code: public void close() throws Exception {
  
          if (blockCount == -1) {
              throw new NullPointerException();
          }
  
          executor.shutdown();
          for (Device device : devices) {
              device.close();
          }
          blockSize = -1;
          blockCount = -1;
          LOGGER.info("Closed " + getName() + ".");
      }
docs: {@inheritDoc}