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 my code:

public class Friday {
    public static void main(String[]args){
        String start = "01/01/2009";
        String end = "12/09/2013";
        String[] startTokens = start.split("/");
        String[] endTokens = end.split("/");
        Calendar  startCal = new GregorianCalendar(Integer.parseInt(startTokens[2]),Integer.parseInt(startTokens[1])-1,Integer.parseInt(startTokens[0]));
        Calendar endCal = new GregorianCalendar(Integer.parseInt(endTokens[2]),Integer.parseInt(endTokens[1])-1, Integer.parseInt(endTokens[0]));

        int startYear = Integer.parseInt(startTokens[2]);
        int endYear = Integer.parseInt(endTokens[2]); 


        int startWeek = startCal.get(Calendar.WEEK_OF_YEAR);
        int endWeek = endCal.get(Calendar.WEEK_OF_YEAR);

        Calendar cal = new GregorianCalendar();
        cal.set(Calendar.DAY_OF_WEEK, Calendar.FRIDAY);
    //  cal.setMinimalDaysInFirstWeek(7);
        ArrayList<String> main = new ArrayList<String>();
        while(startYear <= endYear ){
               cal.set(Calendar.YEAR, startYear);
               System.out.println(cal.getMinimalDaysInFirstWeek());
                if(startYear == endYear){
                    main.addAll(getFridays(startWeek, endWeek, cal));
                }
                else{
                    main.addAll(getFridays(startWeek, 52, cal));
                    startWeek = 1;
                }
                startYear =startYear +1;
        }

        for(String s: main){
            System.err.println(s);
        }
    }
    public static ArrayList<String> getFridays(int startWeek, int endWeek, Calendar cal){
        ArrayList<String> fridays = new ArrayList<String>();
        while(startWeek <= endWeek){
            cal.set(Calendar.WEEK_OF_YEAR, startWeek);
            fridays.add(cal.getTime().toString());
            startWeek = startWeek+1;
        }
        return fridays;
    }
}

Now when I ran the code i noticed that Fridays of 2011 are missing. After some debugging and online browsing i figured that Calendar.WEEK_OF_YEAR is locale specific and I have to use setMinimalDaysInFirstWeek(7) to fix it.

So uncommented the related line in the above code.

From what I understood now first week of year should start from full week of year.

For example Jan 1 2010 is friday. But it should not show up in results as i configured it to treat that week begins from Jan 3rd. But Now i still see the Jan 1 as friday

I am very much confused. Can some one explain why it is happening?

These Stackoverflow articles helped me a bit:

Why dec 31 2010 returns 1 as week of year?

Understanding java.util.Calendar WEEK_OF_YEAR

Answer 1

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]

---

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 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);
    }
}