Inheritence

Inheritance (object-oriented programming)

    
In object-oriented programming, inheritance is the concept that when a class of objects is defined, any subclass that is defined can inherit the definitions of one or more general classes. This means for the programmer that an object in a subclass need not carry its own definition of data and methods that are generic to the class (or classes) of which it is a part. This not only speeds up program development; it also ensures an inherent validity to the defined subclass object (what works and is consistent about the class will also work for the subclass).                                                                                                                                                         In object-oriented programming, inheritance is when an object or class is based on another object (prototypal inheritance) or class (class-based inheritance), using the same implementation. Inheritance in most class-based object-oriented languages is a mechanism in which one object acquires all the properties and behaviours of the parent object. Inheritance allows programmers to: create classes that are built upon existing classes [1], to specify a new implementation to maintain the same behaviour (realizing an interface), to reuse code and to independently extend original software via public classes and interfaces. The relationships of objects or classes through inheritance give rise to a directed graph.
An inherited class is called a subclass of its parent class or super class. The term "inheritance" is loosely used for both class-based and prototype-based programming, but in narrow use is reserved for class-based programming (one classinherits from another), with the corresponding technique in prototype-based programming being instead called delegation(one object delegates to another).
Inheritance should not be confused with subtyping.[2][3] In some languages inheritance and subtyping agree,[a] whereas in others they differ; in general, subtyping establishes an is-a relationship, whereas inheritance only reuses implementation and establishes a syntactic relationship, not necessarily a semantic relationship (inheritance does not ensure behavioral subtyping). To distinguish these concepts, subtyping is also known as interface inheritance, whereas inheritance as defined here is known as implementation inheritance or code inheritance.[4] Still, inheritance is a commonly used mechanism for establishing subtype relationships.[5]
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. Composition implements a has-a relationship, in contrast to the is-a relationship of subtyping.

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