4.24 Java4130: the comparator interface, part 5  (Page 3/4)

(The programs in previous modules that sorted the array did not rearrange the contents of the list. Only the contents of the array were rearranged.)

Thus, the relationship between an element in the list and the index associated with that element can change as a result of the sorting operationshown in Listing 4 .

Following the sort, when an iterator is used to access the elements, the elements will be returned by the iterator in the newly-sorted order.

The Collections class

Despite the similarity of the names, the Collections class is different from the Collection interface. Here is part of what Oracle has to say about the Collections class:

"This class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate oncollections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends."

The sort method

The Collections class provides a large number of very interesting and useful methods, such as binarySearch , copy , reverse , and reverseOrder . (The reverseOrder method will be examined in the next module.)

One of the static methods of the Collections class is the sort method. One overloaded version of the sort method can be used to sort a list into the natural ordering of its elements. Another overloaded version sorts a list according to the order induced by a Comparator .

Here is part of what Oracle has to say about this second version of the sort method that uses a Comparator :

public static void sort(List list, Comparator c) "Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparableusing the specified comparator ...

The specified list must be modifiable, but need not be resizable. This implementation dumps the specified list into an array,sorts the array, and iterates over the list resetting each element from the correspondingposition in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place."

Also uses an array

I find it interesting that the sort method uses an array as an intermediary in the sorting process. However, the difference between thisapproach and the approach involving arrays shown in previous modules is given by the following excerpt from the above quotation:

"iterates over the list resetting each element from the corresponding position in the array"

In other words, after sorting the array, the sort method uses the sorted results in the array to rearrange the positions of the elements in thelist, resulting in a sorted list.

A flexible approach to sorting

Thus, the sort method of the Collections class can be used to sort the elements in a list using whatever set of comparison rules you programinto the compare method of the Comparator object. Furthermore, it doesn't matter how the list is actually implemented so long as it properlyimplements the List interface.

The Comparator

The code in Listing 5 shows the class from which the Comparator object was instantiated.