Polymorphism

By | May 16, 2014

In simple terms, polymorphism lets you treat derived class members just like their parent class’ members. Polymorphism is the process of using an operator or function in different ways for different set of inputs given. More precisely, polymorphism (object-oriented programming theory) is the ability of objects belonging to different types to respond to method calls of the same name, each one according to an appropriate type-specific behavior. The programmer (and the program) does not have to know the exact type of the object in advance, so this behavior can be implemented at run time (this is called late binding or dynamic binding). The different objects involved only need to present a compatible interface to the clients (the calling routines). That is, there must be public methods with the same name and the same parameter sets in all the objects. In principle, the object types may be unrelated, but since they share a common interface, they are often implemented as subclasses of the same parent class. Though it is not required, it is understood that the different methods will also produce similar results (for example, returning values of the same type). In practical terms, polymorphism means that if class B inherits from class A, it doesn’t have to inherit everything about class A; it can do some of the things that class A does differently. This means that the same “verb” can result in different actions as appropriate for a specific class, so controlling code can issue the same command to a series of objects and get appropriately different results from each one.

Overriding and Overloading If a Dog is commanded to speak(), this may emit a Bark. However, if a Pig is commanded to speak(), this may emit an Oink. They both inherit speak() from Animal, but their derived class methods override the methods of the parent class; this is Overriding Polymorphism. Overloading Polymorphism is the use of one method signature, or one operator such as “+”, to perform several different functions depending on the implementation. The “+” operator, for example, may be used to perform integer addition, float addition, list concatenation, or string concatenation. Any two subclasses of Number, such as Integer and Double, are expected to add together properly in an OOP language. The language must therefore overload the concatenation operator, “+”, to work this way. This helps improve code readability.

Advantages of polymorphism Polymorphism allows client programs to be written based only on the abstract interfaces of the objects which will be manipulated (interface inheritance). This means that future extension in the form of new types of objects is easy, if the new objects conform to the original interface. In particular, with object-oriented polymorphism, the original client program does not even need to be recompiled (only relinked) in order to make use of new types exhibiting new (but interface-conformant) behavior. (In C++, for instance, this is possible because the interface definition for a class defines a memory layout, the virtual function table describing where pointers to functions can be found. Future, new classes can work with old, precompiled code because the new classes must conform to the abstract class interface, meaning that the layout of the new class’s virtual function table is the same as before; the old, precompiled code can still look at the same memory offsets relative to the start of the object’s memory in order to find a pointer to the new function. It is only that the new virtual function table points to a new implementation of the functions in the table, thus allowing new, interface-compliant behavior with old, precompiled code.) Since program evolution very often appears in the form of adding types of objects (i.e. classes), this ability to cope with and localize change that polymorphism allows is the key new contribution of object technology to software design.

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