求日期的前一天

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给定一个年月日，返回前一天的日期，年的取值在1812-2017，月的取值为1-12，日的取值为1-31
package Nextdate;import java.util.Scanner;public class NextDate {public static int Leapyear(int year){ if((year%4==0&&year%100!=0)||(year%400==0))              return 1; else return 0;}    public static void main(String args[])    {    int flag;    int preyear=0,premonth=0,preday=0;    System.out.println("请输入此格式日期：年 月 日");    Scanner s=new Scanner(System.in);    int year=s.nextInt();    int month=s.nextInt();    int day=s.nextInt();    flag=Leapyear(year);    if(year<1812||year>2017)    {    System.out.println("年超出了范围");    System.exit(0);    }    if(year>=1812&&year<=2017)    {    if(year==1812&&month==1&&day==1)    {       System.out.println("此为底线日期");       System.exit(0);    }    if(month>12)    {    System.out.println("月份超出了正常值");    System.exit(0);    }    if(month==1||month==3||month==5||month==7||month==8||month==10||month==12)    {    if(day<=0||day>31)    {    System.out.println("该月不存在这一天");    System.exit(0);    }    }    if(month==4||month==6||month==9||month==11)    {    if(day<=0||day>30)    {    System.out.println("该月不存在这一天");    System.exit(0);    }    }    if(month==2)    {    if(flag==1)    {    if(day<=0||day>29)        {        System.out.println("该年二月不存在这一天");        System.exit(0);        }    }    else    {    if(day<=0||day>28)        {        System.out.println("该年二月不存在这一天");        System.exit(0);        }    }    }    if(month==1)    {    if(day>1)    {    preday=day-1;    premonth=month;    preyear=year;    }    else    {    preday=31;    premonth=12;    preyear=year-1;    }    }    if(month==3)    {        if(day==1)    {       if(flag==1)       preday=29;       else       preday=28;       premonth=month-1;       preyear=year;    }        else        {        preday=day-1;        premonth=month;        preyear=year;        }            }    if(month==2||month==4||month==6||month==8||month==9||month==11)    {    if(day==1)    {    preday=31;    preyear=year;    premonth=month-1;    }    else    {    preday=day-1;        premonth=month;        preyear=year;    }    }    if(month==5||month==7||month==10||month==12)    {    if(day==1)    {    preday=30;    preyear=year;    premonth=month-1;    }    else    {    preday=day-1;        premonth=month;        preyear=year;    }    }    System.out.println("前一天为:"+preyear+" "+premonth+" "+preday);    }    }}
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