Default Method in Interface - Java 8

Java 8 comes lots of new features and in one important feature is Default method in Interface. Till Java 7 we can have only abstract methods in interface and we can't have any method definition in interfaces. In Java 8 we can have default method nothing but method with definition, following example will show you how to write those methods.

Next we can ask, why we need Default method in interface?

Lets assume we need to introduce new method in interface and then it breaks all classes implementing those interfaces by compile time error. For example if interface used in hundreds of classes which can break millions of line code by asking to implement new method introduced in Interface.
Even in JDK we can see foreach default method introduced to entire collection classes by giving definition in Iterable interface. Below is the Oracle API docs snapshot for Iterable interface.



Suppose if we don't have default method feature then JDK need to come-up with millions of code change in all collection classes by giving those method implementation.

Next we may ask whats the difference between Java 8 interface and Abstract method?

Interfaces cannot have state associated with them but Abstract classes can have state associated with them.

Next how to solve Multiple Inheritance Ambiguity Problems with default methods?

Since we can implement multiple interface, also we can have same default method (with same name) in 2 different interface. When we implement those interfaces in concrete class compile will give a warning that

"Duplicate default methods named <method_name> with the parameters () and () are inherited from the types <Interface1> and <Interface2>"

To solve this we can Override default method from any 1 interface. Below example will give detail about how to use default method in interface and to solve Ambiguity Problems with 2 interface have same default method.

FirstInterface.java

public interface FirstInterface {

 public void method();
 
 default public String methodName(){
  return "FirstInterface";
 }
}

SecondInterface.java

public interface SecondInterface {
 
 public void method();
 
 public default String methodName(){
  return "SecondInterface";
 }
}

TestClass.java

public class TestClass implements FirstInterface, SecondInterface {

 @Override
 public void method() {
  System.out.println("Hello Java Discover :) ");
 }
 
 /*
  * To solve Ambiguity Problems we can override 
  * default method as below.
  */
 @Override
 public String methodName(){
  return FirstInterface.super.methodName();
 }

 
 public static void main(String[] args) {
  new TestClass().testInterface();
 }
 
 private void testInterface(){
  method();
  System.out.println(FirstInterface.super.methodName());
  System.out.println(SecondInterface.super.methodName());
 }
 
}

OUTPUT:

Hello Java Discover :) 
FirstInterface
SecondInterface

If we see above example 2 interfaces having default method with same name. When we implement those interfaces in our concrete class (TestClass.java) we will get compile time error as

"Duplicate default methods named methodName with the parameters () and () are inherited from the types SecondInterface and FirstInterface"

To overcome this we need to override default method in our class as above. Next we can call default methods by using Interface names as we called in testInterface() method.

FirstInterface.super.methodName()
SecondInterface.super.methodName()