依据出生日期计算年龄

public class DateGetAge
{
    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)
            {
                if (dayOfMonthNow < dayOfMonthBirth)
                    age--;
            }
            else
            {
                age--;
            }
        }
        return age;
    }
}