面向对象的第二次pta作业第四题：求前N天

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner input = new Scanner(System.in);
        int year = input.nextInt();
        int month = input.nextInt();
        int day = input.nextInt();
        int n = input.nextInt();
        
        if(!checkInputValidity(year, month, day, n)) {
            System.out.println("Wrong Format");
            System.exit(0);
        }
        
        
        
        nextDate(year, month, day, n);
        /*nextDate()
        System.out.println("Next date is:"+year+"-"+month+"-"+day);
        也可以*/
    }
    
    public static boolean isLeapYear(int year) {//判断year是否为闰年，返回boolean类型；
        boolean y1 = year%4 == 0;
        boolean y2 = year%100 != 0;
        boolean y3 = year%400 == 0;
        
        if(y1&&y2||y3)
            return true;
        else 
            return false;
    }
    
    
    public static boolean checkInputValidity(int year,int month,int day,int n) {//判断输入日期及n的值是否合法，返回布尔值
        if(year>2020||year<1820||month>12||month<1||day<1||day>31) {
            return false;
        }
        if(n>10||n<(-10))
            return false;
        switch (month) {
        case 4:case 6:case 9:case 11:
            if(day>30) 
                return false;
            break;
        case 2:
            if(isLeapYear(year)&&day>29) 
                return false;
            if(day>28) 
                return false;
            break;
        }
        return true;
    }
    
    public static void nextDate(int year,int month,int day,int n) {  //求输入日期的前n天
        int arr[] = {0,31,28,31,30,31,30,31,31,30,31,30,31};
        
        if(isLeapYear(year))
            arr[2]++;
        if((day-n)<=arr[month]&&(day-n)>=1)//
            day = day-n;
        else if((day-n)>arr[month]&&month<12) {//
            day = day-n-arr[month];
            month++;
        }
        else if((day-n)<1&&month>1) {
            month--;
            day = day-n+arr[month];
        }
        else if((day-n)>arr[month]&&month==12) {
            day = day-n-arr[month];
            month = 1;
            year++;
        }
        else if((day-n)<1&&month==1) {
            month=12;
            day = day-n+arr[month];
            year--;
        }
        System.out.println(n+" days ago is:"+year+"-"+month+"-"+day);
    }
    

}

 

 分不清c和Java的小熊同学被日期代码的3.0版本吓懵了