Java 8 to start now ~ Date time API ~

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Date-time API added in Java 8

When handling date and time in Java7, value retention is separated from Date class and operation is separated from Calendar class, but API group that can be handled collectively in Java8 has been added

Typical class

• LocalDateTime Date time without time zone

• OffsetDateTime Date and time with offset from UTC / Greenwich

• ZonedDateTime Date / time with time zone

• Instant A value that represents a certain time 1970-01-01T 00:00:00 It has a long that represents epoch seconds from Z and an int that represents nanoseconds.

Feature

--Can both hold and manipulate datetime values --Time can be expressed with accuracy up to nanoseconds --Immutable

How to use

The usage of LocalDateTime, OffsetDateTime, and ZonedDateTime is almost the same, so I will explain with LocalDateTime.

Get the current time

LocalDateTime current = LocalDateTime.now();  // -> 2018-03-03T12:09:50.875

Generated by specifying the date and time

LocalDateTime atMin = LocalDateTime.of(2018,3,3,12,1);                  // -> 2018-03-03T12:01
LocalDateTime atSec = LocalDateTime.of(2018,3,3,12,1,23);               // -> 2018-03-03T12:01:23
LocalDateTime atNanoSec = LocalDateTime.of(2018,3,3,12,1,23,123456789); // -> 2018-03-03T12:01:23.123456789

There is a static method to generate by specifying the year, month, day, hour, minute, second, and nanosecond. Fields not specified will be zero The month can be specified by 1 to 12, unlike the Calendar class.

Conversion from Date / Calendar

To convert from Date / Calendar to LocalDateTime, use the newly added .toInstant () method and go through Instant. Get the system default because you need to specify the ZoneId

Calendar now = Calendar.getInstance();
Instant nowInstant = now.toInstant();
LocalDateTime fromCalendar = LocalDateTime.ofInstant(nowInstant, ZoneId.systemDefault());

Get date and time

There is a method to get each year, month, day, hour, minute, second, nanosecond

LocalDateTime current = LocalDateTime.now(); // -> 2018-03-03T12:09:50.875

System.out.println(current.Hour());          // -> 12
System.out.println(current.getMonth());      // -> MARCH
System.out.println(current.getMonthValue()); // -> 3

The getMonth () method returns an enum Month that represents the month.

Date and time manipulation

Operations that the Calendar class was in charge of until Java 7 can be handled directly with LocalDateTime. There is a method that returns LocalDateTime added (plus) and subtracted (minus) for each year, month, day, hour, minute, second, and nanosecond. ** The value of the original instance does not change **

LocalDateTime current = LocalDateTime.now();     // -> 2018-03-03T12:09:50.875

LocalDateTime plusMin = current.plusMinutes(10); // -> 2018-03-03T12:19:50.875
LocalDateTime plusMonth = current.plusMonths(2); // -> 2018-05-03T12:09:50.875
LocalDateTime minusYear = current.minusYears(1); // -> 2017-03-03T12:09:50.875

Formatter

Use the ʻofPattern method of the DateTimeFormatter` class to format the date and time.

LocalDateTime current = LocalDateTime.now();
System.out.println(current.format(DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss.SSS"))); // -> 2018/03/03 12:09:50.875

You can also use constants defined in ISO format

LocalDateTime current = LocalDateTime.now();
System.out.println(current.format(DateTimeFormatter.ISO_DATE));       // -> 2018-03-03
System.out.println(current.format(DateTimeFormatter.ISO_LOCAL_TIME)); // -> 12:09:50.875