Re: Date/Calendar confusion

From:

Lew <lewbloch@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 7 Sep 2012 11:01:36 -0700 (PDT)

Message-ID:

<3931f4ca-a41a-489a-8521-545eaed21742@googlegroups.com>

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