public function scanDir()
    {
        // only possible in directories
        if (!$this->isDir() || !$this->exists()) {
            return null;
        }

        $fileList = [];
        $fileNameList = scandir($this->absoluteFileName);
        foreach ($fileNameList as $fileName) {
            // do not add . and ..
            if ($fileName === "." or $fileName === "..") {
                continue;
            }
            $absoluteFileName = $this->absoluteFileName . "/" . $fileName;
            $fileList [] = new File ($absoluteFileName);
        }

        return $fileList;
    }