The date time of java8 has been updated

Date time class before JDK 1.8

Date It is a date / time class provided by jdk1.0 and holds milliseconds from the epoch. "GMT January 1, 1970 00:00 to the present millisecond" Most of the methods are deprecated (@Deprecated) due to Y2k and performance issues.

Surviving contractors and methods

//Generate
Date d1 = new Date();
Date d2 = new Date(System.currentTimeMillis() + 1);

// before,after
System.out.println(d1.after(d2));
System.out.println(d1.before(d2));

// set,get
d1.setTime(System.currentTimeMillis() + 1);
d2.setTime(System.currentTimeMillis() - 1);
System.out.println(d1.getTime());
System.out.println(d2.getTime());

Date y2k problem

y2k problem

System.out.println(new Date(10, 1, 1));

⇒ It will be January 1, 1910.

Date.java

public Date(int year, int month, int date, int hrs, int min, int sec) {
   int y = year + 1900;　// <-here
   //Abbreviation
}

Calendar A class that improves the defect of Date.java and represents a calendar as the name suggests.

1. The Calendar class has fields for year, month, day, hour, minute, second, and milliseconds. protected int fields[];

2. An abstract class, implemented with the __ (probably) __Tmeplate Method pattern, By implementing subclasses, the differences in calendars used around the world are absorbed. Example: Japanese calendar, lunar calendar

Calendar core API

Calendar generation

For display

private static void show(Calendar cal) {
    System.out.println("Generated calendar: " + cal.getClass().getName());
    System.out.println(
        cal.get(Calendar.YEAR) + "Year"
        + (cal.get(Calendar.MONTH) + 1) + "Month"
        + cal.get(Calendar.DAY_OF_MONTH) + "Day"
        + cal.get(Calendar.HOUR_OF_DAY) + "Time"
        + cal.get(Calendar.MINUTE) + "Minutes"
        + cal.get(Calendar.SECOND) + "Seconds");
}

Generate

Calendar Default calendar= Calendar.getInstance();
show(Default calendar);

Calendar Japanese calendar_Default time zone= Calendar.getInstance(new Locale("ja", "JP", "JP"));
show(Japanese Calendar_Default time zone);

Calendar default region_JST = Calendar.getInstance(TimeZone.getTimeZone("JST"));
show(Default region_JST);

Calendar Japanese calendar_GMT = Calendar.getInstance(TimeZone.getTimeZone("GMT"), new Locale("ja", "JP", "JP"));
show(Japanese Calendar_GMT);

output

Generated calendar: java.util.GregorianCalendar
November 13, 2017 12:20:00
Generated calendar: java.util.JapaneseImperialCalendar
November 13, 2017 12:20:00
Generated calendar: java.util.GregorianCalendar
November 13, 2017 12:20:00
Generated calendar: java.util.JapaneseImperialCalendar
November 13, 2017 3:20:00

1. GregorianCalendar, generated in default region
2. JapaneseImperialCalendar, generated by making the region Japan

Calendar <=> Date

Calendar,Date conversion

// Date to Calendar
Calendar.getInstance().setTime(new Date());
// Calendar to Date
Calendar.getInstance().getTime();

Difference between add () and roll ()

Both add () and roll () are for increasing / decreasing the specified field (year / month / day / hour / minute / second / millisecond).

add()

Calendar calendar = Calendar.getInstance();
calendar.set(2015, Calendar.NOVEMBER,  30);
System.out.println("initial value: " + formatter.format(calendar.getTime()));

calendar.add(Calendar.MONTH, 3);
System.out.println("add : " + formatter.format(calendar.getTime()));

__2015 / 11/30-> add month +3-> 2016/2/29 __

roll()

Calendar calendar = Calendar.getInstance();
calendar.set(2015, Calendar.NOVEMBER,  30);
System.out.println("initial value: " + formatter.format(calendar.getTime()));

calendar.roll(Calendar.MONTH, 3);
System.out.println("roll : " + formatter.format(calendar.getTime()));

__2015 / 11/30-> roll month +3-> 2015/2/28 __

Lenient & Delay

python

SimpleDateFormat formatter = new SimpleDateFormat("yyyy/MM/dd");
Calendar calendar = Calendar.getInstance();
calendar.set(2015, 15,  32);
System.out.println(formatter.format(calendar.getTime()));  // 2016/05/02

calendar.setLenient(false);
calendar.set(2015, 15,  32);
// Exception in thread "main" java.lang.IllegalArgumentException: MONTH
System.out.println(formatter.format(calendar.getTime()));

output

2016/05/02
Exception in thread "main" java.lang.IllegalArgumentException: MONTH

1. Lenient's default is true, so allow 2015/15/32-> 2016/05/02
2. Lenient = falase doesn't allow month = 15
3. __ The exception is raised by get () instead of set. __ __set () is delayed to prevent unnecessary calculations. __

JDK 1.8 datetime class

__ The main subject! !! __ Many date and time classes have been added to the java.time package since JKD 1.8.

Core API

Core API

TemporalAccessor

1. Get the instance with LocalDate that implements TemporalAccessor.
2. Get the year field using it in the ChronoField that implements TemporalField.
3. TemporalQuery implementation Use the returned TemporalQueries.zoneId () to get the precision.

Temporal Temporal inherits from TemporalAccessor and adds a write interface.

1. Addition and subtraction with Period that implements Temporal Amount.
2. Addition and subtraction with ChronoUnit which implements TemporalUnit.
3. Adjust the year field with the ChronoField that implements TemporalField.
4. LocalDate.of (2010, 1, 10) in temporal.with (LocalDate.of (2010, 1, 10)) is cast to Temporal Adjuster.

TemporalAmount

1. Get a instance with Duration that implements Temporal Amount.
2. Get the value with ChronoUnit which implements TemporalUnit.

TemporalField

I'm getting an instance with ChronoField which implements TemporalField.

TemporalUnit

TemporalAdjuster&TemporalAdjusters

Temporal Adjusters offers a variety of time adjustment strategies.

TemporalQuery, TemporalQueries

Example: How many days are left until the end of the year?

python

TemporalQuery<Long> leftDays = temporal -> {
    LocalDate dayOfTarget = LocalDate.from(temporal);
    LocalDate lastDayOfYear = dayOfTarget.with(TemporalAdjusters.lastDayOfYear());
    Period period = dayOfTarget.until(lastDayOfYear);
    return ChronoUnit.DAYS.between(dayOfTarget, lastDayOfYear);
};
System.out.println(
    LocalDate.of(2015, 10, 3).query(leftDays)
);

Temporal Queries offers a variety of access strategies. Example: What is the precision of the object?

LocalDate.of(2015, 10, 3).query(TemporalQueries.precision())

Date time implementation

Instant

1. __Invariant & Threadsafe __ class that represents epoch seconds (time elapsed from 1970-01-01T 00:00:00 Z).
2. Hold up to nanoseconds.
3. It inherits Temporal and TemporalAdjuster.

LocalDate __ No timezone An immutable & thread safe class that represents a date __ (yyyy-mm-dd). It can be used for birthdays.

LocalTime __ An immutable & thread-safe class that represents no timezone __ (HH: mm: ss.SSSSSSSSS).

LocalDateTime It has LocalDate and LocalTime internally, __ No time zone Date Time __ (yyyy-mm-dd HH: mm: ss.SSSSSSSSS) is an immutable & thread-safe class.

LocalDataTime.java

/**
 * The date part.
 */
private final LocalDate date;
/**
 * The time part.
 */
private final LocalTime time;

ZoneDateTime An immutable & thread-safe class that represents __ (HH: mm: ss.SSSSSSSSS) with a timezone. The basic usage seems to be the same as LocalTimeDate.

OffsetDateTime It is a combination of LocalDateTime and Offset.

https://docs.oracle.com/javase/jp/8/docs/api/java/time/OffsetDateTime.html "OffsetDateTime, ZonedDateTime, and Instant all store instants in time series to nanosecond precision. Instant is the simplest and simply represents instant. OffsetDateTime adds the offset from UTC / Greenwich to the instant. And allow you to get the local date / time. ZonedDateTime adds a complete time zone rule. "

OffsetDateTime.java

/**
 * The local date-time.
 */
private final LocalDateTime dateTime;
/**
 * The offset from UTC/Greenwich.
 */
private final ZoneOffset offset;

YearMonth、MonthDay

YearMonth: An immutable & thread-safe class that represents yyyy-mm. MonthDay: An immutable & thread-safe class that represents mm-dd.

Year Year: An immutable & thread-safe class that represents yyyy.

Month、DayOfWeek Month, an enumeration that represents the month DayOfWeek, an enumeration representing the days of the week

Clock __ A class that represents the system time __ with a time zone. The date / time class now () is generated based on this Clock.

Get Clock

Clock clockDefault = Clock.systemDefaultZone();  

UTC clock

Clock clockUTC = Clock.systemUTC();  

Time zone designation clock

Clock clockParis= Clock.system(ZoneId.of("Europe/Paris"));

Clock is a clock, so time advances

• Reason for getting it with gif

Time advances

Clock clock = Clock.systemDefaultZone();
System.out.println(clock.instant());
Thread.sleep(1000 * 2);
System.out.println(clock.instant());

2017-11-13T11:41:27.548Z
2017-11-13T11:41:29.551Z

Get a special watch

Clock that is stopped

Clock fixedClock = Clock.fixed(Instant.now(), ZoneId.of("Asia/Tokyo"));
System.out.println("fixed clock : " + fixedClock.instant());
Thread.sleep(1000 * 2);
System.out.println("fixed clock : " + fixedClock.instant());

result

fixed clock : 2017-11-13T11:46:01.224Z
fixed clock : 2017-11-13T11:46:01.224Z

__⇒ The clock does not advance. __

• __ It seems that it is better to perform UT using a clock that has stopped the test time. __

Second clock

Clock secondsTickClock = Clock.tickSeconds(ZoneId.of("Asia/Tokyo"));
System.out.println("seconds tick clock : " + secondsTickClock.instant());
Thread.sleep(1000 * 2);
System.out.println("seconds tick clock : " + secondsTickClock.instant());

result

seconds tick clock : 2017-11-13T11:52:20Z
seconds tick clock : 2017-11-13T11:52:22Z

__ ⇒ In seconds, do not hold less than seconds __

Minute clock

Clock minutesTickClock = Clock.tickMinutes(ZoneId.of("Asia/Tokyo"));
System.out.println("minutes tick clock : " + minutesTickClock.instant());
Thread.sleep(1000 * 2);
System.out.println("minutes tick clock : " + minutesTickClock.instant());

result

minutes tick clock : 2017-11-13T11:52:00Z
minutes tick clock : 2017-11-13T11:52:00Z

__ ⇒ Minutes, do not hold less than seconds __

Clock advanced by the specified time

Clock nowClock = Clock.systemDefaultZone();
Clock offsetClock = Clock.offset(nowClock, Duration.ofDays(1));
System.out.println("nowClock : " + nowClock.instant());
System.out.println("offsetClock : " + offsetClock.instant());

result

nowClock : 2017-11-13T11:58:25.647Z
offsetClock : 2017-11-14T11:58:25.647Z

__⇒ Clock one day ahead __

TemporalAmount Duration An immutable & thread-safe class that expresses the amount of time.

• Units are Seconds, Nanos

Period An immutable & thread-safe class that expresses the amount of time.

• Units are Years, Months, Days

ZoneId Class that expresses the time zone

python

ZoneId.systemDefault();
ZoneId.getAvailableZoneIds();
ZoneId.of("Asia/Tokyo");

Conversion between datetime classes

Date time format

DateFormat It is provided by jdk1.0 ??, and the format is controlled by the following two elements.

1. LONG,FULL,MEDIUM.SHORT
2. Locale

Won't you use it anymore? !!

SimpleDateFormat It's a familiar date formatter! !! A subclass of DateFormmat that determines the format with a pattern string.

You can set the time zone

SimpleDateFormat s1 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
SimpleDateFormat s2 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
s2.setTimeZone(TimeZone.getTimeZone("Asia/Shanghai"));
System.out.println(s1.format(new Date()));
System.out.println(s2.format(new Date()));

output

2017/11/14 17:26:53
2017/11/14 16:26:53

DateTimeFormatter It was added from JKD1.8 along with the datetime class. It's in the java.time.format package and is like a combination of DateFormat and SimpleDateFormat?

format

DateTimeFormatter[] formatters = {
    DateTimeFormatter.ISO_DATE,
    DateTimeFormatter.ISO_LOCAL_DATE,
    DateTimeFormatter.ISO_LOCAL_DATE_TIME,
    DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL, FormatStyle.MEDIUM),
    DateTimeFormatter.ofPattern("Gyyyy/MM/dd HH::mm:ss.SSS")
};
LocalDateTime localDateTime = LocalDateTime.now();
Arrays.stream(formatters)
    .forEach(
        formatter ->
            System.out.println(formatter.format(localDateTime))
    );
Arrays.stream(formatters)
    .forEach(
        formatter ->
            System.out.println(localDateTime.format(formatter))
    );

parse

 //Date Time#parse
 LocalDate.parse("2015/10/10", DateTimeFormatter.ofPattern("yyyy/MM/dd"));

 // DateTimeFormatter#parse
 DateTimeFormatter.ofPattern("yyyy/MM/dd").parse("2015/10/10");
 DateTimeFormatter.ofPattern("yyyy/MM/dd").parse("2015/10/10", TemporalQueries.localDate());

From a readability standpoint, datetime #parse is recommended.