code: public synchronized void pack() throws IOException {
  		if (!validState) {
  			throw new InvalidStateException();
  		}
  		buf.clear();
  		buf.putLong(value);
  		buf.flip();
  		fc.position(0).write(buf); // go begin and write
  		fc.truncate(8).force(true);
  	}
docs: Write value to file and reduce size to minimal
  
  @throws IOException