Polymorphism (OOP) (Cambridge International A-Level Computer Science)

Polymorphism (OOP)

What is polymorphism?

• In A Level Computer Science , polymorphism is a concept in programming that allows objects to take on different forms or behaviours
• Different objects can share the same name or behaviour but can work in different ways
• It helps make code more flexible, reusable, and easier to maintain
• It allows flexibility and reusability in programming, making it easier to write and manage code
• Objects can be treated as belonging to a common group, even if they belong to different classes, making your code more versatile and adaptable to changes

Example 1 – method overloading

Method Overloading Example 1

• In the example above, all three classes all have a method named move(). Polymorphism allows methods to be declared with the same name but execute different code (in this case printing different messages)
• The override keyword Is used to provide a new implementation for a method that is already defined in the parent class (base class)

Example 2 - method overloading

Method Overloading Example 2

• In the above example both the Motorcycle class and the Car class both inherit from the base class 'Cars'

• Objects from the Motorcycle Class and the Car class can call the startEngines() method which will output "Engines Started!"

• If either of the object types call the displayInfo() method, the program will execute the method from the objects class as it overrides the Vehicle class method

• For example

  • If a motorcycle object calls the displayInfo() method, "I am a Motorcycle!" will be output
  • If a Car object calls the displayInfo() method, "I am a Car!" will be output

Treating objects as common groups

• Polymorphism also allows objects of different classes to be treated as objects of a common superclass or base class

• For example:

  • Vehicle vehicle1 = new Car()
  • Vehicle vehicle2 = new Motorcycle()

• This allows an array of type Vehicle to store both Motorcycle and Car objects rather than in separate data structures

  • If the vehicle1.displayInfo() method is called, it will still output "I am a Car!"
  • If the vehicle2.displayInfo() method is called, it will still output "I am a Motorcycle!"

• This flexibility provided by polymorphism are essential for creating more maintainable and modular code

Programming polymorphism (OOP)

How do you program polymorphism?

Run-time polymorphism

• This happens when a subclass overrides a method defined in a superclass, and the correct version is chosen at run-time, not compile-time

Example scenario: Animal, dog, and cat

Step-by-step logic (pseudocode)

1. Define a superclass called Animal with a method speak() that does nothing
2. Create subclasses called Dog and Cat that inherit from Animal
3. In each subclass, override the speak() method to provide a specific output
4. Write a function make_sound(animal) that accepts an object of type Animal and calls its speak() method
5. Create instances of Dog and Cat, and pass them to make_sound()

• Animal is a base class with a speak() method
• Dog and Cat are subclasses that override the speak() method
• make_sound() takes an Animal object but calls the correct version of speak() depending on whether it’s a Dog or a Cat
• This demonstrates run-time polymorphism – the method call is resolved based on the actual type of the object

Python

• This shows method overriding
• The make_sound() function relies on dynamic method binding

Java

• Recognise use of a superclass reference (Animal animal) pointing to a subclass object
• Polymorphism occurs when animal.speak() dynamically calls the correct overridden method

Worked Example The child class Helicopter inherits from the parent class Vehicle. A helicopter also has a vertical position and changes the vertical position when it increases speed.

The Helicopter method IncreaseSpeed() overrides the method from the parent

class and:

• adds the amount of vertical change to the vertical position
• adds IncreaseAmount to the current speed
• adds the updated current speed to the horizontal position.

The vertical position of a helicopter cannot exceed its maximum height.

The current speed of a helicopter cannot exceed its maximum speed.

Write program code for the method IncreaseSpeed() [4]

Final answer: Answer

• Method header (overriding where required) with no parameter [1 mark]
• Adding vertical change to vertical position … [1 mark]
• …limiting to maximum height [1 mark]
• Repeating/calling/using the code from original for horizontal increase (in every case) [1 mark]

Example program code:

VB.NET

Java

Python