ArrayLists - Building Java Programs: A Back to Basics Approach - page 654

10 this .day = day;
11 }
12
13 // Compares this calendar date to another date.
14 // Dates are compared by month and then by day.
15 public int compareTo(CalendarDate other) {
16 if (month != other.month) {
17 return month - other.month;
18 } else {
19 return day - other.day;
20 }
21 }
22
23 public int getMonth() {
24 return month;
25 }
26
27 public int getDay() {
28 return day;
29 }
30
31 public String toString() {
32 return month + "/" + day;
33 }
34 }
One of the major benefits of implementing the Comparable interface is that it
gives you access to built-in utilities like Collections.sort . As we mentioned
previously, you can use Collections.sort to sort an ArrayList<String> but not
to sort an ArrayList<Point> , because the Point class does not implement
Comparable . The CalendarDate class implements the Comparable interface, so, as
the following short program demonstrates, we can use Collections.sort for an
ArrayList<CalendarDate> :
1 // Short program that creates a list of the birthdays of the
2 // first 5 U.S. Presidents and that puts them into sorted order.
3
4 import java.util.*;
5
6 public class CalendarDateTest {
7 public static void main(String[] args) {
8 ArrayList<CalendarDate> dates =
9 new ArrayList<CalendarDate>();
10 dates.add( new CalendarDate(2, 22)); // Washington
11 dates.add( new CalendarDate(10, 30)); // Adams