Inheritance and Polymorphism

Inheritance lets you create new classes based on existing ones, reusing and extending their behaviour. Polymorphism means a single interface can represent different underlying forms. Together, they are two of the four pillars of object-oriented programming.

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Inheritance with extends

A subclass (child) inherits fields and methods from a superclass (parent):

public class Animal {
    String name;

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

    public void speak() {
        System.out.println(name + " makes a sound");
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);  // call the parent constructor
    }

    @Override
    public void speak() {
        System.out.println(name + " barks");
    }
}

Key Rules

Rule	Detail
Single inheritance only	A class can extend exactly one parent class
super()	Calls the parent constructor — must be the first statement
@Override	Annotation indicating a method overrides a parent method
All classes extend Object	If no extends is specified, the class implicitly extends java.lang.Object

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The super Keyword