Date类

一般用来显示时间

• import java.util.Date;
• String str = new Date().toString();
  以字符串形式返回当前时间：Wed Oct 25 09:55:09 CST 2017
  Date类提供before();/after();用来比较一个时间点是否早于另一个时间点，其他的方法大多数现已不推荐使用

GregorianCalendar类

• public class GregorianCalendar extends Calendar;
• import java.util.GregorianCalendar;
• 该类用来进行日期操作
  GregorianCalendar(int year, int month, int day)(0<=month<=11)
  GregorianCalendar(int year, int month, int day, int hour, int minutes, int seconds)(0<=month<=11)
  用给定的日期和时间构造一个Gregorian日历对象
• void set(int year, int month, int day, int hour, int minutes, int seconds)
  将日期和时间设置为新的值
• void add(int field, int value);
  将常量（如Calendar.DAY,Calendar.DAY_OF_WEEK,Calendar.DAY_OF_MONTH)加上一个数值，该数值可以为负数
• void set(int field, int value);
  将常量（如Calendar.DAY,Calendar.DAY_OF_WEEK,Calendar.DAY_OF_MONTH)设置为一个新的数值
• 不同地区的日期会不一样，比如一个星期的第一天，可以用函数Locale.setDefault(Locale.US);设置为美国日历，或者Locale.CHINA,Locale.ITALY等其他设置。该语句要写在main()方法的第一行。一般虚拟机会根据主机的环境来设置默认地区。

Calendar类

• import java.util.Calendar;
• Calendar calendar = Calendar.getInstance();
  Calendar类和单例挺像，但不是单类，因为Calendar可以有多个不同的对象。不能使用new Calendar();

DateFormatSymbols类

• import java.text.DateFormatSymbols;
String[] getShortWeekdays();,如Mon
  String[] getWeekdays();,如Monday
  String[] getShortMonths();,如Jan
  String[] getShortMonths();，如January
  分别用来返回Locale指定的地区的星期几简称，星期几，月份名简称，月份名，参数为Calendar类的星期，月份常量。如Calendar.MONDAY,Calendar.JANUARY。

代码示范——本月日历

package javawork;

import java.text.DateFormatSymbols;
import java.util.*;

public class CalendarTest {
    public static void main(String[] args){
        //set local date
        Locale.setDefault(Locale.US);
        //construct d as current date
        GregorianCalendar d = new GregorianCalendar();
        
        int today = d.get(Calendar.DAY_OF_MONTH);
        int month = d.get(Calendar.MONTH);
        
        //set d to start date of the month
        d.set(Calendar.DAY_OF_MONTH, 1);
        int weekday = d.get(Calendar.DAY_OF_WEEK);
        
        //get first day of week (Sunday in the U.S.)
        int firstDayOfWeek = d.getFirstDayOfWeek();
        
        //determine the required indentation for the first line
        int indent = 0;
        while(weekday != firstDayOfWeek){
            indent++;
            d.add(Calendar.DAY_OF_MONTH, -1);
            weekday = d.get(Calendar.DAY_OF_WEEK);
        }
        
        //print weekday names
        String[] weekdayNames = new DateFormatSymbols().getShortWeekdays();
        do{
            System.out.printf("%4s", weekdayNames[weekday]);
            d.add(Calendar.DAY_OF_MONTH, 1);
            weekday = d.get(Calendar.DAY_OF_WEEK);
        }while(weekday != firstDayOfWeek);
        System.out.println();
        for(int i = 1; i <= indent; i++){
            System.out.println("    ");
        }
        
        d.set(Calendar.DAY_OF_MONTH, 1);
        do{
            int day = d.get(Calendar.DAY_OF_MONTH);
            System.out.printf("%3d",day);
            
            //mark current day with *
            if(day == today) {
                System.out.print("*");
            }
            else{
                System.out.printf(" ");
            }
            
            d.add(Calendar.DAY_OF_MONTH, 1);
            weekday = d.get(Calendar.DAY_OF_WEEK);
            
            //start a new line at the start of the week
            if(weekday == firstDayOfWeek){
                System.out.println();
            }
        }while(d.get(Calendar.MONTH ) == month);//the loop exits when d is day 1 of next month
        
        //print final end of line if necessary
        if(weekday != firstDayOfWeek){
            System.out.println();
        }
    }
}