code: public Map<Long, QueueItem> getBackupMap() {
          if (backupMap == null) {
              if (itemQueue != null) {
                  backupMap = createHashMap(itemQueue.size());
                  for (QueueItem item : itemQueue) {
                      backupMap.put(item.getItemId(), item);
                  }
                  itemQueue.clear();
                  itemQueue = null;
              } else {
                  backupMap = new HashMap<Long, QueueItem>();
              }
          }
          return backupMap;
      }
docs: Return the map containing queue items when this instance is
  a backup replica.
  The map contains both items that are parts of different
  transactions and items which have already been committed
  to the queue.
  
  @return backup replica map from item ID to queue item