RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v18]

[Brian Burkhalter]

On Mon, 1 Aug 2022 21:54:22 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Resurrection of the proposal to add a method to obtain the filename extension originated in PR [2319](https://github.com/openjdk/jdk/pull/2319).
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8057113: Remove API note

The principal unresolved issue here concerns leading period characters. There is consensus that the extension of a file name string such as `.foo` is `null`, but not as to whether the extension of a file name string such as `...foo` is `null` or `foo`. My intent was that if there were multiple leading periods, then all but the last would be treated as if they were alphabetic, hence the behavior for `...foo` would be the same as for `xy.foo`. I could be persuaded otherwise, and the alternative is likely simpler.

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PR: https://git.openjdk.org/jdk/pull/8066