Access to date and time using fields and units, and date time adjusters.

This package expands on the base package to provide additional functionality for more powerful use cases. Support is included for:

• Units of date-time, such as years, months, days and hours
• Fields of date-time, such as month-of-year, day-of-week or hour-of-day
• Date-time adjustment functions
• Different definitions of weeks

Fields and Units

Dates and times are expressed in terms of fields and units. A unit is used to measure an amount of time, such as years, days or minutes. All units implement TemporalUnit. The set of well known units is defined in ChronoUnit, such as DAYS. The unit interface is designed to allow application defined units.

A field is used to express part of a larger date-time, such as year, month-of-year or second-of-minute. All fields implement TemporalField. The set of well known fields are defined in ChronoField, such as HOUR_OF_DAY. Additional fields are defined by JulianFields, WeekFields and IsoFields. The field interface is designed to allow application defined fields.

This package provides tools that allow the units and fields of date and time to be accessed in a general way most suited for frameworks. Temporal provides the abstraction for date time types that support fields. Its methods support getting the value of a field, creating a new date time with the value of a field modified, and querying for additional information, typically used to extract the offset or time-zone.

One use of fields in application code is to retrieve fields for which there is no convenience method. For example, getting the day-of-month is common enough that there is a method on LocalDate called getDayOfMonth(). However for more unusual fields it is necessary to use the field. For example, date.get(ChronoField.ALIGNED_WEEK_OF_MONTH). The fields also provide access to the range of valid values.

Adjustment and Query

A key part of the date-time problem space is adjusting a date to a new, related value, such as the "last day of the month", or "next Wednesday". These are modeled as functions that adjust a base date-time. The functions implement TemporalAdjuster and operate on Temporal. A set of common functions are provided in TemporalAdjusters. For example, to find the first occurrence of a day-of-week after a given date, use TemporalAdjusters.next(DayOfWeek), such as date.with(next(MONDAY)). Applications can also define adjusters by implementing TemporalAdjuster.

The TemporalAmount interface models amounts of relative time.

In addition to adjusting a date-time, an interface is provided to enable querying via TemporalQuery. The most common implementations of the query interface are method references. The from(TemporalAccessor) methods on major classes can all be used, such as LocalDate::from or Month::from. Further implementations are provided in TemporalQueries as static methods. Applications can also define queries by implementing TemporalQuery.

Weeks

Different locales have different definitions of the week. For example, in Europe the week typically starts on a Monday, while in the US it starts on a Sunday. The WeekFields class models this distinction.

The ISO calendar system defines an additional week-based division of years. This defines a year based on whole Monday to Monday weeks. This is modeled in IsoFields.

Package specification

Unless otherwise noted, passing a null argument to a constructor or method in any class or interface in this package will cause a NullPointerException to be thrown. The Javadoc "@param" definition is used to summarise the null-behavior. The "@throws NullPointerException" is not explicitly documented in each method.

All calculations should check for numeric overflow and throw either an ArithmeticException or a DateTimeException.