What is Polymorphism?
The ability of different objects to respond, each in its own way, to identical messages is called polymorphism.
Polymorphism results from the fact that every class lives in its own namespace. The names assigned within a class definition don’t conflict with names assigned anywhere outside it. This is true both of the instance variables in an object’s data structure and of the object’s methods:
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Just as the fields of a C structure are in a protected namespace, so are an object’s instance variables.
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Method names are also protected. Unlike the names of C functions, method names aren’t global symbols. The name of a method in one class can’t conflict with method names in other classes; two very different classes can implement identically named methods.
Method names are part of an object’s interface. When a message is sent requesting that an object do something, the message names the method the object should perform. Because different objects can have methods with the same name, the meaning of a message must be understood relative to the particular object that receives the message. The same message sent to two different objects can invoke two distinct methods.
The main benefit of polymorphism is that it simplifies the programming interface. It permits conventions to be established that can be reused in class after class. Instead of inventing a new name for each new function you add to a program, the same names can be reused. The programming interface can be described as a set of abstract behaviors, quite apart from the classes that implement them.
Overloading: The terms polymorphism and parameter overloading refer basically to the same thing, but from slightly different points of view. Polymorphism takes a pluralistic point of view and notes that several classes can each have a method with the same name. Parameter overloading takes the point of the view of the method name and notes that it can have different effects depending on the parameters passed to it. Operator overloading is similar. It refers to the ability to turn operators of the language (such as == and + in C) into methods that can be assigned particular meanings for particular kinds of objects. Objective-C implements polymorphism of method names, but not parameter or operator overloading.
For example, suppose you want to report the amount of water used by an Appliance object over a given period of time. Instead of defining an amountConsumed method for the Appliance class, an amountDispensedAtFaucet method for a Faucetclass, and a cumulativeUsage method for a Building class, you can simply define a waterUsed method for each class. This consolidation reduces the number of methods used for what is conceptually the same operation.
Polymorphism also permits code to be isolated in the methods of different objects rather than be gathered in a single function that enumerates all the possible cases. This makes the code you write more extensible and reusable. When a new case comes along, you don’t have to reimplement existing code; you need only to add a new class with a new method, leaving the code that’s already written alone.
For example, suppose you have code that sends a draw message to an object. Depending on the receiver, the message might produce one of two possible images. When you want to add a third case, you don’t have to change the message or alter existing code; you merely allow another object to be assigned as the message receiver.