java根据用户生日计算年龄

/**
* 根据用户生日计算年龄
*/
public static int getAgeByBirthday(Date birthday) {
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) + 1;
int dayOfMonthNow = cal.get(Calendar.DAY_OF_MONTH);

cal.setTime(birthday);
int yearBirth = cal.get(Calendar.YEAR);
int monthBirth = cal.get(Calendar.MONTH) + 1;
int dayOfMonthBirth = cal.get(Calendar.DAY_OF_MONTH);

int age = yearNow - yearBirth;

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