CODEWITHSUNDEEP
javacalendar
user10756738
user10756738

Reputation:

Subtracting one day off Calendar doesn't work

I am trying to build a simple program with java.util.Calendar. When trying to get the weekday before, my output always stays the same. Code and what I tried below:

Calendar c = Calendar.getInstance();
        c.set(Calendar.MONTH, month);
        c.set(Calendar.DAY_OF_MONTH, day);
        c.set(Calendar.YEAR, year);
        int date = c.get(Calendar.DAY_OF_WEEK);
        return new SimpleDateFormat("EEEE").format(date).toUpperCase();

This was my code at the start. I have tried subtracting day by one:

day = day-1;

I have tried adding minus one to both the Day of Month and Day of week field:

c.add(Calendar.DAY_OF_WEEK, -1);
c.add(Calendar.DAY_OF_MONTH, -1);

I think that it has something to do with my SimpleDateFormat, but I am not sure.

Upvotes: 0

Views: 109

Answers (2)

Jim Garrison
Jim Garrison

Reputation: 86774

The reason this fails is that you aren't invoking the method you think you are. There is no SimpleDateFormat#format(int). But SimpleDateFormat extends Format, which declares Format#format(Object). Your int is boxed to Integer and then the relevant code in Format#format(Object) is

    if (obj instanceof Date)
        return format( (Date)obj, toAppendTo, fieldPosition );
    else if (obj instanceof Number)
        return format( new Date(((Number)obj).longValue()),
                      toAppendTo, fieldPosition );
    else
        throw new IllegalArgumentException("Cannot format given Object as a Date");

At this point, since the value you passed is boxed to Integer which extends Number, the second branch of the if-else is taken and your day-of-week value is converted to a date via the constructor. The number is interpreted as a millisecond value and when you subtracted 1 you changed the time by 1 millisecond, not 1 day.

All that said, you should NOT be using Calendar, use the new date/time API in the java.time package.

Upvotes: 1

rzwitserloot
rzwitserloot

Reputation: 103244

I am trying to build a simple program with java.util.Calendar.

That is impossible. Specifically, the 'simple program' part. Nothing that uses Calendar is simple.

The calendar API is horrible; it makes no sense (the first month of the year is... 0, to change values, you have to use int constants, which isn't idiomatic java), and is confused about what it is trying to represent (it's not a calendar, it's a date/time value, or, is it? Is it solarflares time, appointment time, or alarmclock time? It's confused and doesn't know). That's why there is a new API: java.time. Java does not remove stuff even if it is obsolete, because that would break old code. So, the fact that Calendar is still around doesn't mean much.

Use java.time.

I think that it has something to do with my SimpleDateFormat, but I am not sure.

It doesn't. But let's forget about this silly API and use java time instead!

// note that in calendar, january is 0, and that is insane.
// in localdate, it is 1, which is sane.
// thus, assuming you have `month = month - 1;` someplace in your code...
// remove that.

LocalDate date = LocalDate.of(year, month, day);
DayOfWeek day = date.getDayOfWeek();
System.out.println(day);

Wanna go back a day? Okay.

DayOfWeek day = date.minusDays(1).getDayOfWeek();
System.out.println(day);

Upvotes: 3

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