Java Reference
In-Depth Information
Software Engineering Observation 16.5
The collections framework methods are polymorphic. That is, each can operate on objects
that implement specific interfaces, regardless of the underlying implementations.
Method
sort
sorts the elements of a
List
, which must implement the
Comparable
inter-
face
. The order is determined by the natural order of the elements' type as implemented
by a
compareTo
method. For example, the natural order for numeric values is ascending
order, and the natural order for Strings is based on their lexicographical ordering
(Section 14.3). Method
compareTo
is declared in interface
Comparable
and is sometimes
called the
natural comparison method
. The
sort
call may specify as a second argument a
Comparator
object that determines an alternative ordering of the elements.
Sorting in Ascending Order
Figure 16.6 uses
Collections
method
sort
to order the elements of a
List
in
ascending
order (line 17). Method
sort
performs an iterative merge sort (we demonstrated a recur-
sive merge sort in Section 19.8). Line 14 creates
list
as a
List
of
String
s. Lines 15 and
18 each use an
implicit
call to the
list
's
toString
method to output the list contents in
the format shown in the output.
1
// Fig. 16.6: Sort1.java
2
// Collections method sort.
3
import
java.util.List;
4
import
java.util.Arrays;
5
import
java.util.Collections;
6
7
public class
Sort1
8
{
9
public static void
main(String[] args)
10
{
11
String[] suits = {
"Hearts"
,
"Diamonds"
,
"Clubs"
,
"Spades"
};
12
13
// Create and display a list containing the suits array elements
14
List<String> list = Arrays.asList(suits);
15
System.out.printf(
"Unsorted array elements: %s%n"
, list);
16
17
Collections.sort(list);
// sort ArrayList
18
System.out.printf(
"Sorted array elements: %s%n"
, list);
19
}
20
}
// end class Sort1
Unsorted array elements: [Hearts, Diamonds, Clubs, Spades]
Sorted array elements: [Clubs, Diamonds, Hearts, Spades]
Fig. 16.6
|
Collections
method
sort
.
Sorting in Descending Order
Figure 16.7 sorts the same list of strings used in Fig. 16.6 in
descending
order. The example
introduces the
Comparator
interface, which is used for sorting a
Collection
's elements in
