Casting and Interfacses

[Ganesh Pujar]

Hi all

 
class A{}
class B extends A implements E{}//line 1
class C extends A{}
class D extends B{}
interface E{}
public class Question07 {
public static void main(String[] args) {
A a = new D();//line 2
C c = new C();//line 3
E e = (E)a;//line 4
B b = (B)e;//line 5
}
}
 
 
A. The code compiles without error and runs fine. 
B. Compilation error on line 1 because interface E is not yet declared (forward-referencing). 
C. Compilation error on line 4 because class A does not implement interface E. 
D. The cast on line 4 is mandatory. 
E. The cast on line 5 is not mandatory. 
 

According to the answer, the correct answer is B & D 
 

I'm not able to understand it
Plz explane me...

Tq
GMP

[Rajesh MadhanGopal]

I think its option 1 , correct me if am wrong.

[gaurav singhal]

Hi Ganesh,

The ans B is wrong as forward referencing is not in java this code will compile fine the ans should be A) and D).

Gaurav

[Tamara Lopez]

I think the answer should be A & D. It doesnt matter what order the interface is declared in.It could even be in another file.
Order does matter with variables in a class. For example you couldnt say this:
=========
int x = b + c;
int b = 1;
int c = 3;
==========
The casts on both line 4 and 5 are mandatory.On 4 because A doesnt directly implement E. Try E e = new B(); -> This is fine because B directly implements the interface E. But at run time a really is a D so no runtime error. If a was an A = runtime error.

[Edisandro Bessa]

Hi Guys,

Option A is correct : No compile or runtime errors come out.

Options B and C is not correct due to above.

Option D is correct : Although (a) contains a reference to class type D, the compiler doesn't know that. The compiler sees (a) as type of class A, so the cast is mandatory here in otder to allow the code compile.

Option E is incorrect: Same explanation as above.

[Vidhya Hari]

I dont get why the answer is not C...the rules of object reference casting in Philip Heller/Simon Roberts book says,"when castin one class to another or one interface to a class,one of them should be a subclass or implement the other in order for the code to compile ".Here in the example i dont see a chance of it compiling 'cos A does not implement E but has been cast to E,which is wrong.Anyone who is really sure abt this concept,pls help.

[Joyce Lee]

Hi Vidhya,

I dont get why the answer is not C...

Quote from JLS2 - 5.5:

---------------
The detailed rules for compile-time correctness checking of a casting conversion of a value of compile-time reference type S (source) to a compile-time reference type T (target) are as follows:

If S is a class type:

If T is an interface type:
If S is not a final class (�8.1.1), then the cast is always correct at compile time (because even if S does not implement T, a subclass of S might).
---------------

In the sample code posted by Ganesh, class A is the S and interface E is the T. Since class A is not final, it can be extended and its subclasses may implement E.

For example:

class A { }
class B extends A implements E { } 
interface E { }
...
void method(A a) {
 
    // compilation OK - subclasses of A (e.g class B) may implement E
    E e = (E) a; 
}

Example with class A being declared as final:

[B]final[/B] class A { }
interface E { }
...
void method(A a) {
 
   // compilation error - class A cannot be subclassed (no subclasses) and
   // A doesn't implement E
    E e = (E) a; 
}

Example with E as a class:

class A { }
[B]class[/B] E { }
...
void method(A a) {
    
    // compilation error - class E doesn't inherit from class A
    E e = (E) a; 
}

Joyce