About Question enthuware.ocpjp.v8.2.1808 :

[__JJ__]

You should think about what would be an appropriate implementation of the hashCode method if you use this code for the equals method.

Surely the only possible implementation is that all objects get the same hashCode? Equal objects must have an equal hashCode, no?

[admin]

Correct. You got it

[__JJ__]

OK great, thanks very much.

[NickRobinson]

The answer for the second option states "Therefore, you cannot override it with a method whose argument has a different return type." Should this be "reference type" ? The return type is boolean in both cases.

[admin]

Yes, it should be "reference" not "return". Fixed.
thank you for your feedback!

[crazymind]

Correct. You got it

Can I return isbn or name for hashcode in this case? cause they are both subset of variables that equals use.

[admin]

Yes, but always post code to make sure there is no misunderstanding about your question.