Doubt about casting and final classes

[Emanuele Ghe]

Hi,
this code is taken from exam 2 of examlab:

class B extends A{}
final class C extends B{}
class A{
    public static void main(){
        A a = new B();
        B b = new B();
        C c = new C();
        C[] ca = null;
        a = b;
        b = c;
        A a1 = (B)c;
        Runnable r1 = (Runnable)a;
        Runnable[] ra1 = null;
        A r2 = (A)r1;
        A[] aa1 = (A[])r1; //error
        A[] aa2 = (A[])ra1;
        ra1 = (Runnable[])ca;  //error
    }
}

I don't understand why the marked lines give error.

Can someone explain to me ?

Thanks.

[Henry Wong]

It's quite simple. Those lines are reporting an error because the compiler, can determine at compile time, that those assignments are not possible. I think it is more interesting to note why some of the other lines *are* possible.

Henry

[Emanuele Ghe]

Henry Wong wrote:

It's quite simple. Those lines are reporting an error because the compiler, can determine at compile time, that those assignments are not possible...

Henry

Yeah...I understood that by myself. My question is:
Why the compiler can determine that those assignments aren't possible ?

[Henry Wong]

Emanuele Ghe wrote:
Why the compiler can determine that those assignments aren't possible ?

Line 15 is pretty simple. Think about it. It is possible for any array class to implement the runnable interface? And if so... how?

Line 17 is a bit more complicated. Hint. Class C is a final class.

Henry

[Emanuele Ghe]

Henry Wong wrote:
Line 15 is pretty simple. Think about it. It is possible for any array class to implement the runnable interface? And if so... how?

Got it, thanks.

Henry Wong wrote:
Line 17 is a bit more complicated. Hint. Class C is a final class.

Still I am not sure on why/how the compiler is sure that assignment is wrong.

I'll try to explain what I think, please confirm me:
maybe, since class C is final, the compiler knows that the run-time object assigned to the 'c' reference variable can't be a subclass of C implementing runnable ?

But, if we anlyze this assignment:

  A[] aa2 = (A[])ra1;      

Then the compiler can't know for sure that, at run-time, ra1 could be an array object of a class X such that

X exends A implements Runnable

Am I right ?

[Javier Cortes]

Emanuele Ghe wrote:

Henry Wong wrote:
Line 15 is pretty simple. Think about it. It is possible for any array class to implement the runnable interface? And if so... how?

Got it, thanks.

Henry Wong wrote:
Line 17 is a bit more complicated. Hint. Class C is a final class.

Still I am not sure on why/how the compiler is sure that assignment is wrong.

I'll try to explain what I think, please confirm me:
maybe, since class C is final, the compiler knows that the run-time object assigned to the 'c' reference variable can't be a subclass of C implementing runnable ?

But, if we anlyze this assignment:

  A[] aa2 = (A[])ra1;      

Then the compiler can't know for sure that, at run-time, ra1 could be an array object of a class X such that

X exends A implements Runnable

Am I right ?

Right on. Since 'ca' is a reference variable to a final class, and the target to cast is an interface, it follows that in order to avoid the compiler error class C must implement Runnable. Actually there are a couple more rules involved here under the hood, you can check them here: Java Language Specification (links directly to casting conversion).

[Seetharaman Venkatasamy]

read the last post of william in this thread

hth

[Emanuele Ghe]

Seetharaman Venkatasamy wrote:read the last post of william in this thread

hth

Thanks a lot, is very usefull.

[Seetharaman Venkatasamy]

Emanuele Ghe wrote:Thanks a lot, is very usefull.

you are welcome