RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v2]

[Alan Bateman]

On Fri, 29 Jan 2021 23:13:55 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Please review this proposed change to add a method `java.nio.file.Path.getExtension()`. This was initially discussed in the thread http://mail.openjdk.java.net/pipermail/nio-dev/2018-February/004716.html. This method would return the filename extension of the file name of the `Path`. The extension is defined to be the portion of the file name after the last dot `(‘.’)`.
>> 
>> The definitions of file extension for about fifteen platforms and languages were surveyed to try to find a reasonable compromise for the definition of extension. The most common definition was the last segment of the name including and after the last dot. The second definition omitted the last dot from the extension. Java-related platforms all exclude the last dot. (One divergent definition in the internal Java NIO method `AbstractFileTypeDetector.getExtension(String)` defines the extension as the part after the *first* dot.)
>> 
>> All examined cases define the extension to be an empty string if it cannot be determined. None of these cases used `null` to represent an indeterminate extension.
>> 
>> Little in the way of specifying behavior for special cases (consisting mainly of file names with one or more leading dots) was found. Most definitions concern themselves only with the last dot and what comes after it and ignore leading dots altogether. A few definitions ignore a leading dot at the zeroth character. The current proposal ignores a dot at character zero.
>> 
>> The behavior of the proposed method for some example cases is as:
>> 
>> .              -> 
>> ..             -> 
>> .a.b           -> b
>> ......         -> 
>> .....a         -> a
>> ....a.b        -> b
>> ..foo          -> foo
>> test.rb        -> rb
>> a/b/d/test.rb  -> rb
>> .a/b/d/test.rb -> rb
>> foo.           -> 
>> test           -> 
>> .profile       -> 
>> .profile.sh    -> sh
>> ..foo          -> foo
>> .....foo       -> foo
>> .vimrc         -> 
>> test.          -> 
>> test..         -> 
>> test...        -> 
>> foo.tar.gz     -> gz
>> foo.bar.       -> 
>> image.jpg      -> jpg
>> music.mp3      -> mp3
>> video.mp4      -> mp4
>> document.txt   -> txt
>> 
>> If the specification can be agreed upon, then a test will be added to this PR and a corresponding CSR will be filed.
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8057113: Changes pursuant to PR conversation

src/java.base/share/classes/java/nio/file/Path.java line 259:

> 257:      * characters, does not contain a dot, only the first character is a dot,
> 258:      * or the last character is a dot.
> 259:      *

Thanks for bringing this one back. The survey and the current proposal seems reasonable.

The spec will need cover the case that the Path doesn't have a file name (getFileName can return null so I assume the prototype implementation will NPE in that case). Are you planning to return null or the empty String for this case?

"file name string" needs to be replaced with "the String representation of the file name". This is significant aspect of the proposal in that it aligns the method with toString rather than other operations that preserve the platform representation of the path.

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PR: https://git.openjdk.java.net/jdk/pull/2319