Inheritance (OOP) (Cambridge (CIE) A Level Computer Science): Revision Note
Exam code: 9618
Inheritance (OOP)
What is inheritance?
Inheritance is a key concept in object-oriented programming (OOP) that allows a class to inherit the properties and behaviours (methods and attributes) of another class
Inheritance promotes code reuse by allowing derived classes to inherit and utilise the existing code from the base class. This avoids duplicating code and promotes better organization and maintainability
Inheritance establishes an "IS-A" relationship between the base class and the derived class
For example, if you have a base class called Vehicle and a derived class called Car, you can say that "a Car is a Vehicle."
The car class inherits the properties and behaviours associated with being a vehicle
Inheritance involves two main entities:
The base class (also known as the parent class or superclass) and the derived class (also known as the child class or subclass)
The derived class inherits the characteristics of the base class, meaning it can access and use the methods and attributes defined in the base class
Example of a base class and derived classes
Base Class: The base class serves as the blueprint or template from which the derived class inherits
It defines common properties and behaviours that can be shared among multiple derived classes
Derived Class: The derived class inherits the attributes and methods of the base class
It can add additional attributes and methods
If a car object was to be created, it may have the following attributes:
Manufacturer - The company that makes the car
Make -The model of the car
Cost – The price of the car to purchase
IsInsured – Whether or not the car is insured
EngineCapacity – The size of the engine for the car
It may also have access to the following methods:
TurnEngineOn()– To start the car engineTurnEngineOff()– To turn off the car engineSteerLeft()– To turn the car to the leftSteerRight()– To steer the car to the leftGearChange()– To change the gear of the carUnlockDoors()– To unlock the doors to the car
The above methods are only a select few and there could be many more added for extra functionality
In the following code, the super keyword is used in inheritance to refer to the superclass (Base class: Vehicles) and access its members (methods, attributes, or constructors) from within the subclass (Derived Class: Cars)
Programming inheritance
How do you program inheritance?
Pseudocode
Pseudocode for a class created using inheritance
Java
//vehicle class created
public class Vehicle {
//attributes created for the vehicle class
private String manufacturer;
private String make;
private double cost;
//constructor to create objects of type vehicle
public Vehicle(String manufacturer, String make, double cost) {
this.manufacturer = manufacturer;
this.make = make;
this.cost = cost;
}
//method to start the engine of the car
public void turnEngineOn() {
//code to turn the engine on
}
//method to stop the engine of the car
public void turnEngineOff() {
//code to turn the engine off
}
//method to steer the car to the left
public void steerLeft() {
// code to steer the vehicle left
}
//method to steer the car to the right
public void steerRight() {
// code to steer the vehicle right
}
}
//Derived Class (Car)
//the Car class uses the keyword 'extends' to inherit from the Vehicle class
public class Car extends Vehicle {
private boolean isInsured;
private double engineCapacity;
// Constructor to create objects of type Car
public Car(String manufacturer, String make, double cost, boolean isInsured, double engineCapacity) {
//the super keyword is used to inherit the attributes from the base class
super(manufacturer, make, cost);
this.isInsured = isInsured;
this.engineCapacity = engineCapacity;
}
//method to change gear
public void gearChange() {
//code to change the gear of the car
}
//method to unlock the vehicle doors
public void unlockDoors() {
//code to unlock the doors of the car
}
}Python
#Base class (Vehicle)
#Vehicle class created
class Vehicle:
#attributes created for the Vehicle class
def __init__(self, manufacturer, make, cost):
self.manufacturer = manufacturer
self.make = make
self.cost = cost
#method to start the engine of the car
def turn_engine_on(self):
# Code to turn the engine on
#method to stop the engine of the car
def turn_engine_off(self):
# Code to turn the engine off
#method to steer the car to the left
def steer_left(self):
# Code to steer the vehicle left
#method to steer the car to the right
def steer_right(self):
# Code to steer the vehicle right
#Derived Class (Car)
#the Car class uses the keyword parent class name in parenthesis to inherit from the Vehicle class
class Car(Vehicle):
def __init__(self, manufacturer, make, cost, is_insured, engine_capacity):
#the super keyword is used to inherit the attributes from the base class
super().__init__(manufacturer, make, cost)
self.is_insured = is_insured
self.engine_capacity = engine_capacity
#method to change gear
def gear_change(self):
# Code to change the gear of the car
#method to steer the car to the right
def unlock_doors(self):
# Code to unlock the doors of the carWorked 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.
Write program code to declare the class Helicopter. You only need to declare the class and its constructor. You do not need to declare the other methods.
Use your programming language’s appropriate constructor.
All attributes must be private.
If you are writing in Python, include attribute declarations using comments. [5]
Answer
Class header (and end where appropriate) inheriting from Vehicle [1 mark]
3 (private) attribute declarations with data types [1 mark]
Constructor (and end where appropriate) with (min) 5 parameters [1 mark]
Calling parent constructor with appropriate parameters [1 mark]
Initialising
VerticalPositionto 0 andVerticalChangeandMaxHeightto attributes [1 mark]
Example program code:
VB.NET
Class Helicopter
Inherits Vehicle
Private VerticalPosition As Integer
Private VerticalChange As Integer
Private MaxHeight As Integer
Sub New(IDP, MaxSpeedP, IncreaseAmountP, VertChangeP, MaxHeightP)
MyBase.New(IDP, MaxSpeedP, IncreaseAmountP)
VerticalPosition = 0
VerticalChange = VertChangeP
MaxHeight = MaxHeightP
End Sub
End ClassJava
class Helicopter extends Vehicle{
private Integer VerticalPosition;
private Integer VerticalChange;
private Integer MaxHeight;
public Helicopter(String IDP, Integer MaxSpeedP, Integer IncreaseAmountP, Integer VertChangeP, Integer MaxHeightP){
super(IDP, MaxSpeedP, IncreaseAmountP);
VerticalPosition = 0;
VerticalChange = VertChangeP;
MaxHeight = MaxHeightP;
}}Python
class Helicopter(Vehicle):
#VerticalPosition Integer
#VerticalChange Integer
#MaxHeight Integer
def __init__(self, IDP, MaxSpeedP, IncreaseAmountP, VertChangeP, MaxHeightP):
Vehicle.__init__(self,IDP, MaxSpeedP, IncreaseAmountP)
self.__VerticalPosition = 0
self.__VerticalChange = VertChangeP
self.__MaxHeight = MaxHeightP
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