INamedParameters.getAllNamed() returns a list of NamedPairs defined as inner class of PageParameters

In an attempt to create my own implementation of PageParameterEncoder and PageParameters, I encountered the following problem with the API:

The interface INamedParameters defines the method getAllNamed() that returns a list of NamedPairs defined as inner class of PageParameters .

I believe that an interface shouldnt be referencing any element from an implementation to be a true extensible interface.