Exception Handling

[Ravi Dagar]

If I put the highlighted text in try/catch block it is throwing NullPointerException , if I am using command line arguments then also it is showing the same exception.
Please explain!!

public static void main(String []args)
args = null;
args[0] = "test";
System.out.println(args[0]);

[Joe San]

What are you catching in your catch block and why do you think that it should not throw a NullPointerException?

[Bear Bibeault]

It really should be no surprise that if you de-reference something that you just set to null, that it will result in a null pointer exception.

[Ravi Dagar]

Is it so that if we set something to refer to null we cannot refrence it to something other?

[Bear Bibeault]

Sure, you can set the variable to refer to something else, but you haven't.

[fred rosenberger]

of course you can. however "args" is not the same thing as "args[0]".

your 'args' is a box with a number of slots in it (a String array). You then throw the box away with the "args = null".

So when you then try and put something in the first slot of the box, Java is telling you "ummm...you don't have a box any more, so you can't put something in that box's first slot".

[Ravi Dagar]

Okay.Now I am getting it.I actually set the complete refrence variable to null and then assigning its parts some value.
Right?

[Bear Bibeault]

No. Why do you feel the need to set it to null in the first place?

[Ravi Dagar]

Actually I was doing some questions and the code was designed something like this only.
So I just implemented that code only.

[Ravi Dagar]

Hi Fred.
I got to understand your explanation.I actually set the complete args to nulland then I am setting args[0] to some value which obviously throw exception because args[0] does not even exist.

[James Boswell]

Ravi

I would investigate the difference between

args = new String[10]; // you can change 10 to another value but in your case greater than or equal to 1

and

args = null;

Remember, both arrays and Strings are objects.

[Ravi Dagar]

Thank you James.