rohit hitnalikar wrote:
Hello Gurus,
I am not able to understand the following bit from JVM spec v2.0
which talks about permitted narrowing reference conversions:
*From any class type S to any interface type K, provided that S is not
final and does not implement K. (An important special case is that
there is a narrowing conversion from the class type Object to any interface
type.)
Grrr, sorry about the mostly empty reply before...keystroke problems in
Thunderbird.
Ok, here's a simple case:
public class Base {
....
}
public interface AnInterface {
...
}
public class SubBase extends Base implements AnInterface
{
....
}
Base a = new SubBase();
AnInterface b = (AnInterface)a;
At the point of the cast, the compiler is unable to determine if
reference "a" is an object that implements AnInterface. The base class
Base does not, but a superclass might, so this narrowing must be allowed.
However, if Base was final, then it cannot have subclasses and thus this
narrowing cannot succeed.
*
Can somebody please let me know how to correctly interpret this?
In what cases, do we come across the need to use such a feature ? an
example would be most helpful.
This bit is taken from the following links :
http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#32879
or here
http://java.sun.com/docs/books/jls/second_edition/html/conversions.doc.html#25379
thanks in advance,
Rohit
