Queue<UrlQueueImpl<Long>> mockQueue = new LinkedList<>();

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

        // Execute
        service.updateSessionId(oldSessionId, newSessionId);

        // Verify
        verify(dataHelper).getUrlQueueList(oldSessionId);
        verify(dataHelper).addUrlQueueList(newSessionId, mockQueue);

     */
    @Override
    public void updateSessionId(final String oldSessionId, final String newSessionId) {
        // not MT-safe
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(oldSessionId);
        // overwrite
        dataHelper.addUrlQueueList(newSessionId, urlQueueList);
        dataHelper.removeUrlQueueList(oldSessionId);
    }

    /*
     * (non-Javadoc)
     *

     * Creates a new queue if one doesn't exist.
     * @param sessionId the session ID
     * @return the URL queue for the session
     */
    public synchronized Queue<UrlQueueImpl<Long>> getUrlQueueList(final String sessionId) {
        Queue<UrlQueueImpl<Long>> urlQueueList = urlQueueMap.get(sessionId);
        if (urlQueueList == null) {
            urlQueueList = new LinkedList<>();