java根据身份证号或生日计算年龄

 private static final int invalidAge = -1;//非法的年龄，用于处理异常。
/**
    *根据身份证号码计算年龄
     * idNumber 考虑到了15位身份证，但不一定存在
     */

    public static int getAgeByIDNumber(String idNumber) {
        String dateStr;
        if (idNumber.length() == 15) {
            dateStr = "19" + idNumber.substring(6, 12);
        } else if (idNumber.length() == 18) {
            dateStr = idNumber.substring(6, 14);
        } else {//默认是合法身份证号，但不排除有意外发生
            return invalidAge;
        }


        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
        try {
            Date birthday = simpleDateFormat.parse(dateStr);
            return getAgeByDate(birthday);
        } catch (ParseException e) {
            return invalidAge;
        }


    }

    /**
     *根据生日计算年龄
     *  dateStr 这样格式的生日 1990-01-01
     */

    public static int getAgeByDateString(String dateStr) {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
        try {
            Date birthday = simpleDateFormat.parse(dateStr);
            return getAgeByDate(birthday);
        } catch (ParseException e) {
            return -1;
        }
    }


    public static int getAgeByDate(Date birthday) {
        Calendar calendar = Calendar.getInstance();

        //calendar.before()有的点bug
        if (calendar.getTimeInMillis() - birthday.getTime() < 0L) {
            return invalidAge;
        }


        int yearNow = calendar.get(Calendar.YEAR);
        int monthNow = calendar.get(Calendar.MONTH);
        int dayOfMonthNow = calendar.get(Calendar.DAY_OF_MONTH);

        calendar.setTime(birthday);


        int yearBirthday = calendar.get(Calendar.YEAR);
        int monthBirthday = calendar.get(Calendar.MONTH);
        int dayOfMonthBirthday = calendar.get(Calendar.DAY_OF_MONTH);

        int age = yearNow - yearBirthday;


        if (monthNow <= monthBirthday && monthNow == monthBirthday && dayOfMonthNow < dayOfMonthBirthday || monthNow < monthBirthday) {
            age--;
        }

        return age;
    }