static private void validateDirectory (
	    File aDirectory
	  ) throws FileNotFoundException {
	    if (aDirectory == null) {
	      throw new IllegalArgumentException("Directory should not be null.");
	    }
	    if (!aDirectory.exists()) {
	      throw new FileNotFoundException("Directory does not exist: " + aDirectory);
	    }
	    if (!aDirectory.isDirectory()) {
	      throw new IllegalArgumentException("Is not a directory: " + aDirectory);
	    }
	    if (!aDirectory.canRead()) {
	      throw new IllegalArgumentException("Directory cannot be read: " + aDirectory);
	    }
	  }