IS-A relationship

[jon ruane]

im currently studying for my programmers certification and i've come across this question twice, but with opposite answers.

can anyone clear it up just in case it pops up on the real exam.

TRUE or FALSE:

"is-a relationships always require at least two class types"



(i heard that a 'type' can be 'of its own' type i.e Object is-a Object. one class, therefore the is-a relationship statement above is false)

[Peter Chase]

Gosh, what a silly exam question! They're not all like that, are they?

This looks like an example of a degenerate case. Normally, an "is a" relationship is between two different classes. But the degenerate case is where the two classes are the same: "Dog is a Dog", for example. Such a degenerate case is handled correctly by Java, as in "instanceof" operator.

Dog instanceof Animal --> true
Dog instanceof Dog --> true
Dog instanceof Cat --> false

[marc weber]

Originally posted by Peter Chase:
Gosh, what a silly exam question! They're not all like that, are they? ...

Thankfully, no. I wouldn't expect the real exam to be this poorly worded. This example has come up repeatedly in the SCJP forum (where I'm moving this post).

In addition to the single-class issue (Dog IS-A Dog), also consider that an interface is not a class, but it is a type.

So, for example, if Animal is an interface, and Dog is a class that implements Animal, then an instance of Dog IS-AN Animal (and instanceof will return true).