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

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