code: public static long createFileId(long containerId) {
      long id = BlockId.createBlockId(containerId, BlockId.getMaxSequenceNumber());
      if (id == INVALID_FILE_ID) {
        // Right now, there's not much we can do if the file id we're returning is -1, since the file
        // id is completely determined by the container id passed in. However, by the current
        // algorithm, -1 will be the last file id generated, so the chances somebody will get to that
        // are slim. For now we just log it.
        LOG.warn("Created file id -1, which is invalid");
      }
      return id;
    }
docs: Creates an id for a file based on the given id of the container.
  
  @param containerId the id of the container
  @return a file id based on the given container id