To be sure, there's a part of java which is an add on (ie- the plugin), but java is a lot more than an add on to the browser.
In fact, most of the java I run here has nothing to do with a browser; in fact, I even run most applets outside of the browser as they use less memory and generally run faster when run with applet.exe.
The idea that java is only an adjunct to the browser is a particularly Windows way of looking at it, where everything and a ham sandwich gets tossed into a seamless whole. I prefer modularity in my systems...