Java根据用户生日计算年龄

/**
	 * Java根据用户生日计算年龄
	 */
	public int getAgeByBirthday(Date birthday) {
		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) + 1;
		int dayOfMonthNow = cal.get(Calendar.DAY_OF_MONTH);

		cal.setTime(birthday);
		int yearBirth = cal.get(Calendar.YEAR);
		int monthBirth = cal.get(Calendar.MONTH) + 1;
		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 {
				// monthNow>monthBirth 
				age--;
			}
		}
		return age;
	}