B3.2.1 Applying the Concept of Inheritance in OOP to Promote Code Reusability (HL only) Notes

Understanding Inheritance in OOP

What is Inheritance?

It promotes code reusability, reduces duplication, and establishes a hierarchical relationship between classes.

How Inheritance Works

1. Superclass (Parent Class):
   1. Defines shared attributes and methods.
   2. Example: Vehicle has attributes like speed, capacity, and methods like move().
2. Subclass (Child Class):
   1. Inherits attributes and methods from the parent.
   2. Can add its own specialized features.
   3. Example: Car adds numberOfDoors; Aeroplane adds fly().

Benefits of Inheritance

1. Single Inheritance: One parent → one child.
2. Multilevel Inheritance: A child acts as a parent for another child (e.g., Vehicle → Car → ElectricCar).
3. Hierarchical Inheritance: Multiple children share the same parent.
4. Multiple Inheritance: Supported in Python (via multiple parent classes) but not directly in Java (handled using interfaces).

Access Modifiers and Inheritance

1. Public: Accessible everywhere.
2. Protected: Accessible within the package (Java) or subclass (Java/Python).
3. Private: Not inherited; accessible only inside the parent class.
4. Default (Java): Package-level access.

Implementing Inheritance in Java and Python

class Vehicle:
    def __init__(self):
        self.type = "General Vehicle"

    def move(self):
        print("Vehicle is moving...")

class Aeroplane(Vehicle):
    def fly(self):
        print("Aeroplane is flying...")

# Testing inheritance
a = Aeroplane()
print(a.type)   # inherited attribute
a.move()        # inherited method
a.fly()         # subclass method

class Vehicle {
    String type = "General Vehicle";

    void move() {
        System.out.println("Vehicle is moving...");
    }
}

class Aeroplane extends Vehicle {
    void fly() {
        System.out.println("Aeroplane is flying...");
    }
}

public class TestInheritance {
    public static void main(String[] args) {
        Aeroplane a = new Aeroplane();
        System.out.println(a.type);  // inherited attribute
        a.move();                    // inherited method
        a.fly();                     // subclass method
    }
}

Access Modifiers and Inheritance

Understanding Access Modifiers

1. Access modifiers control the visibility of class members (attributes and methods) in Java.
2. Python uses naming conventions to achieve similar effects.
   1. Private: Accessible only within the class.
   2. Protected: Accessible within the class and its subclasses.
   3. Public: Accessible from anywhere.
   4. Default (Java only): Accessible within the same package.

Impact on Inheritance

1. Private Members: Not accessible in subclasses. Use public accessor methods to access them.
2. Protected Members: Accessible in subclasses, promoting code reuse while maintaining encapsulation.
3. Public Members: Fully accessible in subclasses and outside the class hierarchy.

Practical Applications of Inheritance

Code Reusability and Maintenance

1. Reusability: Common code is stored in the superclass, reducing duplication.
2. Maintenance: Changes in the superclass automatically apply to all subclasses, simplifying updates.