Polymorphism (OOP) (OCR A-Level Computer Science): Revision Notes

Polymorphism (OOP)

Overview

Polymorphism is a core concept in Object-Oriented Programming (OOP) that allows objects of different classes to be treated as instances of the same class, typically through a shared parent class. The term "polymorphism" means "many forms," and in programming, it enables a single function, method, or operator to work in different ways depending on the context. Polymorphism promotes flexibility, code reuse, and the ability to extend systems easily, as it allows programmers to write code that can uniformly handle different types of objects.

Types of Polymorphism

Method Overriding (Run-time Polymorphism)

• Definition: Method overriding allows a child class to provide a specific implementation of a method that is already defined in its parent class. This type of polymorphism occurs at runtime, allowing the correct version of the method to be called based on the actual object type.
• Purpose: Run-time polymorphism lets child classes customise or extend the behaviour of inherited methods, making it possible for different classes to define their own unique behaviour while sharing a common interface.

class Animal:
    def make_sound(self):
        return "Some generic animal sound"

class Dog(Animal):
    def make_sound(self):  # Overrides make_sound in Animal
        return "Woof woof"

class Cat(Animal):
    def make_sound(self):  # Overrides make_sound in Animal
        return "Meow"

# Using polymorphism
animals = [Dog(), Cat(), Animal()]
for animal in animals:
    print(animal.make_sound())

Method Overloading (Compile-time Polymorphism)

• Definition: Method overloading allows a class to define multiple methods with the same name but different parameters. In languages like Python, true method overloading is not directly supported, but similar behaviour can be achieved using optional or keyword arguments.
• Purpose: Compile-time polymorphism makes methods more flexible, enabling different behaviours based on input parameters.

class Calculator:
    def add(self, a, b, c=0):
        return a + b + c

calc = Calculator()
print(calc.add(3, 5))       # Outputs: 8
print(calc.add(3, 5, 10))   # Outputs: 18

Polymorphism with Interfaces

• Definition: In OOP languages that support interfaces or abstract classes (like Java), polymorphism is often achieved by defining a set of methods in an interface that multiple classes implement, each with its own specific behaviour.
• Purpose: Interfaces ensure that different classes provide the same methods, allowing polymorphic behaviour based on the interface type.

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):  # Provides specific implementation
        return self.width * self.height

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):  # Provides specific implementation
        return 3.14159 * self.radius ** 2

# Using polymorphism with an interface
shapes = [Rectangle(3, 4), Circle(5)]
for shape in shapes:
    print(shape.area())  # Calls the area method for each shape

Benefits of Polymorphism

1. Flexibility: Polymorphism allows code to work with objects of different types interchangeably, making it adaptable to future changes or additions.
2. Code Reusability: With polymorphism, classes can share methods, reducing redundancy and improving code reusability.
3. Ease of Maintenance: Polymorphic code is more modular and less complex, which improves maintainability. Changes to shared methods can be made in a parent class without modifying subclasses.
4. Extensibility: New subclasses can be added that extend or override existing methods, enabling the programme to expand easily.

Note Summary