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

[Roger Riggs]

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

For a filename to have an extension, then I think there needs to be a non-empty root part.
So:

  ".z",         root = ".z"        ext = null
  "z.",         root = "z"         ext = ""
  "..z",        root = "."         ext = "z"
  "...foo",     root = "..",       ext = "foo"  
  ".....",      root = "....",     ext = ""


Algorithm wise, you can find the last "." and check if there is a non-empty string before it.
Scanning of the prefix is not necessary.

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