This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates 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.