java IS-A relationship exam question confusion

Question

From MasterExam:

Which statements are true? (Choose all that apply)

A. is-a relationship always rely on inheritance
B. is-a relationship always rely on instance variables
C. is-a relationship always require at least two class types
D. is-a relationship always rely on polymorphism
E. is-a relationship are always tightly coupled

Correct answers: A, C, D

I don't see how any of A, C or D are correct.

An Object object IS-A Object. A String object IS-A String. There is only one class type in each of these statements and no inheritance or polymorphism applies.

Is my rationale wrong or are the answers incorrect?

Answer 1

"Relationship" refers to a relationship between two classes. An is-a relationship is a type of relationship that uses inheritance (as opposed to e.g. has-a, which uses composition). For instance, a String is-a Object. A class can't inherit from itself, which implies C. As a side note, a class could have composition (has-a) with itself. E.g. a Person could have another Person as a mother field.

Any time you have inheritance, an instance of the subclass can be used as an instance of the superclass. That's polymorphism, which means D is also right.

Answer 2

Your rationale is slightly off, as this relationship applies to classes, not to objects.

A string IS-A object since String inherits from Object. Similarly a FileOutputStream IS-A OutputStream IS-A Object.

IS-A is a relationship between classes, no between classes and objects.

Answer 3

You need to see it a bit different:

• A cat is-a animal
• A car has-a engine
• A String is-a object
• A String has-a char array

Try to see it that way, it should become clearer now.

Answer 4

Sorry, your rational is wrong. They are talking about two different classes. For example a String is-a Object.

Answer 5

A. That a String is a String is... a tautology, considered obvious. But looking at the API documentation, you find that a java.lang.String is also a java.lang.Object. In fact, all Java classes inherit from java.lang.Object. They inherit basic properties of Object and add some others of their own. That's what the is-a relationship is all about.

C. Again, the tautology about any object being itself is of interest to Zen Buddhists but not language designers and other Computer Scientists. So to have an "is-a" relationship, you need two distinct classes.

D. I'm not so sure here. If I were asked, I'd say polymorphism depends on the "is-a" relationship, not the other way around.

Answer 6

I think points A & C are pretty clear by now... regarding D.though ,it may be technically possible to extend a class with no instance behavior or property..(blank or maybe only with static elements) ...but that in principle kind of defeats the purpose of "extending" classes as you want to inherit behavior/functionality from classes higher up the inheritance tree. So,i would say option D. is correct(almost) :) (if we are not going too technical).btw this can be an interesting discussion.