类内部接口

Question

My code is as following, my question is that what is the meaning of line 1 , if there is not meaning then why does eclipse allows infact gives inner as available option on typing ClassO.

It would have been meaningful if the inner was a static nested class instead of inner interface and if it meaningful as it is then what is the use, what all can i do with it ?

public class InterfaceInsideClass 
{

    public static void main(String[] args) 
    {
        ClassO.inner              **// Line 1**
    }

}

class ClassO
{
    interface inner
    {
        void msg();
    }

    class Two implements inner
    {
        public void msg()
        {
            System.out.println("Class Two");
        }
    }

    class Three
    {
        public void msg()
        {
            System.out.println("Class Three not implementing interface but having same method");
        }
    }

    static class Four
    {
        public void msg()
        {
            System.out.println("Class Four");
        }
    }
}

Answer 1

One on the most common use of static nested classes and nested interface is for another layer of packaging.

If the particular interface is only intended to be implemented within a particular class or its subclasses via by its inner or nested classes then it makes sense to include that interface inside those classes.

Good example is: java.util.Map.Entry .