About Question enthuware.ocajp.i.v7.2.1390 :

[SeoaneR]

The questions asks : What can be inserted in the code below so that it will print UP UP UP?

In relation to ((Tone)s).up();
I get a ClassCastException so their is no print output of UP UP UP.
So why is the answer correct?

[admin]

Do you have "implements Tone" in GoodSpeak? Please make sure that you have typed the code exactly as given in the question.

[chiran12345]

I'm confused, why it is possible to cast s to Tone, ((Tone)s).up();

Per my understanding, we can use parent reference to hold child object and can use interface to hold implemented class object. I used 3 fundamental points to remember in Object Type casting: lets say

A b =(C) d; where A and C= class/interface, and b= name of reference variable, d = reference variable name

1 (compile time checking): the type of d and C must have relation either child to parent vice versa or same type. Otherwise we will get compile time error (CE) saying inconvertible types.

2 (compile time checking): C must be either same or derived type of A, otherwise CE saying incompatible types

3(Runitme Checking): Runtime Object type d must be either same or derived type of C, otherwise RE saying ClassCastException.


So, in our case, s and Tone doesn't have relation, why it is not compile time error? Please enlighten me??

[admin]

Point 1 in your list only talks about classes not interface.
In case of an interface, the compiler checks whether it is possible for a reference to point to an object that implements the interface to which it is being casts. So, when you do ((Tone)s).up();, the compiler checks whether s can point to an object of a class that implements Tone. Here, it is possible because type of s is Speak and it is not a final class. So, s can point to a subclass of Speak that implements Tone.

[chiran12345]

Thank you so much