There are some tools on the web that SOMETIMES do an adequate job of this. One is "Cafe Babe" and an older one is "Mocha". You will have to do a web search to find these, as I don't have links or anything...
Some of the UML tools like
Rose claim to reverse engineer stuff to form models, but I never could get it to work right even on stuff I had source code on (it failed on inner classes and Javadoc comments of all things). I heard TogetherJ works better, but I've never used it.
What are you trying to do, maybe we can help?