JAVA的Date类与Calendar类(常用方法)

1.用java.util.Calender来实现
      Calendar calendar=Calendar.getInstance();   
       calendar.setTime(new Date());

        System.out.println(calendar.get(Calendar.DAY_OF_MONTH));//今天的日期 
      calendar.set(Calendar.DAY_OF_MONTH,calendar.get(Calendar.DAY_OF_MONTH)+1);//让日期加1   
      System.out.println(calendar.get(Calendar.DATE));//加1之后的日期Top

(1)用java.text.SimpleDateFormat和java.util.Date来实现
         Date d=new Date();   
          SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");   
          System.out.println("今天的日期："+df.format(d));   
          System.out.println("两天前的日期：" + df.format(new Date(d.getTime() - 2 * 24 * 60 * 60 * 1000)));   
          System.out.println("三天后的日期：" + df.format(new Date(d.getTime() + 3 * 24 * 60 * 60 * 1000))); 

(2)计算某一月份的最大天数
Calendar time=Calendar.getInstance();

time.clear();

time.set(Calendar.YEAR,year); //year 为 int

time.set(Calendar.MONTH,i-1);//注意,Calendar对象默认一月为0          
int day=time.getActualMaximum(Calendar.DAY_OF_MONTH);//本月份的天数

注：在使用set方法之前，必须先clear一下，否则很多信息会继承自系统当前时间

2.Calendar和Date的转化

(1) Calendar转化为Date

Calendar cal=Calendar.getInstance();

Date date=cal.getTime();

(2) Date转化为Calendar

Date date=new Date();

Calendar cal=Calendar.getInstance();

cal.setTime(date);

3.格式化输出日期时间 （这个用的比较多）

Date date=new Date();

SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");

String time=df.format(date);

System.out.println(time);

4.计算一年中的第几星期

(1)计算某一天是一年中的第几星期

Calendar cal=Calendar.getInstance();

cal.set(Calendar.YEAR, 2006);

cal.set(Calendar.MONTH, 8);

cal.set(Calendar.DAY_OF_MONTH, 3);

int weekno=cal.get(Calendar.WEEK_OF_YEAR);

(2)计算一年中的第几星期是几号

SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");

Calendar cal=Calendar.getInstance();

cal.set(Calendar.YEAR, 2006);

cal.set(Calendar.WEEK_OF_YEAR, 1);

cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);

System.out.println(df.format(cal.getTime()));

输出:

2006-01-02

5.add()和roll()的用法(不太常用)

(1)add()方法

SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");

Calendar cal=Calendar.getInstance();

cal.set(Calendar.YEAR, 2006);

cal.set(Calendar.MONTH, 8);

cal.set(Calendar.DAY_OF_MONTH, 3);

cal.add(Calendar.DATE, -4);

Date date=cal.getTime();

System.out.println(df.format(date));

cal.add(Calendar.DATE, 4);

date=cal.getTime();

System.out.println(df.format(date));

输出：

    2006-08-30

    2006-09-03

(2)roll方法

cal.set(Calendar.YEAR, 2006);

cal.set(Calendar.MONTH, 8);

cal.set(Calendar.DAY_OF_MONTH, 3);

cal.roll(Calendar.DATE, -4);

date=cal.getTime();

System.out.println(df.format(date));

cal.roll(Calendar.DATE, 4);

date=cal.getTime();

System.out.println(df.format(date));

输出：

    2006-09-29

    2006-09-03

可见，roll()方法在本月内循环，一般使用add()方法；

6.计算两个任意时间中间的间隔天数（这个比较常用）

(1)传进Calendar对象

    public int getIntervalDays(Calendar startday,Calendar
endday)...{      
        if(startday.after(endday))...{

            Calendar
cal=startday;

            startday=endday;

            endday=cal;

        }   
        long
sl=startday.getTimeInMillis();

        long
el=endday.getTimeInMillis();

      
        long
ei=el-sl;          
        return
(int)(ei/(1000*60*60*24));

    }
(2)传进Date对象

    public int getIntervalDays(Date startday,Date
endday)...{       
        if(startday.after(endday))...{

            Date
cal=startday;

            startday=endday;

            endday=cal;

        }       
        long
sl=startday.getTime();

        long
el=endday.getTime();      
        long
ei=el-sl;          
        return
(int)(ei/(1000*60*60*24));

    }

(3)改进精确计算相隔天数的方法

    public int getDaysBetween (Calendar d1,
Calendar d2) ...{

        if
(d1.after(d2)) ...{