根据生日自动计算年龄

public static  int getAge(Date birthDay) throws Exception {
        Calendar cal = Calendar.getInstance();
        if (cal.before(birthDay)) { //出生日期晚于当前时间，无法计算
            throw new IllegalArgumentException(
                    "The birthDay is before Now.It's unbelievable!");
        }
        int yearNow = cal.get(Calendar.YEAR);  //当前年份
        int monthNow = cal.get(Calendar.MONTH);  //当前月份
        int dayOfMonthNow = cal.get(Calendar.DAY_OF_MONTH); //当前日期
        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;   //计算整岁数
        if (monthNow <= monthBirth) {
            if (monthNow == monthBirth) {
                if (dayOfMonthNow < dayOfMonthBirth) age--;//当前日期在生日之前，年龄减一
            }else{
                age--;//当前月份在生日之前，年龄减一
            } } return age;
    }