java根據生日精確計算年齡

package getAge;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

/**
 * 根據用戶生日精確計算年齡
 * 用Calender對象取得當前日期對象--從對象中分別取出年月日
 * @author Administrator
 *
 */
public class getAgeByBirthday{
    public static int getAgeByBirth(Date birthday){        
        //Calendar：日歷
        /*從Calendar對象中或得一個Date對象*/
        Calendar cal = Calendar.getInstance();
        /*把出生日期放入Calendar類型的bir對象中，進行Calendar和Date類型進行轉換*/
        Calendar bir = Calendar.getInstance();
        bir.setTime(birthday);
        /*如果生日大於當前日期，則拋出異常：出生日期不能大於當前日期*/
        if(cal.before(birthday)){
            throw new IllegalArgumentException("The birthday is before Now,It's unbelievable");
        }
        /*取出當前年月日*/
        int yearNow = cal.get(Calendar.YEAR);
        int monthNow = cal.get(Calendar.MONTH);
        int dayNow = cal.get(Calendar.DAY_OF_MONTH);
        /*取出出生年月日*/
        int yearBirth = bir.get(Calendar.YEAR);
        int monthBirth = bir.get(Calendar.MONTH);
        int dayBirth = bir.get(Calendar.DAY_OF_MONTH);
        /*大概年齡是當前年減去出生年*/
        int age = yearNow - yearBirth;
        /*如果出當前月小與出生月，或者當前月等於出生月但是當前日小於出生日，那么年齡age就減一歲*/
        if(monthNow < monthBirth || (monthNow == monthBirth && dayNow < dayBirth)){
            age--;
        }
        return age;
    }
    /*main方法測試*/
    public static void main(String[] args){
        SimpleDateFormat sft = new SimpleDateFormat("yyyy-MM-dd");
        String sftBirth = "1980-4-25";
        Date date = null;
        try{
            date = sft.parse(sftBirth);
        }catch(Exception e){
            e.printStackTrace();
        }
        int age = getAgeByBirthday.getAgeByBirth(date);
        System.out.print("年齡=" + age + "歲");
    }
}