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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.
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]
On the other hand, inheritance can be statically type-checked, while delegation generally cannot without generics (although a restricted version of delegation can be statically typesafe [7]). Delegation can be termed "run-time inheritance for specific objects." Here is a pseudocode example in a C#/Java like language:
The relationships are specified in the science of object-oriented design and object interface standards defined by popular use, language designers (Java, C++, Smalltalk, Visual Prolog) and standards committees for software design like the Object Management Group. The class hierarchy can be as deep as needed.
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
Prior to Java 8, Java was not subject to the Diamond problem risk, because it did not support multiple inheritance and interface default methods were not available. JavaFX Script in version 1.2 allows multiple inheritance through the use of mixins. In case of conflict, the compiler prohibits the direct usage of the ambiguous variable or function.
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 ...