@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);
}