Declaration N access control

[Anonymous]

Hi this is the Question from Hardtest
Question
Suppose we have two classes defined as Follows
class Apbase extends Object implements Runnable
class Apderived extends Apbase implements Observer
Given two variables created as Follows
Apbase aBase = new ApBase();
ApDerived aDer = new ApDerived();
which of the Followinf java statements will compile and execute without error?
a. Runnable rn = aDer;
b. Runnable rn2 = (Runnable) aBase;
c. Observer ob = aBase;
d Observer ob2 = (Observer) aBase;
answers given are a and b
But i guess only the correct answer is b,
Becuase my question is when a ADer is upcasted to Runnable rn all the implemented methods Observer cannot be accessible from Runnable rn.hence i assume the answer 'a' will be Wrong??
Please correct me if i am wrong??

[Anonymous]

Doesn't ApDerived also implement (implicitly) Runnable because
its superclass does?
Chengx

[KN]

Hi,
A class can be converted into an interface if that class or any of its superclasses implements that interface. In this case, superclass of ApDerived(i.e, ApBase) implements Runnable. So ApDerived can be converted to Runnable. So, Both A & B are correct.
Hope this helps.
Kiran.

[latha]

A non-final class can be casted to an interface always according to RHC book. Then why not d is correct???pls help me, its sooooo confused.

[satyen sanghavi]

latha ,I think that is true for compilation only not for execution. FYI there are different rules for compilation and execution in casting.