Interfaces just got much better in Java 8

                            

We all know that Interface contains methods and variables, but the methods declared in an interface are by default abstract (only method signature nobody).If a class is implementing an interface it must provide method implementation of the methods in the interface, otherwise, the class must be declared as abstract.

Now imagine a situation where you want to add a new method to an interface later.Modifying one interface breaks all classes that implement the interface.which means adding a single method can break millions of lines of code.

In order to address this problem, Java8 introduced "Default Method" or Defender methods, which allows the user to enter a new method in the interface without breaking existing implementation of these interfaces.It provides flexibility to allow Interface define implementation which will use as default in the situation where a concrete Class fails to provide an implementation for that method.

Let's understand this with an example

	interface OldInterface {
	public void existingMethod();
	default public void DefaultMethod() {
	System.out.println("New default method" + " is added in interface");
	}
	}
	//following class compiles successfully in JDK 8
	public class ClassImpl implements OldInterface {
	@Override
	public void existingMethod() {
	System.out.println("normal method");
	}
	public static void main(String[] args) {
	ClassImpl obj = new ClassImpl ();
	// print “New default method add in interface”
	obj.DefaultMethod();
	}
	}

view raw ClassImpl.java hosted with ❤ by GitHub

When we extend an interface that contains a default method, we can perform following:-

• Not override the default method and will inherit the default method
• Override the default method similar to other methods we override in the subclass.
• Redeclare default method as abstract, which force subclass to override it.

Some References 

1. http://www.oracle.com/technetwork/java/javase/8-whats-new-2157071.html
2. http://cr.openjdk.java.net/~briangoetz/lambda/Defender%20Methods%20v4.pdf