public void test_doneOnNonExistentKey() {
        String key = "non_existent";
        crawlerStatsHelper.done(key);

        assertNull(localLogMsg.get());
    }

    @Test
    public void test_discardOnNonExistentKey() {
        String key = "non_existent";
        crawlerStatsHelper.discard(key);

        assertNull(localLogMsg.get());
    }

    static class TestUrlQueue implements UrlQueue<String> {