Before Java 8, there were two date classes: java.util.Date, java.util.Calendarand after Java 8, a brand new date class was launched java.time.LocalDate. Next, let’s briefly introduce how to use them.
1. java.util.Date
Before talking about the java.util.Date class, let's talk about the method of another class first: System.currentTimeMillis()Return the current time and Greenwich Mean Time 1970-01-01 00:00:00 (Beijing time: 1970-01-01 08: 00:00) in milliseconds.This method is suitable for calculating time differences
// 1626624005531 时间戳
Long time = System.currentTimeMillis();
System.currentTimeMillis()The return type of the method is Long type. Generally used to obtain the execution time difference of a certain method or other. That is: get it once before the start, get it once at the end, and subtract the start time from the end time to get the execution time. like:
public class DateTest {
public void test() {
System.out.println("======test======");
}
public static void main(String[] args) {
DateTest dateTest = new DateTest();
Long startTime = System.currentTimeMillis();
for (int i = 0; i < 10000; i++) {
dateTest.test();
}
Long endTime = System.currentTimeMillis();
Long result = endTime - startTime;
// 55 毫秒
System.out.println(result);
}
}
In the above code, the for loop calls the DateTest#test() method 10,000 times, which takes 55 milliseconds.
Convert timestamps to and from the java.util.Date class:
// Date 类型转换为时间戳
Date date = new Date();
Long time = date.getTime();
// 时间戳转换为 Date 类型
Long long = 1626624005531L;
Date nowTime = new Date(long);
Generally, the java.util.Date class is used together with the java.text.SimpleDateFormat class to format dates.
For example: Convert date to a string in a specific format:
public class DateTest {
public static void main(String[] args) {
String timePattern = "yyyy-MM-dd HH:mm:ss";
SimpleDateFormat format = new SimpleDateFormat(timePattern);
String s = format.format(new Date());
System.out.println(s);
}
}
Convert string to date:
public static void main(String[] args) {
String timePattern = "yyyy-MM-dd HH:mm:ss";
String timeOne = "2021-07-18 22:53:22";
SimpleDateFormat format = new SimpleDateFormat(timePattern);
try {
Date nowDate = format.parse(timeOne);
System.out.println(nowDate);
} catch (ParseException e) {
System.out.println("日期格式化错误！！，日期为：" + timeOne);
}
}
2. java.sql.Date
java.sql.DateIt is a subclass of the java.util.Date class; it is used for SQL statements. It only contains the date without the time part. It is generally used when reading and writing the database, and both the PreparedStament#setDate() parameters of the method and ResultSet#getDate()the method are used java.sql.Date.
How to get the current time using java.sql.Date?
public class DateTest {
public static void main(String[] args) {
java.sql.Date date = new java.sql.Date(System.currentTimeMillis());
// 2021-07-19
System.out.println(date);
}
}
Convert java.util.Date and java.sql.Date to and from each other:
public class DateTest {
public static void main(String[] args) {
Date date = new Date();
java.sql.Date dat = new java.sql.Date(date.getTime());
}
}
3. java.util.Calendar
java.util.Calendar: Calendar class, an abstract class
Calendar instantiation method:
- Create an object of its subclass GregorianCalendar
- Call its static method getInstance()
In fact, these two methods are the same:
public class CalendarTest {
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
// class java.util.GregorianCalendar
System.out.println(calendar.getClass());
}
}
Commonly used methods:
- get(int field): Get a certain time
- set(): set a certain time
- add(): add a certain time at a certain time
- getTime(): Convert Calendar class to Date class
- setTime(): Convert Date class to Calendar class
field values are:
- DAY_OF_MONTH: The day of this month
- DAY_OF_YEAR: The day of the year
- …
Notice:
- When getting the month: January is 0, February is 1,…
- When getting the day of the week: Sunday is 1, Monday is 2,…
get()method
public static void main(String[] args) {
// 获取到的是当前时间
Calendar calendar = Calendar.getInstance();
int i = calendar.get(Calendar.DAY_OF_MONTH);
// 18，表示是这个月的第 18 天（今天是 2021-07-18）
System.out.println(i);
}
set(int field, int value)method
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.DAY_OF_MONTH, 23);
// 23。将当前时间设置成某个时间，即“2021-07-23”
System.out.println(calendar.get(Calendar.DAY_OF_MONTH));
}
add(int field, int value)method
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
// 给当前时间加上 5 天
calendar.add(Calendar.DAY_OF_MONTH, 5);
System.out.println(calendar.get(Calendar.DAY_OF_MONTH));
}
So, what if I want to add 2 months to the current time?
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
// 给当前时间加上 5 天
calendar.add(Calendar.MONTH, 2);
System.out.println(calendar.get(Calendar.MONTH));
}
Then, what about subtracting another 3 days?
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
// 给当前时间加上 5 天
calendar.add(Calendar.MONTH, 2);
// 减去 3 天
calendar.add(Calendar.DAY_OF_MONTH, -3);
System.out.println(calendar.get(Calendar.MONTH));
}
getTime()method
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_MONTH, -3);
Date time = calendar.getTime();
// 获取 3 天前的日期
System.out.println(time);
}
setTime(Date date)method
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
// 获取 3 天前的日期
calendar.add(Calendar.DAY_OF_MONTH, -3);
Date nowTime = new Date();
// 又获取到当前时间
calendar.setTime(nowTime);
}