What is default, defender or extension method of Java 8?

Java 8 introduces a new concept of default method implementation in interfaces. This capability is added for backward compatibility so that old interfaces can be used to leverage the lambda expression capability of Java 8.

For example, ‘List’ or ‘Collection’ interfaces do not have ‘foreach’ method declaration. Thus, adding such method will simply break the collection framework implementations. Java 8 introduces default method so that List/Collection interface can have a default implementation of foreach method, and the class implementing these interfaces need not implement the same.

Syntax

public interface vehicle {
   default void print() {
      System.out.println("I am a vehicle!");
   }
}

Example

Live Demo

public class Java8Tester {
   public static void main(String args[]) {
      Vehicle vehicle = new Car();
      vehicle.print();
   }
}
interface Vehicle {
   default void print() {
      System.out.println("I am a vehicle!");
   }
   static void blowHorn() {
      System.out.println("Blowing horn!!!");
   }
}
interface FourWheeler {
   default void print() {
      System.out.println("I am a four wheeler!");
   }
}
class Car implements Vehicle, FourWheeler {
   public void print() {
      Vehicle.super.print();
      FourWheeler.super.print();
      
      Vehicle.blowHorn();
      System.out.println("I am a car!");
   }
}

Output

I am a vehicle!
I am a four wheeler!
Blowing horn!!!
I am a car!