Because "j" is a member of the class MainActivity, and does not exist in the OnNewItemAddedListener interface.
In general, I strongly question the design of trying to reach into this thing in this way, but from a purely syntactic perspective,
you should be able to cast the OnNewItemAddedListener to a MainActivity (if you're sure it is one!) and then, from the new expression, access the specific features of MainActivity.
E.g.
But please know this a really dreadful approach to any problem. Totally not "good OO". That, however, is a much bigger question, that we probably cannot address here...
HTH
Simon