java中根据出生日期计算年龄

	/**
	 * @author 半知半行
	 */
    public static int getAge(Date birthDay) throws Exception {
    	if(birthDay == null ){
    		throw new IllegalArgumentException(  
                    "The birthDay is null.It's unbelievable!");
    	}
        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;  
    }