1. Date class

java. The util package provides a Date class to encapsulate the current Date and time. The Date class provides two constructors to instantiate a Date object.

constructor

date(): get the number of milliseconds from January 1, 1970

Methods in Date

Long getTime(): get timestamp (i.e. milliseconds)

int compareTo(Date date)

Compare the Date object when this method is called with the specified Date. Returns 0 when the two are equal. The calling object returns a negative number before the specified Date. The calling object returns a positive number after the specified Date.

2. Format

There are two ways: use printf to format the date and use the SimpleDateFormat class

printf format date

Format: start with% t and add conversion character. The specific meanings of conversion symbols are shown in the table below:

Conversion character	explain	Example
c	Include all date and time information	Saturday October 27 14:21:20 CST 2007
F	"Year Month Day" format	2007-10-27
D	"Month / day / year" format	10/27/07
r	"HH:MM:SS PM" format (12 hour system)	02:25:51 PM
T	"HH:MM:SS" format (24 hour system)	14:28:16
R	"HH:MM" format (24 hour system)	14:28

code:

 //Date objects use printf to format dates
 System.out.printf("Current date format:%tF%n",date);
 System.out.printf("Current date format:%tD%n",date);
 System.out.printf("Current date format:%tr%n",date);
 System.out.printf("Current date format:%tT%n",date);
 System.out.printf("Current date format:%tR%n",date);

Effect:

SimpleDateFormat class

Description: SimpleDateFormat class is thread unsafe. The SimpleDateFormat class uses a string as the encoding format.

In this mode, all ASCII letters are reserved as mode letters, which are defined as follows:

letter	describe	Example
G	Epoch mark	AD
y	Four digit year	2001
M	month	July or 07
d	Date of one month	10
h	A.M./P.M. (1~12) format hours	12
H	Hours of the day (0-23)	22
m	Minutes	30
s	Seconds	55
S	Msec	234
E	What day is today?	Tuesday
D	Days of the year	360
F	The day of the week of the month	2 (second Wed. in July)
w	What week of the year	40
W	Week of the month	1
a	A.M./P.M. mark	PM
k	Hours of the day (1-24)	24
K	A.M./P.M. (0~11) format hours	10
z	time zone	Eastern Standard Time
'	Text delimiter	Delimiter
"	Single quotation mark	`

code:

 //Format conversion of date object SimpleDateFormat format
 SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
 String date1 = sdf.format(date);
 System.out.println(date1);
 ​
 //SimpleDateFormat exercise
 //Date conversion format contact
 SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd");
 SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM");
 String date2 = sdf1.format(date);
 String date3 = sdf2.format(date);
 System.out.println(date2);
 System.out.println(date3);

effect:

The SimpleDateFormat class is converted to a Date object

code:

 //Convert SimpleDateFormat to date format. Note: if the format is yyyy MM DD, the conversion is wrong. I changed yyyy to yyyy
 String dateS = "2021-03-21 ";
 SimpleDateFormat sdf3 = new SimpleDateFormat("yyyy-MM-dd");
 try {
     Date date4 = sdf3.parse(dateS);
     System.out.println(sdf3.format(date4));
 } catch (ParseException e) {
     e.printStackTrace();
 }
 ​
 String dates1 = "2021-08-20 10:45:60";
 SimpleDateFormat sdf4 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
 try {
     Date date5 = sdf4.parse(dates1);
     System.out.println(sdf4.format(date5));
 } catch (ParseException e) {
     e.printStackTrace();
 }

effect:

3.calendar Class

The calendar class can obtain specific parts of time, such as hours, minutes and seconds, and can add and subtract dates.

Note: the calendar class is an abstract class.

Understand through code

 //Basic usage of Calendar class for date time calculation
 //Create calendar class
 Calendar calendar = Calendar.getInstance();//This is to get the current time
 Date date5 = calendar.getTime(); //This is to convert calendar to date
 System.out.println(date5);
 ​
 //Create a calendar to specify the time
 Calendar calendar1 = Calendar.getInstance();
 calendar1.set(2021,6-1,12);
 Date cDate = calendar1.getTime();
 SimpleDateFormat CSdf = new SimpleDateFormat("yyyy-MM-dd");
 System.out.println(CSdf.format(cDate));
 ​
 //Set time using set
 Calendar calendar2 = Calendar.getInstance();
 calendar2.set(2021,8-1,20);
 Date cDate1 = calendar2.getTime();
 SimpleDateFormat cSdf = new SimpleDateFormat("yyyy-MM-dd");
 System.out.println(cSdf.format(cDate1));
 ​
 //Use add settings
 Calendar calendar3 = Calendar.getInstance();
 calendar3.add(Calendar.DATE,10); //The parameter of 10 indicates the date ten days after the current date
 Date cDate2 = calendar3.getTime();
 SimpleDateFormat cSdf2 = new SimpleDateFormat("yyyy-MM-dd");
 System.out.println(cSdf2.format(cDate2));
 //Use add to set the time
 Calendar calendar4 = Calendar.getInstance();
 calendar4.add(Calendar.YEAR,2);
 Date cDate3 = calendar4.getTime();
 System.out.println(cDate3);
 ​
 //Acquisition of calendar class object information
 Calendar calendar5 = Calendar.getInstance();
 int year = calendar5.get(Calendar.YEAR);
 int month = calendar5.get(Calendar.MONTH);
 int day = calendar5.get(Calendar.DATE);
 int hour = calendar5.get(Calendar.HOUR);
 int minute = calendar5.get(Calendar.MINUTE);
 int second = calendar5.get(Calendar.SECOND);
 System.out.println(year+" "+month+" "+day+" "+hour+" "+minute+" "+second);