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This article will explain in detail how to set and operate the Date class in Java. Xiaobian thinks it is quite practical, so share it with you as a reference. I hope you can gain something after reading this article.

Date

As the name implies, Date is a class about time. This class is mainly about some settings and operations for time. As you can see from the official documentation, Date class inherits from java.lang.Object class. The related construction methods are as follows:

Construction method:

Date(): parameterless construct, assign a Date object and initialize it, assign it the time it represents, measure to millisecond precision:

import java.util.Date; //Because this class belongs to util package, you need to import Date d = new Date() in advance; //parameterless construct, create object System.out.println(d);

//Output: Fri Jan 14 10:22:58 CST 2022 CST is China Standard Time

//If you use parameterless construction directly, it is actually outputting the current system time.

Date(long date): Parameter structure

First one second equals 1000 milliseconds, and this parametric construct assigns a Date object and initializes it to represent the specified number of milliseconds from a base time called Greenwich Mean Time, which is January 1, 1970, 00:00:00 GMT. That is, you pass in a millisecond, and you increment the time you enter from 00:00:00 on January 1, 1970:

//Date d = new Date(5000); //5000 milliseconds equals 5 seconds System.out.println(d);

//Output: Thu Jan 01 08:00:05 CST 1970

//Why am I counting out 08:00:05 here? Note that mine is CST China Standard Time, China belongs to the East Eight District, so it needs +8

//actually converted to GMT (Greenwich Time) is-8, that is, 00:00:05, no problem

Methods of the Date class:

getTime(): to get the number of milliseconds, its return value type is long, which can convert the date object to a millisecond value

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

//Output: 1642127814408

//This is the millisecond value calculated from 1970-1-1 00:00:00

hashCode(): Returns the hash code value of this object. In fact, this is not very useful. You can understand it.

System.out.println(d.hashCode());

//Output:1450437638 hash value

equals(Object obj): Compare two equal dates

Date d = new Date();Date c = new Date();System.out.println(d.equals(c));

//Output: true, because two objects are created at the same time, both at the current time, because the computer runs fast, not even a millisecond

//So both times are the same

after(Date when): tests returns true if this date is after the specified date, false otherwise

before(Date when) : tests returns true if this date is before the specified date, false otherwise

You can try these two methods yourself.

However, we will find that the Date class cannot output a time format similar to the common one: for example, 2021 01= year 01 13:00, so we need the Calenda class.

Calenda：

calendar class in java.util package, it provides multiple methods to get, set, add calendar field values, more powerful than Date class,

Calendar is an abstract class, so you can't use new directly to create new objects.

So we need to use the GregorianCalendar class, which is also a subclass of Calenda and provides most of the world's standard calendar systems.

Use this class to get the codes for the current year, month, and day of the week:

GregorianCalendar g = new GregorianCalendar(); //Create a GregorianCalendar object int year = g.get(Calendar.YEAR);int month = g.get(Calendar.MONTH); //Use GregorianCalendar get method to get current month int day = g.get(Calendar.DAY_OF_MONTH); //Use GregorianCalendar get method to get current date System.out.println(year+"year"+month+"month"+day+"day");

//Output: October 14, 2022

//Explanation: Months are counted from 0, so 0 represents January and 11 represents December. Month returns 0-11

//Solution:

System.out.println(year+"year"+(month+1)+"month"+day+"day"); //+1 after month

//Output: January 14, 2022

Get Method:

GregorianCalendar can also be used to output more specific time formats:

For example, the output of the month, day, hour, minute and second, this is also the general web page and system will display the time: (complete code)

package DateTest;import java.util.Date;import java.util.Calendar;import java.util.GregorianCalendar;public class test01 { public static void main(String[] args) { Date a = new Date(); GregorianCalendar g = new GregorianCalendar(); int year = g.get(Calendar.YEAR); int month = g.get(Calendar.MONTH); int day = g.get(Calendar.DAY_OF_MONTH); int hour = g.get(Calendar.HOUR_OF_DAY); int min = g.get(Calendar.MINUTE); int second = g.get(Calendar.SECOND); System.out.println(year+"year"+(month+1)+"month"+day+"day"+hour+"hour"+min+"minute"+second+"second"); }}

//Output: January 14, 2022 11:14:36 Is this the ideal format for time output

This is also the time I am writing this blog.

Set method:

The above we use is the get method, of course, there is also a corresponding set method, to set the custom time:

For example, if it is set to January 1, 2020:

//This part of the code is still based on the code of the above get method g.set(Calendar.YEAR,2020);g.set(Calendar.MONTH,0); //Don't forget that the starting value of the month is 0, which means January g.set(Calendar.DAY_OF_MONTH,1);//When the set is finished, use the get method to get it, and then get the modified time.

//Output: January 1, 2020

However, if the month is greater than 11 or the number of days is greater than 30 or 31 days (February is greater than 28 days), then it will automatically advance one:

g.set(Calendar.YEAR,2020);g.set(Calendar.MONTH,0);g.set(Calendar.DAY_OF_MONTH,35); //Date entered 35 days

//Output: February 4, 2020

//Enter January, but the number of days is greater than the number of days in the real month, so one month will be added.

SimpleDateFormat：

However, in the actual application process, the format requirements for time may be different, such as:

2020/01/01 12:00:00

2020-01-01 Week * AM/PM

January 1, 2020 12:00:00----------

So we can't rewrite the code every time we need a different lattice, so Java provides this SimpleDateFormat class:

This class belongs to the text package, so it needs to be imported before use. It inherits from Object, Format and DateFormat classes respectively. Some format symbols of this class are as follows:

Going back, what if we need to print time in a different format? Let's first look at how SimpleDateFormat is constructed:

Let's look at the second one first: SimpleDateFormat(String pattern): Using the given pattern and the default date format symbol, the default FORMAT field constructs a SimpleDateFormat, which means that the object of this time is created according to the format symbol you give.

public static String dateToString(Date d,String pattern){ //Create a new static method SimpleDateFormat s = new SimpleDateFormat(pattern); //parametric structure String time = s.format(d); //format method of SimpleDateFormat: The given Date enters the date/timeline and appends the result to the StringBuffer format given. return time; //Return time}

Code explanation: In fact, this method is passed in a Date class time, and SimpleDateFormat parameter construction method needs a parameter pattern, that is, format, according to the table to see what format you need, and then use the format method.

Complete Code:

import java.text.SimpleDateFormat; //import java.util.Date;public class test03 { public static void main(String[] args) { Date d = new Date(); //Create object Date d required by method String t = dateToString(d,"y-MM-dd HH: mm: ss: E a"); //E represents the day of the week, a represents the morning/afternoon System.out.println(t); //use method }//method area public static String dateToString(Date d,String pattern){ SimpleDateFormat s = new SimpleDateFormat(pattern); String time = s.format(d); return time; }}

If we need to export something else:

"yyy MM MM DD HH mm mm SS S E a"

//Output: January 14, 2022 15:45:06 Friday afternoon

"yyyy/MM/dd HH：mm：ss E a"

//Output: 2022/01/14 15:45:36 Friday afternoon

"MM/dd/yy HH: mm: ss" //even after the year

//Output: 01/14/2022 15:46:24

//You can customize the desired time format according to your needs, combined with the above symbol table ~~~

About "Java Date class how to set and operate time" This article is shared here, I hope the above content can be of some help to everyone, so that you can learn more knowledge, if you think the article is good, please share it to let more people see.

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