Java 判断某一天是这一年的第几天

package Day8_06;

import java.util.*;

public class ClassTest {
    public static void main(String[] args) {
        System.out.print("请输入年：");
        Scanner s1 = new Scanner(System.in);
        int Year = s1.nextInt();
        System.out.print("请输入月：");
        Scanner s2 = new Scanner(System.in);
        int Mon = s2.nextInt();
        System.out.print("请输入日：");
        Scanner s3 = new Scanner(System.in);
        int Day = s3.nextInt();
        int sum = 0;
        
        switch(Mon){
            case 12:sum += 30;
            case 11:sum += 31;
            case 10:sum += 30;
            case 9 :sum += 31;
            case 8 :sum += 31;
            case 7 :sum += 30;
            case 6 :sum += 31;
            case 5 :sum += 30;
            case 4 :sum += 31;
            case 3 :
                if((Year % 4 == 0 && Year % 100 != 0) || (Year % 100 == 0 && Year % 400 == 0)){
                    System.out.println("这一年是闰年！");
                    sum += 29;
                }
                else{
                    System.out.println("这一年是平年！");
                    sum += 28;
                }
            case 2 :sum += 31;
            case 1 :sum += Day;
        }
        System.out.println(Year + "年" + Mon + "月" + Day + "日,是这一年的第：" + sum + "天");
    }
}