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An aggregation is a kind of association that models a part/whole relationship between an aggregate (whole) and a group of related components (parts). A composition, also called a composite aggregation, is a kind of aggregation that models a part/whole relationship between a composite (whole) and a group of exclusively owned parts.
Furthermore, there is hardly a difference between aggregations and associations during implementation, and the diagram may skip aggregation relations altogether. [9] Aggregation can occur when a class is a collection or container of other classes, but the contained classes do not have a strong lifecycle dependency on the container. The contents ...
Misuses of composition and aggregation. In object-oriented programming this relationship can be represented with a Unified Modeling Language Class diagram. This has-a relationship is also known as composition. As you can see from the Class Diagram on the right a car "has-a" carburetor, or a car is "composed of" a carburetor.
Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should favor polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) over inheritance from a base or parent class. [2]
Object composition is used to represent "has-a" relationships: every employee has an address, so every Employee object has access to a place to store an Address object (either directly embedded within itself or at a separate location addressed via a pointer).
Inheritance is contrasted with object composition, where one object contains another object (or objects of one class contain objects of another class); see composition over inheritance. In contrast to subtyping’s is-a relationship, composition implements a has-a relationship.
Also take the point about the carburetor. The diagram in the article describes this relationship as composition - but I would have thought the life-line of a carburetor is not implicitly tied to the life-line of the car (which would be the definition of composition), and so this too would be aggregation. Ultimately: I agree.
The bridge uses encapsulation, aggregation, and can use inheritance to separate responsibilities into different classes. When a class varies often, the features of object-oriented programming become very useful because changes to a program's code can be made easily with minimal prior knowledge about the program. The bridge pattern is useful ...