RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v3]

[Brian Burkhalter]

On Mon, 1 Feb 2021 18:54:54 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8057113: Handle getFileName() == null; revise spec
>
> Marked as reviewed by rriggs (Reviewer).

**Alternate Specifications**

Alternate specifications are possible. Ignoring more precise naming, e.g., `getExtensionOrElseEmptyString()` for the original version above, some possible options are:

1. `String getExtension(`) returns the extension or the empty string if no extension found
2. `String getExtension()` returns the extension or `null` if no extension found
3. `String getExtension(String default)` returns the extension or `default` if no extension found (may be `null`)
4. `Optional<String> getExtension()` returns the extension, if any, contained in an `Optional`

Option 1 is that originally proposed. Option 2 is identical but returns null instead of the empty string if there is no extension. This is consistent with other methods in `Path` such as `getRoot()`, `getParent()`, and `getFileName()`. Option 3 allows the possibility to supply the value to be returned if no extension is found. Option 4 wraps the extension string or `null` in an `Optional`.

If one wanted to be able to retain the internal structure of the extension as a `Path`, then one could instead define two methods:

`Path Path.getExtension()`
`String Path.getExtensionString()`

as part of the point is the user not being obliged to call `toString()`. Here `getExtension()` would be implemented in the platform-specific `Path` classes, and `getExtensionString()` would be a default method in the `Path` interface.

In all cases, when the `Path` represents a root component, then the extension is indeterminate (empty or `null`, depending).

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