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));
    }
}