简单的可视化日历系统

根据提示输入年份和月份，自动显示出日历界面【带有年月验证】

package com.tiger.collection;

import java.util.Scanner;

public class Calendar {

	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);

		System.out.println("请输入年份:");	
		int year = sc.nextInt();	
		//验证年份是输入否正确
		while (year < 1990 || year > 2020) {
			System.out.println("输入的年份不合法，请重新输入！年份区间[1990-2020]");
			year = sc.nextInt();
		}

		System.out.println("请输入月份:");
		int month = sc.nextInt();
		//验证月份是否输入正确
		while (month <= 0 || month > 12) {
			System.out.println("输入的月份不合法，请重新输入！月份区间[1-12]");
			month = sc.nextInt();
		}

		boolean a = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
		//从1900到用户输入[sum = (年+月)]份之前的总天数sum.
		int sum=0;
		for(int i=1900;i