Reading a big Java project - advice?

This is sort of amorphous, but:
I'm starting a new job soon, and will be working on a fairly large (40 packages, several hundred classes) project. It's an awful lot of material to cover, and I have only a couple of weeks to get a handle on things.
The project is a network client, and to make matters worse they use a bunch of external packages (aside from the 40 from the project itself)
My question is, could anyone recommend any effective strategies in dealing with something like this?  I tried starting at the main class and following each branch as I get to it (ie, every import, method, etc) but it seems fairly hopeless as it is. It took me two hours to get through two lines of import (especially when I realized that two of the imported classes actually import each other. I didn't even know you can do that...)
Please help.