Collections and Generics - SCJP Sun Certified Programmer for Java Platform SE6 - page 465

Comparator Sorting
The other version of the sort method in Collections does not require its elements to
implement Comparable . Instead, it uses a separate object of type Comparator to determine
the ordering:
public static <T> void sort(List<T> list, Comparator<? super T> c)
To use this version of Collections.sort , you need to write a class that implements the
generic Comparator interface, making sure the generic type of the Comparator is T or a
parent of T (based on the <? super T> in the method signature). The Comparator interface
declares the following method:
int compare(T o1, T o2)
Defi ne this method so that it returns 0 if o1 equals o2 , a positive number if o1 is greater
than o2 , and a negative number if o1 is less than o2 . The ordering is totally up to you, so
the logic can be anything you defi ne. For example, suppose we have the following class
named Reverse that implements the Comparator interface with an Integer generic type:
public class Reverse implements java.util.Comparator<Integer> {
public int compare(Integer o1, Integer o2) {
return o2 - o1;
}
}
The logic in Reverse is the opposite of the natural order of Integer . For example, 10 is
less than 5 using the logic of Reverse . To use this Comparator , create an instance and pass
it into the sort method along with the List object to be sorted. Study the following code
and see if you can determine its output:
List<Integer> list = new ArrayList<Integer>();
list.add(-5);
list.add(12);
list.add(7);
list.add(7);
list.add(30);
Collections.sort(list, new Reverse());
for(Integer i : list) {
System.out.println(i);
}