Joda Time - Go back 1 year, adjusting for specific day-of-week

Question

I want to go 1 year back with joda time, but I don't want to go from 15 February 2010 to 15 February 2009 but rather, if 15 Februari 2010 is a Monday for example, then I want to get to the closest Monday of 2009.

At the moment I am using this:

int dayNumber = Integer.parseInt(iDate.dayOfWeek().getAsString());

if(dayNumber == 1) { // start of week
    println(iDate.plusYears(-1).plusDays(1).dayOfWeek().getAsString());
}

Which works fine for now but there is probably safer way that allows you to go back easily 5 years for example.

Answer 1

I would use Joda Time methods to go back 1 year and then have a little switch statement that moves it to the appropriate day. Can't tell if you want to go to monday or not from your question. Doing your own calculations is going to miss important things like leap years as @Klas pointed out.

iDate = iDate.minusYears(1);
switch (iDate.getDayOfWeek()) {
    case 1:
        // monday so no change
        break;
    case 2:
        iDate = iDate.minusDays(1);
        break;
    case 3:
        iDate = iDate.minusDays(2);
        break;
    ...
}

If you want to get fancy you can build a little grid to speed it up.

// days in joda time are 1 based so 0 won't be used here
private static final int[] daysShift = new int[] = {0, 0, -1, -2, -3, 3, 2, 1};
...
iDate = iDate.minusYears(1);
// based on the current day, shift us by a certain number of days, + or -
iDate.plusDays(daysShift[iDate.getDayOfWeek()]);

Answer 2

tl;dr

LocalDate.now( ZoneId.of( "Pacific/Auckland" ) )
         .minusYears( 1 )
         .previousOrSame( DayOfWeek.MONDAY )

java.time

The Joda-Time project is now in maintenance mode , with the team advising migration to the java.time classes.

LocalDate

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec .

Specify a proper time zone name in the format of continent/region , such as America/Montreal , Africa/Casablanca , or Pacific/Auckland . Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );

Year ago

The java.time classes can do math.

LocalDate yearAgo = today.minusYears( 1 ) ;

Day of week

Determine today's day-of-week.

DayOfWeek dow = today.getDayOfWeek() ;

To adjust into different date-time values, use a TemporalAdjuster implementation. One for your purpose is found in TemporalAdjusters class.

LocalDate previousOrSameDay = yearAgo.with( TemporalAdjusters.previousOrSame( dow ) ;

Or get the following day-of-week.

LocalDate nextOrSameDay = yearAgo.with( TemporalAdjusters.nextOrSame( dow ) ;

You need to define your rules for going to next or previous.

Set< DayOfWeek > previous = EnumSet.range( DayOfWeek.MONDAY , DayOfWeek.THURSDAY ) ;  // Collect Mon, Tue, Wed, and Thu.
Set< DayOfWeek > next = EnumSet.complementOf​( previous) ;  // Collect Fri, Sat, Sun.

Then test your day-of-week.

LocalDate ld ;
if( previous.contains( dow ) ) {
    ld = yearAgo.with( TemporalAdjusters.previousOrSame( dow ) ;
} else if ( next.contains( dow ) ) {
    ld = yearAgo.with( TemporalAdjusters.nextOrSame( dow ) ;
} else {
    // Handle unexpected error.
    System.out.println( "Unexpectedly reached IF-ELSE for day-of-week: " + dow ) ;
}

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date , Calendar , & SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to the java.time classes.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations. Specification is JSR 310 .

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use ThreeTenABP… .

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .

Answer 3

To start, #getAsString()

returns the value converted to a String using Integer.toString

so it's rather silly to convert to a string, and then immediately convert back:

int dayNumber = iDate.dayOfWeek().get();
// or
int dayNumber = iDate.getDayOfWeek();