Tuesday, 24 June 2008

Throw null.

« Dont put a lot in IN. | Main

Eugene posed an interesting question about Alex' blog entry on why the following code does not require main method to throw throwable -

  public class Weird {  
    public static void main(String[] args) {  
      throw null;  
    }  
  }

The fact that it throws a NPE has been discussed on Alex' blog.
But the reason(or my hypothesis) why the compile dos not require you to add a Throwable to throws clause is as follows -

The null reference can always be cast to any reference type.
Checked Exceptions should be declared in the throws clause.
RuntimeExceptions and Descendants are exempt from the above requirement(2)
Rephrasing 2 and 3 as "if the type is not an instance of RuntimeException", which translates to the following code -
```
if(!(type instanceof RuntimeException)) {
 //check throws clause for validity
}else {
 //dont check.
}
```
and we can see that 'null' case would go to the else block.

Q.E.D. ? (you decide)

Posted by sid at 12:27 PM in Thoughts on Java/J2EE

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