code: private static String getRelativePathName(String absolutePathName,
        String volume) {
      
      absolutePathName = normalizePath(absolutePathName);
      // Get the file names
      if (!absolutePathName.startsWith(volume)) {
        return null;
      }
      // Get rid of the volume prefix
      String fileName = absolutePathName.substring(volume.length());
      if (fileName.charAt(0) == Path.SEPARATOR_CHAR) {
        fileName = fileName.substring(1);
      }
      return fileName;
    }
docs: Get the relative path name with respect to the root of the volume.
  @param absolutePathName The absolute path name
  @param volume Root of the volume.
  @return null if the absolute path name is outside of the volume.