Java — Inheritance

Basic inheritance

class Animal {
    String name;

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

class Dog extends Animal {
    void bark() {
        System.out.println(name + " is barking");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.name = "Rex";
        dog.eat();   // Inherited from Animal
        dog.bark();  // Dog-specific
    }
}

Method overriding

class Animal {
    void speak() {
        System.out.println("The animal speaks");
    }
}

class Dog extends Animal {
    @Override
    void speak() {
        System.out.println("The dog barks");
    }
}

class Cat extends Animal {
    @Override
    void speak() {
        System.out.println("The cat meows");
    }
}

super keyword

class Person {
    String name;

    Person(String name) {
        this.name = name;
    }

    void greet() {
        System.out.println("Hello, I'm " + name);
    }
}

class Employee extends Person {
    String company;

    Employee(String name, String company) {
        super(name);
        this.company = company;
    }

    @Override
    void greet() {
        super.greet();
        System.out.println("I work at " + company);
    }
}

Constructors

class Base {
    Base() {
        System.out.println("Base constructor");
    }
}

class Derived extends Base {
    Derived() {
        super(); // Must be first statement
        System.out.println("Derived constructor");
    }
}

Protected access

class Parent {
    protected int x = 10;
    private int y = 20;
}

class Child extends Parent {
    void show() {
        System.out.println(x);  // OK: protected
        // System.out.println(y); // Error: private
    }
}

Final classes and methods

final class Immutable {
    private final int value;

    Immutable(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

// class Child extends Immutable {} // Error: cannot extend final class

class Parent {
    final void show() {
        System.out.println("Cannot override");
    }
}

class Child extends Parent {
    // void show() {} // Error: cannot override final method
}

Object class

class Person {
    String name;

    Person(String name) {
        this.name = name;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Person other = (Person) obj;
        return name.equals(other.name);
    }

    @Override
    public int hashCode() {
        return name.hashCode();
    }

    @Override
    public String toString() {
        return "Person{name='" + name + "'}";
    }
}

Mini Practice

Write Java code that:

1. Creates a parent class and child class
2. Overrides a method with @Override
3. Uses super to call parent constructor
4. Demonstrates final class and methods

Up Next

In the next lesson, you'll learn about Interfaces — defining contracts in Java.