Could be...of course, it could be a LinkedList or some other implementation of List (a Queue implementation with a length limit of ten seems probable to me). Suffice to say, it would be an implementation of java.util.List -- unless, of course, your phone only stores the last ten unique numbers, in which case a java.util.Set might be more appropriate. (Of course, you would want a Set that also provides List-type capibilities). Your phone book would be an implementation of a java.util.Map--probably a java.util.SortedMap, since it not only stores a Name:Number::Key:Value relationship, but it sorts on the Name for display.
Piscis Babelis est parvus, flavus, et hiridicus, et est probabiliter insolitissima raritas in toto mundo.