B Smith-Mannschott wrote:
> On Wed, Jul 1, 2009 at 11:20, Ben Schulz<ya...@gmx.net> wrote:
>
>>> The problem with Java's null is the instance of bottom in the static
>>> type system, but outside the dynamic type system. Classic puzzler:
>>>
>>> public boolean puzzle(String par) {
>>> return (par instanceof String);
>>>
>>> }
>>>
>>> How to get a false? Pass a null. The type systems don't match.
>>>
>> Yes, this really should return true for null, however since (right
>> now, in Java) null is allowed everywhere, I don't mind so much.
>>
>
> No, it shouldn't return true for null. You're confusing *instanceof*
> with some hypothetical iscompatiblewithtype. Null is not an *instance*
> of anything. There's *nothing* there. It's null.
>
> Sorry, but the sloppy way null is used in typical Java code is one of
> my pet peeves.
>
> // Ben /* oh no, name clash ;-) */
>
I agree: the dynamic type system is right IMO. Null is not an instance
of anything since it doesn't obey any specifications. For example I
expect to be able to call toString() on any object type in Java, but
null won't be too friendly if I do that.
While the compiler treats the value like an instance of the bottom type,
semantically it is not.
Peter
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