compareTo method
Returns sort order as negative, 0, or positive
Implementation
@override
int compareTo(Object that) {
HDate x = that as HDate;
if (year < x.year) {
return -1;
} else if (year > x.year) return 1;
if (month < x.month) {
return -1;
} else if (month > x.month) return 1;
if (day < x.day) {
return -1;
} else if (day > x.day) return 1;
return 0;
}