Java extends Keyword
Usage and Examples

extends Keyword in Java

The extends keyword in Java allows one class to inherit the fields and methods of another class. This is Java's way of implementing single inheritance. Inheritance is one of the core pillars of object-oriented programming (OOP), and extends is the gateway to it.

What Does extends Do?

When one class extends another, it means the child class (also called the subclass or derived class) gains all the accessible properties and behaviors (methods) of the parent class (also called the superclass).

Syntax

class ParentClass {
    // fields and methods
}

class ChildClass extends ParentClass {
    // additional fields and methods
}

Example 1: Simple Inheritance

class Animal {
    void makeSound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.makeSound(); // inherited method
        d.bark();      // Dog's own method
    }
}

Animal makes a sound
Dog barks

Explanation

In this example, the class Dog inherits the method makeSound() from Animal. It can use both its own methods and the ones from its parent.

Example 2: Method Overriding

class Animal {
    void makeSound() {
        System.out.println("Generic animal sound");
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Cat(); // polymorphism
        a.makeSound();
    }
}

Meow

Explanation

The subclass Cat overrides the makeSound() method. Even though the reference is of type Animal, the actual object is Cat. So, the overridden method in Cat is executed. This is dynamic method dispatch at play.

Example 3: Inheriting Fields

class Vehicle {
    int speed = 60;
}

class Car extends Vehicle {
    String brand = "Toyota";
}

public class Main {
    public static void main(String[] args) {
        Car car = new Car();
        System.out.println(car.speed); // inherited field
        System.out.println(car.brand); // Car's own field
    }
}

60
Toyota

Best Practices

• Use extends when there's a clear "is-a" relationship. For example, a Dog is an Animal.
• Favor composition over inheritance when the relationship isn't so clear-cut.
• Always annotate overridden methods with @Override to catch mistakes at compile time.

What You Cannot Do

• Java supports only single inheritance using classes. A class cannot extend more than one class.
• To achieve multiple inheritance-like behavior, use interfaces with the implements keyword.