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.
You need to understand difference between Comparable and Comparator.
Comparable implementations provide a natural ordering for a class, which allows objects of that class to be sorted automatically.
Comparator is an object that encapsulates an ordering. You will use Comparator when you want to sort some objects in an order other than their natural ordering or when you want to sort some objects that don't implement Comparable.