This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You wouldn't usually use a Comparator with a List. A List "remembers" the order you insert the elements, and if you follow omar al kababji's hint, you can select a different insertion ordering.
A Comparator is used with some Sets so as to order the elements in the order of the elements' "value". Look at Comparator and this Set interface.