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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.
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.
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 ...
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).
Acquaintance is a weaker relationship than aggregation and suggests much looser coupling between objects, which can often be desirable for maximum maintainability in designs. The authors employ the term 'toolkit' where others might today use 'class library', as in C# or Java.
Scientists thought that Lake Enigma was frozen from top to bottom. Then they discovered that water—and mysterious lifeforms—existed 11 meters below the surface.
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.