6.1. answer these questions: a) when should abstract classes be used ? give a general answer and an example. b) explain when an abstract class cannot be used in a java program and an interface is the only choice. c) the section class used in a text editor application includes a text attribute, contains a collection of objects of class section or its subclasses, and has a method called display that displays the text of this section object and of all other sections referenced in its collection. what design pattern does class section implement ? explain.
