Java 8 comes with a much improved and much required change in the way date and time is handled. Almost all of us have experienced the pains of working with date in java. Most of us have switched to Joda Time, but java 8 has changed that with a much cleaner and more extensive API. Before we look at the API lets look at the date and time concepts. The java date follows the Gregorian Calendar rules. The classes that represent the date and time concepts are present in the java.time package. The important classes in this package are:
- java.time.Period: This class represents the date part of the datetime. It represents the date part in terms of days, months and years. for example 1 year, 2 months and 5 days
- java.time.Duration: This class represents the time part of the datetime. It represents the time part in terms of seconds and nanoseconds. for example ’29 seconds’
- java.time.Instant: This represents a particular instant in the timeline. It stores the number of seconds through epoch time and also has another field that stores the nanoseconds
- java.time.LocalDate: This stores the date part of date-time in terms of years-months-days. It does not store the TimeZone. The class is immutable.
- java.time.LocalTime: This class stores the time part of date time without any TimeZone info.
- java.time.LocalDateTime: This class stores the LocalDate and LocalTime but no TimeZone
- java.time.ZonedDateTime: This class stores the LocalDateTime and the TimeZone info as a ZoneOffset object. The class has access to ZoneRules which can be used to convert to local time
- java.time.ZoneOffset:This stores the time zone offset from UTC. The zone rules are stored in ZoneId.
- java.time.OffsetDateTime:This stores the local datetime with the offset. This class does not have information about Zone Rules.
Lets look at examples for these, later in this tutorial we will look at ways to format and parse time.
Creating a local date
Instant now = Instant.now(); //2014-09-20T14:32:33.646Z
This creates a new time instant. The instant does not have timezone info. If you print the instant it prints the time with UTC time zone.
System.out.println(now.getEpochSecond()); // prints 1411137153
The epoch seconds is the number of seconds since 1970-01-01T00:00:00Z
Adding time to an Instant
Instant tomorrow = now.plus(1,ChronoUnit.DAYS); // prints 2014-09-20T14:32:33.646Z
the plus function allows adding time intervals. The time intervals can be NANOS, MICROS, MILLIS, SECONDS, MINUTES, HOURS, HALF_DAYS, DAYS
Subtracting time from an Instant
Instant yesterday = now.minus(1,ChronoUnit.HALF_DAYS); // prints 2014-09-20T03:38:33.860Z
The minus function allows subtracting time from an instant. It allows the same time intervals as plus.
Comparing two instants
System.out.println(now.compareTo(tomorrow)); // prints -1
The compare function can be used to compare two dates. It returns -1 if the date that is passed is after , 1 if it is before
check if one instant is after another
System.out.println(now.isAfter(yesterday));// prints true
create a LocalDateTime
LocalDateTime localDateTime = LocalDateTime.now(); System.out.println(localDateTime); // prints 2014-09-28T13:01:40.556
Note that this gets the time in the local time zone
Convert LocalDateTime to datetime in particular zone
System.out.println(localDateTime.atZone(ZoneId.of("America/New_York"))); // prints 2014-09-28T13:07:31.207-04:00[America/New_York]
This creates and instance of ZonedDateTime
Get the day of week from DateTime
System.out.println(DayOfWeek.from(localDateTime)); // prints SUNDAY. // (yes, i am working on a sunday :-( )
Get the day of year from DateTime
System.out.println(localDateTime.get(ChronoField.DAY_OF_YEAR)); // prints 271
The other fields that can be returned are MINUTE_OF_HOUR, MINUTE_OF_DAY, HOUR_OF_AMPM, HOUR_OF_DAY, AMPM_OF_DAY, DAY_OF_WEEK, DAY_OF_MONTH, DAY_OF_YEAR, MONTH_OF_YEAR, YEAR, OFFSET_SECONDS (offset from UTC).
Get the LocalDate part of LocalDateTime
System.out.println(localDateTime.toLocalDate()); // prints 2014-09-29
Get the LocalTime part of LocalDateTime
System.out.println(localDateTime.toLocalTime()); // prints 22:26:30.146
Create a LocalDateTime from year, month, day, hour, min
System.out.println(LocalDateTime.of(2014, 10, 1, 10, 0)); // prints 2014-10-01T10:00
Create LocalDateTime by parsing a string
LocalDateTime parsedLocalDateTime = LocalDateTime.parse("2014-01-01T11:00");
Create LocalDateTime in different timezone
System.out.println(LocalDateTime.now(ZoneId.of("UTC"))); // prints 2014-09-29T17:07:26.653 (the local timezone in UTC)
Create LocalDateTime from an Instant and a timezone
Instant now = Instant.now(); System.out.println(LocalDateTime.ofInstant(now, ZoneId.of("UTC"))); 2014-09-29T17:09:19.644
Create a ZonedDateTime
ZonedDateTime zonedDateTime = ZonedDateTime.now(); 2014-09-29T22:41:24.908+05:30[Asia/Calcutta]
Find the difference between two times in specific units
System.out.println(zonedDateTime.until(ZonedDateTime.parse("2014-09-29T22:41:00-10:00"), ChronoUnit.HOURS)); // prints the difference between the current zonedDateTime and the zonedatetime parsed from the above string
Get the offset for the current zonedDateTime
System.out.println(zonedDateTime.getOffset()); // prints the offset e.g. +10:00
Parsing or formatting a datetime using DateTimeFormatter
System.out.println(zonedDateTime.format(DateTimeFormatter.ofPattern("'The' dd 'day of' MMM 'in year' YYYY 'and zone is' z"))); // prints The 29 day of Sep in year 2014 and zone is IST
Converting ZoneDateTime to another zone.
There are two ways to do this. In the first method, the code maintains the same instant but changes the timezone. In the second method the code just changes the timezone but the localtime is constant. Look at this example.
System.out.println(zonedDateTime); System.out.println(zonedDateTime.toInstant()); System.out.println(zonedDateTime.withZoneSameInstant(ZoneId.of("America/Chicago"))); System.out.println(zonedDateTime.withZoneSameLocal(ZoneId.of("America/Chicago"))); // prints //System.out.println(zonedDateTime); //System.out.println(zonedDateTime.toInstant()); //System.out.println(zonedDateTime.withZoneSameInstant(ZoneId.of("America/Chicago"))); //System.out.println(zonedDateTime.withZoneSameLocal(ZoneId.of("America/Chicago")));
The above examples give some of the method present in the DateTime API. There are many other methods but this should suffice for basic usage. If you have a specific problem then do drop us a mail or put in your comments and we will be happy to answer.