java中根据出生日期计算年龄

[code]	/**
* @author 半知半行
*/
public static int getAge(Date birthDay) throws Exception {
if(birthDay == null ){
throw new IllegalArgumentException(
"The birthDay is null.It's unbelievable!");
}
Calendar cal = Calendar.getInstance();

if (cal.before(birthDay)) {
throw new IllegalArgumentException(
"The birthDay is before Now.It's unbelievable!");
}
int yearNow = cal.get(Calendar.YEAR);
int monthNow = cal.get(Calendar.MONTH);
int dayOfMonthNow = cal.get(Calendar.DAY_OF_MONTH);
cal.setTime(birthDay);

int yearBirth = cal.get(Calendar.YEAR);
int monthBirth = cal.get(Calendar.MONTH);
int dayOfMonthBirth = cal.get(Calendar.DAY_OF_MONTH);

int age = yearNow - yearBirth;

if (monthNow <= monthBirth) {
if (monthNow == monthBirth) {
if (dayOfMonthNow < dayOfMonthBirth) age--;
}else{
age--;
}
}
return age;
}