous says:

03/07/2012 at 12:07 pm

unclear definitions
Anonymous says:

03/07/2012 at 8:28 am

Had confusion with all these terms..your way of explanation is very simple and straight forward..thanks
Kausik says:

06/06/2012 at 3:25 pm

It will be great if you have some real life example on this..

Thanks Kausik
Anonymous says:

06/06/2012 at 1:37 pm

bhiya tussi to cha gaye…
manikesh says:

05/06/2012 at 5:15 pm

I was screwed in a interview for all these.. thanks for sharing…really good one..
Anonymous says:

05/06/2012 at 11:41 am

thank u …………
jpnaidu says:

28/05/2012 at 8:18 pm

very helpful and easy understanding
Hakkim says:

27/05/2012 at 8:20 pm

Content is very good and the site is also looking so good keep it up sir,…
Vijay says:

26/05/2012 at 2:39 pm

Really a useful stuff to understand object relation ship.
Minesh says:

25/05/2012 at 5:27 pm

Clear explanation with simplicity!
Thanks
Jigar says:

25/05/2012 at 3:07 am

Thanks!!:) It was really worthy.. Short & to the point.. Clarifying examples played vital role in understanding.. ‘Thanks’ once again..
_Js
Vijay Arige says:

23/05/2012 at 12:46 pm

Sir…it clearly differentiated the terms for a novice programmer.
Parshant says:

22/05/2012 at 5:44 pm

sir…you should use email validation in comment box…..
and your site content is awesome….i loved it….
manojbharal says:

19/05/2012 at 3:02 pm

Could you please share with java code examples.

A implements B is realization
A extends B is generalization

AM I right?
Pratik Jain says:

12/05/2012 at 8:28 pm

Nice article.
Adding some example, hope till will support the article.
There are four kinds of Class relationships

Association: uses a
Ex:a Class Man uses a Class Pen
Aggregation: has a
Ex:a Class Man has a Class Car ( Car is still there when Man die )
Composition: owns a
Ex:a Class Man owns a Class Heart ( When Man die, Heart die )
Inheritance: is a
Ex:a Class Man is a Class Human ( Man is a Human )
A relationship between classes of objects

Inheritance>Composition>Aggregation>Association
Priyabrat says:

08/05/2012 at 3:10 pm

I Think U are right Hikmat.
Hikmat Jaber says:

08/05/2012 at 2:08 am

In defining the Composition in top of page, I think the sentence should be
“…, if the container object cannot exist without the existence of contained object, then it is called composition”
instead of “…, if the contained object cannot exist without the existence of container object, then it is called composition”
nitish says:

08/05/2012 at 1:55 am

very lucid and helpful. thank you.
Ganesh says:

29/04/2012 at 12:27 pm

ijust hv a doubt about composition ,as a example “A circle is composed of points”.
then i think points are exist without circle ,,but according to u points r not exist without circle ..plz explain.?
Abdul Hadi Ibrahimi says:

08/04/2012 at 9:59 pm

I love your blog and find helpful…thanks for your great passion! You people make the world really great!
Pavan says:

07/04/2012 at 3:39 pm

Nice Article Joe
Anonymous says:

06/04/2012 at 3:09 pm

nice explanation..
Ganesh says:

06/04/2012 at 11:05 am

Thanks a lot Joe, Very Useful information with simple language and good examples.
SanjayK says:

06/04/2012 at 9:25 am

Thanks a lot for such an easy explanation. Examples are also easy to understand and correlate.
Ravi says:

02/04/2012 at 12:07 am

give me answer plz..
Justifying the statement ,”inheritance is a special case of Generalization”.
zeeshan says:

27/03/2012 at 5:06 pm

Very nice content. I like it. The best site and very comprehensiveness information I ever found.
Gajendra says:

26/03/2012 at 11:06 pm

Nice explanation yaar
VVV says:

26/03/2012 at 3:08 pm

Good explanations….
Mohammad Arif says:

14/03/2012 at 10:52 pm

Thanks very much nice information about Generalization, Aggregation, Association, Composition, Dependency,, only one time i have refereed to it but absolutely i got******
sourabh singhai says:

14/03/2012 at 10:48 pm

good site thanks
sabarish says:

13/03/2012 at 2:35 pm

Thanks for your inputs
SAGAR_D says:

09/03/2012 at 9:33 pm

GOOD and NICE and SIMPLE Explanation……..
Rupesh says:

08/03/2012 at 9:50 pm

Great and a very simple way to explain a confusing subject. Thanks a lot for this Post.
priya says:

14/02/2012 at 2:22 pm

nice explanation. very useful. thanks
YoFo says:

13/02/2012 at 7:06 pm

Good Summary. Thanks a lot!
A simple class diagram to explain the direction of the relationship would be great, if can be added
himanshu says:

11/02/2012 at 8:32 pm

please explain more sir,with example..
samy says:

10/02/2012 at 11:37 am

awesome
venkysays:

26/12/2011 at 2:08 am

really helping me a lot….
u r dng a great work.Thank You
Nikhil says:

25/12/2011 at 11:26 am

Really good man easy to understand
Pradip Lavania says:

22/12/2011 at 5:47 am

Hi,

Superb Explanation sir .
Rohit Ojha says:

20/12/2011 at 3:51 pm

Nice way of information.
Thanks………………
Anonymous says:

16/12/2011 at 3:32 pm

really clears the confusion.
TSKarthicsays:

13/12/2011 at 6:50 pm

Wonderful Job Joe. Keep it up.
Chandra says:

15/11/2011 at 3:33 pm

Good article. One of the questions mentioned above, was related to manager,supervisor and staff. Certainly there is a generalization relationship with Staff as superclass and manager and supervisor classes are subclasses inheriting from Staff. Though attributes could be same in manager and supervisor when it comes behaviour(methods) manager and supervisor classes are specialized with inheriting the generic class i.e., Staff.
Joesays:

10/11/2011 at 4:56 am

@Sanchit

I feel very happy when I get to know that my blog helps a student.

More than anything, I maintain this to help students succeed in their exams / interviews.
Sanchit says:

09/11/2011 at 8:14 pm

Thanks..!! M sure your examples gonna help in tomorrows exam.
bluepicaso says:

06/11/2011 at 1:26 pm

Nice one
thank you
jay says:

04/11/2011 at 4:36 pm

identify in detail association and aggregation
siva says:

31/10/2011 at 10:31 am

Very good explanation
Subir says:

30/10/2011 at 5:49 pm

Really simple and excellent explanation. Easy to understand for beginners…!!!
Imrana Aminu Muhd says:

30/10/2011 at 2:34 pm

Questions
1. Identify and briefly explain the objectives, classes, method and attributes for a hotel reservation system.
2. Draw a use cases diagram to represent a hotel reservation system.
3. Draw a class diagram that models hotel reservation system data structure, clearly representing the objects, classes, associations, generalization and aggregations.

Thanks
Imrana Aminu Muhd says:

30/10/2011 at 2:18 pm

Questions
1. Identify and briefly explain the objectives, classes, method and attributes for a hotel reservation system.
2. Draw a use case diagram to represent a hotel reservation system.
3. Draw a class diagram that models hotel reservation system data structure, clearly representing the objects, classes, associations, generalization and aggregations.

Thanks
Arvind kumar says:

28/10/2011 at 1:05 pm

Very nice explanation about class diagram terms…
mayur bhalekar says:

24/10/2011 at 6:52 pm

very nice artical!!
Anonymous says:

24/10/2011 at 4:01 pm

A very good explanation..I never seen before like this type explanation about this concept
cheng kuyleang says:

24/10/2011 at 3:30 pm

Thanks you very much for share good knowledge for me .
CT says:

24/10/2011 at 10:23 am

Hi, may I seek a clarification about generalization concept. Let’s say I have a two kinds of staff, Manager and Supervisor and both have the same attributes (name,contact number). So, is it possible to say that this has a generalization relationship – Staff as the superclass, Manager and Supervisor as its subclasses althought there’s no difference in terms of the attributes between Manager and Supervisor?
abi says:

17/10/2011 at 10:39 am

Very nice post.. i have never experienced such
simple examples for explaining such complex terms like aggregation and composition
i thought so far.. Keep posting…
Arun K Deo says:

30/09/2011 at 5:08 pm

You mentioned in this article – Abstraction is specifying the framework and hiding the implementation level information. Abstraction never says about hiding anything. Let’s take an Example of Employee with Super class and Manager and Salesman bening subclass of Employee. So if Employee may provide various implentation in common, but can’t have implementation for calculateBonus because this is fully dependet on nature of employee subclass, and hence Employee would have calculateBonus as abstract. So overall, abstraction is isolation of common essentioal behaviour and supress unimportant behaviours. Abstraction never meanse hiding anything. Hiding of implementation from accidental access is Encapsulation.

Thanks
Arun Deo
Chandraprakash says:

24/09/2011 at 3:27 am

Nice explanation on the topics of association, aggregation and composition with quite undrestandable language.
Mani says:

25/08/2011 at 5:55 pm

Dear sir, I need more explanation and example about abstraction… How to apply in c…? i use data structure linked list concept..
Ramyashree says:

25/08/2011 at 7:43 am

very good explanation on composition and aggregation relationships
NextGeneration says:

22/08/2011 at 4:17 pm

Nice Article, Much useful for a beginner
Prakash says:

22/08/2011 at 6:58 am

Superb Article !!!!!!!!!
Om Behera says:

16/08/2011 at 5:45 am

Nice Article Joe..Like very much the way you describe ,Must say Very good Article for the beginners …!!!
shubhsays:

15/08/2011 at 2:18 pm

explanation also easy for preparing answerssssssssssss
shubh says:

15/08/2011 at 2:16 pm

it is so easy and interesting
Kintesh says:

09/08/2011 at 9:02 am

thanks for simple explanation of association, aggregation, composition. Good Work…Thank you..
Anonymous says:

06/08/2011 at 2:44 pm

Very Good article… Thank you..
Thirupathi says:

03/08/2011 at 4:43 pm

nice explanation.. thiru
Zahid Nasim says:

29/07/2011 at 10:16 am

Thanks dude.. its really so simple to understand these stuffs…….. really gr8 work.
Ramakrishna Chandragirisays:

23/07/2011 at 4:11 pm

Useful information with good explanation well done Joseph..
mowlika says:

19/07/2011 at 7:37 am

it helped me alot sir………thank u sir
pravinth says:

17/07/2011 at 9:49 am

Good Artcile. it helps.. Thanks..!
Baravil Barivare says:

15/07/2011 at 7:10 pm

This article is very helpful and well explained. Thanks
niranjan says:

15/07/2011 at 9:02 am

simply super….very useful Thanks for posting..
Parmod says:

08/07/2011 at 6:56 am

Very easily explained and easy to understand all the definition. Keep it up
Girish Varma says:

24/05/2011 at 4:34 pm

multiplicity is no given here :) pls add

thanks
Varma says:

18/05/2011 at 4:56 pm

could you please provide some code samples for association, aggregation and other all oops concepts.

Thanks
Varma
Rajasekhar says:

13/04/2011 at 2:22 am

thanks much. very useful information
ankana says:

13/04/2011 at 7:51 am

thankz sir , very nice content
KARTHIKEYAN A says:

09/04/2011 at 5:17 pm

Wow., Awesome work., Thankz,
Keep up the Good work.,
ray says:

06/04/2011 at 6:21 pm

thank you for you very clear work joe.
Su says:

29/03/2011 at 3:29 pm

Very nice article. I am facing a unique situation where we have modeled a test case with two compositions between the same two entities. Is this realistic? If so, can you give me a real world example for this?
sadish says:

16/03/2011 at 3:20 pm

Great Work !!!
Keep going on !!!

All the best !!
sambha says:

14/03/2011 at 8:01 pm

simple and understanble…
Thanks a lot
kondalarao says:

09/03/2011 at 3:29 am

it is simple and very easy to understand
thank you,
viji says:

12/01/2011 at 5:32 pm

Nice.. Thank You..
Prabhat says:

03/01/2011 at 8:59 am

Very nice page.
It contains all required information about association, aggregation, composition.
thanks a lot.
Prabhat.
Kareem Afifiy says:

22/12/2010 at 10:49 am

Thanks for these information. Great definition for aggregation n composition
Sanjay Upadhyay says:

17/12/2010 at 6:49 am

I was always get confused for these relationship but as you defined these relationship it became easy to understand. Thanks buddy :)
Vijay Agalcha says:

10/12/2010 at 2:26 pm

thank you very much for sharing your knowledge. very useful content. I learnt the concept very well to be applied in OOAD.
kamatchi sundaram says:

03/12/2010 at 9:05 am

Hi,

The definition given in this site is very clean and neat and simple and very understandable instead of blah..blah…

Cheers !!!
KM
Abhijit Boruah says:

27/11/2010 at 1:34 pm

jst superb…think u r bst faculty for starters..i bliv in it…”with strong foundationz u can aim for the sky.”
RenuNaidu..... says:

27/11/2010 at 1:41 am

i searched most of the definitions for these association,aggregation and composition…u cleared my doubt …u people are explaining like baby feeding thank you……
Chetan Jadhav says:

14/11/2010 at 5:38 pm

Terse explanation of Association, Aggregation and Composition..!!! :) Thanks a lot… :)

Looking forward for your next posts…
Narayan says:

09/11/2010 at 10:15 am

Good explaination mate
Casper says:

09/11/2010 at 9:03 am

Thank you for addressing all relevant items.. little bit more code would be marvelous. still great. CDW
Salman Khan says:

06/11/2010 at 12:02 am

Thank You Very Much… It Was Very Helpfull…
Amit Shekhar says:

03/11/2010 at 10:28 am

Thanks Joe for the wonderful post. It is really quite useful.Most of the times we tend to get confused with these terms as all sound the same. Few more additions would make it better:
a) Class diagrams
b) Code for Aggregation and Composition
Mutayyab Shah says:

26/10/2010 at 6:05 pm

Sir. i m n trouble..if u post full code .which contain aggregation and compostion.then it will be very clear to all of us.according to programing point of view.
and realy good work u have done.we appriciate ur work…
plz post full code waiting 4 ur reply
swathi says:

15/08/2010 at 12:37 pm

Hi ,

thnaks for that information.

but i think i havnet understand that last.(Dependency)
does it comes with arrow or just —- lines. pls post me
Prashant S Shivashimpi says:

20/07/2010 at 11:02 am

Really very good and straight forward description about the Association, Aggregation, Composition, Abstraction, Generalization, Realization, Dependency. Thank you so much..

Its very handy too..

Cheers,
Prashant.
Joesays:

14/07/2010 at 5:13 am

Thanks Jyotilal for the aggregation example. Actually its otherway around.

A car has a stereo system. A stereo system can exist without a car. There exists aggregation between car and stereo.

Don’t go too technical into electronics and say a car stereo needs a car. Just for an example!
Jyotilal says:

13/07/2010 at 5:59 pm

An example for aggregation is missing and it could be like:

A car has a stereo system.A car can exist without a stereo system. There exists aggregation between car and stereo.
himanshu says:

28/06/2010 at 7:24 am

very nice content to start

Thanks!
Rajesh says:

28/06/2010 at 7:12 am

Thank you..
Looking forward for your next post…
Gift vincy says:

28/06/2010 at 12:12 pm

Now these basic can be easily differentiated, after read this post

Comments are closed.