casting of references

[mohammed mustafa]

can any one explain me the concept behind explicit casting ; in one of khalids mocks there was a question on casting
class A {}
class B extends A {}
class C extends A {}
public class Q3ae4 {
public static void main(String a[])
{
A x=new A();
B y=new B();
C z=new C();
// insert statement here
}
}
Choices :
(i) x=y;
(ii)z=x;
(iii) y=(B)x;
(iv) z=(C)y;
(v) y=(A)y;
I could not understand why can't we cast like the choice 'iii'
above since y is the reference to the sub class of A

[Pravin Panicker]

Look
You can downcast only if you've upcasted somewhere.
So,
this will compile all right
y = (B)x
But at runtime it will throw a ClassCastException.
Simply because you cannot cast an Object to something
it never was.
This would run :
A x = new B();
y = (A)x;
Hope this clarifies your doubt!

[Cherry Mathew]

Pravin,
I think u r making a mistake here this wont compile
A x = new B();
y = (B)x;
please check this is valid at runtime but it wont reach that stage it will be rejected at compile time.
Mohammed, see u r trying to store a super class into a subclass this is not allowed bcoz consider this case
u r deriving sub from super and u have added some extra functions in the subclass and when u call the function using the subclass variable compiler will pass it bcoz it cant checkk the object contained in the variable but at runtime it will give problems and java doesnt allow this thatz the beauty of java.
Hi , Jane Ajith, Bill,mahana dont find u guys in the forum anymore what happened.
Luv Cherry
[This message has been edited by Cherry Mathew (edited January 18, 2001).]

[Weigang Gu]

This involves assignment conversion and explicit casting.
And remember, for assignment conversion of x to y; x must be an instanceof Y (class for object y). If you want to do the other way, you need explicit casting.
For assignment conversion, there is only one check made by compiler. For above example, if you assign y to x, a complier error will occur.
For explicit casting, there are actually two checks. One is made by compiler and one made by runtime. For above example, x = (X) y is okay for compiler b/c y is converted to x by casting. This is syntaxically right to compiler. However, during runtime, another check was made to see if it is possible to convert y to X
and you know you can not (because y is not an instanceof X).
Hope this explains it.

Originally posted by mohammed mustafa:
can any one explain me the concept behind explicit casting ; in one of khalids mocks there was a question on casting
class A {}
class B extends A {}
class C extends A {}
public class Q3ae4 {
public static void main(String a[])
{
A x=new A();
B y=new B();
C z=new C();
// insert statement here
}
}
Choices :
(i) x=y;
(ii)z=x;
(iii) y=(B)x;
(iv) z=(C)y;
(v) y=(A)y;
I could not understand why can't we cast like the choice 'iii'
above since y is the reference to the sub class of A