nested interface

[Hari priya]

Hello,
I am not sure if my question belongs to the advance forum but I am hoping to get some good advices here..
I have seen the following section in my code base .

public class A{
public interface B{
}
}

public class C implements A.B{
}

Isn't this same as -

public class A {
}
public class B extends A{
}
and C extends B

what is the advantage of using this kind of nested interface? Will C have the properties of A and B? What does C implements A.B mean exactly??
Kindly advise.

[David Newton]

Isn't this same as ...

No; implementing an interface is not the same thing as extending a class.

What does C implements A.B mean exactly?

It means that C implements the A.B interface--there's no difference between implementing an interface and implementing a nested interface--they're both just interfaces.

[Hari priya]

Thanks David!

Can you also answer these questions-

what is the advantage of using this kind of nested interface? Will C have the properties of A and B?

[Seetharaman Venkatasamy]

i dont know what is the advantage of this kind of interface.

Coming to your 2nd question : whether C has the properties of A and B ?

try below program

class A{
	int a = 10;
	interface B{
		void print();
		int a = 9;
	}
}

public class C implements A.B{
	public void print(){
		System.out.println(a);
	}	
	public static void main(String[] args){
		
		try{
			new C().print();
			 
		}catch(Exception e){
			e.printStackTrace();
		}
	}	
}
 

[Rob Spoor]

Hari priya wrote:what is the advantage of using this kind of nested interface? Will C have the properties of A and B?

If interface B is only used in combination with interface A you could use a nested interface. This is an indication that interface B belongs somehow to interface A. Map.Entry is one such example.

C will have no combination with A anymore; interface B is implicitly static, and therefore the only real link between A and B is the namespace. A Map.Entry implementation does not need to know anything about the Map interface. For instance, java.util.AbstractMap.SimpleEntry is a static nested class has just one link to AbstractMap - and that's a static utility method that could just as well been omitted:

private static boolean eq(Object o1, Object o2) {
    return o1 == null ? o2 == null : o1.equals(o2);
}