Why can I throw null in Java?

Question

When running this:

public class WhatTheShoot {

    public static void main(String args[]){
        try {
            throw null;
        } catch (Exception e){
            System.out.println(e instanceof NullPointerException);
            System.out.println(e instanceof FileNotFoundException);
        }
    }
}

The response is:

true  
false

Which was fairly stunning for me. I would have thought this would net a compile-time error.

Why can I throw null in Java, and why does it upcast it to a NullPointerException?

(Actually, I don't know if it is an "upcast", given I'm throwing null)

Aside from a really really stupid interview question (please nobody ask this in an interview) I cannot see any reason to throw null. Maybe you want to be fired, but that's... I mean, why else would anyone throw null?

Fun fact IntelliJ IDEA 12 tells me that my line, e instanceof NullPointerException, will always be false. Which isn't true at all.

Answer 1

It looks like it's not that null is treated as a NullPointerException, but that the act of attempting to throw null itself throws a NullPointerException.

In other words, throw checks that its argument is nonnull, and if it is null, it throws a NullPointerException.

JLS 14.18 specifies this behavior:

If evaluation of the Expression completes normally, producing a null value, then an instance V' of class NullPointerException is created and thrown instead of null. The throw statement then completes abruptly, the reason being a throw with value V'.