Interesting open-source APIs (Java) for self-education?
api, java
Solution
I think that google collections is a great place to start. Josh Bloch advised the development of a lot of it, and it's a very well done API. While Spring is great, it's a little hard to know where to start. A good introduction to google collections is "coding in the small with google collections" (I can't post the actual link because of stackoverflow spam filter).
Problem
Which is your top 3 open-source APIs (in Java) you recommend as an example of well-designed piece of art? That would be code that you had pleasure to browse through and got some insights from it. Any problem domain acceptable. Emphasis here is on educational/study quality of code, complexity level - intermediate to top. Thanks a lot for responses.