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

[David Alayachew]

Hello Cay,

Thank you for your response!

I certainly can agree that the advice given by Schildt, putting aside
whether or not it is correct, is simply not helpful to the reader at all.
What benefit does one get from mixing & and pattern-matching? That's
actually part of what I wanted to know by originally posting this question
- I couldn't think of a reason, but wanted to know if there actually was
one.

Also, it was surprising to hear such harsh words spoken about the author. I
have heard this persons praises sung for years, so I am sorry to hear that.

Since it looks like I was wrong, I will say as much in the SO thread. If
anyone else reading has something they can add on the subject, I would
appreciate it!

Thank you for your time and help!
David Alayachew




On Wed, Nov 15, 2023 at 4:55 PM Cay Horstmann <cay.horstmann at gmail.com>
wrote:

> Here is my unofficial clarification.
>
> Herbert Schildt is wrong when he says that "the right side of the & will
> not necessarily be evaluated". It will be. The difference between && and &
> with boolean operands is that & will evaluate both operands, but && will
> not evaluate the right operand if the left one is false.
>
> You are right that in this context, it is plausible to think that iObj
> could have been declared.
>
> Nevertheless, why use the & operator? Herbert Schildt could/should tell
> his readers that there is no reason to use & other than with bit patterns.
> Admittedly it is legal to use & with boolean operands in the very uncommon
> situation of a side effect in the second operand. But that's subtle and may
> well be surprising to readers of your code.
>
> The Java Language Specification lays out rules to trace the scope of
> instanceof pattern definitions with && || ! and ?: operators. See
> https://docs.oracle.com/javase/specs/jls/se21/html/jls-6.html#jls-6.3.
>
> There are no rules for & and | operators. I think that's because they were
> never intended for boolean logic but only for bit patterns (and perhaps
> unfortunately, side effects in boolean conditions). And I wholeheartedly
> agree with the decision not to add that complexity to the language rules.
>
> My advice is to stay away from & and | for boolean operands. They were
> meant to fiddle with bits. For sure, don't use instanceof with those
> operators. With && and ||, and ! and ?:, the JLS rules are sensible and
> unsurprising.
>
> Cheers,
>
> Cay
>
> PS. Many years ago, a C FAQ had this statement (
> https://www.lysator.liu.se/c/c-faq/c-5.html): The cost [of the C standard
> document] is $130.00 from ANSI . . .the Annotated ANSI C Standard, with
> annotations by Herbert Schildt . . . sells in the U.S. for approximately
> $40. It has been suggested that the price differential between this work
> and the official standard reflects the value of the annotations.
>
>
>
>
>
> On 15/11/2023 03.34, David Alayachew wrote:
> > 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 <mailto: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
> <
> 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
> >
>
> --
>
> Cay S. Horstmann | http://horstmann.com | mailto:cay at horstmann.com
>
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