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org.joda.time package




Provides support for dates, times, time zones, durations, intervals, and
partials. This package aims to fully replace the Java Date,
Calendar, and TimeZone classes. This implementation
covers both the Gregorian/Julian calendar system and the ISO8601
standard. Additional calendar systems and extensions can be created as well.


The ISO8601 standard is the international standard for dates, times, durations,
and intervals. It defines text representations, the first day of the week as
Monday, and the first week in a year as having a Thursday in it. This standard
is being increasingly used in computer interchange and is the agreed format for
XML. For most uses, the ISO standard is the same as Gregorian, and is thus the
preferred format.


Interfaces

The main API concepts are defined by interfaces:


ReadableInstant - an instant in time
ReadableDateTime - an instant in time with field accessors such as dayOfWeek
ReadablePartial - a definition for local times that are not defined to the millisecond, such as the time of day
ReadableDuration - a duration defined in milliseconds
ReadablePeriod - a time period defined in fields such as hours and minutes
ReadableInterval - a period of time between two instants


ReadWritableInstant - an instant that can be modified
ReadWritableDateTime - a datetime that can be modified
ReadWritablePeriod - a time period that can be modified
ReadWritableInterval - an interval that can be modified


These define the public interface to dates, times, periods, intervals and durations.
As with java.util.Date and Calendar, the design is
millisecond based with an epoch of 1970-01-01.
This should enable easy conversions.


Implementations

The basic implementation of the ReadableInstant interface is
Instant. This is a simple immutable class that stores the
millisecond value and integrates with Java Date and Calendar.
The class follows the definition of the millisecond instant fully, thus
it references the ISO8601 calendar system and UTC time zone.
If you are dealing with an instant in time but do not know, or do not want to specify,
which calendar system it refers to, then you should use this class.


The main implementation class for datetimes is the DateTime class.
This implements the ReadableDateTime interface, providing
convenient methods to access the fields of the datetime. Conversion methods
allow integration with the Java Date and Calendar classes.


Like Instant, DateTime is immutable, and it
can be used safely in a multi-threaded environment. 
In order to be fully immutable, key clases are declared as final.
Abstract superclasses are provided should you need to define your own implementations.


The concrete implementations of the ReadWritable... interfaces are
named the same as their immutable counterparts, but with a "Mutable" prefix.
For example, MutableDateTime implements
ReadWritableDateTime, making datetime editing easy.
Note that it is possible to use the immutable DateTime for modifying datetimes,
however each modification method returns a new instance of DateTime.


Interface usage

The interfaces in Joda-Time are not designed to operate in the same way as those
in the Java Collections Framework (List/Map/Set etc).
The Joda-Time interfaces represent a core subset of the functionality available
via the actual classes.
Thus, much of the work of an application will probably use methods on the class, not
on the interface.
Your application must determine whether it should define dates in terms of the
interfaces, or in terms of the classes.


The interfaces provide simple methods to access an instance of the immutable class,
which is implemented either via typecast or object creation.
Thus, if you hold a reference to a ReadableInstant, and you call the method
toDateTime(), the same instance will be returned (typecast) if it
already was a DateTime.

Chronologies and Fields

In order to enable the package to be easily extended, each field of the
datetime, such as the month, is calculated by an implementation of
DateTimeField. Likewise, duration fields are calculated by
specialized DurationField instances. If desired, users can write
their own implementations to retrieve an unusual field from the millisecond
value.


The datetime and duration fields that together represent a calendar system are
grouped into a Chronology. The chronology represents all the
information to convert from a millisecond value to human understandable fields
in a specific calendar system. Chronologies are provided for ISO,
Gregorian/Julian (GJ), Buddhist, Coptic and Ethiopic.
More implementations are sought from the community.


The chronology and field classes are singletons.
This design results in a low overhead on the date and time classes.
The Java Calendar class performs poorly because it has many internal
fields that are constantly kept in sync.
This design only calculates fields when required, resulting in lightweight and
simple date time classes.


When reviewing the library for the first time, it is easy to mistake the number
of classes with complexity.
The library is in fact clearly divided between user packages and implementation packages
in the javadoc.
Most users will should not need to be concerned with the back-end implementation.


Partials

Partials are like instants, except they do not completely specify a point in
time. The main interface is ReadablePartial.


The main implementations are:

LocalTime - A class storing a local time without a date
LocalDate - A class storing a local date without a time
LocalDateTime - A class storing a local datetime
Partial - A class storing any combination of datetime fields, such as dayOfMonth and dayOfWeek

For consistency, the API of each partial class is similar to that of an instant class.


All partial implementations represent a local time, in other words without a time zone.
Thus, to convert a partial to an instant (which does contain a time zone) requires adding a zone.


Formatting

Formatting is provided by the format subpackage. Comprehensive
support is provided for outputting dates and times in multiple formats. A
pattern similar to Java SimpleDateFormat can be used, but a more
advanced programmatic technique is available via the builder classes.