Java8 Enhancement to interfaces

Java8 added new features to interfaces, that you can provide default method implementations to interfaces using default keyword, and you can add static methods also.

Default Methods

Default methods enables to add new functionality to the existing interfaces, and ensure binary compatibility with code written for older versions of those interfaces.

By using default keyword you can specify a default implementation for a method in interface. All method declarations in an interface, including default methods, are implicitly public.

Example

interface Car{
 void swim();
 void drive();
}

class MyCar implements Car{
 public void swim(){
  System.out.println("I can swim");
 }
 
 public void drive(){
  System.out.println("I can Drive");
 }
}

Lets suppose, the Car interface adds new functionality like that all cars can fly.A new abstract method 'fly' added to Car interface like below.

interface Car{
 void swim();
 void drive();
 void fly();
}

Previously MyCar class implemented the Car interface, now with this updated version of Car interface, if you tries to compile MyClass.java, compiler throws below error since, MyCar is not providing implementation for 'fly'.

MyCar.java:1: error: MyCar is not abstract and does not override abstract method
 fly() in Car
class MyCar implements Car{
^
1 error

Here default methods plays a role to ensure binary compatibility with code written for older versions of those interfaces.

interface Car{
 void swim();
 void drive();
 default void fly(){
  System.out.println("I Can Fly");
 }
}

We are providing default implementation for the fly method in Car interface. So it makes your previously existed classes which implements Car interface works fine.

class MyCar implements Car{
 public void swim(){
  System.out.println("I can Swim");
 }
 
 public void drive(){
  System.out.println("I can Drive");
 }
}

class CarTest{
 public static void main(String args[]){
  MyCar car1 = new MyCar();
  
  car1.swim();
  car1.drive();
  car1.fly();
 }
}

Output

I can Swim
I can Drive
I Can Fly

Extending Interfaces That Contain Default Methods

When you extend an interface that contains default methods you have 3 possibilities.

1. If interface 'B' extend interface 'A', and not providing the implementations for default methods in A, then extended interface 'B' inherit the default method.

interface A{
 void print();
 
 default void show(){
  System.out.println("I am in show");
 }
}

interface B extends A{
 void display();
}

class C implements B{
 public void display(){
 }
   
 public void print(){
 }
}

class Test{
 public static void main(String args[]){
  C obj1 = new C();
  obj1.show();
 }
}

Output

I am in show

2. If interface 'B' extend interface 'A', and re-declare the default methods, then default methods became abstract.

interface A{
 void print();
 
 default void show(){
  System.out.println("I am in show");
 }
}

interface B extends A{
 void display();
 void show();
}

Now any class that is going to implement the interface B, has to provide the implementation for the method show also.

class C implements B{
 public void display(){
 }
   
 public void print(){
 }
}

When you tries to compile the above program compiler throws below error, since class 'C' is not providing the implementation for the method 'show'.

C.java:1: error: C is not abstract and does not override abstract method show()in B
class C implements B{
^
1 error

To make the program works fine, add the implementation for the method 'show' in the class 'C'.

class C implements B{
 public void display(){
 }
   
 public void print(){
 }
 
 public void show(){
  System.out.println("I am in show");
 }
}

class Test{
 public static void main(String args[]){
  C obj1 = new C();
  obj1.show();
 }
}

Output

I am in show

3. If interface 'B' extend interface 'A', and redefine the default methods, then default methods are overridden.

interface A{
 void print();
 
 default void show(){
  System.out.println("I am in interface A");
 }
}

interface B extends A{
 void display();
 default void show(){
  System.out.println("I am in interface B");
 }
}

class C implements B{
 public void display(){
 }
   
 public void print(){
 }
}

class Test{
 public static void main(String args[]){
  B obj1 = new C();
  A obj2 = obj1;
  
  obj2.show();
 }
}

Output

I am in interface B

Interface : static Methods

In addition to default methods, you can define static methods in interfaces. All method declarations in an interface, including static methods, are implicitly public.

interface A{
 void print();
 
 static void show(){
  System.out.println("I am in interface A");
 }
 
}

interface B extends A{
 void display();
 static void show(){
  System.out.println("I am in interface B");
 }
}

class Test{
 public static void main(String args[]){
  B.show();
  A.show();
 }
}

Output

I am in interface B
I am in interface A

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