nextDay、beforeDay以及根据nextDay（beforeDay）求解几天后的日期，几天前的日期和两个日期之间的天数

实现代码：

package com.corejava.chap02;

public class Date {
private int year;
private int month;
private int day;

public Date(int year, int month, int day) {
this.year = year;
this.month = month;
this.day = day;
}

public void display(){
System.out.println(year + "/" + month + "/" +day);
}

//isLeapYear
private boolean isLeapYear(){

if(year % 400 == 0) return true;
else if(year%4 == 0 && year % 100 != 0) return true;
else return false;
}
//The max days of the month
private int MaxofMonth(int month){
int max = 0;
switch(month){
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12: max = 31;break;
case 4:
case 6:
case 9:
case 11: max = 30;break;
case 2:if(isLeapYear()) max = 29;else max = 28;break;
default : System.out.println("Date error");//
}
return max;
}

public Date nextDay(){
int nday = day +1;
int nmonth = month;
int nyear = year;
if (nday > MaxofMonth(month)){
nmonth = month +1;
if (nmonth > 12){
nyear += 1;
return new Date(nyear,1,1);
} else {
return new Date(year,nmonth,1);
}
} else {
return new Date(year,month,nday);
}

}

public Date afterDay(int n) {
/*
Date temp = new Date(year,month,day);
for (int i=0; i<n; i++){
temp = temp.nextDay();
}
return temp;
*/

Date temp = null;

if (n==0) temp = new Date(year,month,day);
else if (n<0) temp = afterDay(n+1).previousDay();
else temp = afterDay(n-1).nextDay();
return temp;
}

public Date previousDay(){
int pday = day - 1;
int pmonth = month;
int pyear = year;
if(pday <= 0) {
pmonth = month - 1;
if(pmonth < 1){
pyear = year - 1;
pmonth = 12;
}
pday = MaxofMonth(pmonth);
}
return new Date(pyear,pmonth,pday);
}
private int compare(Date date) {
if(date.year == this.year){
if(date.month == this.month) {
if(date.day == this.day) return 0; // more true
else if(date.day > this.day) return 1;
else return -1;
}
else if(date.month > this.month) return 1;
else return -1;
}
else if(date.year > this.year) return 1;
else return -1;
}
public int sub(Date date) {
Date sdate = date;
int count = 0;
if(compare(date)==0);
else if(compare(date) == 1){
for(;compare(sdate)==1;){
count ++;
sdate = sdate.previousDay() ;
}
} else {
for(;compare(sdate)==-1;){
count ++;
sdate = sdate.nextDay();
}
}
return count;
}
}

验证代码：

package com.corejava.chap02;

public class TestDate{
public static void main(String[] args){
Date date = new Date(2012,12,30);
Date nextDate = date.nextDay();
Date nextNextDay = nextDate.nextDay();

System.out.println("------first Date------");
date.display();
System.out.println("------Next Day------");
nextDate.display();
System.out.println("------Next Next Day------");
nextNextDay.display();

System.out.println("------after 5 days------");
date.afterDay(5).display();

System.out.println("------previousday of 2013/1/1------");
Date previous = nextDate.previousDay();
previous.display();

System.out.println("------before 5 days------");
date.afterDay(-5).display();

Date d = new Date(2012,12,29);
System.out.println("------Test sub()------");
System.out.print("compare with :");
d.display();
System.out.println(date.sub(d));
}
}