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well written java open source projects (for learning)?
I want to improve my design and programming skills by understanding design & code of open source projects. I downloaded hadoop,groovy but they are very difficult to follow. I am not having a clue of how to follow this code without having a high level overview of the design. Any suggestions??
Thank you.
The Apache foundation has a whole bunch of high-quality projects to study. (Answer included here because the duplicate question does not contain it)
How big a project? If you're just starting, then maybe starting with something small is a good idea. I've contributed to a Mp3 player for java - javalayer - mostly in the area of separating the code and applying class-level design (e.g. separate stream reader, decoder, audio output.)
If you want a big project, then look at Spring.
I recommend checking out http://nemo.sonarsource.org/, the site contains a lot of open source Java projects run against Sonar, which is a build quality tool. Probably a good idea to check out a project out with a low amount of code and technical debt.
As a side note I am not affiliated in anyway with Sonar.
Investigate Google Collections. Many well-thought solutions can be found there.
Fitnesse in the pet project of Uncle Bob, who wrote the "Clean Code" book. It is not too big and you can more or less easily see what's it is doing as it is a Wiki and a test framework, which gives a nice set of moving, but visible parts.