Returns an object that uniquely identifies the given file, or
null if a file key is not available. On some platforms or file systems
it is possible to use an identifier, or a combination of identifiers to
uniquely identify a file. Such identifiers are important for operations
such as file tree traversal in file systems that support symbolic links or file systems
that allow a file to be an entry in more than one directory. On UNIX file
systems, for example, the device ID and inode are
commonly used for such purposes.
The file key returned by this method can only be guaranteed to be unique if the file system and files remain static. Whether a file system re-uses identifiers after a file is deleted is implementation dependent and therefore unspecified.
File keys returned by this method can be compared for equality and are
suitable for use in collections. If the file system and files remain static,
and two files are the
null file keys, then their file keys are equal.