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FAQ (What are Java applets ? )

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Frequently Asked Questions
About Java applets

A Java applet is a small program written in the Java Language.
In generally the applet program has the *.class extension.
When the browser encounters the tag the applet is downloaded.
After dowloading applet is executed in an software emulated computer.
This is called the Java Virtual Machine (=JVM).
Because off the Java standard this JVM is on all computer types the same.
So the applet will also run on an UNIX , MacOS , OS/2 or whatever brand or type.
The output of the applet is displayed on your html page.

JavaScript is used to control the layout of a page.
A Java applet just displays it output in a box on the page.
So the goal of the two are completly different.
A Java applet is a real program.
Because of that is capable of doing things JavaScript can't.
But it can't control the layout of a page.

Most browsers doo support Java without a plug in.
These are the 32 bit versions of
- Netscape 2.x
- Netscape 3.x
- Netscape 4.x
- Ms Explorer 3.x (the newer 16 bit version also supports Java)
- Ms Explorer 4.x
So you can say 95% of the browsers do support Java without plug in.
There is a plug in for the 16 bit Ms Explorer 3.x but later relaeses do already have it build in.
The 16 bit versions are written for the 16 bit operating systems Windows 3.1x.

It is impotant that you add some html tags between the applet tags.
These are ignored by Java capable browsers but executed by non-Java browsers.

First thing to do is look at the Java console.
Many program related information is displayed here.
Most browsers do have a Java console.
In Netscape it is started from a pull down menu.
In Ms Explorer 4.x you will have to do :
view,internet options, advanced and then enable the console , restart.
Versions of Ms Explorer 3.x don't have a console they use a log file.
If you are using different versions of the class file then close and restart the browser.
The java cache is then cleared , otherwise the browser will use the old class file from the cache.
The java cache is a seperate cache , it isn't the same cache where files from visited pages are stored.
On some browser suites you also must close the other components like the mailing program to clear the cache.
A browser isn't build as a testing machine for java class files.

Probably you are trying to run an applet written with a version 1.1.x interface.
This isn't supported on all browsers.
All the applet I made are written with an 1.02 interface.
This is the oldest ineterface and supported on all Java browsers.
The 1.1.x interface is backwards compatible with 1.02.
Only the newest version 4.x browsers do support it.
On some of them you have to do a patch to activate the newer interface.

An applet is executed on your machine in a Java Virtual Machine (=JVM).
This Java virtual machine is nothing more than a software emulated computer.
Because it is executed on your machine there are many Security restrictions implemented.
You do not want an applet to erase or spy on your harddisk.
Some browsers do have are more riged in this then others.
I will try to explain the most important ones:
- Applets can only connect back to the host they originated from.
Done to prevent people of making a spying applet.
If you visit a web page on the net then the originated host is the computer on the internet.
It can't even connect to other computers on the internet.
It then also can't get files or read files from you harddisk.
If you start an applet from a html page on your harddisk then the originated host is your computer.
The applet then can't connect to computers on the enternet.
This is done to prevent spying
- Applets can't write to files.
To prevent it from damaging files on your system.
But it also means that it can't write files to computers on the internet.
This unfortunatly also restricts the usefullness of applets.
Note : The Explorers 3.01 3.02 4.0 do not allow an applet to read files from you local drive at all.
Even if started from the local drive.
There is a solution for this from the Microsoft web site.
There where to many complains from people that couldn't test there applets localy.

It means that the browser can't get the applet class file.
The applet program is a file with the *.class extension.
Sometimes the program is splitt up in several class files.
Perhaps you dit forget to upload the class file to the internet.
Normaly it is expected to located in the same directory as the html file.
The file names are case sensitive , check the letter casing.
Check if the browser can find the the applet by typing the url direcly.
for example http://www.nedernet.nl/~rvdb/Clock.class .
When found you will be prompted for download.

There could be several things wrong.
First look at the Java console see question 5.
Then also look at question 6,7,8 to see if they are relevant.
If these don't help the most likely it is caused by the letter cassing.
A Windows system isn't case sensitive but a UNIX system is.
The computer of your provider is most likely a UNIX.
So check the cassing of all the file names the applet uses.
Some FTP porgrams do change the lettercassing.
FTP (=File Transvere Protocol) programs are used to put the files to the internet.
Also check to see if all files are available on the net.
Perhaps you forget to upload an background picture or whatever the applet needed.
You must upload the class file as a binary file it isn't plain text.

This is the most common error to occur.
It means that the applet wants to work with an object it doesn't have.
For example draw an background image that hasn't been loaded.
Convert a string to upper or lower case that it doesn't have.
The object is null that is it doesn't exist.
It could be a bug in the applet program.
It can also be caused by a wrong applet parameter tag in the html source.
The names of the parameters are case sensitive.
Perhaps you did forget a parmater tag.
The Java console often gives some usefull information see question 5.
If that doesn't help you will have to conact the author of the applet.

This can happens in some of the older browsers Netscape 2.0x and 3.x.
The effect you see is dithering.
It is caused by the default settings of these browsers.
You can change this with pull down , general preferences , images.
It only happens with *.gif pictures not with *.jpg
Workarrounds are:
- convert the picture to jpg.
- be sure the gif contains (uses) 256 colors
- convert the gif to a 125 color pallete
The values of the RGB collors can be ( 0,64,128,192,255)
These are the values Netscape converts the colors to.
This solution isn't usefull for photo like images.

This means that the loaded class file is corrupted.
Most likely caused because you uploaded the class file as plain text.
You must upload it as binary.

This is caused by a bug in the Windows 95 system.
The Java machine doesn't have a pointing hand available.
It will probably be solved (internal workarround) in the next Java versions.
Mostly because of this I use the hair cross in stead of the pointing hand.

To speed op drawing many times a technique called offscreen painting is used.
A picture is first created in memory and then displayed on the applet.
Otherwise you would see the picture being build up.
On some browsers mostly Netscape these offscreen images can't be transparent.
This technique isn't alway necessary to use.
Also note that the background from an applet isn't the same as the html page.
By default it is gra y.

First look at the Java console for add itional information (question 5).
- If it complains about a securety Exception then the applet probably can't connect.
This is because the computer the applet is on only does html pages.
(also read question 7)
Some providers do use a seperate computer for their email.
In that case it can't connect to an SMTP (mail) port.
- It also can complain about a refusing email relay.
Some people try to use other ones system to sent email for them.
Mostly this are the people that sent bulk email.
So your providers computer is configered not to relay emails.
A good thing.
But it still should accept message to local users.
If it doesn't something is wrong in the configuration of the computer.
A workarround that often works is to remove the providers name from the recipient address.
So for example gets
.
If the server name is omited it must be a local user.

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