This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
I think that's just your decision. In my application customer id must be exactly 8, otherwise a message is shown. That's why I stored it in a String and not an Integer But less than 8 digits will be fine too (I believe Roberto implemented it that way)