Get all Fridays in a date Range in Java

Question

I recently came across a task where i have to get all Fridays in a date range. I wrote a small piece of code and was surprised see some strange behaviour.

Below is m

Answer 1

Here a solution based on new stream-features of Java-8 and using my library Time4J (v4.18 or later):

String start = "01/01/2009";
String end = "12/09/2013";

ChronoFormatter<PlainDate> f =
    ChronoFormatter.ofDatePattern("dd/MM/yyyy", PatternType.CLDR, Locale.ROOT);

PlainDate startDate =
    f.parse(start).with(PlainDate.DAY_OF_WEEK.setToNextOrSame(Weekday.FRIDAY));
PlainDate endDate = f.parse(end);

Stream<PlainDate> fridays =
    DateInterval.stream(Duration.of(1, CalendarUnit.WEEKS), startDate, endDate);

fridays.forEachOrdered(System.out::println);

// output
2009-01-02 
2009-01-09 
... 
2013-08-30 
2013-09-06

// other example: list of fridays in ISO-8601-format
List<String> result =
  DateInterval.between(startDate, endDate)
    .stream(Duration.of(1, CalendarUnit.WEEKS))
    .map((date) -> date.toString()) // or maybe use dd/MM/yyyy => f.format(date)
    .collect(Collectors.toList());

By the way, Java-9 will offer a similar solution (but with exclusive end date boundary), see also this enhancement-issue.

Answer 2

Here is an easier method, using the wonderful http://www.joda.org/joda-time/ library:

String start = "01/01/2009";
String end = "12/09/2013";
DateTimeFormatter pattern = DateTimeFormat.forPattern("dd/MM/yyyy");
DateTime startDate = pattern.parseDateTime(start);
DateTime endDate = pattern.parseDateTime(end);

List<DateTime> fridays = new ArrayList<>();

while (startDate.isBefore(endDate)){
    if ( startDate.getDayOfWeek() == DateTimeConstants.FRIDAY ){
        fridays.add(startDate);
    }
    startDate = startDate.plusDays(1);
}

at the end of this, you'd have the fridays in the fridays array. Simple?

Or if you want to speed things up, once you have gotten a friday, you can switch from using days, to using weeks:

String start = "01/01/2009";
String end = "12/09/2013";
DateTimeFormatter pattern = DateTimeFormat.forPattern("dd/MM/yyyy");
DateTime startDate = pattern.parseDateTime(start);
DateTime endDate = pattern.parseDateTime(end);

List<DateTime> fridays = new ArrayList<>();
boolean reachedAFriday = false;
while (startDate.isBefore(endDate)){
    if ( startDate.getDayOfWeek() == DateTimeConstants.FRIDAY ){
        fridays.add(startDate);
        reachedAFriday = true;
    }
    if ( reachedAFriday ){
        startDate = startDate.plusWeeks(1);
    } else {
        startDate = startDate.plusDays(1);
    }
}

Answer 3

First off, I would not bother with weeks. Set the Calendar to the beginning of the range, and figure out which DOW it is, then increment to get to the next Friday, then simply loop adding 7 days until you are at the end of the range.

Actually, since you are always only going forward, should be something like:

int daysToAdd = FridayDOW - currentDOW;
if (daysToAdd < 0) daysToAdd += 7; 
Date startDate = currentDate.add(Calendar.DAYS, daysToAdd);

Yeah, like that.

Ok, actually, for kicks, here it is in Java 8:

@Test
public void canFindAllFridaysInRange(){
    start = LocalDate.of(2013, 5, 10);
    end = LocalDate.of(2013, 8,30);

    DayOfWeek dowOfStart = start.getDayOfWeek();
    int difference = DayOfWeek.FRIDAY.getValue() - dowOfStart.getValue();
    if (difference < 0) difference += 7;

    List<LocalDate> fridaysInRange = new ArrayList<LocalDate>();

    LocalDate currentFriday = start.plusDays(difference);
    do {
        fridaysInRange.add(currentFriday);
        currentFriday = currentFriday.plusDays(7);
    } while (currentFriday.isBefore(end));

    System.out.println("Fridays in range: " + fridaysInRange);
}

Got to love the new date classes!! Of course a lambda would condense this further.

Answer 4

tl;dr

someLocalDate.with(                                      // Date-only value without time-of-day and without time zone, represented by `LocalDate` class.
    TemporalAdjusters.nextOrSame ( DayOfWeek.FRIDAY ) )  // Moving from one `LocalDate` object to another, to find the next Friday unless the starting date is already a Friday.
)                                                        // Return a `LocalDate` object.

java.time

The other Answers are outdated. The old java.util.Date/.Calendar classes have been supplanted in Java 8 and later by the new java.time framework. Joda-Time library is excellent, continues to be maintained, and even inspired java.time. But the Joda-Time team recommends moving on to java.time as soon as is convenient.

LocalDate

The java.time classes include LocalDate for a date-only value without time-of-day nor time zone. See Tutorial.

First parse your input strings to get LocalDate objects.

String inputStart = "01/01/2009";
String inputStop = "12/09/2013";  // 258 Fridays.
// String inputStop = "01/01/2009";  // 0 Friday.
// String inputStop = "01/02/2009";  // 1 Friday.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern ( "MM/dd/yyyy" );

LocalDate start = LocalDate.parse ( inputStart , formatter );
LocalDate stop = LocalDate.parse ( inputStop , formatter );

In your own code, try-catch for exception in case of bad inputs. And verify that stop is indeed the same or later than start.

TemporalAdjusters

The java.time framework includes the TemporalAdjuster interface as a way of shifting date-time values. For example, getting the next or same Friday for any particular date. On your starting date, call with(TemporalAdjuster adjuster) and pass a pre-defined implementation of a TemporalAdjuster from the class TemporalAdjusters (note the plural s). See Tutorial.

List<LocalDate> fridays = new ArrayList<> ();  // Collect each Friday found.
LocalDate nextOrSameFriday = start.with ( TemporalAdjusters.nextOrSame ( DayOfWeek.FRIDAY ) );
// Loop while we have a friday in hand (non-null) AND that friday is not after our stop date (isBefore or isEqual the stop date).
while ( ( null != nextOrSameFriday ) & (  ! nextOrSameFriday.isAfter ( stop ) ) ) {
    fridays.add ( nextOrSameFriday );  //  Remember this friday.
    nextOrSameFriday = nextOrSameFriday.plusWeeks ( 1 );  // Move to the next Friday, setting up for next iteration of this loop.
}

Dump to console.

System.out.println ( "From: " + start + " to: " + stop + " are " + fridays.size () + " Fridays: " + fridays );

From: 2009-01-01 to: 2013-12-09 are 258 Fridays: [2009-01-02, 2009-01-09, 2009-01-16, 2009-01-23, 2009-01-30, 2009-02-06, 2009-02-13, 2009-02-20, 2009-02-27, 2009-03-06, 2009-03-13, 2009-03-20, 2009-03-27, 2009-04-03, 2009-04-10, 2009-04-17, 2009-04-24, 2009-05-01, 2009-05-08, 2009-05-15, 2009-05-22, 2009-05-29, 2009-06-05, 2009-06-12, 2009-06-19, 2009-06-26, 2009-07-03, 2009-07-10, 2009-07-17, 2009-07-24, 2009-07-31, 2009-08-07, 2009-08-14, 2009-08-21, 2009-08-28, 2009-09-04, 2009-09-11, 2009-09-18, 2009-09-25, 2009-10-02, 2009-10-09, 2009-10-16, 2009-10-23, 2009-10-30, 2009-11-06, 2009-11-13, 2009-11-20, 2009-11-27, 2009-12-04, 2009-12-11, 2009-12-18, 2009-12-25, 2010-01-01, 2010-01-08, 2010-01-15, 2010-01-22, 2010-01-29, 2010-02-05, 2010-02-12, 2010-02-19, 2010-02-26, 2010-03-05, 2010-03-12, 2010-03-19, 2010-03-26, 2010-04-02, 2010-04-09, 2010-04-16, 2010-04-23, 2010-04-30, 2010-05-07, 2010-05-14, 2010-05-21, 2010-05-28, 2010-06-04, 2010-06-11, 2010-06-18, 2010-06-25, 2010-07-02, 2010-07-09, 2010-07-16, 2010-07-23, 2010-07-30, 2010-08-06, 2010-08-13, 2010-08-20, 2010-08-27, 2010-09-03, 2010-09-10, 2010-09-17, 2010-09-24, 2010-10-01, 2010-10-08, 2010-10-15, 2010-10-22, 2010-10-29, 2010-11-05, 2010-11-12, 2010-11-19, 2010-11-26, 2010-12-03, 2010-12-10, 2010-12-17, 2010-12-24, 2010-12-31, 2011-01-07, 2011-01-14, 2011-01-21, 2011-01-28, 2011-02-04, 2011-02-11, 2011-02-18, 2011-02-25, 2011-03-04, 2011-03-11, 2011-03-18, 2011-03-25, 2011-04-01, 2011-04-08, 2011-04-15, 2011-04-22, 2011-04-29, 2011-05-06, 2011-05-13, 2011-05-20, 2011-05-27, 2011-06-03, 2011-06-10, 2011-06-17, 2011-06-24, 2011-07-01, 2011-07-08, 2011-07-15, 2011-07-22, 2011-07-29, 2011-08-05, 2011-08-12, 2011-08-19, 2011-08-26, 2011-09-02, 2011-09-09, 2011-09-16, 2011-09-23, 2011-09-30, 2011-10-07, 2011-10-14, 2011-10-21, 2011-10-28, 2011-11-04, 2011-11-11, 2011-11-18, 2011-11-25, 2011-12-02, 2011-12-09, 2011-12-16, 2011-12-23, 2011-12-30, 2012-01-06, 2012-01-13, 2012-01-20, 2012-01-27, 2012-02-03, 2012-02-10, 2012-02-17, 2012-02-24, 2012-03-02, 2012-03-09, 2012-03-16, 2012-03-23, 2012-03-30, 2012-04-06, 2012-04-13, 2012-04-20, 2012-04-27, 2012-05-04, 2012-05-11, 2012-05-18, 2012-05-25, 2012-06-01, 2012-06-08, 2012-06-15, 2012-06-22, 2012-06-29, 2012-07-06, 2012-07-13, 2012-07-20, 2012-07-27, 2012-08-03, 2012-08-10, 2012-08-17, 2012-08-24, 2012-08-31, 2012-09-07, 2012-09-14, 2012-09-21, 2012-09-28, 2012-10-05, 2012-10-12, 2012-10-19, 2012-10-26, 2012-11-02, 2012-11-09, 2012-11-16, 2012-11-23, 2012-11-30, 2012-12-07, 2012-12-14, 2012-12-21, 2012-12-28, 2013-01-04, 2013-01-11, 2013-01-18, 2013-01-25, 2013-02-01, 2013-02-08, 2013-02-15, 2013-02-22, 2013-03-01, 2013-03-08, 2013-03-15, 2013-03-22, 2013-03-29, 2013-04-05, 2013-04-12, 2013-04-19, 2013-04-26, 2013-05-03, 2013-05-10, 2013-05-17, 2013-05-24, 2013-05-31, 2013-06-07, 2013-06-14, 2013-06-21, 2013-06-28, 2013-07-05, 2013-07-12, 2013-07-19, 2013-07-26, 2013-08-02, 2013-08-09, 2013-08-16, 2013-08-23, 2013-08-30, 2013-09-06, 2013-09-13, 2013-09-20, 2013-09-27, 2013-10-04, 2013-10-11, 2013-10-18, 2013-10-25, 2013-11-01, 2013-11-08, 2013-11-15, 2013-11-22, 2013-11-29, 2013-12-06]

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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.

Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.

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
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). 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 5

This code will print all dates having Friday.

public class Friday {
   public static void main(String[] args) throws ParseException {
     String start = "01/01/2013";
     String end = "12/01/2013";
     SimpleDateFormat dateFormat=new SimpleDateFormat("dd/MM/yyyy");
     Calendar scal=Calendar.getInstance();
     scal.setTime(dateFormat.parse(start));
     Calendar ecal=Calendar.getInstance();
     ecal.setTime(dateFormat.parse(end));

     ArrayList<Date> fridayDates=new ArrayList<>();

     while(!scal.equals(ecal)){
         scal.add(Calendar.DATE, 1);
         if(scal.get(Calendar.DAY_OF_WEEK)==Calendar.FRIDAY){
             fridayDates.add(scal.getTime());
         }
     }

     System.out.println(fridayDates);
 }
}

Answer 6

with Lamma Date :

List<Date> fridays = Dates.from(2015, 12, 1).to(2016, 1, 1).byWeek().on(DayOfWeek.FRIDAY).build();
for (Date friday: fridays) {
    System.out.println(friday);
}