1. 输入任何一年，2020则输出该年的所有月历表
  2. 输入任何年月，2020 11，则输出该月的月历表
  3. 输入任何年月日，2020 11 14，则输出该天是星期几

  • 已知年月日，求星期几

    int w=(day+2*month+3*(month+1)/5+year+year/4-year/100+year/400+1)%7;

  • 判断是闰年

    (year%400 == 0 ) || (year%4==0 && year%100 != 0)

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import java.util.Scanner;

public class Demo {

	int choice;
	int year = 0;
	int month=0;
	int day;
	int m_d[] = new int[13];
	boolean is_rn;

	public void calendar() {
		while(true) {
		System.out.println("Enter a letter from three choices marked the 1), 2) and 3): ");
		System.out.println("1. year");
		System.out.println("2. year month");
		System.out.println("3. year month day");
		Scanner input = new Scanner(System.in);
		choice = input.nextInt();

		System.out.println("Enter the format you have just chosen: ");
		switch(choice) {
		case 1:
			
			year = input.nextInt();
			this.creat_calendar();
			this.print_year();
			break;
		case 2:
			
			year=input.nextInt();
			month=input.nextInt();
			this.creat_calendar();
			this.print_month();
			break;

		case 3:
			
			year=input.nextInt();
			month=input.nextInt();
			day=input.nextInt();
			this.print_week();
			break;
		default:
			System.out.println("The number is invalid.");
			break;
		
		}
	}
	}
	
	public void creat_calendar() {
		if((year%400 == 0 ) || (year%4==0 && year%100 != 0))
			is_rn=true;
		else
			is_rn=false;
		for(int i = 1; i <= 12; i++) {
			
			if(i==1 || i==3 ||
					i == 5 || i == 7 || i == 8 || i == 10 || i == 12)
				day=31;
			else if(i == 4 || i == 6 || i == 9 || i == 11 )
				day=30;
			else if(i == 2) {
				if(is_rn)
					day=29;
				else
					day=28;
			}
			m_d[i] = day;
			}
		
	}

	public void print_year() {
		
		for(int i = 1; i <=12; i++) {
			System.out.println("the " + (i) +" month");
			for(int j = 1; j <= m_d[i]; j++) {
				if(j % 7 ==0 )
					System.out.println();
				System.out.print(j + "\t");
			}
			System.out.println();
		}
	}
	public void print_month() {
		System.out.println("the " + month + " month");
		for(int i = 1; i <= m_d[month]; i++) {
			if(i % 7 ==0)
				System.out.println();
			System.out.print(i + "\t");
		}
		System.out.println();
	}
	
	public void print_week() {
		int w=(day+2*month+3*(month+1)/5+year+year/4-year/100+year/400+1)%7;
		System.out.println("Today is " + w + "th day of a week");

	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Demo d = new Demo();
		d.calendar();
		
	}

}