简单的日历制作

import java.util.*;
import java.text.*;
class Demo 
{
    public static void main(String[] args) 
    {
        long weekday = 0;
        int c = 0;
        Scanner scan = new Scanner(System.in);
        String str = "1900-1-1";//1900-1-1是星期一
        System.out.println("请输入年月(年-月):");
        String str1 = scan.next();
        String str2 = str1 + "-1";//为了和1900-1-1到输入的时间计算出差多少天而加上字符"-1"
        String str3 = null;
        long yearDays = charge(str,str2);//计算出1900-1-1到输入的时间相差的天数
        weekday = yearDays%7+1;//计算出一号一星期几
        System.out.println(weekday);
        String str4 = str1.substring(5);//截取字符，截取到后面的月份
        int num = 0;
        num=Integer.valueOf(str4); 
        String str5 = str1.substring(0,4);
        
        int num1 = 0;
        num1 = Integer.valueOf(str5);//把截取到的字符月份转换为整形
        switch (num)
        {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            c = 31;
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            c = 30;
            break;
        case 2:
            {
                //判断是否为闰年
                if ((num1%4 == 0 && num1%100 != 0) || num1 % 400 ==0)
                {
                    c = 29;
                }else {
                    c = 28;
                }
            }
            break;
        }
        System.out.println("星期日\t"+"星期一\t"+"星期二\t"+"星期三\t"+"星期四\t"+"星期五\t"+"星期六\t");
          if(weekday%7!=0){//判断星期天是不是一号，是就不打印*号
             for (int i = 1; i <= weekday ; i++)//打印*号
            {

                System.out.print("  *"+"\t");
            
            }
        }
        int day=0;
        for (int i = (int)weekday  ; i < c+ (int)weekday ; i++ )//打印每天的号数
        {
            day++;//每天号数记录
            if ((i) % 7 == 0)
            {
                System.out.println();
            }
            System.out.print("  "+day+"\t");
        }
    }

    public static long charge(String date1,String date2){
        
        //做时间差
        //将String转为Date做计算
        SimpleDateFormat sd = new SimpleDateFormat("yyyy-MM-dd");
        long charge = 0;
        try{
            Date d1 = sd.parse(date1);  
            Date d2 = sd.parse(date2);
            //时间也是以毫秒为单位。
            charge = (d2.getTime() - d1.getTime()) / (1000*24*60*60);
        }catch(ParseException e){
            System.out.println(e);
        }
        return charge;
    }
}