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The new CTO strongly prohibits the use of calendar?

2022-07-27 21:20:00 【Hollis Chuang】

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Java 8 It has been widely used , But it is still used Java Calendar Processing time and date , Not only poor performance , Very cutting code, very redundant , You can't use Java 8 What's new API Do you ？ therefore CTO Forced , Must use Java 8 Processing date , Otherwise, they will be dismissed . Here's the finishing 18 A way to deal with dates , It can be collected , It must be useful .

Java Processing date 、 The calendar and time approach has long been criticized by the community , take java.util.Date Set to variable type , as well as SimpleDateFormat Non-thread-safe makes its application very limited .

new API be based on ISO Standard calendar system ,java.time All classes under the package are immutable and thread safe .

Example 1:Java 8 Get today's date

Java 8 Medium LocalDate Used to indicate the date of the day . and java.util.Date Different , It has only date , Not including time . Use this class when you just need to represent dates .

package com.shxt.demo02;

import java.time.LocalDate;

public class Demo01 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println(" Today's date :"+today);
    }
}

Example 2:Java 8 Get the year 、 month 、 Daily information

package com.shxt.demo02;

import java.time.LocalDate;

public class Demo02 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        int year = today.getYear();
        int month = today.getMonthValue();
        int day = today.getDayOfMonth();

        System.out.println("year:"+year);
        System.out.println("month:"+month);
        System.out.println("day:"+day);

    }
}

Example 3:Java 8 Processing specific dates

We use the static factory method now() It's very easy to create the date of the day , You can also call another useful factory method LocalDate.of() Create any date , This method needs to be introduced in 、 month 、 Daily parameters , Return the corresponding LocalDate example . The advantage of this method is that it doesn't get old again API Design mistakes , For example, the year starts from 1900, From the month 0 open First wait .

package com.shxt.demo02;

import java.time.LocalDate;

public class Demo03 {
    public static void main(String[] args) {
        LocalDate date = LocalDate.of(2018,2,6);
        System.out.println(" Custom date :"+date);
    }
}

Example 4:Java 8 Determine whether two dates are equal

package com.shxt.demo02;

import java.time.LocalDate;

public class Demo04 {
    public static void main(String[] args) {
        LocalDate date1 = LocalDate.now();

        LocalDate date2 = LocalDate.of(2018,2,5);

        if(date1.equals(date2)){
            System.out.println(" Time is equal ");
        }else{
            System.out.println(" Time is not equal ");
        }

    }
}

Example 5:Java 8 Check for periodic events like birthdays

package com.shxt.demo02;

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

public class Demo05 {
    public static void main(String[] args) {
        LocalDate date1 = LocalDate.now();

        LocalDate date2 = LocalDate.of(2018,2,6);
        MonthDay birthday = MonthDay.of(date2.getMonth(),date2.getDayOfMonth());
        MonthDay currentMonthDay = MonthDay.from(date1);

        if(currentMonthDay.equals(birthday)){
            System.out.println(" It's your birthday ");
        }else{
            System.out.println(" Your birthday hasn't arrived yet ");
        }

    }
}

As long as the date of the day matches the birthday , Congratulations will be printed no matter what year . You can integrate the program into the system clock , See if you'll be reminded on your birthday , Or write a unit test to check whether the code is running correctly .

Example 6:Java 8 Get the current time

package com.shxt.demo02;

import java.time.LocalTime;

public class Demo06 {
    public static void main(String[] args) {
        LocalTime time = LocalTime.now();
        System.out.println(" Gets the current time , No date :"+time);

    }
}

You can see that the current time only contains time information , No date

Example 7:Java 8 Get the current time

By adding hours 、 branch 、 Seconds to calculate the future time is common .Java 8 In addition to the benefits of immutable types and thread safety , It also provides a better plusHours() Methods to replace add(), And it's compatible . Be careful , These methods return a whole new LocalTime example , Because of its immutability , When you return, you must assign a value with a variable .

package com.shxt.demo02;

import java.time.LocalTime;

public class Demo07 {
    public static void main(String[] args) {
        LocalTime time = LocalTime.now();
        LocalTime newTime = time.plusHours(3);
        System.out.println(" The time after three hours is :"+newTime);

    }
}

Example 8:Java 8 How to calculate the date in a week

And the last example 3 Hours later, the time is similar to , This example calculates the date in a week .LocalDate Date does not contain time information , its plus() Method to add days 、 Zhou 、 month ,ChronoUnit Class declares these time units . because LocalDate It's the same type , When you return, you must assign a value with a variable .

package com.shxt.demo02;

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class Demo08 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println(" Today's date is :"+today);
        LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
        System.out.println(" The date a week later is :"+nextWeek);

    }
}

You can see that the new date is 7 God , It's a week . You can add... In the same way 1 Months 、1 year 、1 Hours 、1 Minutes or even a century , More options are available Java 8 API Medium ChronoUnit class

Example 9:Java 8 Calculate the date one year ago or one year later

utilize minus() Method calculate the date one year ago

package com.shxt.demo02;

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class Demo09 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();

        LocalDate previousYear = today.minus(1, ChronoUnit.YEARS);
        System.out.println(" Date a year ago  : " + previousYear);

        LocalDate nextYear = today.plus(1, ChronoUnit.YEARS);
        System.out.println(" A year later :"+nextYear);

    }
}

Example 10:Java 8 Of Clock Clock class

Java 8 Added one Clock The clock class is used to get the timestamp at the time , Or date time information in the current time zone . Used to use System.currentTimeInMillis() and TimeZone.getDefault() All places are available Clock Replace .

package com.shxt.demo02;

import java.time.Clock;

public class Demo10 {
    public static void main(String[] args) {
        // Returns the current time based on your system clock and set to UTC.
        Clock clock = Clock.systemUTC();
        System.out.println("Clock : " + clock.millis());

        // Returns time based on system clock zone
        Clock defaultClock = Clock.systemDefaultZone();
        System.out.println("Clock : " + defaultClock.millis());

    }
}

Example 11: How to use Java Judge whether the date is earlier or later than another date

Another common operation in the work is how to judge whether a given date is greater than or less than a day ？ stay Java 8 in ,LocalDate Class has two kinds of methods isBefore() and isAfter() Used to compare dates . call isBefore() When the method is used , Returns... If the given date is less than the current date true.

package com.shxt.demo02;

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;


public class Demo11 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();

        LocalDate tomorrow = LocalDate.of(2018,2,6);
        if(tomorrow.isAfter(today)){
            System.out.println(" The date after :"+tomorrow);
        }

        LocalDate yesterday = today.minus(1, ChronoUnit.DAYS);
        if(yesterday.isBefore(today)){
            System.out.println(" Previous date :"+yesterday);
        }
    }
}

Example 12:Java 8 Processing time zone

Java 8 It's not just a separation of date and time , Time zones are also separated . Now there are a series of separate classes like ZoneId To handle specific time zones ,ZoneDateTime Class to represent the time in a time zone . This is in Java 8 It used to be GregorianCalendar Class . The following example shows how to convert the time in this time zone to the time in another time zone .

package com.shxt.demo02;

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;

public class Demo12 {
    public static void main(String[] args) {
        // Date and time with timezone in Java 8
        ZoneId america = ZoneId.of("America/New_York");
        LocalDateTime localtDateAndTime = LocalDateTime.now();
        ZonedDateTime dateAndTimeInNewYork  = ZonedDateTime.of(localtDateAndTime, america );
        System.out.println("Current date and time in a particular timezone : " + dateAndTimeInNewYork);
    }
}

Example 13: How to express a fixed date such as the expiration of a credit card , The answer lies in YearMonth

And MonthDay Examples of checking for repeat events are similar ,YearMonth Is another composite class , Used to indicate the expiry date of a credit card 、FD due date 、 Expiration date of futures options, etc . You can also use this class to get How many days are there in the month ,YearMonth Example of lengthOfMonth() Method returns the number of days in the current month , In judging 2 Monthly 28 Heaven is still 29 The weather is very useful .

package com.shxt.demo02;

import java.time.*;

public class Demo13 {
    public static void main(String[] args) {
        YearMonth currentYearMonth = YearMonth.now();
        System.out.printf("Days in month year %s: %d%n", currentYearMonth, currentYearMonth.lengthOfMonth());
        YearMonth creditCardExpiry = YearMonth.of(2019, Month.FEBRUARY);
        System.out.printf("Your credit card expires on %s %n", creditCardExpiry);
    }
}

Example 14: How to be in Java 8 Check leap year

package com.shxt.demo02;

import java.time.LocalDate;

public class Demo14 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        if(today.isLeapYear()){
            System.out.println("This year is Leap year");
        }else {
            System.out.println("2018 is not a Leap year");
        }

    }
}

Example 15: Calculate the number of days and months between the two dates

A common date operation is to calculate the number of days between two dates 、 Weeks or months . stay Java 8 Can be used in the java.time.Period Class to do the calculation .

In the following example , We calculated the number of months between the day and the day to come .

package com.shxt.demo02;

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

public class Demo15 {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();

        LocalDate java8Release = LocalDate.of(2018, 12, 14);

        Period periodToNextJavaRelease = Period.between(today, java8Release);
        System.out.println("Months left between today and Java 8 release : "
                + periodToNextJavaRelease.getMonths() );


    }
}

Example 16: stay Java 8 Get the current timestamp

Instant Class has a static factory method now() Will return the current timestamp , As shown below ：

package com.shxt.demo02;

import java.time.Instant;

public class Demo16 {
    public static void main(String[] args) {
        Instant timestamp = Instant.now();
        System.out.println("What is value of this instant " + timestamp.toEpochMilli());
    }
}

The time stamp information contains both date and time , This sum java.util.Date It's like . actually Instant Class is really equivalent to Java 8 Previous Date class , You can use Date Classes and Instant Class's own transformation methods transform each other , for example ：Date.from(Instant) take Instant convert to java.util.Date,Date.toInstant() Will be Date Class to Instant class .

Example 17:Java 8 How to use a predefined formatting tool to parse or format dates

package com.shxt.demo02;

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class Demo17 {
    public static void main(String[] args) {
        String dayAfterTommorrow = "20180205";
        LocalDate formatted = LocalDate.parse(dayAfterTommorrow,
                DateTimeFormatter.BASIC_ISO_DATE);
        System.out.println(dayAfterTommorrow+"   The formatted date is :  "+formatted);
    }
}

Example 18: String conversion date type

package com.shxt.demo02;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class Demo18 {
    public static void main(String[] args) {
        LocalDateTime date = LocalDateTime.now();

        DateTimeFormatter format1 = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
        // Date to string 
        String str = date.format(format1);

        System.out.println(" Date to string :"+str);

        DateTimeFormatter format2 = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
        // String to date 
        LocalDate date2 = LocalDate.parse(str,format2);
        System.out.println(" The date type :"+date2);

    }
}

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