What is the difference between aggregation composition and dependency

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What is the difference between aggregation  composition and dependency

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Aggregation and composition are almost completely identical except that composition is used when the life of the child is completely controlled by the parent   Aggregation    Car - gt  Tires   The Tires can be taken off of the Car object and installed on a different one   Also  if the car gets totaled  the tires do not necessarily have to be destroyed   Composition    Body - gt  Blood Cell   When the Body object is destroyed the BloodCells get destroyed with it   Dependency  A relationship between two objects where changing one may affect the other

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Aggregation implies a relationship where the child can exist independently of the parent  Example  Class  parent  and Student  child   Delete the Class and the Students still exist   Composition implies a relationship where the child cannot exist independent of the parent  Example  House  parent  and Room  child   Rooms don t exist separate to a House   The above two are forms of containment  hence the parent-child relationships    Dependency is a weaker form of relationship and in code terms indicates that a class uses another by parameter or return type   Dependency is a form of association

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Aggregation  - separable part to whole  The part has a identity of its own  separate from what it is part of  You could pick that part and move it to another object   real world examples  wheel -  car  bloodcell -  body    Composition - non-separable part of the whole  You cannot move the part to another object  more like a property   real world examples  curve -  road  personality -  person  max speed -  car  property of object -  object    Note that a relation that is an aggregate in one design can be a composition in another  Its all about how the relation is to be used in that specific design   dependency - sensitive to change   amount of rain -  weather  headposition -  bodyposition   Note   Bloodcell  -  Blood  could be  Composition  as Blood Cells can not exist without the entity called Blood   Blood  -  Body  could be  Aggregation  as Blood can exist without the entity called Body

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Containment  - Here to access inner object we have to use outer object  We can reuse the contained object  Aggregation  - Here we can access inner object again and again without using outer object

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One object may contain another as a part of its attribute    document contains sentences which contain words  Computer system has a hard disk  ram  processor etc    So containment need not be physical  e g   computer system has a warranty

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Aggregation and composition are terms that most people in the OO world have acquired via UML  And UML does a very poor job at defining these terms  as has been demonstrated by  for example  Henderson-Sellers and Barbier   What is This Thing Called Aggregation     Formalization of the Whole-Part Relationship in the Unified Modeling Language    I don t think that a coherent definition of aggregation and composition can be given if you are interested in being UML-compliant  I suggest you look at the cited works   Regarding dependency  that s a highly abstract relationship between types  not objects  that can mean almost anything

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An object associated with a composition relationship will not exist outside the containing object  Examples are an Appointment and the owner  a Person  or a Calendar  a TestResult and a Patient   On the other hand  an object that is aggregated by a containing object can exist outside that containing object  Examples are a Door and a House  an Employee and a Department   A dependency relates to collaboration or delegation  where an object requests services from another object and is therefor dependent on that object  As the client of the service  you want the service interface to remain constant  even if future services are offered

[oop] What is the difference between aggregation, composition and dependency?

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