For all other paths, sun.nio.fs.UnixPath implementation (its getParent() and getFileName() methods) seems largely compatible with dirname and basename utilities, standard for any UNIX system.
Still, for the root of a UNIX file system, both Paths.get("/").getParent() and Paths.get("/").getFileName() return null's, which seems inconsistent:
$ basename /
/
$ dirname /
/
Moreover, getFileName() behaves differently from the older java.io.File implementation:
Paths.get("/").toFile().getName();
returns an empty string.
Why?
Why?
Because the javadoc says so. Question already links to the javadoc, so you just have to read it:
getParent() - "Returns the parent path, or null if this path does not have a parent."
The root directory doesn't have a parent, obviously.
getFileName() - "Returns a path representing the name of the file or directory, or null if this path has zero elements."
There are no "elements" in the path /, where "element" is defined as the text between separators.