Java--Date operations

Calendar calendat = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
String dateStr = sdf.format(calendar.getTime());

2.String to Calendar

String str="2012-5-27";
SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
Date date =sdf.parse(str);
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);

3.Date conversion to String

SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
String dateStr=sdf.format(new Date());

4.String conversion Date

String str="2012-5-27";
SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
Date date= sdf.parse(str);

5.Date to Calendar

Calendar calendar = Calendar.getInstance();
calendar.setTime(new java.util.Date());

6.Calendar converted to Date

Calendar calendar = Calendar.getInstance();
java.util.Date date =calendar.getTime();

7.Convert String to Timestamp

Timestamp ts = Timestamp.valueOf("2012-1-14 08:11:00");

8.Date to TimeStamp

SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String time = df.format(new Date());
Timestamp ts = Timestamp.valueOf(time);

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9 code examples:

import java.text.SimpleDateFormat; 
import java.util.Calendar; 
import java.util.Date; 
   
public class HelloWorld { 
      
    public static void main(String[] args) { 
          
        //指定时间输出格式 
        SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"); 
         
        Date dt = new Date(); 
        System.out.println("当前时间：" + sdf.format(dt)); 
          
        Calendar rightNow = Calendar.getInstance(); 
        rightNow.setTime(dt);         
          
        rightNow.add(Calendar.YEAR,-1);//当时日期减1年 
        System.out.println("减1年：" + sdf.format(rightNow.getTime())); 
          
        rightNow.add(Calendar.MONTH,3);//(在刚才的结果上)再加3个月 
        System.out.println("再加3个月：" + sdf.format(rightNow.getTime())); 
          
        rightNow.add(Calendar.DAY_OF_YEAR,10);//(在刚才的结果上)再加10天         
        System.out.println("再加10天：" + sdf.format(rightNow.getTime())); 
          
  
    } 
} 
　　输出结果：

当前时间：2013-02-21 09:40:49
减1年：2012-02-21 09:40:49
再加3个月：2012-05-21 09:40:49
再加10天：2012-05-31 09:40:49

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10. Disassemble the date:

1 Date dt = new Date();
2 Calendar calendar = Calendar.getInstance();
3 calendar.setTime(dt);
4 System.out.println("Current date:" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(dt));// Current date: 2013-02-28 16:22:37
5 System.out.println("Year:" + calendar.get(Calendar.YEAR));//Year: 2013
6 System.out.println("Month:" + (calendar.get(Calendar.MONTH)+1));//Month: 2 - Note: The month is from 0 Starting
7 System.out.println("Day:" + calendar.get(Calendar.DATE));//Day: 28
8 System.out.println("Hour (12-hour format):" + calendar.get(Calendar.HOUR));//Hour: 4 (12-hour format)
9 System. out.println("Hour (24-hour format):" + calendar.get(Calendar.HOUR_OF_DAY));//Hour: 16 (24-hour format)
10 System.out. println("Minutes:" + calendar.get(Calendar.MINUTE));//Minutes:30
11 System.out.println("Seconds:" + calendar.get (Calendar.SECOND));//Seconds: 19
12 System.out.println("this month" + calendar.get(Calendar.WEEK_OF_MONTH) + "week" );//The 5th week of this month
13 System.out.println("This year" + calendar.get(Calendar.WEEK_OF_YEAR) + "week");/ /9th week of this year

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11. Find the date n days later

Use the Calendar class. There is a method in this class that can add any number of days to the time to get the corresponding date.

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
public class NDays { 
    public static void main(String[] args) { 
      Date date = new Date();   
      System.out.println((new SimpleDateFormat("yyyy-MM-dd")).format(date)); 
      Calendar cal = Calendar.getInstance(); 
      cal.setTime(date);  
      cal.add(Calendar.DATE, 100);  
      System.out.println((new SimpleDateFormat("yyyy-MM-dd")).format(cal.getTime()));
    }
}

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class date {
    
    public static void main(String[] args) throws ParseException {
        System.out.println(datechange(new GregorianCalendar()));//将输入的Calendar转化成yyyy-mm-dd格式的String
        System.out.println(String.valueOf(dateminus("2012-9-1")));//求出输入的yyyy-mm-dd格式的String与今天的差值
        System.out.println(dateadd(90));//计算出今天起，输入的int天后的日期
    }

12.JAVA date addition and subtraction operations.

12.1. Use java.util.Calender to implement

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

12.2. Use java.text.SimpleDateFormat and java.util.Date to implement

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)));

12.3 Implementation using GregorianCalendar

GregorianCalendar gc=new GregorianCalendar(); 
gc.setTime(new Date); 
gc.add(field,value); 
//value为正则往后,为负则往前 
//field取1加1年,取2加半年,取3加一季度,取4加一周，取5加一天.... 

/*
*java中对日期的加减操作
*gc.add(1,-1)表示年份减一.
*gc.add(2,-1)表示月份减一.
*gc.add(3.-1)表示周减一.
*gc.add(5,-1)表示天减一.
*以此类推应该可以精确的毫秒吧.没有再试.大家可以试试.
*GregorianCalendar类的add(int field,int amount)方法表示年月日加减.
*field参数表示年,月.日等.
*amount参数表示要加减的数量.
*
* UseDate.java 测试如下:
*/
package temp.util;

import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.text.SimpleDateFormat;
public class UseDate {

    Date d=new Date();
    GregorianCalendar gc =new GregorianCalendar();
    SimpleDateFormat sf  =new SimpleDateFormat("yyyy-MM-dd");

    public String getYears()
    {
        gc.setTime(d);
        gc.add(1,+1);
        gc.set(gc.get(Calendar.YEAR),gc.get(Calendar.MONTH),gc.get(Calendar.DATE));

        return sf.format(gc.getTime());
    }

    public String getHalfYear()
    {
        gc.setTime(d);
        gc.add(2,+6);
        gc.set(gc.get(Calendar.YEAR),gc.get(Calendar.MONTH),gc.get(Calendar.DATE));

        return sf.format(gc.getTime());
    }
    public String getQuarters()
    {
        gc.setTime(d);
        gc.add(2,+3);
        gc.set(gc.get(Calendar.YEAR),gc.get(Calendar.MONTH),gc.get(Calendar.DATE));

        return sf.format(gc.getTime());
    }

    public String getLocalDate()
    {
        return sf.format(d);
    }


    public static  void  main(String[] args)
    {
        UseDate ud= new UseDate();
        System.out.println(ud.getLocalDate());
        System.out.println(ud.getYears());
        System.out.println(ud.getHalfYear());
        System.out.println(ud.getQuarters());
    }

}


//----------------------------------------------------
GregorianCalendar gc=new GregorianCalendar();
        
        try {
            gc.setTime( new SimpleDateFormat("yyyyMM").parse("200901"));
            gc.add(2, -Integer.parseInt("7"));
        } catch (ParseException e) {
            e.printStackTrace();
        }
        System.out.println(new SimpleDateFormat("yyyyMM").format(gc.getTime())); 

运行结果：200806

13. Calculate the number of days between two dates?

SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");     
long to = df.parse("2008-4-25").getTime();     
long from = df.parse("2008-1-20").getTime();     
System.out.println((to - from) / (1000 * 60 * 60 * 24));

//The following calculates the date difference (target day-today)

//缺陷：S传入时必须已经满足yyyy-mm-dd的格式。
    public static long dateminus(String S) throws ParseException{  
        DateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        Date dt=format.parse(S);
             
        Calendar endDate=Calendar.getInstance();
        endDate.setTime(dt);
        Long e=endDate.getTimeInMillis();
        
        Calendar currentDate=Calendar.getInstance();
        currentDate.setTime(new Date());
        Long c=currentDate.getTimeInMillis(); 
        Long t=e-c;
        return format(t);  
    }
    
    public static long format(long ms) {//将毫秒数换算成x天x时x分x秒x毫秒  
        int ss = 1000;
        int mi = ss * 60;
        int hh = mi * 60;
        int dd = hh * 24;
        long day = ms / dd+1;
        return day;
    }

//The following calculates the date sum and calculates the date one day from today.

public static String dateadd(int day){
        Calendar c=new GregorianCalendar();
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        String s=String.valueOf(datechange(c));
        String S=null;
    
            Date d;
            try {
                d = df.parse(s);
                for (int i = 0; i < day; i++) {
                    d=new Date(d.getTime()+60*60*24*1000);//由于当day大于25时，会溢出（int放不下），所以只能每次只加1天，加day次。
                }
            S=df.format(d);
            } catch (ParseException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }


        return S;
    }

//The following converts the incoming Calendar type data into yyyy-mm-dd format

public static String datechange(Calendar c){
        return String.valueOf(c.get(c.YEAR))
                +"-"+(String.valueOf(c.get(c.MONTH)+1).length()==1?"0"+String.valueOf(c.get(c.MONTH)+1):String.valueOf(c.get(c.MONTH)+1))
                +"-"+(String.valueOf(c.get(c.DAY_OF_MONTH)).length()==1?"0"+String.valueOf(c.get(c.DAY_OF_MONTH)):String.valueOf(c.get(c.DAY_OF_MONTH)));
    }

//The following will truncate the date with hours, minutes and seconds read from the database into the form of yyyy-mm-dd

public static String dateshort(String S){
        return S.substring(0,10);
    }
    
    public static int getyear(String S){
        return Integer.parseInt(S.substring(0, 4));
    }
}

Method of taking a certain day (ignoring the specific time)

select * from table where DATE(reg_time)="2014-10-22"
//        String startTimeString = date+" 00:00:00";
//        String ednTimeString = date+" 23:59:59";