java 根据生日计算年龄

// 根据生日计算年龄，判断了年月日，比简单的年份上计算更精准
public static int getAge(Date birthDay) throws Exception {
        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) {
                //monthNow==monthBirth
                if (dayOfMonthNow < dayOfMonthBirth) {
                    age--;
                } else {
                    //do nothing
                }
            } else {
                //monthNow>monthBirth
                age--;
            }
        } else {
            //monthNow<monthBirth
            //donothing
        }

        return age;
    }