I believe that any parameter you pass to an applet will get passed in as a
string. What you do with that string is up to you. You could use it as a string, use a built-in method to convert it to a number or date or have a parse routine to convert the string into something more complicated.
Applets can access files on server so you can also pass a filename to the applet if what you're trying to pass is difficult or inconvenient to embed in the html.