interface questions

[Niyas Ahmed Sheikh]

Hi,

I know we can't create an object for an interface. But the I am very confused with the flow of the below program.

[CODE]
class A{}
class B extends A implements E{} //line 1
class C extends A{}
class D extends B{}
interface E{}

public static void main(String args[])
{
A a = new D(); //line 2(I know this, we creating the subclass
object with super class refernce)
C c = new C(); //line 3

E e = (E)a; // line 4 (What we are doing in this step)
B b = (B)e; // line 5 ( - do - )
}
}

To select:
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. The cast on the line 4 is mandatory
d. The cast on the line 5 is not mandatory.

What is forward-referencing?

[Anand Bala]

http://forum.java.sun.com/thread.jspa?threadID=398001&messageID=1731551#1731551

[Niyas Ahmed Sheikh]

Thanks bala. But I think I need more help regd to my question. As for as, forward referencing, I learnt it is not possible to have forward reference for variables, but we can have for methods. Any other details or coding regd this.

Note: Alos please answer my question related to the pgm given.

[Marilyn de Queiroz]

class A{}
class B extends A implements E{}  //line 1
class C extends A{}
class D extends B{}
interface E{}
 
class TestA {
 public static void main(String args[])
 {
   A a = new D();  //line 2(I know this, we creating the subclass  
                            object with super class refernce)     
   C c = new C();  //line 3
  
   E e = (E)a; // line 4 (What we are doing in this step)
   B b = (B)e; // line 5 (       - do -                 ) 
 }
}

Line 4: Unnecessary cast since 'a' "is-a" E slready.
The correct answer is "a. The code compiles without error and runs fine." (if you put the main() method inside a class as I have done)

[Niyas Ahmed Sheikh]

hi,

Thanks for the reply.
So if we want to check whether the cast is necessary or not, we have to check the "is-a" condidtion. Am I right?

[Philip Heller]

Niyas wrote, "So if we want to check whether the cast is necessary or not, we have to check the "is-a" condidtion. Am I right?"

Yes!

(Much more on converting & casting in Chapter 4 of "Complete Java 2 Certification".)

-- Phil