    @Test
    public void testFolderSize() throws IOException {

        File dir = Files.createTempDir();
        assertEquals(Helper.folderSize(dir), 0);
        // write to a file
        // see https://stackoverflow.com/a/2885224
        byte data[] = new byte[100];
        FileOutputStream out = new FileOutputStream(dir.toString() + "/test.txt");
        out.write(data);
        out.close();
        long size = Helper.folderSize(dir);
        assertEquals(size, 100);
    }