instanceof with primitive type

[Brian Goetz]

Unfortunately the behavior of instanceof and switch has always been 
different on this matter; instanceof evaluates to false when presented 
with a null, and switch throws NPE when presented with a null.  (They 
both dislike nulls, but one is more tolerant than the other.)

When we added pattern support to switch, we extended this behavior to 
support `null` case labels.  If a switch has a `case null`, then when 
given a null selector, the appropriate case will be run, rather than the 
switch completing abruptly with NPE.  This preserves compatibility with 
existing code, while enabling null to be treated as "just another value" 
if desired.

So the NPE you are seeing is not coming from the unboxing operation in 
`case byte x`, but before we even enter the switch, because the first 
thing switch does is evaluate the selector, and if that succeeds, checks 
for null, and if null, and no `case null` present, the switch completes 
abruptly with NPE.


On 11/8/2023 6:08 PM, Ella Ananeva wrote:
>
> Thank you very much for addressing this issue, Angelos!
>
> I have another question. What is our approach to treating scenarios 
> when we use instanceof of null type with cast?
>
> Byte b = null;
>
> If (!(b instanceof byte))  System.our.println(“Yes”); //prints “Yes”
>
> In the spec,
>
> A reference conversion may not complete normally, namely:
>
>   * An unboxing conversion may raise a NullPointerException
>
> Here, no exception is thrown., so the behavior is as if we had a null 
> type instanceof with a reference type in the pattern. But we have a 
> primitive type in the pattern, so I would assume there should be some 
> conversion going on? Precisely the scenario described in the 
> assertion? BTW if I do a similar thing in switch, there will be an 
> exception:
>
> Byte y = null;
> try {
> switch (y) {
> case byte x: System./out/.println("converted");
> default: System./out/.println("not converted");
>     }
> } catch (NullPointerException ex) {
> System./out/.println("Threw exception");
> }
>
> This code prints “Threw exception.”
>
> Shouldn’t the behavior of switch and instanceof be similar in this 
> situation?
>
> Thank you,
>
> Ella
>
> *From: *Angelos Bimpoudis <angelos.bimpoudis at oracle.com>
> *Date: *Wednesday, November 8, 2023 at 2:56 PM
> *To: *Ella Ananeva <ella.ananeva at oracle.com>, amber-dev at openjdk.org 
> <amber-dev at openjdk.org>
> *Subject: *Re: instanceof with primitive type
>
> Hello Ella,
>
> Indeed. The bullet you showed is now deleted from both. The line you 
> have put in bold, replaces the other two.
>
> Thanks for bringing this up.
>
> Aggelos
>
> ------------------------------------------------------------------------
>
> *From:*amber-dev <amber-dev-retn at openjdk.org> on behalf of Ella 
> Ananeva <ella.ananeva at oracle.com>
> *Sent:* 08 November 2023 19:18
> *To:* amber-dev at openjdk.org <amber-dev at openjdk.org>
> *Subject:* instanceof with primitive type
>
> Hi team,
>
> Reading JEP 455 spec 
> <https://cr.openjdk.org/~abimpoudis/instanceof/jep455-20231030/specs/instanceof-jls.html#jls-15.20.2>, 
> I see this:
>
> *The type of the expression */*RelationalExpression*/*can be a 
> reference type, a primitive type* *or the null type.*
>
> And then, later:
>
> The following rules apply when |instanceof| is the pattern match operator:
>
>   * The type of the expression /RelationalExpression/ must be a
>     reference type or the null type, or a compile-time error occurs.
>
> But I would expect that primitive types in patterns should be good, 
> too. And in the freshest version of JEP_455 JDK 
> <https://mach5.us.oracle.com/mdash/buildIds/2023-11-06-1633093.angelos.bimpoudis.dev>, 
> both examples compile and print “Yes”:
>
> Integer x = 5;
> if (x instanceof int y) System./out/.println("Yes");
>
> int x = 5;
> if (x instanceof Integer y) System./out/.println("Yes");
>
> Shouldn’t the spec be updated?
>
> Thank you,
>
> Ella Ananeva
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/amber-dev/attachments/20231108/d56eb273/attachment-0001.htm>