About Question enthuware.ocpjp.v7.2.1305 :

[maurezen]

"It will not compile" is listed as a correct answer.

Why exactly the code will not compile?

The method Book#add(Object) does not override ArrayList<Book>#add, that's right. It overloads it, which is perfectly fine.

[admin]

All generic related information is eliminated in the compiled code. So typing ArrayList to Book in BookList doesn't mean BookList now has add(Book ) method. The compiled code of BookList still has add(Object ) method that it inherits from ArrayList.

So, when you try to add add(Object ) method in BookList, the compiler thinks that you are overriding the same add(Object ) method and complains that it violates the BookList class because it is typed to Book and not Object.

[maurezen]

Got it after I tried to compile the code.

Compiler actually does not think that BookList#add overrides ArrayList#add, it knows that it doesn't and complains about these methods having the same type erasure (which is why this is not overloading):

Test.java:7: error: name clash: add(Object) in BookList and add(E) in ArrayList
have the same erasure, yet neither overrides the other
public boolean add(Object o){
^
where E is a type-variable:
E extends Object declared in class ArrayList
1 error

I think it would be helpful to correct the explanation in ETS somehow to include the point about the same erasure.

[admin]

Yes, we will add it to the explanation.
thank you for your feedback!