src/chrono/ChronoLocalDate.js
/*
* @copyright (c) 2016, Philipp Thuerwaechter & Pattrick Hueper
* @copyright (c) 2007-present, Stephen Colebourne & Michael Nascimento Santos
* @license BSD-3-Clause (see LICENSE in the root directory of this source tree)
*/
import {ChronoField} from '../temporal/ChronoField';
import {ChronoUnit} from '../temporal/ChronoUnit';
import {TemporalQueries} from '../temporal/TemporalQueries';
import {Temporal} from '../temporal/Temporal';
import {LocalDate} from '../LocalDate';
/**
* A date without time-of-day or time-zone in an arbitrary chronology, intended
* for advanced globalization use cases.
* <p>
* <b>Most applications should declare method signatures, fields and variables
* as {@link LocalDate}, not this interface.</b>
* <p>
* A {@code ChronoLocalDate} is the abstract representation of a date where the
* {@code Chronology chronology}, or calendar system, is pluggable.
* The date is defined in terms of fields expressed by {@link TemporalField},
* where most common implementations are defined in {@link ChronoField}.
* The chronology defines how the calendar system operates and the meaning of
* the standard fields.
*
* <h4>When to use this interface</h4>
* The design of the API encourages the use of {@code LocalDate} rather than this
* interface, even in the case where the application needs to deal with multiple
* calendar systems. The rationale for this is explored in the following documentation.
* <p>
* The primary use case where this interface should be used is where the generic
* type parameter {@code <C>} is fully defined as a specific chronology.
* In that case, the assumptions of that chronology are known at development
* time and specified in the code.
* <p>
* When the chronology is defined in the generic type parameter as ? or otherwise
* unknown at development time, the rest of the discussion below applies.
* <p>
* To emphasize the point, declaring a method signature, field or variable as this
* interface type can initially seem like the sensible way to globalize an application,
* however it is usually the wrong approach.
* As such, it should be considered an application-wide architectural decision to choose
* to use this interface as opposed to {@code LocalDate}.
*
* <h4>Architectural issues to consider</h4>
* These are some of the points that must be considered before using this interface
* throughout an application.
* <p>
* 1) Applications using this interface, as opposed to using just {@code LocalDate},
* face a significantly higher probability of bugs. This is because the calendar system
* in use is not known at development time. A key cause of bugs is where the developer
* applies assumptions from their day-to-day knowledge of the ISO calendar system
* to code that is intended to deal with any arbitrary calendar system.
* The section below outlines how those assumptions can cause problems
* The primary mechanism for reducing this increased risk of bugs is a strong code review process.
* This should also be considered a extra cost in maintenance for the lifetime of the code.
* <p>
* 2) This interface does not enforce immutability of implementations.
* While the implementation notes indicate that all implementations must be immutable
* there is nothing in the code or type system to enforce this. Any method declared
* to accept a {@code ChronoLocalDate} could therefore be passed a poorly or
* maliciously written mutable implementation.
* <p>
* 3) Applications using this interface must consider the impact of eras.
* {@code LocalDate} shields users from the concept of eras, by ensuring that {@code getYear()}
* returns the proleptic year. That decision ensures that developers can think of
* {@code LocalDate} instances as consisting of three fields - year, month-of-year and day-of-month.
* By contrast, users of this interface must think of dates as consisting of four fields -
* era, year-of-era, month-of-year and day-of-month. The extra era field is frequently
* forgotten, yet it is of vital importance to dates in an arbitrary calendar system.
* For example, in the Japanese calendar system, the era represents the reign of an Emperor.
* Whenever one reign ends and another starts, the year-of-era is reset to one.
* <p>
* 4) The only agreed international standard for passing a date between two systems
* is the ISO-8601 standard which requires the ISO calendar system. Using this interface
* throughout the application will inevitably lead to the requirement to pass the date
* across a network or component boundary, requiring an application specific protocol or format.
* <p>
* 5) Long term persistence, such as a database, will almost always only accept dates in the
* ISO-8601 calendar system (or the related Julian-Gregorian). Passing around dates in other
* calendar systems increases the complications of interacting with persistence.
* <p>
* 6) Most of the time, passing a {@code ChronoLocalDate} throughout an application
* is unnecessary, as discussed in the last section below.
*
* <h4>False assumptions causing bugs in multi-calendar system code</h4>
* As indicated above, there are many issues to consider when try to use and manipulate a
* date in an arbitrary calendar system. These are some of the key issues.
* <p>
* Code that queries the day-of-month and assumes that the value will never be more than
* 31 is invalid. Some calendar systems have more than 31 days in some months.
* <p>
* Code that adds 12 months to a date and assumes that a year has been added is invalid.
* Some calendar systems have a different number of months, such as 13 in the Coptic or Ethiopic.
* <p>
* Code that adds one month to a date and assumes that the month-of-year value will increase
* by one or wrap to the next year is invalid. Some calendar systems have a variable number
* of months in a year, such as the Hebrew.
* <p>
* Code that adds one month, then adds a second one month and assumes that the day-of-month
* will remain close to its original value is invalid. Some calendar systems have a large difference
* between the length of the longest month and the length of the shortest month.
* For example, the Coptic or Ethiopic have 12 months of 30 days and 1 month of 5 days.
* <p>
* Code that adds seven days and assumes that a week has been added is invalid.
* Some calendar systems have weeks of other than seven days, such as the French Revolutionary.
* <p>
* Code that assumes that because the year of {@code date1} is greater than the year of {@code date2}
* then {@code date1} is after {@code date2} is invalid. This is invalid for all calendar systems
* when referring to the year-of-era, and especially untrue of the Japanese calendar system
* where the year-of-era restarts with the reign of every new Emperor.
* <p>
* Code that treats month-of-year one and day-of-month one as the start of the year is invalid.
* Not all calendar systems start the year when the month value is one.
* <p>
* In general, manipulating a date, and even querying a date, is wide open to bugs when the
* calendar system is unknown at development time. This is why it is essential that code using
* this interface is subjected to additional code reviews. It is also why an architectural
* decision to avoid this interface type is usually the correct one.
*
* <h4>Using LocalDate instead</h4>
* The primary alternative to using this interface throughout your application is as follows.
* <p><ul>
* <li>Declare all method signatures referring to dates in terms of {@code LocalDate}.
* <li>Either store the chronology (calendar system) in the user profile or lookup
* the chronology from the user locale
* <li>Convert the ISO {@code LocalDate} to and from the user's preferred calendar system during
* printing and parsing
* </ul><p>
* This approach treats the problem of globalized calendar systems as a localization issue
* and confines it to the UI layer. This approach is in keeping with other localization
* issues in the java platform.
* <p>
* As discussed above, performing calculations on a date where the rules of the calendar system
* are pluggable requires skill and is not recommended.
* Fortunately, the need to perform calculations on a date in an arbitrary calendar system
* is extremely rare. For example, it is highly unlikely that the business rules of a library
* book rental scheme will allow rentals to be for one month, where meaning of the month
* is dependent on the user's preferred calendar system.
* <p>
* A key use case for calculations on a date in an arbitrary calendar system is producing
* a month-by-month calendar for display and user interaction. Again, this is a UI issue,
* and use of this interface solely within a few methods of the UI layer may be justified.
* <p>
* In any other part of the system, where a date must be manipulated in a calendar system
* other than ISO, the use case will generally specify the calendar system to use.
* For example, an application may need to calculate the next Islamic or Hebrew holiday
* which may require manipulating the date.
* This kind of use case can be handled as follows:
* <p><ul>
* <li>start from the ISO {@code LocalDate} being passed to the method
* <li>convert the date to the alternate calendar system, which for this use case is known
* rather than arbitrary
* <li>perform the calculation
* <li>convert back to {@code LocalDate}
* </ul><p>
* Developers writing low-level frameworks or libraries should also avoid this interface.
* Instead, one of the two general purpose access interfaces should be used.
* Use {@link TemporalAccessor} if read-only access is required, or use {@link Temporal}
* if read-write access is required.
*
* <h3>Specification for implementors</h3>
* This interface must be implemented with care to ensure other classes operate correctly.
* All implementations that can be instantiated must be final, immutable and thread-safe.
* Subclasses should be Serializable wherever possible.
* <p>
* Additional calendar systems may be added to the system.
* See {@link Chronology} for more details.
* <p>
* In JDK 8, this is an interface with default methods.
* Since there are no default methods in JDK 7, an abstract class is used.
*/
export class ChronoLocalDate extends Temporal {
isSupported(fieldOrUnit) {
if (fieldOrUnit instanceof ChronoField) {
return fieldOrUnit.isDateBased();
} else if (fieldOrUnit instanceof ChronoUnit) {
return fieldOrUnit.isDateBased();
}
return fieldOrUnit != null && fieldOrUnit.isSupportedBy(this);
}
query(query) {
if (query === TemporalQueries.chronology()) {
return this.chronology();
} else if (query === TemporalQueries.precision()) {
return ChronoUnit.DAYS;
} else if (query === TemporalQueries.localDate()) {
return LocalDate.ofEpochDay(this.toEpochDay());
} else if (query === TemporalQueries.localTime() || query === TemporalQueries.zone() ||
query === TemporalQueries.zoneId() || query === TemporalQueries.offset()) {
return null;
}
return super.query(query);
}
adjustInto(temporal) {
return temporal.with(ChronoField.EPOCH_DAY, this.toEpochDay());
}
}
