java计算多少天后的日期

    /**
     *  计算多少天后的日期
     * @param date   手动输入日期
     * @param days   多少天后
     * @return
     * @throws ParseException
     */
    public static String addDate(String date,int days) throws ParseException {
        Calendar calendar = Calendar.getInstance();
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        Date parse = format.parse(date);
        calendar.setTime(parse);
        int year = calendar.get(Calendar.YEAR);
        int month = calendar.get(Calendar.MONTH);
        int day = calendar.get(Calendar.DAY_OF_MONTH);
        // days  多少天后的日期
        int newDay = day+days;
        calendar.set(Calendar.YEAR,year);
        calendar.set(Calendar.MONTH,month);
        calendar.set(Calendar.DAY_OF_MONTH,newDay);
        return format.format(calendar.getTime());
    }

 

有的时候我会想，再过个100天是几月几日呢？也许你可以翻翻日历，但是如果是1000天、10000天以后呢？为什么我们不写个小程序来专门处理这个问题呢？似乎也不是很复杂……

于是这篇博客应运而生，其实计算的逻辑很简单，就是使用计算机模拟日期计数，比较有特色的地方在于switch-case的“非常规”运用——
/ *
* 文件名：nextdate.c
* 作者：于子豪
* 描述：计算多少天后的日期
* 日期：2019-7-30
  */
#include <stdio.h>
typedef struct date
{
	int year;
	int month;
	int day;
}Date;

//是否是闰年判断
int isbissextile(int year)
{
	if(((year%100!=0)&&(year%4==0))||year%400==0)
	  return 1;
        else return -1;
}

Date todate(Date today, int days)
{
	int day=today.day+days;
	while(1) 
	{
		switch(today.month)
		{
			case 1:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 2:
			  if(day*isbissextile(today.year)>29) {day-=29;today.month++;}
			  else if(day*isbissextile(today.year)<-28) {day-=28;today.month++;}
			  else {today.day=day;return today;}
			case 3:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 4:
			  if(day>30) {day-=30;today.month++;}
			  else {today.day=day;return today;}
			case 5:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 6:
			  if(day>30) {day-=30;today.month++;}
			  else {today.day=day;return today;}
			case 7:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 8:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 9:
			  if(day>30) {day-=30;today.month++;}
			  else {today.day=day;return today;}
			case 10:
			  if(day>31) {day-=31;today.month++;}
			  else {today.day=day;return today;}
			case 11:
			  if(day>30) {day-=30;today.month++;}
			  else {today.day=day;return today;}
			case 12:
			  if(day>31) {day-=31;today.month=1;today.year++;}
			  else {today.day=day;return today;}
			default:break;
		}
	}
	
}

//测试程序
int main(int argc, char *argv[])
{
	Date today={1999,1,11};
        int days=117+365;
        printf("今天是%d.%d.%d：经过%d天后是：",today.year,today.month,today.day,days);
	Date nextday=todate(today,days);
	printf("%d.%d.%d\n",nextday.year,nextday.month,nextday.day);
	return 0;
}

最后，附上一个C-Free5的压缩包：

https://download.csdn.net/download/qq_45467083/11453165

public class Demo3{
 public static void main(String[] args){
   
//  方法1
  f1();
// 方法2
  f2();
 }
 
 public static void f1(){
//  用calendar计算两个日期相隔多少天
  String s1="2011-9-26";
  String s2="2012-7-27";
  
  Calendar calendar1=Calendar.getInstance();
  Calendar calendar2=Calendar.getInstance();
  calendar1.set(2011, 8, 26); //注意月事从0-11；所以8代表9月
  calendar2.set(2012, 6, 27);
  
        long longs=(calendar2.getTimeInMillis()-calendar1.getTimeInMillis()); 
     int in=(int)(longs/(24*60*60*1000));
     System.out.println(in+"天数");
 }
// 计算加上N天后的日期。
 public static void f2(){
 String s3="2011-8-26";
 Calendar calendar=Calendar.getInstance();
// 加上305天后的日期。
 calendar.set(2011, 8, 26+305);
// 方法一
 System.out.println(calendar.get(calendar.YEAR)+"-"+(calendar.get(calendar.MONTH)+1)+"-"+calendar.get(calendar.DAY_OF_MONTH));
//  方法二 
 SimpleDateFormat simp=new SimpleDateFormat("yyyy-MM-dd");
  String date=simp.format(calendar.getTime());
  System.out.println(date);
 }
}

由输入的日期得到若干天后的日期 
输入格式 

输入4个整数y,m,d,k,表示现在的日期，k为多少天后 
输出格式 

输出格式按照yyyy-mm-dd的格式输出k天后的日期

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Scanner;
public class Main {
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);
        int y = scanner.nextInt();
        int m = scanner.nextInt();
        int d = scanner.nextInt();
        int k = scanner.nextInt();
        Calendar ca = Calendar.getInstance();
        SimpleDateFormat simpleDateFormat=new SimpleDateFormat("yyyy-MM-dd");
        ca.set(Calendar.YEAR,y);
        ca.set(Calendar.MONTH,m-1);//java中月份为0到11
        ca.set(Calendar.DAY_OF_MONTH,d);
        ca.add(Calendar.DAY_OF_MONTH, k);
        System.out.println(simpleDateFormat.format(ca.getTime()));
    }
}