Collections and Generics - SCJP Sun Certified Programmer for Java Platform SE6 - page 462

public static <T> void sort(List<T> list, Comparator<? super T> c) sorts the
given List according to the ordering of the given Comparator . All elements in list
must be mutually comparable.
Notice that in the fi rst sort method the generic T is declared as <T extends
Comparable<? super T>> , meaning the list object passed in must contain elements that
implement the Comparable interface (because of extends Comparable ) or whose parent class
implements Comparable (based on <? super T> ).
The second sort method does not have this restriction because the sorting is based on a
Comparator , which is a separate object.
We discuss the difference between Comparable sorting and Comparator sorting next,
starting with a discussion on the Comparable sort method.
Comparable Sorting
Let's look at an example of the fi rst sort method shown previously that uses the natural
ordering of objects. Recall that the natural ordering refers to the behavior of the compareTo
method in the Comparable interface, so the elements being sorted must implement the
Comparable interface. The following program sorts a list of Character objects in their
natural order. Study the following program and see if you can determine its result:
1. import java.util.*;
2.
3. public class CharacterSorter {
4. public static void main(String [] args) {
5. char [] chars = args[0].toCharArray();
6. List<Character> list = new ArrayList<Character>();
7. for(char c : chars) {
8. list.add(c);
9. }
10. Collections.sort(list);
11. for(Character c : list) {
12. System.out.print(c + “ “);
13. }
14. }
15. }
The CharacterSorter program sorts an ArrayList of Character objects. Here is the
sequence of events of the program:
1. Line 5 converts the first command-line argument into a char array.
2. Line 6 creates a new ArrayList<Character> , and the for-each loop on line 7 adds
each char in chars to the list. Because of autoboxing, each char is wrapped in a
Character object.