How to use concurrenthashmap in executorcompletionservice?

List<Map<Long, Node>> aNodeMapList = new ArrayList<>();
Map<String, List<Map<String, Object>>> cacheRingMap=new ConcurrentHashMap<>();
for (Ring startRing : startRings) {
       for (Ring endRing : endRings) {
           Map<String, Object> nodeMapResult = getNodeMapResult(startRing, endRing,cacheRingMap);
           Map<Long, Node> nodeMap = (Map<Long, Node>) nodeMapResult.get("nodeMap");
           if (nodeMap.size() > 0) {
               aNodeMapList.add(nodeMap);
           }
       }
   }

int totalThreadCount= startRings.size()*endRings.size();
ExecutorService threadPool2 =  Executors.newFixedThreadPool(totalThreadCount>4?4:2);
CompletionService<Map<String, Object>> completionService = new ExecutorCompletionService<Map<String, Object>>(threadPool2);
for (Ring startRing : startRings) {
        for (Ring endRing : endRings) {
            completionService.submit(new Callable<Map<String, Object>>() {
                    @Override
                    public Map<String, Object>  call() throws Exception {
                        return getNodeMapResult(startRing, endRing,cacheRingMap);
                    }
                });
        }
    }

    for(int i=0;i<totalThreadCount;i++){
    Map<String, Object> nodeMapResult=completionService.take().get();
    Map<Long, Node> nodeMap = (Map<Long, Node>) nodeMapResult.get("nodeMap");
    if (nodeMap.size() > 0) {
        aNodeMapList.add(nodeMap);
    }
    }