Comments on: throw null

By: Neal Gafter

Neal Gafter — Sat, 28 Jun 2008 21:12:05 +0000

Although null is assignable to every reference type, the type of null is not itself a reference type. It was our intent that the requirement that the expression in a throw statement be a reference type was to be removed from the third edition of the JLS, but that change never actually made it into the published version. Thus, this is a javac compiler bug which I introduced in SE 5.

By: Eric Burke

Eric Burke — Wed, 25 Jun 2008 11:47:03 +0000

The comments are funny…arguing about whether to throw NPE or IAE is silly, because *both* are acceptable. The fact is, the documentation for these exceptions leaves much open to interpretation. It is perfectly valid to argue that a null parameter is an illegal argument, and that completely agrees with the JavaDoc comments for IAE. You can also find examples and documentation that suggest NPE is appropriate. This is a classic code style argument all over again. Trust me folks, the most important thing is to fail fast, it really doesn’t matter if you choose IAE or NPE. Either way, your program will abort and which of these two exceptions you choose won’t make it any harder to diagnose the problem.

By: Reinier Zwitserloot

Reinier Zwitserloot — Tue, 24 Jun 2008 12:02:16 +0000

@Eugene: You manually throw an NPE so that the NPE happends in the right place. Lets say I have a constructor with 1 parameter, “String foo”, and part of the contract is that foo isn’t null. But in the constructor, I just store foo, I don’t do anything with it – but in almost any other method, I dereference foo.

I could just write ‘this.foo = foo’ in my constructor and be safe in the knowledge that almost any further usage of my object will result in NPEs automatically, but this is bad coding. The ‘bug’ is setting foo to null in the first place, THATS where your debugging efforts should start (when I call new Doohickey(bar), where bar is null, that’s the bug, not when I later call doohickey.xyzzy() – the NPE stack trace of that event is useless).

By: contrapunctus

contrapunctus — Tue, 24 Jun 2008 10:22:22 +0000

it compiles because null can simply cast into System.Exception:)

By: Luigi R. Viggiano

Luigi R. Viggiano — Tue, 24 Jun 2008 09:22:55 +0000

In my opinion the NullpointerException could be thrown when the JVM tries to call methods on the null reference, as it was a real Throwable instance. Possibly at the fillInStackTrace() method.

This produces the same output (and explains what I think happens):

public class Weird {

public static void main(String[] args) {
((Throwable)null).fillInStackTrace();
}

}