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Polymorphism

What

Polymorphism:

The ability of different objects to respond, each in its own way, to identical messages is called polymorphism. -- Object-Oriented Programming with Objective-C, Apple

Polymorphism allows you to write code targeting superclass objects, use that code on subclass objects, and achieve possibly different results based on the actual class of the object.

Assume classes Cat and Dog are both subclasses of the Animal class. You can write code targeting Animal objects and use that code on Cat and Dog objects, achieving possibly different results based on whether it is a Cat object or a Dog object. Some examples:

  • Declare an array of type Animal and still be able to store Dog and Cat objects in it.
  • Define a method that takes an Animal object as a parameter and yet be able to pass Dog and Cat objects to it.
  • Call a method on a Dog or a Cat object as if it is an Animal object (i.e., without knowing whether it is a Dog object or a Cat object) and get a different response from it based on its actual class e.g., call the Animal class's method speak() on object a and get a "Meow" as the return value if a is a Cat object and "Woof" if it is a Dog object.

Polymorphism literally means "ability to take many forms".

How

Three concepts combine to achieve polymorphism: substitutability, operation overriding, and dynamic binding.

  • Substitutability: Because of substitutability, you can write code that expects objects of a parent class and yet use that code with objects of child classes. That is how polymorphism is able to treat objects of different types as one type.
  • Overriding: To get polymorphic behavior from an operation, the operation in the superclass needs to be overridden in each of the subclasses. That is how overriding allows objects of different subclasses to display different behaviors in response to the same method call.
  • Dynamic binding: Calls to overridden methods are bound to the implementation of the actual object's class dynamically during the runtime. That is how the polymorphic code can call the method of the parent class and yet execute the implementation of the child class.

Exercises:

Concepts related to polymorphism