Gets the chronology of this date.
Gets the chronology of this date.
The Chronology represents the calendar system in use.
The era and other fields in ChronoField are defined by the chronology.
the chronology, not null
Gets the value of the specified field as a long.
Gets the value of the specified field as a long.
This queries the date-time for the value for the specified field. The returned value may be outside the valid range of values for the field. If the date-time cannot return the value, because the field is unsupported or for some other reason, an exception will be thrown.
Implementations must check and handle all fields defined in ChronoField.
If the field is supported, then the value of the field must be returned.
If unsupported, then a DateTimeException must be thrown.
If the field is not a ChronoField, then the result of this method
is obtained by invoking TemporalField.getFrom(TemporalAccessor)
passing this as the argument.
Implementations must not alter either this object.
the field to get, not null
the value for the field
if numeric overflow occurs
DateTimeExceptionif a value for the field cannot be obtained
Returns the length of the month represented by this date, as defined by the calendar system.
Returns the length of the month represented by this date, as defined by the calendar system.
This returns the length of the month in days.
the length of the month in days
Returns an object of the same type as this object with the specified period added.
Returns an object of the same type as this object with the specified period added.
This method returns a new object based on this one with the specified period added.
For example, on a LocalDate, this could be used to add a number of years, months or days.
The returned object will have the same observable type as this object.
In some cases, changing a field is not fully defined. For example, if the target object is a date representing the 31st January, then adding one month would be unclear. In cases like this, the field is responsible for resolving the result. Typically it will choose the previous valid date, which would be the last valid day of February in this example.
If the implementation represents a date-time that has boundaries, such as LocalTime,
then the permitted units must include the boundary unit, but no multiples of the boundary unit.
For example, LocalTime must accept DAYS but not WEEKS or MONTHS.
Implementations must check and handle all units defined in ChronoUnit.
If the unit is supported, then the addition must be performed.
If unsupported, then a DateTimeException must be thrown.
If the unit is not a ChronoUnit, then the result of this method
is obtained by invoking TemporalUnit.addTo(Temporal, long)
passing this as the first argument.
Implementations must not alter either this object or the specified temporal object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the amount of the specified unit to add, may be negative
the unit of the period to add, not null
an object of the same type with the specified period added, not null
if numeric overflow occurs
DateTimeExceptionif the unit cannot be added
Calculates the period between this date and another date as a ChronoPeriod.
Calculates the period between this date and another date as a ChronoPeriod.
This calculates the period between two dates. All supplied chronologies
calculate the period using years, months and days, however the
ChronoPeriod API allows the period to be represented using other units.
The start and end points are this and the specified date.
The result will be negative if the end is before the start.
The negative sign will be the same in each of year, month and day.
The calculation is performed using the chronology of this date. If necessary, the input date will be converted to match.
This instance is immutable and unaffected by this method call.
the end date, exclusive, which may be in any chronology, not null
the period between this date and the end date, not null
if numeric overflow occurs
DateTimeExceptionif the period cannot be calculated
Calculates the period between this temporal and another temporal in terms of the specified unit.
Calculates the period between this temporal and another temporal in terms of the specified unit.
This calculates the period between two temporals in terms of a single unit.
The start and end points are this and the specified temporal.
The result will be negative if the end is before the start.
For example, the period in hours between two temporal objects can be
calculated using startTime.until(endTime, HOURS).
The calculation returns a whole number, representing the number of complete units between the two temporals. For example, the period in hours between the times 11:30 and 13:29 will only be one hour as it is one minute short of two hours.
There are two equivalent ways of using this method.
The first is to invoke this method directly.
The second is to use TemporalUnit#between(Temporal, Temporal):
// these two lines are equivalent between = thisUnit.between(start, end); between = start.until(end, thisUnit);The choice should be made based on which makes the code more readable.
For example, this method allows the number of days between two dates to be calculated:
val daysBetween: Long = DAYS.between(start, end); // or alternatively val daysBetween: Long = start.until(end, DAYS);
Implementations must begin by checking to ensure that the input temporal
object is of the same observable type as the implementation.
They must then perform the calculation for all instances of ChronoUnit.
A DateTimeException must be thrown for ChronoUnit
instances that are unsupported.
If the unit is not a ChronoUnit, then the result of this method
is obtained by invoking TemporalUnit.between(Temporal, Temporal)
passing this as the first argument and the input temporal as
the second argument.
In summary, implementations must behave in a manner equivalent to this code:
// check input temporal is the same type as this class
if (unit instanceof ChronoUnit) {
// if unit is supported, then calculate and return result
// else throw DateTimeException for unsupported units
}
return unit.between(this, endTemporal);
The target object must not be altered by this method.
the end temporal, of the same type as this object, not null
the unit to measure the period in, not null
the amount of the period between this and the end
if numeric overflow occurs
DateTimeExceptionif the period cannot be calculated
Returns an object of the same type as this object with the specified field altered.
Returns an object of the same type as this object with the specified field altered.
This returns a new object based on this one with the value for the specified field changed.
For example, on a LocalDate, this could be used to set the year, month or day-of-month.
The returned object will have the same observable type as this object.
In some cases, changing a field is not fully defined. For example, if the target object is a date representing the 31st January, then changing the month to February would be unclear. In cases like this, the field is responsible for resolving the result. Typically it will choose the previous valid date, which would be the last valid day of February in this example.
Implementations must check and handle all fields defined in ChronoField.
If the field is supported, then the adjustment must be performed.
If unsupported, then a DateTimeException must be thrown.
If the field is not a ChronoField, then the result of this method
is obtained by invoking TemporalField.adjustInto(Temporal, long)
passing this as the first argument.
Implementations must not alter either this object or the specified temporal object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the field to set in the result, not null
the new value of the field in the result
an object of the same type with the specified field set, not null
if numeric overflow occurs
DateTimeExceptionif the field cannot be set
Adjusts the specified temporal object.
Adjusts the specified temporal object.
This adjusts the specified temporal object using the logic encapsulated in the implementing class. Examples might be an adjuster that sets the date avoiding weekends, or one that sets the date to the last day of the month.
There are two equivalent ways of using this method.
The first is to invoke this method directly.
The second is to use Temporal#with(TemporalAdjuster):
// these two lines are equivalent, but the second approach is recommended temporal = thisAdjuster.adjustInto(temporal); temporal = temporal.with(thisAdjuster);It is recommended to use the second approach,
with(TemporalAdjuster),
as it is a lot clearer to read in code.The implementation must take the input object and adjust it.
The implementation defines the logic of the adjustment and is responsible for
documenting that logic. It may use any method on Temporal to
query the temporal object and perform the adjustment.
The returned object must have the same observable type as the input object
The input object must not be altered. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable temporal objects.
The input temporal object may be in a calendar system other than ISO.
Implementations may choose to document compatibility with other calendar systems,
or reject non-ISO temporal objects by TemporalQueries#chronology() querying the chronology.
This method may be called from multiple threads in parallel. It must be thread-safe when invoked.
the temporal object to adjust, not null
an object of the same observable type with the adjustment made, not null
if numeric overflow occurs
DateTimeExceptionif unable to make the adjustment
Combines this date with a time to create a ChronoLocalDateTime.
Combines this date with a time to create a ChronoLocalDateTime.
This returns a ChronoLocalDateTime formed from this date at the specified time.
All possible combinations of date and time are valid.
the local time to use, not null
the local date-time formed from this date and the specified time, not null
Compares this date to another date, including the chronology.
Compares this date to another date, including the chronology.
The comparison is based first on the underlying time-line date, then
on the chronology.
It is "consistent with equals", as defined by Comparable.
For example, the following is the comparator order:
2012-12-03 (ISO) 2012-12-04 (ISO) 2555-12-04 (ThaiBuddhist) 2012-12-05 (ISO)Values #2 and #3 represent the same date on the time-line. When two values represent the same date, the chronology ID is compared to distinguish them. This step is needed to make the ordering "consistent with equals".
If all the date objects being compared are in the same chronology, then the
additional chronology stage is not required and only the local date is used.
To compare the dates of two TemporalAccessor instances, including dates
in two different chronologies, use ChronoField#EPOCH_DAY as a comparator.
the other date to compare to, not null
the comparator value, negative if less, positive if greater
Checks if this date is equal to another date, including the chronology.
Checks if this date is equal to another date, including the chronology.
Compares this date with another ensuring that the date and chronology are the same.
To compare the dates of two TemporalAccessor instances, including dates
in two different chronologies, use ChronoField#EPOCH_DAY as a comparator.
the object to check, null returns false
true if this is equal to the other date
Formats this date using the specified formatter.
Formats this date using the specified formatter.
This date will be passed to the formatter to produce a string.
The default implementation must behave as follows:
return formatter.format(this);
the formatter to use, not null
the formatted date string, not null
if an error occurs during printing
Gets the value of the specified field as an int.
Gets the value of the specified field as an int.
This queries the date-time for the value for the specified field. The returned value will always be within the valid range of values for the field. If the date-time cannot return the value, because the field is unsupported or for some other reason, an exception will be thrown.
Implementations must check and handle all fields defined in ChronoField.
If the field is supported and has an int range, then the value of
the field must be returned.
If unsupported, then a DateTimeException must be thrown.
If the field is not a ChronoField, then the result of this method
is obtained by invoking TemporalField.getFrom(TemporalAccessor)
passing this as the argument.
Implementations must not alter either this object.
the field to get, not null
the value for the field, within the valid range of values
if numeric overflow occurs
DateTimeExceptionif the value is outside the range of valid values for the field
Gets the era, as defined by the chronology.
Gets the era, as defined by the chronology.
The era is, conceptually, the largest division of the time-line.
Most calendar systems have a single epoch dividing the time-line into two eras.
However, some have multiple eras, such as one for the reign of each leader.
The exact meaning is determined by the Chronology.
All correctly implemented Era classes are singletons, thus it
is valid code to write date.getEra() == SomeEra.NAME).
the chronology specific era constant applicable at this date, not null
A hash code for this date.
A hash code for this date.
a suitable hash code
Checks if this date is after the specified date ignoring the chronology.
Checks if this date is after the specified date ignoring the chronology.
This method differs from the comparison in #compareTo in that it
only compares the underlying date and not the chronology.
This allows dates in different calendar systems to be compared based
on the time-line position.
This is equivalent to using date1.toEpochDay() > date2.toEpochDay().
the other date to compare to, not null
true if this is after the specified date
Checks if this date is before the specified date ignoring the chronology.
Checks if this date is before the specified date ignoring the chronology.
This method differs from the comparison in #compareTo in that it
only compares the underlying date and not the chronology.
This allows dates in different calendar systems to be compared based
on the time-line position.
This is equivalent to using date1.toEpochDay() < date2.toEpochDay().
the other date to compare to, not null
true if this is before the specified date
Checks if this date is equal to the specified date ignoring the chronology.
Checks if this date is equal to the specified date ignoring the chronology.
This method differs from the comparison in #compareTo in that it
only compares the underlying date and not the chronology.
This allows dates in different calendar systems to be compared based
on the time-line position.
This is equivalent to using date1.toEpochDay() == date2.toEpochDay().
the other date to compare to, not null
true if the underlying date is equal to the specified date
Checks if the year is a leap year, as defined by the calendar system.
Checks if the year is a leap year, as defined by the calendar system.
A leap-year is a year of a longer length than normal. The exact meaning is determined by the chronology with the constraint that a leap-year must imply a year-length longer than a non leap-year.
The default implementation uses Chronology#isLeapYear(long).
true if this date is in a leap year, false otherwise
Checks if the specified unit is supported.
Checks if the specified unit is supported.
This checks if the date-time can be queried for the specified unit.
If false, then calling the #plus(TemporalAmount) plus and #minus(TemporalAmount) minus
methods will throw an exception.
Implementations must check and handle all fields defined in ChronoUnit.
If the field is supported, then true is returned, otherwise false
If the field is not a ChronoUnit, then the result of this method
is obtained by invoking TemporalUnit.isSupportedBy(Temporal)
passing this as the argument.
Implementations must not alter this object.
the unit to check, null returns false
true if this date-time can be queried for the unit, false if not
Checks if the specified field is supported.
Checks if the specified field is supported.
This checks if the date-time can be queried for the specified field.
If false, then calling the #range(TemporalField) range and #get(TemporalField) get
methods will throw an exception.
Implementations must check and handle all fields defined in ChronoField.
If the field is supported, then true is returned, otherwise false
If the field is not a ChronoField, then the result of this method
is obtained by invoking TemporalField.isSupportedBy(TemporalAccessor)
passing this as the argument.
Implementations must not alter this object.
the field to check, null returns false
true if this date-time can be queried for the field, false if not
Returns the length of the year represented by this date, as defined by the calendar system.
Returns the length of the year represented by this date, as defined by the calendar system.
This returns the length of the year in days.
The default implementation uses #isLeapYear() and returns 365 or 366.
the length of the year in days
Returns an object of the same type as this object with the specified period subtracted.
Returns an object of the same type as this object with the specified period subtracted.
This method returns a new object based on this one with the specified period subtracted.
For example, on a LocalDate, this could be used to subtract a number of years, months or days.
The returned object will have the same observable type as this object.
In some cases, changing a field is not fully defined. For example, if the target object is a date representing the 31st March, then subtracting one month would be unclear. In cases like this, the field is responsible for resolving the result. Typically it will choose the previous valid date, which would be the last valid day of February in this example.
If the implementation represents a date-time that has boundaries, such as LocalTime,
then the permitted units must include the boundary unit, but no multiples of the boundary unit.
For example, LocalTime must accept DAYS but not WEEKS or MONTHS.
Implementations must behave in a manor equivalent to the default method behavior.
Implementations must not alter either this object or the specified temporal object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the amount of the specified unit to subtract, may be negative
the unit of the period to subtract, not null
an object of the same type with the specified period subtracted, not null
if numeric overflow occurs
DateTimeExceptionif the unit cannot be subtracted
Returns an object of the same type as this object with an amount subtracted.
Returns an object of the same type as this object with an amount subtracted.
This adjusts this temporal, subtracting according to the rules of the specified amount.
The amount is typically a Period but may be any other type implementing
the TemporalAmount interface, such as Duration.
Some example code indicating how and why this method is used:
date = date.minus(period); // subtract a Period instance date = date.minus(duration); // subtract a Duration instance date = date.minus(workingDays(6)); // example user-written workingDays method
Note that calling plus followed by minus is not guaranteed to
return the same date-time.
Implementations must not alter either this object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the amount to subtract, not null
an object of the same type with the specified adjustment made, not null
if numeric overflow occurs
DateTimeExceptionif the subtraction cannot be made
Returns an object of the same type as this object with an amount added.
Returns an object of the same type as this object with an amount added.
This adjusts this temporal, adding according to the rules of the specified amount.
The amount is typically a Period but may be any other type implementing
the TemporalAmount interface, such as Duration.
Some example code indicating how and why this method is used:
date = date.plus(period); // add a Period instance date = date.plus(duration); // add a Duration instance date = date.plus(workingDays(6)); // example user-written workingDays method
Note that calling plus followed by minus is not guaranteed to
return the same date-time.
Implementations must not alter either this object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the amount to add, not null
an object of the same type with the specified adjustment made, not null
if numeric overflow occurs
DateTimeExceptionif the addition cannot be made
Queries this date-time.
Queries this date-time.
This queries this date-time using the specified query strategy object.
Queries are a key tool for extracting information from date-times. They exists to externalize the process of querying, permitting different approaches, as per the strategy design pattern. Examples might be a query that checks if the date is the day before February 29th in a leap year, or calculates the number of days to your next birthday.
The most common query implementations are method references, such as
LocalDate::from and ZoneId::from.
Further implementations are on TemporalQueries.
Queries may also be defined by applications.
Implementations of this method must behave as follows:
public <R> R query(TemporalQuery<R> type) {
// only include an if statement if the implementation can return it
if (query == TemporalQueries.zoneId()) return // the ZoneId
if (query == TemporalQueries.chronology()) return // the Chrono
if (query == TemporalQueries.precision()) return // the precision
// call default method
return super.query(query);
}
the type of the result
the query to invoke, not null
the query result, null may be returned (defined by the query)
if numeric overflow occurs
DateTimeExceptionif unable to query
Gets the range of valid values for the specified field.
Gets the range of valid values for the specified field.
All fields can be expressed as a long integer.
This method returns an object that describes the valid range for that value.
The value of this temporal object is used to enhance the accuracy of the returned range.
If the date-time cannot return the range, because the field is unsupported or for
some other reason, an exception will be thrown.
Note that the result only describes the minimum and maximum valid values and it is important not to read too much into them. For example, there could be values within the range that are invalid for the field.
Implementations must check and handle all fields defined in ChronoField.
If the field is supported, then the range of the field must be returned.
If unsupported, then a DateTimeException must be thrown.
If the field is not a ChronoField, then the result of this method
is obtained by invoking TemporalField.rangeRefinedBy(TemporalAccessorl)
passing this as the argument.
Implementations must not alter either this object.
the field to query the range for, not null
the range of valid values for the field, not null
if the range for the field cannot be obtained
Converts this date to the Epoch Day.
Converts this date to the Epoch Day.
The ChronoField#EPOCH_DAY Epoch Day count is a simple
incrementing count of days where day 0 is 1970-01-01 (ISO).
This definition is the same for all chronologies, enabling conversion.
the Epoch Day equivalent to this date
Outputs this date as a String.
Outputs this date as a String.
The output will include the full local date and the chronology ID.
the formatted date, not null
Returns an adjusted object of the same type as this object with the adjustment made.
Returns an adjusted object of the same type as this object with the adjustment made.
This adjusts this date-time according to the rules of the specified adjuster.
A simple adjuster might simply set the one of the fields, such as the year field.
A more complex adjuster might set the date to the last day of the month.
A selection of common adjustments is provided in TemporalAdjusters.
These include finding the "last day of the month" and "next Wednesday".
The adjuster is responsible for handling special cases, such as the varying
lengths of month and leap years.
Some example code indicating how and why this method is used:
date = date.with(Month.JULY); // most key classes implement TemporalAdjuster date = date.with(lastDayOfMonth()); // static import from TemporalAdjusters date = date.with(next(WEDNESDAY)); // static import from TemporalAdjusters and DayOfWeek
Implementations must not alter either this object. Instead, an adjusted copy of the original must be returned. This provides equivalent, safe behavior for immutable and mutable implementations.
the adjuster to use, not null
an object of the same type with the specified adjustment made, not null
if numeric overflow occurs
DateTimeExceptionif unable to make the adjustment
A date without time-of-day or time-zone in an arbitrary chronology, intended for advanced globalization use cases.
Most applications should declare method signatures, fields and variables as ` LocalDate`, not this interface.
A
ChronoLocalDateis the abstract representation of a date where theChronology chronology, or calendar system, is pluggable. The date is defined in terms of fields expressed byTemporalField, where most common implementations are defined inChronoField. The chronology defines how the calendar system operates and the meaning of the standard fields.When to use this interface
The design of the API encourages the use of
LocalDaterather 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.The primary use case where this interface should be used is where the generic type parameter
<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.When the chronology is defined in the generic type parameter as ? or otherwise unknown at development time, the rest of the discussion below applies.
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
LocalDate.Architectural issues to consider
These are some of the points that must be considered before using this interface throughout an application.
1) Applications using this interface, as opposed to using just
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.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
ChronoLocalDatecould therefore be passed a poorly or maliciously written mutable implementation.3) Applications using this interface must consider the impact of eras.
LocalDateshields users from the concept of eras, by ensuring thatgetYear()returns the proleptic year. That decision ensures that developers can think ofLocalDateinstances 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.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.
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.
6) Most of the time, passing a
ChronoLocalDatethroughout an application is unnecessary, as discussed in the last section below.False assumptions causing bugs in multi-calendar system code
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.
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.
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.
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.
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.
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.
Code that assumes that because the year of
date1is greater than the year ofdate2thendate1is afterdate2is 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.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.
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.
Using LocalDate instead
The primary alternative to using this interface throughout your application is as follows.
LocalDate.LocalDateto and from the user's preferred calendar system during printing and parsingThis 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.
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.
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.
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:
LocalDatebeing passed to the methodLocalDateDevelopers writing low-level frameworks or libraries should also avoid this interface. Instead, one of the two general purpose access interfaces should be used. Use
TemporalAccessorif read-only access is required, or useTemporalif read-write access is required.Specification for implementors
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.
Additional calendar systems may be added to the system. See
Chronologyfor more details.