4

I'm trying to iterate in my Java program over all weeks between two dates (the end date being today). First, I get the starting date:

Calendar start = Calendar.getInstance();
start = data.getFirstDate(users, threads);

So far, so good. The start date is correct and I can work with it. Now I iterate:

Calendar current = start;
while(current.before(Calendar.getInstance()) {
    // Do something
    current.add(Calendar.DATE, 7);
}

Well, this kind of works. I start at 2002/8/23, then comes 2002/8/30, then 2002/9/7... UNTIL 2002/11/30. The date after that is 2003/0/6, which is neither correct nor even a valid date!

What am I doing wrong? I tried current.add(Calendar.DATE, 7), current.add(Calendar.WEEK_OF_YEAR, 1), current.add(Calendar.DAY_OF_YEAR, 7) and two other ways. Using current.roll(Calendar.DATE, 7) does not work because I stay in the same month. Using GregorianCalendar has no effect as well.

Any suggestions would be greatly appreciated!

Thanks Julian

Julian
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  • FYI, the troublesome old date-time classes such as [`java.util.Date`](https://docs.oracle.com/javase/9/docs/api/java/util/Date.html), [`java.util.Calendar`](https://docs.oracle.com/javase/9/docs/api/java/util/Calendar.html), and `java.text.SimpleDateFormat` are now [legacy](https://en.wikipedia.org/wiki/Legacy_system), supplanted by the [java.time](https://docs.oracle.com/javase/9/docs/api/java/time/package-summary.html) classes built into Java 8 & Java 9. See [Tutorial by Oracle](https://docs.oracle.com/javase/tutorial/datetime/TOC.html). – Basil Bourque Jan 22 '18 at 02:13

4 Answers4

11

The month field in the Calendar API is 0-based not 1-based. So 0 stands for January. Don't ask me why.

hleinone
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1

If you're seeing 0 as a month, that's January since months are 0 index based. 0 is January and 11 is December.

darioo
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  • @Joeri: Thanks. I misread the question, wrote a wrong answer and before I could edit it, I was downvoted. Gotta be more careful in the future ;-) – darioo Dec 02 '10 at 12:26
1

I think your interpretation of the dates is incorrect. The month field is zero-based, i.e. JANUARY is 0. So, 2002/11/30 is DECEMBER 30th.

Edric
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0

tl;dr

LocalDate.now().plusWeeks( 1 ).isBefore( stopDate )

java.time

The modern answer uses java.time classes rather than Calendar.

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.

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

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 );
LocalDate weekLater = today.plusWeeks( 1 ) ;

Get your starting date. You may specify the month by a number, with sane numbering 1-12 for January-December, unlike the crazy zero-based numbering in the legacy class. 

LocalDate start = LocalDate.of( 2017 , 2 , 23 ) ;  // Month is 1-12 for January-December.

Or, better, use the Month enum objects pre-defined, one for each month of the year.

LocalDate start = LocalDate.of( 2017 , Month.FEBRUARY , 23 ) ;

From there, looping is just basic Java, using the LocalDate object’s comparison methods: isBefore, isAfter, and isEqual.

LocalDate ld = start ;
while( ld.isBefore( today ) ) {
    ld = ld.plusWeeks( 1 ) ;
    …
}

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?

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.

Basil Bourque
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