Dark horse notes - common date API

Catalog

1. Date and time

1.1Date

1.2SimpleDateFormat

1.3Calendar

2.JDK8 Add date class

2.1 summary ,LocalTime / LocalDate / LocalDateTime

2.2Instant

2.3DateTimeFormatter

2.4Duration / Period（LocalDateTime / LocalDate）

2.5ChronoUnit

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1. Date and time

1.1Date

Date Class Overview ：Date Class represents the date and time information of the current system .

Date Constructor

Date The common method of

 

  Case study ： Please calculate the current time and go back 1 Hours 121 What is the time after seconds .

public class DateDemo1 {
    public static void main(String[] args) {
        // 1、 Create a Date Class object ： Represents the current date and time object of the system 
        Date d = new Date();
        System.out.println(d);

        // 2、 Gets the value of the time in milliseconds 
        long time = d.getTime();
        System.out.println(time);
//        long time1 = System.currentTimeMillis();
//        System.out.println(time1);

        System.out.println("----------------------------");
        // 1、 Get the current time 
        Date d1 = new Date();
        System.out.println(d1);

        // 2、 Go back at the current time  1 Hours   121s
        long time2 = System.currentTimeMillis();
        time2 += (60 * 60 + 121) * 1000;

        // 3、 Convert the time millisecond value into the corresponding date object .
        // Date d2 = new Date(time2);
        // System.out.println(d2);

        Date d3 = new Date();
        d3.setTime(time2);
        System.out.println(d3);

    }
}

summary ：

1、 How date objects are created , How to get the value of time in milliseconds ？

public  Date();
public long getTime();

2、 How to restore the time millisecond value to the date object ？

public Date(long time);
public void setTime(long time);

1.2SimpleDateFormat

SimpleDateFormat Class action ： You can complete the formatting operation of date and time .

Constructors

  formatting method

import java.text.SimpleDateFormat;
import java.util.Date;

/**
     The goal is ：SimpleDateFormat Use of simple date formatting class 
     Format time 
     Parsing time 

 */
public class SimpleDateFormatDemo01 {
    public static void main(String[] args) {
        // 1、 Date object 
        Date d = new Date();
        System.out.println(d);

        // 2、 Format this date object  ( Specify the final format )
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd Japan  HH:mm:ss EEE a");
        // 3、 Start formatting the date object into your favorite string form 
        String rs = sdf.format(d);
        System.out.println(rs);

        System.out.println("----------------------------");

        // 4、 Format time in milliseconds 
        //  demand ： Excuse me, 121 What's the time in seconds 
        long time1 = System.currentTimeMillis() + 121 * 1000;
        String rs2 = sdf.format(time1);
        System.out.println(rs2);

        System.out.println("------------ Parse string time , Next code ---------------");
    }
}

analytic method

 

The common pattern correspondence of formatted time form is as follows ：

  Case study ： Please calculate 2021 year 08 month 06 Japan 11 spot 11 branch 11 second , turn back and proceed 2 God 14 Hours 49 branch 06 What's the time in seconds .

import javax.xml.crypto.Data;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class SimpleDateFormatDemo2 {
    public static void main(String[] args) throws ParseException {
        //  The goal is :  Learn how to use SimpleDateFormat Parse the string time to become a date object .
        //  There is a time  2021 year 08 month 06 Japan  11:11:11  Back up  2 God  14 Hours  49 branch  06 What's the time in seconds .
        // 1、 Get the string time into the program 
        String dateStr = "2021 year 08 month 06 Japan  11:11:11";

        // 2、 Parse the time string into a date object （ The point of this section ）: The form must be exactly the same as the form of the parsed time , Otherwise, an error is reported during runtime parsing ！
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd Japan  HH:mm:ss");
        Date d = sdf.parse(dateStr);

        // 3、 turn back and proceed 2 God  14 Hours  49 branch  06 second 
        long time = d.getTime() + (2L*24*60*60 + 14*60*60 + 49*60 + 6) * 1000;

        // 4、 Formatting this time in milliseconds is the result 
        System.out.println(sdf.format(time));
    }
}

summary ：

1、SimleDateFormat What time forms can be formatted ？

Date Date object , Time in milliseconds .

2、SimpleDateFormat How to parse string time ？

Through a way ： public Date parse​(String source).

Get a string time , call SimpleDateFormat Constructor where the time format must be exactly the same as that of the parsed string , Finally using public final String format（Date date） Format time .

1.3Calendar

Calendar summary

Calendar Represents the calendar object corresponding to the current date of the system .

Calendar Is an abstract class , You can't create objects directly .

Calendar Method of calendar class to create calendar object ：

Calendar Common methods

  Be careful ：calendar Is a variable Date object , Once modified, the time represented by its object itself will change .

 

import javax.xml.crypto.Data;
import java.util.Calendar;
import java.util.Date;

/**
     The goal is ： Calendar class Calendar Use , You can get more information .

    Calendar Represents the calendar object corresponding to the current date of the system .
    Calendar Is an abstract class , You can't create objects directly .
    Calendar Syntax for calendar class to create calendar objects ：
        Calendar rightNow = Calendar.getInstance();
    Calendar Methods ：
        1.public static Calendar getInstance():  Return an object of calendar class .
        2.public int get(int field)： Get the information of a field in the date .
        3.public void set(int field,int value)： Modify a field information of the calendar .
        4.public void add(int field,int amount)： Add... To a field / Decrease the specified value 
        5.public final Date getTime():  Get the current date object .
        6.public long getTimeInMillis():  Get the current time in milliseconds 
     Summary ：
         remember .
 */
public class CalendarDemo{
    public static void main(String[] args) {
        // 1、 Get the calendar object of the system at the moment 
        Calendar cal = Calendar.getInstance();
        System.out.println(cal);

        // 2、 Get calendar information :public int get(int field)： Get the information of a field in the date .
        int year = cal.get(Calendar.YEAR);
        System.out.println(year);

        int mm = cal.get(Calendar.MONTH) + 1;
        System.out.println(mm);

        int days = cal.get(Calendar.DAY_OF_YEAR) ;
        System.out.println(days);

        // 3、public void set(int field,int value)： Modify a field information of the calendar .
        // cal.set(Calendar.HOUR , 12);
        // System.out.println(cal);

        // 4.public void add(int field,int amount)： Add... To a field / Decrease the specified value 
        //  Excuse me, 64 What time is the day after 
        cal.add(Calendar.DAY_OF_YEAR , 64);
        cal.add(Calendar.MINUTE , 59);

        //  5.public final Date getTime():  Get the current date object .
        Date d = cal.getTime();
        System.out.println(d);

        //  6.public long getTimeInMillis():  Get the current time in milliseconds 
        long time = cal.getTimeInMillis();
        System.out.println(time);

    }
}

2.JDK8 Add date class

2.1 summary ,LocalTime / LocalDate / LocalDateTime

summary ： from Java 8 Start ,java.time The package provides a new date and time API, The main types involved are ：

 

Newly added API Strictly distinguish the time 、 Local date 、 Local time , also , It is more convenient to calculate the date and time .

secondly , new API Almost all types of are invariant types （ and String The use of is similar to ）, You can use it safely without worrying about being modified .

LocalDate、LocalTime、LocalDateTime：

They indicate the date , Time , Date time object , Instances of their classes are immutable objects .

The three of them build objects and API It's all universal .

The way to build objects is as follows ：

LocalDate、LocalTime、LocalDateTime Access to information API：

 

import java.time.LocalDate;
import java.time.Month;

public class Demo01LocalDate {
    public static void main(String[] args) {
        // 1、 Get local date object .
        LocalDate nowDate = LocalDate.now();
        System.out.println(" Today's date ：" + nowDate);// Today's date ：

        int year = nowDate.getYear();
        System.out.println("year：" + year);


        int month = nowDate.getMonthValue();
        System.out.println("month：" + month);

        int day = nowDate.getDayOfMonth();
        System.out.println("day：" + day);

        // The day of the year 
        int dayOfYear = nowDate.getDayOfYear();
        System.out.println("dayOfYear：" + dayOfYear);

        // week 
        System.out.println(nowDate.getDayOfWeek());
        System.out.println(nowDate.getDayOfWeek().getValue());

        // month 
        System.out.println(nowDate.getMonth());//AUGUST
        System.out.println(nowDate.getMonth().getValue());//8

        System.out.println("------------------------");
        LocalDate bt = LocalDate.of(1991, 11, 11);
        System.out.println(bt);// Directly input the corresponding month, year and day 
        System.out.println(LocalDate.of(1991, Month.NOVEMBER, 11));// In contrast, the above only replaced the month with the enumeration 
    }
}

Modify the relevant API：

LocalDateTime A combination of  LocalDate and  LocalTime The method inside , So here's just  LocalDate and  LocalTime For example .

These methods return a new instance reference , because LocalDateTime 、LocalDate 、LocalTime It's all immutable .

 

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.MonthDay;

public class Demo04UpdateTime {
    public static void main(String[] args) {
        LocalTime nowTime = LocalTime.now();
        System.out.println(nowTime);// current time 
        System.out.println(nowTime.minusHours(1));// An hour ago 
        System.out.println(nowTime.minusMinutes(1));// A minute ago 
        System.out.println(nowTime.minusSeconds(1));// A second ago 
        System.out.println(nowTime.minusNanos(1));// A nanosecond ago 

        System.out.println("----------------");

        System.out.println(nowTime.plusHours(1));// An hour later 
        System.out.println(nowTime.plusMinutes(1));// A minute later 
        System.out.println(nowTime.plusSeconds(1));// In a second 
        System.out.println(nowTime.plusNanos(1));// In a nanosecond 

        System.out.println("------------------");
        //  Immutable object , Each modification produces a new object ！
        System.out.println(nowTime);

        System.out.println("---------------");
        LocalDate myDate = LocalDate.of(2018, 9, 5);
        LocalDate nowDate = LocalDate.now();

        System.out.println(" It's today 2018-09-06 Do you ？ " + nowDate.equals(myDate));// It's today 2018-09-06 Do you ？ false
        System.out.println(myDate + " Whether in " + nowDate + " Before ？ " + myDate.isBefore(nowDate));//2018-09-05 Whether in 2018-09-06 Before ？ true
        System.out.println(myDate + " Whether in " + nowDate + " after ？ " + myDate.isAfter(nowDate));//2018-09-05 Whether in 2018-09-06 after ？ false

        System.out.println("---------------------------");
        //  Judge whether today is your birthday 
        LocalDate birDate = LocalDate.of(1996, 8, 5);
        LocalDate nowDate1 = LocalDate.now();

        MonthDay birMd = MonthDay.of(birDate.getMonthValue(), birDate.getDayOfMonth());
        MonthDay nowMd = MonthDay.from(nowDate1);

        System.out.println(" Is today your birthday ？ " + birMd.equals(nowMd));// Is today your birthday ？ false
    }
}

2.2Instant

Instant Time stamp ：JDK8 Getting timestamps is very simple , More functions .Instant Class consists of a static factory method now() You can return the current timestamp .

A timestamp is a time stamp that contains a date and time , And java.util.Date Is very similar , in fact Instant Just like that. JDK8 Former Date

Instant and Date These two classes can be converted .

import java.time.Instant;
import java.time.LocalDate;
import java.time.ZoneId;
import java.util.Date;

public class Demo05Instant {
    public static void main(String[] args) {
        // 1、 Get one Instant Timestamp object 
        Instant instant = Instant.now();
        System.out.println(instant);

        // 2、 What about the timestamp of the system at the moment ？
        Instant instant1 = Instant.now();
        System.out.println(instant1.atZone(ZoneId.systemDefault()));

        // 3、 How to return Date object 
        Date date = Date.from(instant);
        System.out.println(date);

        Instant i2 = date.toInstant();
        System.out.println(i2);
    }
}

2.3DateTimeFormatter

 DateTimeFormatter： stay JDK8 in , Introduced a new date and time formatter DateTimeFormatter.

Both positive and negative can call format Method .

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
public class Demo06DateTimeFormat {
    public static void main(String[] args) {
        //  Local moment    Date time   object 
        LocalDateTime ldt = LocalDateTime.now();
        System.out.println(ldt);

        //  analysis / formatter 
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss EEE a");
        //  Forward formatting 
        System.out.println(dtf.format(ldt));
        //  Reverse formatting 
        System.out.println(ldt.format(dtf));

        //  Parse string time 
        DateTimeFormatter dtf1 = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        //  Parse the current string time to become a local date time object 
        LocalDateTime ldt1 = LocalDateTime.parse("2019-11-11 11:11:11" ,  dtf1);
        System.out.println(ldt1);
        System.out.println(ldt1.getDayOfYear());
    }
}

2.4Duration / Period（LocalDateTime / LocalDate）

Period：

stay Java8 in , We can use the following classes to calculate the date interval difference ：java.time.Period

Mainly Period Class method getYears(),getMonths() and getDays() To calculate , It can only be accurate to date .

be used for LocalDate Comparison between .

import java.time.LocalDate;
import java.time.Period;

public class Demo07Period {
    public static void main(String[] args) {
        //  Current local   Specific date 
        LocalDate today = LocalDate.now();
        System.out.println(today);//

        //  For your birthday   Specific date 
        LocalDate birthDate = LocalDate.of(1998, 10, 13);
        System.out.println(birthDate);

        Period period = Period.between(birthDate, today);// The second parameter minus the first parameter 

        System.out.println(period.getYears());
        System.out.println(period.getMonths());
        System.out.println(period.getDays());
    }
}

Duration：

stay Java8 in , We can use the following classes to calculate the time interval difference ：java.time.Duration.

A method of measuring the amount of time using time-based values is provided .

be used for LocalDateTime Comparison between . It can also be used for  Instant Comparison between .

import java.time.Duration;
import java.time.LocalDateTime;

public class Demo08Duration {
    public static void main(String[] args) {
        //  Local date time object .
        LocalDateTime today = LocalDateTime.now();
        System.out.println(today);

        //  Date and time of birth object 
        LocalDateTime birthDate = LocalDateTime.of(2021,8
                ,06,01,00,00);

        System.out.println(birthDate);

        Duration duration = Duration.between(  birthDate , today);// The second parameter minus the first parameter 

        System.out.println(duration.toDays());// The number of days between two times 
        System.out.println(duration.toHours());// Hours of difference between two times 
        System.out.println(duration.toMinutes());// The number of minutes between two times 
        System.out.println(duration.toMillis());// The number of milliseconds between two time differences 
        System.out.println(duration.toNanos());// Nanoseconds of two time differences 
    }
}

summary ：

1、 Duration:  Used to calculate two “ Time ” interval .
2、 Period:      Used to calculate two “ date ” interval .

2.5ChronoUnit

java.time.temporal.ChronoUnit

ChronoUnit Class can be used to measure a period of time in a single unit of time , This tool class is the most complete , Can be used to compare all time units .

import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;

public class Demo09ChronoUnit {
    public static void main(String[] args) {
        //  Local date time object ： At the moment 
        LocalDateTime today = LocalDateTime.now();
        System.out.println(today);

        //  Birthday time 
        LocalDateTime birthDate = LocalDateTime.of(1990,10,1,
                10,50,59);
        System.out.println(birthDate);

        System.out.println(" Years of difference ：" + ChronoUnit.YEARS.between(birthDate, today));
        System.out.println(" The difference in the number of months ：" + ChronoUnit.MONTHS.between(birthDate, today));
        System.out.println(" The number of weeks of difference ：" + ChronoUnit.WEEKS.between(birthDate, today));
        System.out.println(" Days of difference ：" + ChronoUnit.DAYS.between(birthDate, today));
        System.out.println(" The difference in hours ：" + ChronoUnit.HOURS.between(birthDate, today));
        System.out.println(" The difference in scores ：" + ChronoUnit.MINUTES.between(birthDate, today));
        System.out.println(" The number of seconds apart ：" + ChronoUnit.SECONDS.between(birthDate, today));
        System.out.println(" Millisecond difference ：" + ChronoUnit.MILLIS.between(birthDate, today));
        System.out.println(" Microseconds of difference ：" + ChronoUnit.MICROS.between(birthDate, today));
        System.out.println(" The nanosecond difference ：" + ChronoUnit.NANOS.between(birthDate, today));
        System.out.println(" The difference of half a day ：" + ChronoUnit.HALF_DAYS.between(birthDate, today));
        System.out.println(" The difference of ten years ：" + ChronoUnit.DECADES.between(birthDate, today));
        System.out.println(" A century apart （ a hundred years ） Count ：" + ChronoUnit.CENTURIES.between(birthDate, today));
        System.out.println(" The number of millennia ：" + ChronoUnit.MILLENNIA.between(birthDate, today));
        System.out.println(" The number of different eras ：" + ChronoUnit.ERAS.between(birthDate, today));
    }
}