基础JAVA编程题目求大神详细解答，谢谢！

import java.util.Scanner;
public class Days {

public static void main(String[] args) {
int years,month;
Scanner reader=new Scanner(System.in);
years=reader.nextInt();
month=reader.nextInt();
switch(month){
case 1:System.out.println("31");break;
case 2:if((years%400==0)||(years%100!=0&&years%4==0))
System.out.println("29");
else System.out.println("28");
break;

case 3:System.out.println("31");break;
case 4:System.out.println("30");break;
case 5:System.out.println("31");break;
case 6:System.out.println("30");break;
case 7:System.out.println("31");break;
case 8:System.out.println("31");break;
case 9:System.out.println("30");break;
case 10:System.out.println("31");break;
case 11:System.out.println("30");break;
case 12:System.out.println("31");break;

}
}

}