runtime exception

[samdeep aarzoo]

class A
{
}
class B extends A
{

public static void main(String args[])

{
A s = new A();
B p = (B) s ;
}
}

compiles fine but why runtime exception
please help

[Kedar Dravid]

You get an exception, because a reference of subclass cannot possibly refer to a supercalss object at runtime.

[Shaila Vijaya Raja]

From your question it's clear that B is the subclass of A

so that

B Bref = new B();
A Aref = Bref;

is possible.

But as we know

A Aref = new A();
B Bref = Aref; This is not possible which leads to a compile time error.

Actually a superclass object cannot be referred by a subclass reference, eventhough you cast it will compilebut leads a runtime error .

Hope you understand..

With Regards
Shaila Vijaya Raja

[Srinivasa Raghavan]

To make the concept easy think about this.

java.lang.String extends Object.

So we can say all String's are Object , but all Object's may not be a String.
[ July 16, 2005: Message edited by: Srinivasa Raghavan ]

[samdeep aarzoo]

thats cool Srinivasa Raghavan

[Ramakrishna Nalla]

Trying to assign a superclass reference variable to subclass reference variable (child=parent)...is said to be downcasting...and it is possible at run time...

class A
{
}
 
class B extends A
{
 
public static void main(String args[])
 
{
A s = new A();
s=new B(); //to avoid exception...
B p = (B) s ;
}
}

And you have to rememmber :
downcasting an superclass reference variable to an subclass reference
variable is possible only... if the super class variable actually refers to an instance of subclass or an subclass of the subclass....down the hierarchy...

Hope U clear now..

Cheers...