preface
As a matter of fact, Java8 did not pay much attention to the Date API. For example, the popular Date class was not included in a special package, and SimpleDateFormat, a popular Date formatting utility class, was included in the java.text package.
Even SimpleDateFormat may cause an exception in the case of multithreaded access, the JDK does this If Multiple Threads Access a format concurrently, it must be synchronized.
The methods for manipulating dates in these classes are also confusing and hard to master, and many of the methods are outdated.
The world has long been bitter old! (No wonder I didn’t do well in this area before. There was a reason.)
After Java8, it’s much easier to manipulate dates. Let’s take a look.
Date/time
There are three common classes for dates and times:
- LocalDateTime: indicates the default date format, YYYY-MM-DD HH: MM :ss.fff
- LocalDate: yyyy – MM – dd
- The LocalTime: HH: mm: ss. FFF
LocalDateTime = LocalDate + LocalTime
LocalDate is the year, month and day class
LocalTime is the time, minute and second class
It should be noted that the default time zone of these three classes is the local time zone (the default time zone of the system). Due to different geographical locations, the time expression is also different. Currently, The time zone of Shanghai, Asia/Shanghai, is used in China
From LocalDateTime, you can easily obtain LocalDate and LocalTime
LocalDateTime now = LocalDateTime.now();
LocalDate localDate = now.toLocalDate();
LocalTime localTime = now.toLocalTime();
Copy the codeLocalDate can also be converted to LocalDateTime by adding LocalTime
LocalDateTime localDateTime = localDate.atTime(localTime);
Copy the codeThe LocalTime versa
LocalDateTime localDateTime = localTime.atDate(localDate);
Copy the codeThe methods of the three date classes are similar. Here is an example of LocalDateTime
LocalDateTime now = LocalDateTime.now();
System.out.println("You can also specify a time:" + LocalDateTime.of(2020.4.15.9.30.2));
System.out.println("Current moment:" + now);
System.out.println(Current year: + now.getYear());
System.out.println("Current month:" + now.getMonthValue());
System.out.println("Current date:" + now.getDayOfMonth());
System.out.println("Present tense:" + now.getHour());
System.out.println("Current minute:" + now.getMinute());
System.out.println("Current second:" + now.getSecond());
System.out.println("Plus 2 years:" + now.plusYears(2l));
System.out.println("Minus 3 months:" + now.minusMonths(3l));
System.out.println("Minus 50 days:" + now.minusDays(50l));
System.out.println("Basically realize socialist modernization by 2035." + now.withYear(2035));
System.out.println("Each calculation will return a new object, the original object is not affected." + now);
Copy the codeThese simple API calls you know, have an impression, when coding, there are code prompts, programming obstacles are relatively small. And date, time, timestamp mutual conversion and formatting, is the focus, this aspect is not familiar with, will cause a relatively large programming obstacles.
The time zone
Java8 provides the ZoneId to identify the time zone used by each country
You can use the systemDefault() method to get the default time zone for the current system, such as Asia/Shanghai
ZoneId zoneId = ZoneId.systemDefault();
Copy the codeGet all time zones
Set<String> zoneIds = ZoneId.getAvailableZoneIds();
Copy the codeGets the time zone object based on the time zone name
ZoneId zoneId = ZoneId.of("Asia/Shanghai");
Copy the codeDate time with time zone, ZonedDateTime, which differs from LocalDateTime by adding the time zone at the end. There is no derivative ZonedDate or ZonedTime.
For example, get the date and time with the current time zone and print it
System.out.println(ZonedDateTime.now());
Copy the codePrint result:
2020-07-07T15:17:23.737+ 08:00[Asia/Shanghai]
Copy the codeThe time stamp
The time stamp in the computer refers to the total number of seconds from 00 00 00 00 00 GMT on 01 01 1970 to the present day.
Timestamps are represented by Instant classes
Timestamp of Unix year 1
Instant.EPOCH
Copy the codeGets the current timestamp
Instant now = Instant.now();
Copy the codeConvert the timestamp to the total number of seconds
long second = now.getEpochSecond();
Copy the codeConvert the time to the total millisecond value
long second = now.toEpochMilli();
Copy the codeGetEpochSecond (); toEpochMilli(); toEpochMilli()
The timestamp generates LocalDateTime with the time zone
Instant now = Instant.now();
LocalDateTime dateTime = LocalDateTime.ofInstant(now, ZoneId.systemDefault());
Copy the codeSample diagram of the transformation
formatting
DateTimeFormatter is the latest date formatter class that provides a number of predefined time formats, in contrast to SimpleDateFormat, which was previously used for formatting.
Using LocalDateTime as an example, here are some simple date-time formatting operations:
LocalDateTime now = LocalDateTime.now();
System.out.println("yyyy-MM-dd HH:mm:ss.fff : " + now.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
System.out.println("yyyy-MM-dd : " + now.format(DateTimeFormatter.ISO_LOCAL_DATE));
System.out.println("HH:mm:ss.fff : " + now.format(DateTimeFormatter.ISO_LOCAL_TIME));
// Custom formatting
System.out.println(now.format(DateTimeFormatter.ofPattern("Yyyy yyyy MM dd day HH: MM :ss")));
Copy the codePrint result:
yyyy-MM-dd HH:mm:ss.fff : 2020-07-07T19:21:24.728 // No formatting
yyyy-MM-dd : 2020-07-07
HH:mm:ss.fff : 19:21:24.728
2020years07month07day19:21:24
Copy the codeYou can see it up here
ISO_LOCAL_DATE_TIME = ISO_LOCAL_DATE + ISO_LOCAL_TIME, where ISO refers to the abbreviation of the International Organization for Standardization
The DateTimeFormatter can also format timestamps, although it is important to note that a time zone must be specified; without a time zone, the formatter will not know how to instantly convert to a human date-time field, so an exception will be thrown.
Sample code:
DateTimeFormatter formatter = DateTimeFormatter.ISO_LOCAL_DATE_TIME.withZone(ZoneId.systemDefault());
System.out.println(formatter.format(Instant.now()));
Copy the codePrint result:
2020-07-07T19:42:28.805
Copy the codeThe timestamp custom formatting is the same as LocalDateTime above, but more detail.
Period of time
Duration (Duration); Duration (Duration); Duration (Duration);
- Duration: time seconds minutes
- Period: year month day
The methods of both classes are similar, which we demonstrate with Duration:
Get the time period between current and future time:
LocalTime now = LocalTime.now();
LocalTime hour = now.plusHours(2);
LocalTime minute = now.plusMinutes(10);
LocalTime second = now.plusSeconds(30);
System.out.println(Duration.between(now, hour));
System.out.println(Duration.between(now, minute));
System.out.println(Duration.between(now, second));
Copy the codePrint result:
PT2H
PT10M
PT30S
Copy the codeNote that the default printed result is in the ISO date format, where H in PT2H stands for Hour, M for Minute, and S for Second
Other example operations:
LocalTime now = LocalTime.now();
LocalTime hour = now.plusHours(2);
Duration duration = Duration.between(now, hour);
System.out.println("The number of seconds to get the time range:" + duration.getSeconds());
System.out.println("Get the number of milliseconds for the time period:" +duration.toMillis());
System.out.println("Number of days to obtain time range:" +duration.toDays());
Copy the codePrint result:
The number of seconds to get the period:7200Get the number of milliseconds for the time period:7200000Number of days to obtain a time range:0
Copy the codeThe Duration class also has related methods for adding and subtracting minutes and seconds, but I won’t go into detail here.
regulator
LocalDateTime, Duration, Instant, etc., although many methods of adding and subtracting dates have been provided, they are still relatively limited, and it is difficult to meet the needs of the next Sunday and the next wedding birthday.
To do that, we can use a modifier to adjust the date.
The date adjuster class is TemporalAdjuster, which is an interface. Java8 also provides a tool class for implementing TemporalAdjusters. The use of TemporalAdjusters is as follows:
LocalDateTime now = LocalDateTime.now();
System.out.println("Current time :"+now);
System.out.println("Next Sunday :" + now.with(TemporalAdjusters.next(DayOfWeek.SUNDAY)));
System.out.println("First day of next month :" + now.with(TemporalAdjusters.firstDayOfNextMonth()));
System.out.println("Next business day :" + now.with(l -> {
LocalDateTime dateTime = (LocalDateTime) l;
// Get the day of the week
DayOfWeek dayOfWeek = dateTime.getDayOfWeek();
if (DayOfWeek.FRIDAY.equals(dayOfWeek)){
// Friday plus three days equals working days
return dateTime.plusDays(3);
}else if(DayOfWeek.SATURDAY.equals(dayOfWeek)){
Saturday plus two days
return dateTime.plusDays(2);
}
// Add one day for everything else
return dateTime.plusDays(1);
}));
Copy the codePrint result:
Current time:2020-07-08T00:10:05.549Next Sunday:2020-07-12T00:10:05.549First day of next month:2020-08-01T00:10:05.549Next working day:2020-07-09T00:10:05.549
Copy the codeThe DayOfWeek class has seven constant values from MONDAY to Sunday, such as MONDAY,TUESDAY,WEDNESDAY, and so on.
The Date conversion
The Date class currently has many outdated and unrecommended methods that you might consider replacing if you use it in your project.
The new Date API can use the timestamp millisecond value to new a Date object through the Date class constructor
As follows:
long milli = Instant.now().toEpochMilli();
Date date = new Date(milli);
Copy the codePay special attention! This constructor passes values in milliseconds
Date can be converted to a timestamp using the toInstant method
Instant instant = new Date().toInstant();
Copy the code