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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]
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.
In software engineering, the delegation pattern is an object-oriented design pattern that allows object composition to achieve the same code reuse as inheritance. In delegation, an object handles a request by delegating to a second object (the delegate). The delegate is a helper object, but with the original context.
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 ...
Object composition using UML properties to compose objects. In UML modeling, objects can be conceptually composed, independently of the implementation with a programming language. There are four ways of composing objects in UML: property, association, aggregation and composition: [4] A property represents an attribute of the class.
Traits come originally from the programming language Self [5] and are supported by the following programming languages: AmbientTalk : Combines the properties of Self traits (object-based multiple inheritance) and Smalltalk 's Squeak traits (requiring explicit composition of traits by the programmer).
In general, the further down in the hierarchy a class appears, the more specialized its behavior. When a message is sent to an object, it is passed up the inheritance tree starting from the class of the receiving object until a definition is found for the method. This process is called upcasting.
Composite Pattern implementation in Java; Composite pattern description from the Portland Pattern Repository; Composite pattern in UML and in LePUS3, a formal modelling language; Class::Delegation on CPAN "The End of Inheritance: Automatic Run-time Interface Building for Aggregated Objects" by Paul Baranowski