Java Programmers FAQ

[Peter van der Linden]

Here are the DIFFS for the latest Java FAQ.

11c11
- Last-modified: 1999/03/16
---
+ Last-modified: 1999/03/26
91,93c92,93
- RMI).
- If your interest is Java, leave the downrev J++ book back on the
- shelf.
---
+ RMI). J++ does not implement Java 2. If your interest is Java,
+ leave the downrev J++ book back on the shelf.
1094c1094,1097
- There are some good Java games applets at
---
+ There are some excellent Java games applets at
+ http://www.javaarcade.com Check out the pinball -- dig that crazy
+ rhythm, man.
+ There are more good Java games applets at
+
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- Many people think that contracts prohibiting the distribution of a
- third party's products are somewhat sleazy. Such contracts are also in
- restraint of competition and illegal when used by a monopoly. This is
- why Microsoft is facing mounting legal problems in the United States,
- Italy, Brazil, and the European Union.
---
+ Many people think that contracts between "A" and "B",
+ restricting/discouraging "B" from distributing "C's" products are
+ sleazy. ["A" is Microsoft, "B" is AOL, many ISPs, computer vendors,
+ etc. "C" is Netscape"]. Such contracts are in restraint of competition
+ and illegal when used by a monopoly. This is why Microsoft is facing
+ mounting legal problems in the United States, Italy, Brazil, and the
+ European Union.
1970c1982
- $ js pack/*.java # works
---
+ $ jc pack/*.java # works
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- This policy actually gives you more consistent results than on a system
- where floating point output is deliberately rounded to make the output
- "pretty".
---
+ You are actually getting more accuracy than on systems (like C,C++)
+ that suppress the less significant digits.
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+ 20. (Sect. 15) How do I "timeout" a socket read, as the "select" function
+ in Unix does?
+
+ [*] There are two timeouts at issue here.
+ Use the ServerSocket.setSoTimeout for timing out your 'accept()' calls.
+
+ For a select() like mechanism, you can poll InputStreams of your
+ connections for available() > 0
+
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+
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- (application) that could append itself to some other Java program. That
- is just normal file processing, no different to a program written in C,
- C++, Fortran, COBOL, etc. What makes PC viruses dangerous is when they
- have a hidden means of travelling between places, such as being
- attached to code that is automatically executed, like boot sector code,
- or Word macro initialization files.
---
+ (application) that could append itself to some other Java program. They
+ termed it "Strange Brew". That is just normal file processing, no
+ different to a program written in C, C++, Fortran, COBOL, etc. What
+ makes PC viruses dangerous is when they have a hidden means of
+ travelling between places, such as being attached to code that is
+ automatically executed, like boot sector code, or Word macro
+ initialization files. No one has yet produced a virus that does this in
+ Java.
+
+ In March 1999, reports surfaced of a virus called BeanHive. Again this
+ was not Java-related, but relied on the browser user explicitly
+ accepting an unknown certificate as trusted when prompted. If the user
+ allowed that, then any code from that source will be uploaded and run
+ without further input from the user. Don't remove security for
+ untrusted code simply because it asks for it, is the lesson here.
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+ See also http://www.hi.is/~logir/logi.crypto/
8095,8098c8126
- Java to get the Julian date, written by Joe Preston: Watch those month
- numbers, this expects month to be in range 1..12. Java Date dumbly uses
- 0..11. (This code is being rewritten for the FAQ. I include the current
- code as a placeholder to remind me to replace it).
---
+ Java to get the Julian date, written by Paul Hill.
8100,8118c8128,8129
+ /**
+ * This Calendar provides Julian Day and "Calendar Day" methods,
+ * The Calendar Day starts at _local_ midnight.
+ *
+ * @author Paul A. Hill
+ * @version 1.1
+ * @since JDK 1.1.7 3/99
+ * @see java.util.Calendar
+ */
+ public class FAQCalendar extends GregorianCalendar implements Cloneable {
+ /**
+ ** this constant and the ones that follow are actually private
+ ** in the GregorianCalendar, so we'll redefine them here.
+ **/
+ public static final long
+ ONE_SECOND = 1000,
+ ONE_MINUTE = ONE_SECOND * 60,
+ ONE_HOUR = ONE_MINUTE * 60;
+ /**
+ * this next constant and the others with it have their uses,
+ * but watch out for DST days and the weeks that contain them.
+ * Note also that 1/1/1970 doesn't start on the first day of the week.
+ */
+ protected static final long
+ ONE_DAY = ONE_HOUR * 24,
+ ONE_WEEK = ONE_DAY * 7;
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+ /** The number of days from Jan 1, 4713 BC (Proleptic Julian)
+ ** to 1/1/1970 AD (Gregorian). 1/1/1970 is time 0 for a java.util.Date.
+ **/
+ public static final long JULIAN_DAY_OFFSET = 2440588L;
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- The ACM algorithms aren't online (they should be). Look for this one in
- Communications of the ACM, page 444, vol 6, issue 8, Aug 1963. There's
- a follow up in CACM, p661, vol 7, issue 11, Nov 1964 by D.K. Oppenheim.
- The magazine algorithm is in Algol 60 for all you amateur historians.
- The code to convert back from Julian to DD MM YY is:
---
+ /**
+ * Same as GregorianCalendar(). Creates a calendar with the time set to
+ * the moment it was created.
+ *
+ Note: for Brevety I have not provided all of the other
+ * constructors that you can find in GregorianCalendar.
+ *
+ * @see java.util.GregorianCalendar#GregorianCalendar
+ */
+ public FAQCalendar() {
+ super();
+ }
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- public void CalendarDate(
- long jdays /* Input julian day #. */
- )
- {
- long temp;
- int wkyear, wkmonth, wkday;
---
+ /**
+ * A calendar day as defined here, is like the Julian Day but it starts
+ * and ends at _local_ 12 midnight, as defined by the timezone which is
+ * attached to this calendar. This value is useful for comparing
+ * any date in the calendar to any other date to determine how many days
+ * between them, i.e. tomorrow - today = 1
+ *
+ * @return the calendar day (see above).
+ * @see #getCalendarDay
+ */
+ public long getCalendarDay() {
+ TimeZone tz = getTimeZone();
+ // Figure the exact offset for the exact time of day.
+ int offset = tz.getOffset( get( ERA ), get( YEAR ),
+ get( MONTH ), get( DAY_OF_MONTH ), get( DAY_OF_WEEK ),
+ (int)((long)get( HOUR_OF_DAY ) * ONE_HOUR +
+ get( MINUTE ) * ONE_MINUTE +
+ get( SECOND ) * ONE_SECOND ) );
+ return round( ONE_DAY, getTime().getTime() + offset ) + JULIAN_DAY_OFFSET;
+ }
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- jdays -= 1721119L; /* Remove days before 00-00-0000. */
---
+ /**
+ * Sets the date in the calendar to 00:00 (midnight) on the local calendar day.
+ * See getCalendarDay for the definition of calendar day as used in this class.
+ *
+ * @param calendarDay the day to set the calendar to.
+ * @see #setCalendarDay
+ * @see java.util.TimeZone#getRawOffset
+ */
+ public void setCalendarDay( long calendarDay ) {
+ // Set to the beginning of the Julian day.
+ // Then add in the difference to make it 00:00 local time.
+ setJulianDay( calendarDay );
+ setTimeInMillis( getTime().getTime() - getTimeZone().getRawOffset() );
+ // we may have gone slightly too far, because we used the
+ // raw offset (diff between Standard time to GMT/UT, instead of the
+ // actual value for this day, so during DLS we may be at 1 AM or whatever
+ // the local DLS offset is), so we'll just drop back to midnight.
+ set( HOUR_OF_DAY, 0 );
+ }
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- wkyear = (int)((4L * jdays - 1L) / 146097L); /* Get century. */
- jdays = 4L * jdays - 1L - 146097L * wkyear; /* And subtract it out. */
- temp = jdays / 4L;
---
+ /**
+ * Finds the number of days after 12/31/4312 BC 24:00 GMT on a proleptic
+ * Julian Calendar (i.e. extending the Julian Calendar into pre-history)
+ * to the current time.
+ *
+ The Astronomers Julian Day begins at noon. The Julian Day used here
+ * sometimes called the Chronologists or Historians Julian Day
+ * starts at midnight. For more information see
+ * http://www.magnet.ch/serendipity/hermetic/cal_stud/jdn.htm#astronomical
+ *
+ Note: This routine does NOT take into consideration
+ * leap seconds.
+ *
+ * @return the day number of the current time from 1/
+ * @see #getCalendarDay
+ */
+ public long getJulianDay() {
+ return round( ONE_DAY, getTime().getTime() ) + JULIAN_DAY_OFFSET;
+ }
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- jdays = (4L * temp + 3L) / 1461L; /* Get century offset. */
- wkyear = (int)(100L * wkyear + jdays); /* Add it into year. */
- temp = (4L * temp + 3L) - 1461L * jdays; /* And subtract it out. */
- temp = (temp + 4L) / 4L;
---
+ /**
+ * Sets the current date contained in this calendar to exactly
+ * 00:00 GMT on the date defined by the Julian Day provided.
+ *
+ * @param julianDay the Julian Day to set the calendar to
+ * @see #setCalendarDay
+ */
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- wkmonth = (unsigned)((5L * temp - 3L) / 153L); /* Get month. */
- temp = 5L * temp - 3L - 153L * wkmonth; /* And subtract it out. */
- wkday = (unsigned)((temp + 5L) / 5L);
---
+ public void setJulianDay( long julianDay ) {
+ setTimeInMillis( ( julianDay - JULIAN_DAY_OFFSET ) * ONE_DAY );
+ }
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- if (wkmonth < 10) /* Shift Jan & Feb back into first 2 months. */
- wkmonth += 3;
- else
- {
- wkmonth -= 9;
- wkyear++;
---
+ /**
+ * This is a utility routine for rounding (toward negative) to the nearest
+ * multiple of the conversion factor.
+ *
+ BUG? Why is this different than the formula given in
+ * java.util.GregorianCalendar private millisToJulianDay?
+ *
+ * @param conversion typically one of the constants defined
+ * above ONE_MINUTE, ONE_DAY etc.
+ * @param fractions the value to convert expressed in the same units
+ * as the conversion factor (i.e milliseconds).
+ *
+ * @return the value divided by the conversion factor, rounded to the negative.
+ * @see java.util.Calendar
+ */
+ protected static final long round( long conversion, long fractions ) {
+ long wholeUnits;
+
+ // round toward negative:
+ // For secs rounded to minutes -61/60=-1, -60/60=-1, -59/60=0,
+ // but we want -2, -1, -1 not -1,-1,0
+ // or month 0..11 => year 1; -12..-1 => 0; -24..-13 => -1
+
+ if ( fractions >= 0 ) {
+ wholeUnits = fractions / conversion;
+ }
+ else {
+ wholeUnits = ( fractions + 1 )/ conversion - 1;
+ }
+ return wholeUnits;
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- set(wkyear, wkmonth - 1, wkday);
- } // CalendarDate
---
+ } // FAQCalendar
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- Again, watch those month numbers!
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- Nat Pryce, Jeff Luszcz, Brent Callaghan, Neil Allen, Joe Preston, Tim Bell
---
+ Nat Pryce, Jeff Luszcz, Brent Callaghan, Neil Allen, Joe Preston, Tim Bell,
+ Rajesh,
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