This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
certainly it is a business rule. in fact my Data class contains no reference to rooms what so ever. So it can easily be used to write customers or hotels to a similar database file. You could also decide not to implement this business rule.