Simple Question

[first2drown]

I'm reviewing for my mid term which I will be taking this evening. On the practice exam, the following question is given:
Given the following, which line is illegal, assuming that class Watch extends class Clock?
Clock A = new Clock();
Watch B = new Watch();
A=B;
B=A;

On the answer key, he states that B=A; is the illegal statement because a Clock is not necessarily a Watch...but that feels wrong. I feel that in this, A=B; would be the illegal statement. But I also understand that maybe I don't fully understand how an assignment operator would work in this case. Could someone please explain this to me?

[r035198x]

Maybe you could tell us why you feel that A =B is wrong. Think memory locations. A memory location that holds a Clock can hold a Watch because a watch IS-A Clock. That is what the inheritance is-A relationship is all about.

Now suppose we create another Clock, say QuartzTimer. Our QuartzTimer would extend Clock because we said that it IS another Clock.
Now a memory location that can hold a Watch cannot hold a QuartzTimer This is because a QuartzTimer IS NOT a Watch. Since a QuartzTimer is Clock, we have shown that a memory location that holds a Watch does not hold all Clocks.
A and B are pointers to memory locations to Clock and Watch objects.

[JosAH]

@first2drown
Your variable B can point to any clock you can easily carry around: a watch (at your wrist maybe). You most certainly can't make it point to a huge clock; you need a variable A for that; so concluding: you can't say B=A because that could accidentally make your B point to such a huge beast. (see the attachment for a typical "not a watch" ;-)

kind regards,

Jos