code: public static IdGenerator getInstance(long nodeId) {
          IdGenerator idGen = cache.get(nodeId);
          if (idGen == null) {
              idGen = new IdGenerator(nodeId);
              idGen.init();
              IdGenerator existing = cache.putIfAbsent(nodeId, idGen);
              idGen = existing != null ? existing : idGen;
          }
          return idGen;
      }
docs: Gets an {@link IdGenerator} instance for a node.
  
  @param nodeId
  @return