code: public void stopBookie(int bookieIndex) {
          Preconditions.checkState(this.servers.size() > 0, "No Bookies initialized. Call startAll().");
          Preconditions.checkState(this.servers.get(0) != null, "Bookie already stopped.");
          val bk = this.servers.get(bookieIndex);
          bk.shutdown();
          this.servers.set(bookieIndex, null);
          log.info("Bookie {} stopped.", bookieIndex);
      }
docs: Stops the BookieService with the given index.
  
  @param bookieIndex The index of the bookie to stop.