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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Copyright (c) 2012, Stephen Colebourne & Michael Nascimento Santos
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
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* * Neither the name of JSR-310 nor the names of its contributors
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/**
* <p>
* The main API for dates, times, instants, and durations.
* </p>
* <p>
* The classes defined here represent the principle date-time concepts,
* including instants, durations, dates, times, time-zones and periods.
* They are based on the ISO calendar system, which is the <i>de facto</i> world
* calendar following the proleptic Gregorian rules.
* All the classes are immutable and thread-safe.
* </p>
* <p>
* Each date time instance is composed of fields that are conveniently
* made available by the APIs. For lower level access to the fields refer
* to the {@code java.time.temporal} package.
* Each class includes support for printing and parsing all manner of dates and times.
* Refer to the {@code java.time.format} package for customization options.
* </p>
* <p>
* The {@code java.time.chrono} package contains the calendar neutral API
* {@link java.time.chrono.ChronoLocalDate ChronoLocalDate},
* {@link java.time.chrono.ChronoLocalDateTime ChronoLocalDateTime},
* {@link java.time.chrono.ChronoZonedDateTime ChronoZonedDateTime} and
* {@link java.time.chrono.Era Era}.
* This is intended for use by applications that need to use localized calendars.
* It is recommended that applications use the ISO-8601 date and time classes from
* this package across system boundaries, such as to the database or across the network.
* The calendar neutral API should be reserved for interactions with users.
* </p>
*
* <h3>Dates and Times</h3>
* <p>
* {@link java.time.Instant} is essentially a numeric timestamp.
* The current Instant can be retrieved from a {@link java.time.Clock}.
* This is useful for logging and persistence of a point in time
* and has in the past been associated with storing the result
* from {@link java.lang.System#currentTimeMillis()}.
* </p>
* <p>
* {@link java.time.LocalDate} stores a date without a time.
* This stores a date like '2010-12-03' and could be used to store a birthday.
* </p>
* <p>
* {@link java.time.LocalTime} stores a time without a date.
* This stores a time like '11:30' and could be used to store an opening or closing time.
* </p>
* <p>
* {@link java.time.LocalDateTime} stores a date and time.
* This stores a date-time like '2010-12-03T11:30'.
* </p>
* <p>
* {@link java.time.ZonedDateTime} stores a date and time with a time-zone.
* This is useful if you want to perform accurate calculations of
* dates and times taking into account the {@link java.time.ZoneId}, such as 'Europe/Paris'.
* Where possible, it is recommended to use a simpler class without a time-zone.
* The widespread use of time-zones tends to add considerable complexity to an application.
* </p>
*
* <h3>Duration and Period</h3>
* <p>
* Beyond dates and times, the API also allows the storage of periods and durations of time.
* A {@link java.time.Duration} is a simple measure of time along the time-line in nanoseconds.
* A {@link java.time.Period} expresses an amount of time in units meaningful
* to humans, such as years or days.
* </p>
*
* <h3>Additional value types</h3>
* <p>
* {@link java.time.Month} stores a month on its own.
* This stores a single month-of-year in isolation, such as 'DECEMBER'.
* </p>
* <p>
* {@link java.time.DayOfWeek} stores a day-of-week on its own.
* This stores a single day-of-week in isolation, such as 'TUESDAY'.
* </p>
* <p>
* {@link java.time.Year} stores a year on its own.
* This stores a single year in isolation, such as '2010'.
* </p>
* <p>
* {@link java.time.YearMonth} stores a year and month without a day or time.
* This stores a year and month, such as '2010-12' and could be used for a credit card expiry.
* </p>
* <p>
* {@link java.time.MonthDay} stores a month and day without a year or time.
* This stores a month and day-of-month, such as '--12-03' and
* could be used to store an annual event like a birthday without storing the year.
* </p>
* <p>
* {@link java.time.OffsetTime} stores a time and offset from UTC without a date.
* This stores a date like '11:30+01:00'.
* The {@link java.time.ZoneOffset ZoneOffset} is of the form '+01:00'.
* </p>
* <p>
* {@link java.time.OffsetDateTime} stores a date and time and offset from UTC.
* This stores a date-time like '2010-12-03T11:30+01:00'.
* This is sometimes found in XML messages and other forms of persistence,
* but contains less information than a full time-zone.
* </p>
*
* <h3>Package specification</h3>
* <p>
* Unless otherwise noted, passing a null argument to a constructor or method in any class or interface
* in this package will cause a {@link java.lang.NullPointerException NullPointerException} to be thrown.
* The Javadoc "@param" definition is used to summarise the null-behavior.
* The "@throws {@link java.lang.NullPointerException}" is not explicitly documented in each method.
* </p>
* <p>
* All calculations should check for numeric overflow and throw either an {@link java.lang.ArithmeticException}
* or a {@link java.time.DateTimeException}.
* </p>
*
* <h3>Design notes (non normative)</h3>
* <p>
* The API has been designed to reject null early and to be clear about this behavior.
* A key exception is any method that takes an object and returns a boolean, for the purpose
* of checking or validating, will generally return false for null.
* </p>
* <p>
* The API is designed to be type-safe where reasonable in the main high-level API.
* Thus, there are separate classes for the distinct concepts of date, time and date-time,
* plus variants for offset and time-zone.
* This can seem like a lot of classes, but most applications can begin with just five date/time types.
* <ul>
* <li>{@link java.time.Instant} - a timestamp</li>
* <li>{@link java.time.LocalDate} - a date without a time, or any reference to an offset or time-zone</li>
* <li>{@link java.time.LocalTime} - a time without a date, or any reference to an offset or time-zone</li>
* <li>{@link java.time.LocalDateTime} - combines date and time, but still without any offset or time-zone</li>
* <li>{@link java.time.ZonedDateTime} - a "full" date-time with time-zone and resolved offset from UTC/Greenwich</li>
* </ul>
* <p>
* {@code Instant} is the closest equivalent class to {@code java.util.Date}.
* {@code ZonedDateTime} is the closest equivalent class to {@code java.util.GregorianCalendar}.
* </p>
* <p>
* Where possible, applications should use {@code LocalDate}, {@code LocalTime} and {@code LocalDateTime}
* to better model the domain. For example, a birthday should be stored in a code {@code LocalDate}.
* Bear in mind that any use of a {@linkplain java.time.ZoneId time-zone}, such as 'Europe/Paris', adds
* considerable complexity to a calculation.
* Many applications can be written only using {@code LocalDate}, {@code LocalTime} and {@code Instant},
* with the time-zone added at the user interface (UI) layer.
* </p>
* <p>
* The offset-based date-time types {@code OffsetTime} and {@code OffsetDateTime},
* are intended primarily for use with network protocols and database access.
* For example, most databases cannot automatically store a time-zone like 'Europe/Paris', but
* they can store an offset like '+02:00'.
* </p>
* <p>
* Classes are also provided for the most important sub-parts of a date, including {@code Month},
* {@code DayOfWeek}, {@code Year}, {@code YearMonth} and {@code MonthDay}.
* These can be used to model more complex date-time concepts.
* For example, {@code YearMonth} is useful for representing a credit card expiry.
* </p>
* <p>
* Note that while there are a large number of classes representing different aspects of dates,
* there are relatively few dealing with different aspects of time.
* Following type-safety to its logical conclusion would have resulted in classes for
* hour-minute, hour-minute-second and hour-minute-second-nanosecond.
* While logically pure, this was not a practical option as it would have almost tripled the
* number of classes due to the combinations of date and time.
* Thus, {@code LocalTime} is used for all precisions of time, with zeroes used to imply lower precision.
* </p>
* <p>
* Following full type-safety to its ultimate conclusion might also argue for a separate class
* for each field in date-time, such as a class for HourOfDay and another for DayOfMonth.
* This approach was tried, but was excessively complicated in the Java language, lacking usability.
* A similar problem occurs with periods.
* There is a case for a separate class for each period unit, such as a type for Years and a type for Minutes.
* However, this yields a lot of classes and a problem of type conversion.
* Thus, the set of date-time types provided is a compromise between purity and practicality.
* </p>
* <p>
* The API has a relatively large surface area in terms of number of methods.
* This is made manageable through the use of consistent method prefixes.
* <ul>
* <li>{@code of} - static factory method</li>
* <li>{@code parse} - static factory method focussed on parsing</li>
* <li>{@code get} - gets the value of something</li>
* <li>{@code is} - checks if something is true</li>
* <li>{@code with} - the immutable equivalent of a setter</li>
* <li>{@code plus} - adds an amount to an object</li>
* <li>{@code minus} - subtracts an amount from an object</li>
* <li>{@code to} - converts this object to another type</li>
* <li>{@code at} - combines this object with another, such as {@code date.atTime(time)}</li>
* </ul>
* <p>
* Multiple calendar systems is an awkward addition to the design challenges.
* The first principle is that most users want the standard ISO calendar system.
* As such, the main classes are ISO-only. The second principle is that most of those that want a
* non-ISO calendar system want it for user interaction, thus it is a UI localization issue.
* As such, date and time objects should be held as ISO objects in the data model and persistent
* storage, only being converted to and from a local calendar for display.
* The calendar system would be stored separately in the user preferences.
* </p>
* <p>
* There are, however, some limited use cases where users believe they need to store and use
* dates in arbitrary calendar systems throughout the application.
* This is supported by {@link java.time.chrono.ChronoLocalDate}, however it is vital to read
* all the associated warnings in the Javadoc of that interface before using it.
* In summary, applications that require general interoperation between multiple calendar systems
* typically need to be written in a very different way to those only using the ISO calendar,
* thus most applications should just use ISO and avoid {@code ChronoLocalDate}.
* </p>
* <p>
* The API is also designed for user extensibility, as there are many ways of calculating time.
* The {@linkplain java.time.temporal.TemporalField field} and {@linkplain java.time.temporal.TemporalUnit unit}
* API, accessed via {@link java.time.temporal.TemporalAccessor TemporalAccessor} and
* {@link java.time.temporal.Temporal Temporal} provide considerable flexibility to applications.
* In addition, the {@link java.time.temporal.TemporalQuery TemporalQuery} and
* {@link java.time.temporal.TemporalAdjuster TemporalAdjuster} interfaces provide day-to-day
* power, allowing code to read close to business requirements:
* </p>
* <pre>
* LocalDate customerBirthday = customer.loadBirthdayFromDatabase();
* LocalDate today = LocalDate.now();
* if (customerBirthday.equals(today)) {
* LocalDate specialOfferExpiryDate = today.plusWeeks(2).with(next(FRIDAY));
* customer.sendBirthdaySpecialOffer(specialOfferExpiryDate);
* }
*
* </pre>
*
* @since 1.8
*/
package java.time;