Groups > comp.lang.java.programmer > #18594

Re: Date/Calendar confusion

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Subject	Re: Date/Calendar confusion
From	Lew <lewbloch@gmail.com>
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Xref	csiph.com comp.lang.java.programmer:18594

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 John B. Matthews wrote:
> Ulrich Scholz  wrote:
>> calendar2.set(Calendar.YEAR, calendar2.get(Calendar.YEAR) + 1970);
>> calendar2.set(Calendar.MONTH, calendar2.get(Calendar.MONTH) + 1);
>> calendar2.set(Calendar.DAY_OF_MONTH,
>>      calendar2.get(Calendar.DAY_OF_MONTH) + 1);
> 
> Note that Calendar.JANUARY is not 1. Use clear() to set some or all 
> fields to a known (undefined, !isSet()) state.

DANGER!
http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html#clear()
"Sets all the calendar field values and the time value (millisecond offset from 
the Epoch) of this Calendar undefined."

http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html#clear(int)
"Sets the given calendar field value and the time value (millisecond offset from 
the Epoch) of this Calendar undefined."

These set the fields to *undefined* - not zero-equivalents.

I have seen bugs in production caused by a programmer confusing 'clear()' 
with 'set(field, 0)'.
 
> public static void main(String[] args) {
>     TimeZone timeZone = TimeZone.getTimeZone("GMT");
>     SimpleDateFormat f = new SimpleDateFormat(
>         "yyyy-MMM-dd HH:mm:ss.SSS Z");
> 
>     Calendar calendar1 = Calendar.getInstance(timeZone);
> 
>     System.out.println(f.format(calendar1.getTime()));
> 
>     calendar1.clear();

Dangerous. You need to do something to set that 'Calendar' instance
to a consistent state now.

>     System.out.println(calendar1.getTimeInMillis()); // 0
> 
>     Calendar calendar2 = Calendar.getInstance(timeZone);
> 
>     System.out.println(f.format(calendar2.getTime()));
> 
>     calendar2.set(Calendar.YEAR, 1970);
>     calendar2.set(Calendar.MONTH, Calendar.JANUARY);
>     calendar2.set(Calendar.DAY_OF_MONTH, 1);
> 
>     calendar2.clear(Calendar.HOUR);

Better: 'calendar2.set(Calendar.HOUR, 0);'

>     calendar2.clear(Calendar.MINUTE);
>     calendar2.clear(Calendar.SECOND);
>     calendar2.clear(Calendar.MILLISECOND);
> 
>     System.out.println(calendar2.getTimeInMillis()); // 0
> }

-- 
Lew

Thread

Date/Calendar confusion Ulrich Scholz <d7@thispla.net> - 2012-09-06 01:03 -0700
  Re: Date/Calendar confusion nogales <nogales.manuel@gmail.com> - 2012-09-06 02:29 -0700
  Re: Date/Calendar confusion Lew <lewbloch@gmail.com> - 2012-09-06 10:21 -0700
  Re: Date/Calendar confusion "John B. Matthews" <nospam@nospam.invalid> - 2012-09-06 20:53 -0400
    Re: Date/Calendar confusion Lew <lewbloch@gmail.com> - 2012-09-07 11:01 -0700
      Re: Date/Calendar confusion "John B. Matthews" <nospam@nospam.invalid> - 2012-09-07 21:02 -0400
        Re: Date/Calendar confusion Lew <lewbloch@gmail.com> - 2012-09-07 18:44 -0700
          Re: Date/Calendar confusion "John B. Matthews" <nospam@nospam.invalid> - 2012-09-08 09:12 -0400
            Re: Date/Calendar confusion Lew <lewbloch@gmail.com> - 2012-09-08 16:51 -0700

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