String url = "https://example.com/page";
        Queue<UrlQueueImpl<Long>> mockQueue = new LinkedList<>();

        when(dataHelper.getUrlQueueList(sessionId)).thenReturn(mockQueue);

        // Execute
        service.add(sessionId, url);

        // Verify
        assertEquals(1, mockQueue.size());
        UrlQueueImpl<Long> added = mockQueue.peek();
        assertNotNull(added);