Oh, and another thing - even though your post doesn't specifically ask this, your title does - how to detect Java availability.
The applet tag itself can include a param that checks for the installed version of Java. Different browsers react differently to this when the desired version isn't installed, but most present the user with an opportunity to download and install it.
Also, as with iframe tag for external pages and the object tag for Flash, the applet tag should (I'm not certain of this) allow one to include alternative content within it that will appear if the browser doesn't support the tag itself, ex:
Code:
<applet whatever="whatever">
<param name="whatever" value="whatever">
- - - Alternative content that can include HTML tags here - - -
</applet>
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