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Groups > comp.lang.java.programmer > #39435

Re: Character.isIdentifierIgnorable

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On 4/24/2020 6:46 PM, Jeff Higgins wrote:
> Recently I wanted to implement a system of names and a hierarchic
> namespace. My first thought was to use Java names, simply ignoring
> the Java language semantics. So, investigating what exactly is a Java
> name I came across this paragraph in the JLS Chapter 3.8.
> 
> quote
> Two identifiers are the same only if, after ignoring characters that are
> ignorable, the identifiers have the same Unicode character for each 
> letter or digit. An ignorable character is a character for which the 
> method Character.isIdentifierIgnorable(int) returns true. Identifiers 
> that have the same external appearance may yet be different.
> unquote
> 
> So, in my system of names I don't have to follow exactly follow the JLS,
> but thinking more about this paragraph I began to wonder and hope that
> someone here could help me understand.
> 
> What is the motivation for including these ignorable characters in
> Java identifiers?
> 
> Where exactly in the Java environment (tools, language, standard 
> libraries, etc.) are two identifiers checked according to the above 
> specification?
> 
> For instance I can do:
> 
> class IdentifierTest {
>    class _\u0001 {}
>    class _\u0001\u0001 {}
> }
> 
> and javac doesn't complain, which is understandable because those are
> two different names - but then again these are the same identifier if
> the ignorable \u001 are ignored - and even more curious - in Java later 
> than 8 the underscore is a keyword so if the \u0001's are ignored then
> I have two classes with the simple name of _?

My javac complains.

:-)

C:\Work>type Z.java
class Z {
   class _\u0001 {}
   class _\u0001\u0001 {}
}
C:\Work>javac -version
javac 1.8.0_192

C:\Work>javac Z.java
Z.java:2: warning: '_' used as an identifier
   class _\u0001 {}
         ^
   (use of '_' as an identifier might not be supported in releases after 
Java SE
8)
Z.java:3: warning: '_' used as an identifier
   class _\u0001\u0001 {}
         ^
   (use of '_' as an identifier might not be supported in releases after 
Java SE
8)
Z.java:3: error: class Z._ is already defined in class Z
   class _\u0001\u0001 {}
   ^
1 error
2 warnings

Arne

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Thread

Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 18:46 -0400
  Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-24 18:54 -0400
    Re: Character.isIdentifierIgnorable Martin Gregorie <martin@mydomain.invalid> - 2020-04-24 23:07 +0000
    Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 20:12 -0400
      Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-24 20:29 -0400
        Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 20:37 -0400
          Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-24 22:24 -0400
            Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 23:36 -0400
              Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-25 12:57 -0400
                Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-25 13:01 -0400
                Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-25 13:39 -0400
                Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-25 13:51 -0400
                Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-25 14:36 -0400
            Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 23:41 -0400
    Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 20:25 -0400
      Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-24 20:32 -0400
        Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-24 20:42 -0400
          Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-25 09:53 -0400
            Re: Character.isIdentifierIgnorable Arne Vajhøj <arne@vajhoej.dk> - 2020-04-25 12:55 -0400
              Re: Character.isIdentifierIgnorable Jeff Higgins <invalid@invalid.org> - 2020-04-25 13:43 -0400

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