Java Inheritance

extends Keyword

Inheritance allows a child class (subclass) to acquire the fields and methods of a parent class (superclass) using the extends keyword. Java supports single inheritance - a class can extend only one class, but can implement multiple interfaces.

Note: Every class in Java implicitly extends java.lang.Object - the root of the class hierarchy.

extends & super keyword

// Parent class (superclass)
class Animal {
    String name;
    int age;

    public Animal(String name, int age) {
        this.name = name;
        this.age  = age;
    }

    public void eat() {
        System.out.println(name + " is eating.");
    }

    public void sleep() {
        System.out.println(name + " is sleeping.");
    }
}

// Child class (subclass) - inherits Animal
class Dog extends Animal {
    String breed;

    public Dog(String name, int age, String breed) {
        super(name, age);   // calls Animal's constructor
        this.breed = breed;
    }

    public void bark() {
        System.out.println(name + " says: Woof!");
    }

    // Method overriding - redefine parent behavior
    @Override
    public void eat() {
        super.eat();  // call parent version first
        System.out.println(name + " wags its tail while eating.");
    }
}

public class Inheritance {
    public static void main(String[] args) {
        Dog dog = new Dog("Rex", 3, "Labrador");
        dog.eat();    // overridden version
        dog.sleep();  // inherited from Animal
        dog.bark();   // Dog-specific method
        System.out.println(dog.breed);  // Labrador
    }
}

Method Overriding & @Override

A subclass can provide its own implementation of a method defined in the parent class. Rules for overriding:

Same method name, same parameter list, same return type (or covariant).
Access modifier cannot be more restrictive than the parent's.
Always use @Override annotation - the compiler will catch mistakes.
static, private, and final methods cannot be overridden.

final Class and final Method

final class & final method

class Vehicle {
    // final method - cannot be overridden by subclasses
    public final void startEngine() {
        System.out.println("Engine started.");
    }

    public void accelerate() {
        System.out.println("Vehicle accelerating.");
    }
}

class Truck extends Vehicle {
    @Override
    public void accelerate() {
        System.out.println("Truck accelerating slowly.");
    }
    // Cannot override startEngine() - it's final
}

// final class - cannot be extended
final class ImmutablePoint {
    final int x;
    final int y;

    public ImmutablePoint(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

// class SubPoint extends ImmutablePoint {}  // ERROR: cannot extend final class

public class FinalDemo {
    public static void main(String[] args) {
        Truck t = new Truck();
        t.startEngine();   // Engine started.
        t.accelerate();    // Truck accelerating slowly.

        ImmutablePoint p = new ImmutablePoint(3, 4);
        System.out.println("Point: (" + p.x + ", " + p.y + ")");
    }
}

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Java Inheritance

extends Keyword

Method Overriding & @Override

final Class and final Method

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