Page 879 - AP Computer Science A, 7th edition

students are either graduates or undergraduates. Having the Student class abstract guarantees that you won’t create an instance of a Student (who is neither a graduate nor an undergraduate). Choice E has a major design flaw: making Graduate an abstract class means that you can’t create any instances of Graduate objects. Disaster! If the keyword abstract is removed from choice E, it becomes a fine design, as good as that in choice B. Once Student has implemented all the common PersonalInformation methods, these are inherited by each of the subclasses.