This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
To create the innner class you would need an instance of the containing class but there is no such thing: it's an interface. Ignoring that, what benefit would you derive with an inner class instance? Access to package and private scope fields and methods? There are none.