Java Primer





Here's a little primer on the Java programming
sub-language...thanks to

Putting Java Applets on Your Website



Setting up JAVA applets, though perhaps daunting the first
time you do it, is really quite simple. Today we're going
to walk you through the basics; and believe it or not, you
can get all this sorted out in under ten minutes. Let's
just dive in and do it!

JAVA applets use only two types of HTML tags -- both easy
to deal with.

The <APPLET> tag basically just tells the browser what
applet.class file to use, and how wide and high the applet
should be.

There are additional (optional) attributes you can set up,
too; but in simplest use, that's all there is to this tag,
and usually all you will need.

The <PARAM> tag is likewise simple -- it NAMES a parameter
the JAVA applet needs to run, and provides a VALUE for that

Though a given applet may have anywhere from no PARAM tags
to dozens, still, every PARAM tag takes the very same
simple form: just a NAME, and a VALUE.

Let's have a look at those two tags in greater detail. And
please, remember the maxim: "This is
simple -- don't make it complicated!"



Here's the framework of a simple HTML tag set for putting
an applet into your page:

<APPLET CODE="filename.class" WIDTH="400" HEIGHT="200">
(parameters go here - more about them presently)

The CODE="filename.class" contains the name of the applet's
class file. The class file is a small executable which does
the real work of the applet.

For newcomers to JAVA, there are two important things to
remember about the class file: Put the class file in the
same place on your server as the HTML page calling it; and
make certain you send the class file up in binary format --
never ASCII!

Forgetting to send the class file up or sending it up in
the wrong format is responsible for about 90% of all
problems encountered while setting up applets.

Next, let's look at how to set the applet size on the page.
WIDTH="400" and HEIGHT="200" would cause the applet to
appear 400 pixels wide and 200 pixels high on your page. If
you want the applet to be a different size, change these
values, just like you would for an image.

Following the <APPLET> tag you will then insert the <PARAM>
tags, and -- don't forget! -- close off this tag set with a
mating </APPLET> tag.



As we said, the <PARAM> tags go between the <APPLET> and
</APPLET> tags, as in this sample code:

<APPLET CODE="filename.class" WIDTH="400" HEIGHT="200">
<PARAM NAME="IMAGE1" VALUE="thisimage.gif">
<PARAM NAME="IMAGE2" VALUE="thatimage.jpg">

The very first thing you will notice is that <PARAM> tags
absolutely do not, ever, have a mating end tag. <PARAM>
tags are among the few HTML tags that do not.

As for what they do, parameter tags tell the applet how it
is to behave and what resources it will use (for instance,
in this example, how fast the applet will run, and what
image files it will use).

A key point for newcomers to remember is that, unlike HTML
tags and JavaScript methods or properties, a parameter's
NAME is absolutely not standard. The person who builds the
applet decides what the parameter names will be, so study
the applet's documentation carefully.

However, the syntax and use of <PARAM> tags is regular, and
very simple.

The NAME="whatever" specifies the parameter to be set, and
its corresponding VALUE="whatever" says what its value is
to be.

In the example above, the SPEED parameter is being set to a
value of 100 (probably in milliseconds, but you would check
the applet documentation to find out). Likewise, the IMAGE1
and IMAGE2 parameters would tell this applet to use
"thisimage.gif" and "thatimage.jpg" respectively for its
image resources.

And that's it for the parameters! No matter how many
parameters, they'll all follow the same simple pattern.



Now comes the easiest part. To insert applet code into an
HTML page, you simply copy everything from <APPLET> through
</APPLET> into the area of your page HTML, wherever
you would like the applet to appear.

To make this clearer, you can think of everything from
<APPLET> to </APPLET> as one block, and insert the whole
block into your page just like you would, say, an image

For layout control, note that you can put the entire
<APPLET> ... </APPLET> block into an individual cell in a
table; as in this example, which would show a 5-pixel red
border around the applet:

<table cellpadding="5" bgcolor="#FF0000">
<APPLET CODE="filename.class" WIDTH="400" HEIGHT="200">
<PARAM NAME="IMAGE1" VALUE="thisimage.gif">
<PARAM NAME="IMAGE2" VALUE="thatimage.jpg">

Tip: Note how we pulled the </td> tag right against the
</APPLET> tag, by the way -- that solves a shortcoming of
Netscape when it comes to proper centering.



For newcomers to JAVA, always put everything related to the
JAVA applet in the same place (directory) on your server.

What has to go up? Your HTML page, of course. But also be
sure any image files or other resources the applet uses are
properly sent up. If they aren't there, most applets will
be unable to start.

Finally, and of special note, the applet class file must be
sent up. We can't say it often enough: Make sure the class
file is on the server, where it should be, and make certain
you sent it up in binary format.



The single most common cause for problems with JAVA applets
is either neglecting to send up the class file, or sending
it up in ASCII (text) format, instead of binary.

If you see an error in the browser status bar like "class
whatever.class not found" or "class whatever.class could
not be loaded", send the class file up again, and watch
your FTP client to be sure it goes up in binary.

The second most common problem is forgetting to send up
resources the applet needs, such as image files or text
files. Obviously, make sure they're on the server, in the
right place, and sent in the appropriate formats.

The third most common problem is, you didn't proofread your
code! Remember, check it three times, and when you're
absolutely sure it's right? Check it again.



There is a truly stunning array of JAVA applets available,
thousands upon thousands of them, that perform myriad tasks
and functions -- from basic text scrollers to striking
display applets and powerful animation tools.

Once you get the basic dance steps down, JAVA applets are
straightforward installations, and a great way to add
interest, functionality, and versatility to your pages.

Even better, many JAVA applets are absolutely free for the
taking, so don't miss a great opportunity to add some flash
for zero cash!