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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]
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 ...
Like inheritance, delegation allows the sending object to modify the original behavior, but is susceptible to problems analogous to the fragile base class; while forwarding provides stronger encapsulation and avoids these problems; see composition over inheritance. [1]
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.
Reportedly, Java inventor James Gosling has spoken against implementation inheritance, stating that he would not include it if he were to redesign Java. [19] Language designs that decouple inheritance from subtyping (interface inheritance) appeared as early as 1990; [ 21 ] a modern example of this is the Go programming language.
Composition over inheritance: "Favor 'object composition' over 'class inheritance'." (Gang of Four 1995:20) The authors claim the following as advantages of interfaces over implementation: clients remain unaware of the specific types of objects they use, as long as the object adheres to the interface
This is an example of a violation of the composition over inheritance principle in the Java platform libraries, since in computer science, a vector is generally not a stack. [16] Composition would have been more appropriate in this scenario.
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