Our Navigator 3.0 release will allow java and javascript to call into plugins. Since plugins are native code, you will be able to freely mix C and Java. Of course you will have to get the user to install your plugin on their disk.
That's the problem, installing the plugin. I (and some others, I think) was hoping that it would be possible to build powerful crypto applets and put them up on web pages. That way everyone with a java enabled copy of Netscape could use a remailer or send crypted mail without having to download, install, and configure software. If people have to download and install a plugin to use a java mixmaster applet, why not just download and install a native mixmaster client? Of course there are other reasons to use java -- platform independence, for example. But it's the user's ability to download and run applets just by jumping to a web page that has everyone excited. With that gone (for crypto), java loses a lot of its lustre (again, for crypto work).