Getting started with Java lesson 100 - calendar's getInstance method

Monkeys learn to program 2020-11-13 02:42:03
getting started java lesson calendar


Unknowingly to the 100 Class is over , Code words are really tired , After finishing writing, check again and again whether there is a clerical error , So as not to cause trouble to everyone .

There are still many classes in the future , It's really starting from scratch , All the way to the frame .

I wonder if I can finish writing , The more people you pay attention to, the faster .

Recently it's faster because of the stock , The stock still holds 50 What a class looks like .

Write at most if you don't eat or drink in a day 10 Class hours .

It's a little verbose , Please leave me a message if you have any idea .

problem

    Calendar Class is an abstract class , It is for a specific moment with a set of YEAR、MONTH、DAY_OF_MONTH、HOUR Waiting for the conversion between calendar fields provides some methods , And provides some methods for operating calendar fields .

     This case requires obtaining Calendar Class , After getting the instance, do the following :

    1) obtain Calendar The actual type of instance , And the output .

    2) take Calendar Object to Date object , And output the Date object .

    3) Use GregorianCalendar Building objects , The corresponding date of this object is 2013 year 12 month 25 Japan , take GregorianCalendar Object to Date object , And output the Date object .

programme

    1) First , Use Calendar Class getInstance Method to get Calendar Class object , Then use the following code to get the actual type of the object , And the output , The code is as follows :

System.out.println(c.getTime());

    2) Use Calendar Class getTime Method , Can be Calendar Object to Date object , then , Output this Date Class object .

    3) Use GregorianCalendar The build date is 2013 year 12 month 25 The object of the day , then ,GregorianCalendar Class getTime Methods will GregorianCalendar Object to Date object , And output the Date object .

step

     To implement this case, follow the steps below .

Step one : Create test methods

     First, create a new class TestCalendar; then , Create a new test method under this class testGetInstance, The code is as follows :

 package day03;
    
    import java.util.Calendar;
    import java.util.GregorianCalendar;
    import org.junit.Test;
    
    public class TestCalendar{
     /**
      * test getInstance Method
      */
      @Test
      public void testGetInstance(){
      
      }
    }

Step two : establish Calendar Class

     First , Use Calendar Class getInstance Method to get Calendar Class object , Then use the following code to get the actual type of the object , And the output , The code is as follows :

 package day03;
    
    import java.util.Calendar;
    import java.util.GregorianCalendar;
    import org.junit.Test;
    
    public class TestCalendar{
     /**
      * test getInstance Method
      */
      @Test
      public void testGetInstance(){
          Calendar c=Calendar.getInstance();
          // Output Calendar The actual type of object
          System.out.println(c.getClass().getName());
      }
    }

Step three : function

     function testGetInstance Method , The console output is as follows :

    java.util.GregorianCalendar

     You can see from the output of the console , object c The actual type is GregorianCalendar, This is because , There are many ways of dating in history , They are so different , For example, a person's birthday is " 7-Jul ", So one possibility is Yang ( Male ) July 7 of the calendar , It can also be Yin ( farmers ) July 7 of the calendar . In order to unify the timing , The world usually chooses the most popular 、 The most common calendar :Gregorian Calendar( Gregorian calendar ), That's what we call the Gregorian calendar .

    Calendar Class itself is an abstract class , It's a template for all calendar classes , And provides some common methods for all calendars ; But it can't instantiate objects directly by itself , The program can only create Calendar Instances of subclasses ,Java Provide a GregorianCalendar class , A subclass representing Gregorian calendar , It represents what we usually call the Gregorian calendar .Calendar Class provides several static getInstance Method to get Calendar object , These methods are based on TimeZone、Locale To create Calendar object .

     The getInstance Method just uses the default TimeZone、Locale To create Calendar object , You can see the default... From the output TimeZone、Locale Created Calendar The actual type of object is GregorianCalendar.

Step four : take Calendar Object to Date object

 package day03;
    
    import java.util.Calendar;
    import java.util.GregorianCalendar;
    import org.junit.Test;
    
    public class TestCalendar{
     /**
      * test getInstance Method
      */
      @Test
      public void testGetInstance(){
          Calendar c=Calendar.getInstance();
          // Output Calendar The actual type of object
          System.out.println(c.getClass().getName());
          
          //getTime Method returns the corresponding Date object
          System.out.println(c.getTime());
      }
    }

Step five : Use GregorianCalendar Build date object

     Use GregorianCalendar The build date is 2013 year 12 month 25 The object of the day , then ,GregorianCalendar Class getTime Methods will GregorianCalendar Object to Date object , And output the Date object , The code is as follows :

 package day03;
    
    import java.util.Calendar;
    import java.util.GregorianCalendar;
    import org.junit.Test;
    
    public class TestCalendar{
     /**
      * test getInstance Method
      */
      @Test
      public void testGetInstance(){
          Calendar c=Calendar.getInstance();
          // Output Calendar The actual type of object
          System.out.println(c.getClass().getName());
          
          //getTime Method returns the corresponding Date object
          System.out.println(c.getTime());
          
          // establish GregorianCalendar object
          GregorianCalendar c1=new GregorianCalendar(2013,Calendar.DECEMBER,25);    
          System.out.println(c1.getTime());
      }
    }

Step six : function

     function testGetInstance Method , The console output is as follows :

 java.util.GregorianCalendar
    Mon Jan 20 10:40:39 CST 2014
    Wed Dec 25 00:00:00 CST 2013

     Looking at the above code, you can see that Calendar Objects and GregorianCalendar Object to Date object .

     Summing up the previous cases, we can know that the expression of time can be Date、long perhaps Calendar, The mutual transformation between them is shown in the figure -1 Shown .

image.png

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