Other folks may already have seen this before but GrepCode allows you to conveniently view the source code for a whole slew of projects. So if you want to see how java.util.HashMap or org.apache.commons.io.FileUtils is implemented, it’s the place to go.
Thanks,
I haven’t seen it before.
Now I will finally be able to choose the right ones as I will be able to understand whats going on and why one is slower than the other.
The presentation of source code is amazing. I wanted such a presentation for a few months, but couldn’t imagine it being that easy to read. I really want to see this (the documentation in html form mixed with the original source code) built into IDEs.
Using a shortcut to toggle modes, of course, and not a WYSIWYG editor 100% of the time. (For anyone that might want to implement it.) 
Then put a search function that looks for classes in shared libraries and open projects and automatically switch to this mode whenever a read only file is opened.