How to do get list of calendar dates in array from two calendar dates in java

Question

I am trying to figure out find out list of calendar dates from two specific dates. But I am getting only within a same year not fix two dates.

What I have done:

    private ArrayList<Calendar> weekendList = null;
    public void findWeekendsList(long startDate, long endDate)
    {
        weekendList = new ArrayList();
        Calendar calendarStart = null;
        Calendar calendarEnd=null;

        calendarStart= Calendar.getInstance();
        calendarStart.setTimeInMillis(startDate);



        /*calendarEnd= Calendar.getInstance();
        calendarEnd.setTimeInMillis(endDate);
       */

        // The while loop ensures that you are only checking dates in the current year

       while(calendar.get(Calendar.YEAR) == Calendar.getInstance().get(Calendar.YEAR)){
            // The switch checks the day of the week for Saturdays and Sundays
            switch(calendar.get(Calendar.DAY_OF_WEEK)){
                case Calendar.SATURDAY:
                case Calendar.SUNDAY:

                    weekendList.add(calendar);
                    break;
            }

            // Increment the day of the year for the next iteration of the while loop
            calendar.add(Calendar.DAY_OF_YEAR, 1);
        }
    }

I am getting 102 weekends and it was like jan 9 to dec 31 but i want to today date(jan 9, 2018) to up to next year( like jan 9 2019), all the weekends in arrayList.

If anyone have any idea, that would great help for me.

Answer 1

So, immediately, you have a number of issues which stand out...

• Using an out of date API. Calendar should be avoided at all costs, it's troublesome, clumsy and all way to easy to screw up
• The while loop is looping only so long as the year is the same, so it's ignoring the endDate altogether
• Because Calendar is mutable, all you are producing is a List with the same date/time value contained within it
• You algorithm considers a weekend to be both "Saturday" or "Sunday", where normally, I'd consider a weekend to be the time between "Friday" and "Monday" - Don't know if this is deliberate on your part, but it stands out to me.

Since Java 8 included a newer Date/Time API, you should start using it. If you're not using Java 8+ (and I'd be asking some serious questions as to why not), you should be using a more reliable API - JodaTime comes to mind, but there is also a compatible back port of the Java 8 Date/Time for earlier JDK/JVMs

Now, having said all that, I'd do something more like...

public List<LocalDate> findWeekendsBetween(long startTime, long endTime) {
    LocalDate startDate = LocalDate.ofEpochDay(startTime);
    LocalDate endDate = LocalDate.ofEpochDay(endTime);

    System.out.println("Starting from " + startDate);
    System.out.println("Ending at " + endDate);

    List<LocalDate> weekends = new ArrayList<>(25);
    while (startDate.isBefore(endDate) || startDate.equals(endDate)) {
        switch (startDate.getDayOfWeek()) {
            case SATURDAY:
                weekends.add(startDate);
                startDate = startDate.plusDays(2);
                break;
            default:
                startDate = startDate.plusDays(1);
        }
    }
    return weekends;
}

In fact, I'd even change it so that callers were required to pass you LocalDate values...

public List<LocalDate> findWeekendsBetween(LocalDate startDate, LocalDate endDate) {

as long is ambiguous.

The above algorithm is inclusive of the startTime and endTime and considers a "weekend" to be the time between "Friday" and "Monday", so it will only return "Saturday" values

Then you could call it using something like...

LocalDate startDate = LocalDate.now();
LocalDate endDate = startDate.plusMonths(1);

List<LocalDate> weekends = findWeekendsBetween(startDate.toEpochDay(), endDate.toEpochDay());
System.out.println("Found " + weekends.size() + " Saturday/Sundays");
for (LocalDate date : weekends) {
    System.out.println(date);
}

(as you can see, I'm to lazy to calculate a fixed point in time and just use LocalDate and convert it to a long value)

Which in my testing printed out something like...

Starting from 2018-01-09
Ending at 2018-02-09
Found 4 Weekends
2018-01-13
2018-01-20
2018-01-27
2018-02-03

but you have only added Saturday, it has to be Sundays also.

Yes, as I said, I made it accept only Saturday, as for me, a weekend is inclusive of Saturday AND Sunday.

It would be easily fixed by simply including SUNDAY into the switch statement...

switch (startDate.getDayOfWeek()) {
    case SATURDAY:
    case SUNDAY:
        weekends.add(startDate);
        break;
    default:
        startDate = startDate.plusDays(1);
}

I think, it might be 52*2=104 list size.

I've not tested a full year, I've only checked a month's worth. You could just change the end date, something like...

LocalDate endDate = startDate.plusYears(1);