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

This question already has answers here: What is the difference between association, aggregation and composition? (21 answers) Closed 9 years ago.

What is the difference between ag开发者_如何学运维gregation, composition and dependency?

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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 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 -> 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 -> 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 - 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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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.

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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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One object may contain another as a part of its attribute.

1. document contains sentences which contain words.
2. 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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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.