Method isValid

ch2/YoilTellerMVC.java:40–45

(int year, int month, int day)

    }
    
    private boolean isValid(int year, int month, int day) {    
    	if(year==-1 || month==-1 || day==-1) 
    		return false;
    	
    	return (1<=month && month<=12) && (1<=day && day<=31); // 간단히 체크 
    }
}