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In aggregation, the object may only contain a reference or pointer to the object (and not have lifetime responsibility for it). Sometimes aggregation is referred to as composition when the distinction between ordinary composition and aggregation is unimportant. The above code would transform into the following UML Class diagram:
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]
In object-oriented programming, association defines a relationship between classes of objects that allows one object instance to cause another to perform an action on its behalf. This relationship is structural , because it specifies that objects of one kind are connected to objects of another and does not represent behaviour .
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.
Using inheritance is recommended mainly when adding to the functionality of existing components, reusing most of the old code and adding relatively small amounts of new code. To the authors, 'delegation' is an extreme form of object composition that can always be used to replace inheritance.
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.
Class diagram showing Aggregation between two classes. Here, a Professor 'has a' class to teach. Aggregation is a variant of the "has a" association relationship; aggregation is more specific than association. It is an association that represents a part-whole or part-of relationship. As shown in the image, a Professor 'has a' class to teach.
The doctrine of composition over inheritance advocates implementing has-a relationships using composition instead of inheritance. For example, instead of inheriting from class Person, class Employee could give each Employee object an internal Person object, which it then has the opportunity to hide from external code even if class Person has ...