计算某天是星期几的java代码

没有用网上的公式。原理是已知公元1年1月1日是星期一，计算任意日期与这一天之间一共有多少天，再判断是星期几。开始一直有问题，后来发现平年的2月应该是28天，而不是我写的30天，—_—#！

import java.util.Scanner;
public class Test {
    public static void main(String argv[]){
        while(true){
        System.out.println("请输入日期:");            
        Scanner sc=new Scanner(System.in);
        int y=sc.nextInt();
        int m=sc.nextInt();
        int d=sc.nextInt();    
        System.out.println("这一天是星期"+weekday(y, m, d));
        }
    }
    public static boolean isLeapYear(int year){
        boolean flag = false;

        if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
            flag = true;
        else
            flag = false;
        return flag;
    }
    
    public static String weekday(int year,int month,int day){
        long amount=0;
        for(int i=1;i<year;i++){
            if(isLeapYear(i))
                amount++;
        }
        amount+=365*(year-1);
        for(int i=1;i<month;i++){
            amount+=daysofMonth(year, i);
        }
        amount+=day;
        int week=(int) (amount%7);        
        String w="";
        switch(week){
        case 0:w="日";break;
        case 1:w="一";break;
        case 2:w="二";break;
        case 3:w="三";break;
        case 4:w="四";break;
        case 5:w="五";break;        
        case 6:w="六";break;                
        }
        return w;        
    }
    private static int daysofMonth(int year,int month){        
        switch(month){
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:return 31;
        case 4:
        case 6:
        case 9:
        case 11:return 30;
        case 2: if(isLeapYear(year))
            return 29;
        else
            return 28;
        default :return 0;
        }        
    }    
}