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java IS-A relationship exam question confusion

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?

Solution

  • 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.