Exception

[Chaitali Deshpande]

hi
I have problem with the following code
import java.io.*;
class A {
A() throws Exception {
System.out.prinln ("Executing class A constructor");
throw new IOException();
}
}
public class B extends A {
B() throws Exception{
System.out.prinln ("Executing class B constructor");
}
public static void main ( String args[] ) {
try {
A a = new B();
} catch ( Exception e) {
System.out.println( e.getMessage() );
}
}
}
the o/p is
Executing class A constructor
null
My problem is why do i get null in o/p
Thanks in advance

[Son Le]

Chaitali,
During the construction of an instance of B, exception is thrown from A() constructor. (B() construtor calls default constructor in A()) Since you don't specify the message, the default is null.
Try the following code for clarification

class A { 
A() throws Exception 
{ 
System.out.println ("Executing class A constructor"); 
throw new IOException("OK");  ////// 
} 
} 

public class B extends A 
{
B() throws Exception
{ 
System.out.println ("Executing class B constructor"); 
} 
public static void main ( String args[] ) 
{
try 
{
A a = new B(); 
} catch ( Exception e) 
{
System.out.println( e.getMessage() ); 
} 
} 
} 

"OK" will print at the end.
SonLe
[This message has been edited by Son Le (edited January 17, 2001).]
[This message has been edited by Son Le (edited January 17, 2001).]

[Chaitali Deshpande]

Thank You.
I have one more Question regarding the above program
Is there any other way by which derived class can
handle exceptions thrown by constructor of parent class

[Wong KK]

Can anyone tell me why the above code runs A's constructor instead of B's?
A a = new B();
For me, it should be interpreted as new a B instance and then implicit cast it to an A instance. So, B's constructor should be run.
Thx. and Regards.
KK.

[Bob Moranski]

It has to do with "constructor chaining". When you call the no args constructor of a subclass, an implicit super() is called within the subclass' constructor.

[Bob Moranski]

It has to do with "constructor chaining". When you call the no args constructor of a subclass, an implicit super() is called within the subclass' constructor.