ArrayLists - Building Java Programs: A Back to Basics Approach - page 653

the difference between month and other.month , because it will be negative when
month is less than other.month , it will be 0 when they are equal, and it will be positive
when month is greater than other.month . So, we can simplify the code as follows:
// still uses only the month, but more compact
public int compareTo(CalendarDate other) {
return month - other.month;
}
It is a good idea to keep things simple when you can, so this version is preferable
to the original one. It returns slightly different values than the earlier version, but it
satisfies the contract of the Comparable interface just as well. However, the code still
has a problem.
Consider, for example, a comparison of April 1 (4/1) and April 5 (4/5). The current
version of compareTo would subtract the months and return a value of 0 , indicating that
these two dates are equal. However, the dates aren't equal: April 1 comes before April 5.
The day of the month becomes important only when the months are equal. If the
months differ, we can use the months to determine order. Otherwise (when the
months are equal), we must use the day of the month to determine order. This com-
mon ordering principle is present in many tasks. We can implement it as follows:
public int compareTo(CalendarDate other) {
if (month != other.month) {
return month - other.month;
} else {
return day - other.day;
}
}
It is still possible for this code to return 0 . Suppose that we have two
CalendarDate objects that both store the date April 5 (4/5). The months are equal,
so the program returns the difference between the days. That difference is 0, so the
program returns 0 , which correctly indicates that the two dates are equal.
Here is a complete CalendarDate class with the compareTo method, two acces-
sor methods, and a toString method:
1 // The CalendarDate class stores information about a single
2 // calendar date (month and day but no year).
3
4 public class CalendarDate implements Comparable<CalendarDate> {
5 private int month;
6 private int day;
7
8 public CalendarDate( int month, int day) {
9 this .month = month;