My First Java Applet

by Neil Rieck

Unlike many Java Applets Calendars which are based upon the "Date" function,
this one is based upon the "GregorianCalendar" function which was introduced in JDK1.1

The "correction date" is defaulted to October 15, 1582.
where Pope Gregory authorized the removal of 10 days.
(along with special leap-year cases for 100 and 400 year boundaries)

click here if the applet doesn't initialize

Gregorian Calendar Notes:

Easter was originally linked to Passover, but in the year 325 AD at the council at Nicaea, it was decided that Easter would be celebrated on the first Sunday following the first full moon on, or after, the vernal equinox (the passing from winter into spring). In the year 325, the vernal equinox was assumed to be fixed at March 21.

Due to inaccuracies in the Julian calendar, by the 16th century the vernal equinox was occurring on March 11. This caused problems for the church in Rome because most Christians outside of Rome were using the calendar date of "March 21" to calculate the day of Easter rather than the "vernal equinox" event. (sometimes bad weather combined with a lack of local astronomers make the calendar method the only practical choice). This had the effect of pushing the celebration day of Easter closer toward summer.

Note: Christmas was a relatively unimportant church event in those days and was created only to obscure pagan rituals like The Feast of Saturnalia etc. The changing of these calendars is one reason why eastern Europe celebrates Christmas on Jan 6 but I'll leave that for another time partly because that correction involves 12 days.

Pope Gregory XIII assigned the problem to a Jesuit astronomer named Christopher Schlussel (a.k.a. Clavius) who suggested that the equinox problem could be solved by removing 10 days from the current year but from that time forward, the Christian world should adopt the idea proposed by Aloysius Lilius. The plan was approved by Pope Gregory who decreed that October 4, 1582  would be followed by October 15. The peasants revolted thinking that their lives had been shortened by that much. The new calendar was named the Gregorian calendar after the pope who approved the changes.

This so-called "continental calendar" wasn't adopted by England, or its colonies including America, until 1752. At that time, 11 days^3b needed to be removed. (10 for the original correction, 1 for 1600, 0 for 1700)

Superscript Notes:

1. The Romans thought the Earth required 365.25 days to orbit the Sun and so added an extra day to February every fourth year. These "extra day" calendars are called "leap years" because the calendar leaps to catch up with Earth's orbit position in the sky.
    
2. The Earth actual requires 365.242193 days to orbit the Sun rather than 365.25 as is commonly believed. Skipping a leap year every century gets us much closer. Skipping the century exclusion every 400 years gets us really close.
   
   Here are the Gregorian leap year rules which only employ positive logic:

   rule	test	result	leap year?	calendar
   1	current year/400	= 0 <> 0	yes (so exit) ??? (do next test)	Gregorian
   2	current year/100	= 0 <> 0	no (so exit) ??? (do next test)	Gregorian
   3	current year/4	= 0 <> 0	yes no	Julian

   Note: Since rule #1 takes precedent over rule #2, then 2000 was a leap year.
   
   These Gregorian rules employ negative logic to reduce computing overhead:
   1. if not a fourth year then exit immediately (LY=no)
   2. if not a century then exit immediately (LY=yes)
   3. make final decision based upon  400-year test  

   rule	test	result	leap year?	calendar
   1	current year/4	<> 0  = 0	no  (so exit) ??? (so do next test)	Julian
   2	current year/100	<> 0  = 0	yes (so exit) ??? (so do next test)	Gregorian
   3	current year/400	<> 0  = 0	no yes	Gregorian

    
3. Correction Calculations (1582 is the main reference point):  

   	Year 2	Year 1	year difference	sec/year error	total seconds error	equiv days	notes
   a	1582 AD	325 AD	1257	x 676	=  847218	09.805763889	Europe
   b	1752 AD	325 AD	1427	x 676	=  961798	11.1319213	England and America

   Note: 11 minutes and  14 s = 676 seconds

More Calendar Links:

• calendar, time, and y2k concerns

Debugging Java Applet Problems

User Tips:

• Debugging Java applet problems require that you look for helpful messages in the Java console.
• With JVM 1.4 you'll see a Java icon on the right hand side of the Windows task bar. Right click on the icon and then click "Show Console" on the pop-up menu.
• With all versions of JVM, you can pull up the console by choosing:
  Start (Button) -> Settings -> Control Panel -> Java Plug-in
• With Netscape Navigator you may be using the internal JVM without knowing it so you should only bring up the console like so:
  Communicator (Menu) -> Tools -> Java Console

Possible Failure Modes:

• Applet didn't initialize
  1. Your Netscape browser is not Java enabled.
     1. Click: Edit (menu) -> Preferences -> Advanced -> Enable Java
  2. Your Internet Explorer browser is not Java enabled.
     1. Click: Tools (menu) -> Internet Options -> Security
        1. If slider is visible, make sure it is not set to Max
        2. If slider is not visible, click the custom button, search for "Microsoft VM", make sure "Disable Java" is not selected
     2. Click: Tools (menu) -> "Manage Add-ons..."
        1. Make sure "Sun Java Console: is not set to "disabled"
  3. your "JVM 1.3" (or 1.2) expects applets to have 3 control methods but the applet just downloaded was written using "SDK 1.4" and contains only one control method
      
• Applet didn't complete initialization
  1. your "JVM 1.1" (or earlier) only supports general graphics but the applet was written using Swing graphics which requires "JVM 1.2" or higher. When this happens you might see the following message in the Java console or message bar:
          # Applet exception: error: java.lang.NoClassDefFoundError: javax/swing/JApplet
     You'll probably see this with "Netscape Navigator 4.any" which has built in support for JVM 1.1.5 (which is a good reason to keep your browser up-to-date)
  2. your JVM supports an earlier version of a class but the applet was compiled with a later version. When this happens you might see the following message in the Java console or message bar:
          # Applet exception: error: java.lang.ClassFormatError: Bad major version number

Click here to download a free JVM from the Sun web site.
Note: "JVM 1.3"  and "JVM 1.4" only support "Netscape Navigator 6" and any version of Internet Explorer. You shouldn't be using any version of Netscape Navigator that is pre version 6 (unless the web page uses the <Object> tag rather than the <Applet> tag)

Programmer Tips:

In a business environment your clients are usually on a high speed intranet and it's not unreasonable to to expect that everyone will have the latest browser and JVM (Java Virtual Machine). In this environment you may use the latest Java programming techniques and the latest JVMs. However, on the public internet you must assume that many people are using:

• older machines with smaller memories
• older browsers (like Netscape 4)
• earlier JVMs (like version 1.1 which is built into Netscape 4)
• older dialup technology like 28.8 kb or 56 kb modems (which means they won't be downloading a new browser or JVM just to run your applet)

For this reason if you don't want to ostracize certain segments of the public, you must program using JDK 1.1 which means you must use general graphics classes rather than Swing graphics etc.

Please note that this might be a little harder than it sounds. The calendar applet above was first written using SDK 1.3 with Swing graphics. To modify it for use with SDK 1.1 I had to do the following:

• drop the "J" prefix from each graphic class
• change code written for class Rectangle2D to just Rectangle
• find a suitable replacement for Font method getFontRenderContext()
• write some code to replace GregorianCalendar method getActualMaximum(Calendar.DAY_OF_MONTH)

Click here to download older SDK packages from Sun's archive.

Some older browsers like Netscape 4.x have a built in JVM 1.1 while other browsers don't support Java at all. You can force a browser to use an external JVM by replacing the <Applet> tags on your web pages with <Object> tags. Fortunately, this conversion is done for you when you run a program called HtmlConverter in the JDK bin directory.

Legend:

JAR	Java Archive
Java	An object oriented programming language produced by Sun
JavaScript	A browser scripting language produced by Netscape (not related to Java in any way)
JDK	Java Development Kit
JRE	Java Run-time Environment (JVM and support files but no compiler or debugger)
JVM	Java Virtual Machine
SDK	Software Development Kit
Swing	2nd generation graphics

Java Rev Numbers:

SDK (JDK)	Marketing	Basic Features	Miscellaneous Notes
1.0	1.0	Introduction
1.1	1.1	Graphics 1.1 features	Built into Netscape Navigator 4.x
1.2	Java2	Swing Graphics 1.2 features	Swing graphic classes contain a 'J' prefix. eg. Panel becomes JPanel, Applet becomes JApplet, etc.
1.3	Java2	CORBA ORB 1.3 features	JApplet requires 3 methods: start(), stop(), init() a second Timer() method was added Support for Open-JVM Interface (Netscape 6)
1.4	Java2	Unicode 3, XML 1.4 features	JApplet requires 1 method: init()

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Neil Rieck
Kitchener - Waterloo - Cambridge, Ontario, Canada.