In your own words to define...

Interface - An interface in Java is a group of related methods with empty bodies. That, to me, means that it is where a programmer has a starting point where they can begin to define objects and their behavior.

Abstract Class - In Java, it is defined as a class that is missing definitions for one or more methods. It is a class that is defined by a subclass where youcan define the class.

Inner Class - An inner class is nested within anotherclass.