Sunday, July 31, 2016

Java 8: Implementing a custom TemporalAdjuster

TemporalAdjusters, introduced in Java 8's new Date and Time API, allow you to perform complex date manipulations. For example, you can adjust a date to the next Friday, or to the last day of the month. There are already several pre-defined TemporalAdjusters, which can be accessed using the static factory methods in the TemporalAdjusters class, as shown below:

import java.time.DayOfWeek;
import java.time.LocalDate;
import static java.time.temporal.TemporalAdjusters.*;

LocalDate date = LocalDate.of(2016, 7, 30);
LocalDate nextFriday = date.with(nextOrSame(DayOfWeek.FRIDAY)); // 2016-08-05
LocalDate monthEnd = date.with(lastDayOfMonth()); // 2016-07-31

If you can't find a suitable TemporalAdjuster, it's quite easy to create your own, by implementing the TemporalAdjuster interface.

Here is an example of a custom TemporalAdjuster, which moves the date forward to the next working day:

public class NextWorkingDay implements TemporalAdjuster {
  public Temporal adjustInto(Temporal temporal) {
    DayOfWeek dayOfWeek = DayOfWeek.of(temporal.get(ChronoField.DAY_OF_WEEK));
    int daysToAdd = dayOfWeek == DayOfWeek.FRIDAY ? 3 :
                   (dayOfWeek == DayOfWeek.SATURDAY ? 2 : 1);
    return, ChronoUnit.DAYS);

Since TemporalAdjuster is a functional interface, you could use a lambda expression, but it is very likely that you will want to use this adjuster in other parts of your code, so it is better to encapsulate the logic in a proper class, which can then be re-used.

To define a TemporalAdjuster with a lambda expression, it is more convenient to use the ofDateAdjuster static factory method because it allows you work with a LocalDate object instead of a low level Temporal.

static TemporalAdjuster NEXT_WORKING_DAY = TemporalAdjusters.ofDateAdjuster(
  date -> {
   DayOfWeek dayOfWeek = date.getDayOfWeek();
   int daysToAdd = dayOfWeek == DayOfWeek.FRIDAY ? 3 :
                  (dayOfWeek == DayOfWeek.SATURDAY ? 2 : 1);
   return date.plusDays(daysToAdd);

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