Want some official clarification on a quirk about pattern-matching for instanceof

[David Alayachew]

Bumping this one up since I didn't receive a response.

On Fri, Nov 10, 2023 at 11:40 AM David Alayachew <davidalayachew at gmail.com>
wrote:

> Hello Amber Dev Team,
>
> Someone on StackOverflow raised an excellent question about
> Pattern-Matching for instanceof, and I would like to get a response from
> one of you to include in the answer. Here is the link.
>
>
> https://stackoverflow.com/questions/77453336/instanceof-pattern-matching-in-java-not-compiling#77453336
>
> To summarize, the book that they were reading (Java: The Complete
> Reference, 12th Edition by Herbert Schildt) had the following quote.
>
> -----QUOTE_START---- (with minor modifications for readability)
>
> ```java
> Number myOb = Integer.valueOf(9);
> int count = 10;
>
> //                 vv---- Conditional AND Operator
> if ( (count < 100) && myOb instanceof Integer iObj)
> {
>
>     iObj = count;
>
> }
> ```
>
> The above fragment compiles because the if block will execute only when
> both sides of the && are true. Thus, the use of iObj in the if block is
> valid. However, a compilation error will result if you tried to use the &
> rather than the &&, as shown below.
>
> ```java
> Number myOb = Integer.valueOf(9);
> int count = 10;
>
> //                 v----- Bitwise Logical AND Operator
> if ( (count < 100) & myOb instanceof Integer iObj)
> {
>
>     iObj = count;
>
> }
> ```
>
> In this case, the compiler cannot know whether or not iObj will be in
> scope in the if block because the right side of the & will not necessarily
> be evaluated.
>
> ----QUOTE_END----
>
> When compiling the second example, it is exactly as the author says, we
> get told that the variable may not necessarily be in scope. Here is the
> error I get using OpenJDK 22 Early Access.
>
> ```java
> $ java --version
> openjdk 22-ea 2024-03-19
> OpenJDK Runtime Environment (build 22-ea+20-1570)
> OpenJDK 64-Bit Server VM (build 22-ea+20-1570, mixed mode, sharing)
>
> $ javac --version
> javac 22-ea
>
> $ cat abc.java
> public class abc
> {
>
>
>     public static void main(String[] args)
>     {
>
>         Number myOb = Integer.valueOf(9);
>
>         int count = 10;
>
>         if ( (count < 100) & myOb instanceof Integer iObj )
>         {
>
>             iObj = count;
>
>         }
>
>     }
>
> }
>
> $ javac abc.java
> abc.java:15: error: cannot find symbol
>                 iObj = count;
>                 ^
>   symbol:   variable iObj
>   location: class abc
> 1 error
>
> ```
>
> I feel like I have a very good idea of why this might be the case, but I
> lack the terminology to put it into words correctly. Could someone help me
> out?
>
> Thank you for your time and help!
> David Alayachew
>
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