Nested Interfaces

[Arthur Blair]

It is possible to declare an interface like this:

    
public class ExampleClass {
    
    public void anotherMethod() {
        //
    }
    
    private interface InnerInterface {
        public void aMethod();
    }
    
}

...but why would you want to have a nested interface and what are it's implications?

Is it only available to nested classes?

[Ajith Kallambella]

Yes, you can declare a nested interface like that. One use would be to limit the visibiilty of such a type.

[Arthur Blair]

Thanks for the reply. Perhaps I didn't make myself clear in my post. I understand that is it possible to declare an interface in this way. I'm interested in what the implications are of creating such an interface.

If I have a class in the same package, it won't be able implement the inner interface. So can I assume that this inner interface is only available to classes that are members of the enclosing class?

...and why would you need to do this? Can someone provide a real world example?

[Hamid Virani]

You may have another nested class or an inner class which may implement that.
For E.g.

  
public class ExampleClass {
    public void anotherMethod() {
    //    
    }        
    private interface InnerInterface { 
       public void aMethod();
    } 
    private class InnerClass implements InnerInterface {
        public void aMethod() {
         //some implementation
         }
    }
}

[Valentin Jacquemin]

I don't see the advantage of it....

I don't speak English very well sorry...

[Mark Spritzler]

Personally, if it needed to have an inner interface because of the number of inner class that needed to implement, I would run away as fast as I can, because I feel it is going overboard, and would rather it be a more maintainable file, not 100,000 lines in one file (exaggeration there).

But you could create a whole inner factory with many objects implementing the same inner interface, then the code that uses the inner factory can code to an interface, in theory.

Mark

[santhosh sharma]

hi arthur,
I dont think nested interfaces can be declared as private
Nested interfaces are implicitly abstract and public

santhosh sharma (scjp 1.4 100%)

[Ajith Kallambella]

In fact, unlike top level interfaces that are implicitly public, nested interfaces can be declared private. Here's what JLS says

The access modifiers protected and private pertain only to member interfaces within a directly enclosing class declaration (�8.5) and are discussed in �8.5.1.

[Arthur Blair]

Santhosh,

Thats not correct. A top-level interface is implicitly abstract, but can be default or public. If it is public it needs to be in a file with the same name as the interface plus the extension .java

However, If an interface is nested in another class, it's access modifier can be private, default, protected or public. While if an interface is nested within another interface, it can be public or default.

...which brings me to my next question. Why on earth would you would to have an interface nested within another interface?

Arthur

[santhosh sharma]

Hi arthur ,
I agree with you , there is no point in allowing an interface to be declared inside another interface , but hey

please check ,

interface a
{
private interface b{}
}

is giving me a compile time error
[illegal combination of access modifiers public and private]

santhosh sharma (scjp 1.4 100%)