Java Inheritance: IS-A Hierarchy, 'extends' & the 'super' Keyword

Inheritance is an essential pillar of Object-Oriented Programming that allows a new class (known as the child / subclass / derived class) to inherit the state fields and behavioral methods of an existing class (known as the parent / superclass / base class).

The 3 Supported Types of Class Inheritance in Java

• 1. Single Inheritance: A single subclass inherits from a single superclass (B extends A).
• 2. Multilevel Inheritance: A class derives from a child class, creating a chain of inheritance (C extends B, and B extends A).
• 3. Hierarchical Inheritance: Multiple distinct subclasses inherit from a single parent class (B extends A and C extends A).

Why Multiple Class Inheritance is Forbidden: The Diamond Problem

Suppose Class B and Class C both inherit from Class A and override display(). If Class D were permitted to extend both B and C, the compiler would have no way to determine which version of display() to inherit. To preserve language simplicity and safety, Java prohibits multiple class inheritance.

Practical Demonstration: Inheritance with the super Keyword

Let us examine a vehicle hierarchy showcasing extends and the three roles of super:

// Superclass (Parent)
class Vehicle {
    protected String registrationNumber;
    protected int maxSpeed;

    // Parent Constructor
    public Vehicle(String regNum, int maxSpeed) {
        this.registrationNumber = regNum;
        this.maxSpeed = maxSpeed;
    }

    public void startEngine() {
        System.out.println("Vehicle engine ignited. Max speed: " + maxSpeed + " km/h");
    }
}

// Subclass (Child)
class ElectricCar extends Vehicle {
    private int batteryCapacityKwh;

    public ElectricCar(String regNum, int maxSpeed, int batteryCapacity) {
        // 1. Invoking Parent Constructor using super()
        super(regNum, maxSpeed);
        this.batteryCapacityKwh = batteryCapacity;
    }

    // Overriding Parent Method
    @Override
    public void startEngine() {
        // 2. Invoking Parent's Original Method using super.
        super.startEngine();
        System.out.println("Electric inverter active! Battery: " + batteryCapacityKwh + " kWh");
    }
}

public class InheritanceDemo {
    public static void main(String[] args) {
        ElectricCar tesla = new ElectricCar("MH-12-EV-9999", 250, 85);
        tesla.startEngine();
    }
}