Good Java code to read for purposes of learning? [closed]

Question

A common recommendation is to read the code of excellent programmers. I'm looking for recommendations for Java code to read for purposes of learning. I've worked through code in Tomcat and found that helpful. Am now looking for recommendations for other open-source code that is particularly well written/commented. My interest are in mainstream Java SE apps (and not looking for Swing/JavaFX/UI code). I have about three years' working experience in Java and so looking for that extra polish, rather than language basics.

Answer 1

Google Guava code is refined and mostly bullet proof: http://code.google.com/p/guava-libraries/source/browse/#git

You can start from base classes: http://code.google.com/p/guava-libraries/source/browse/#git%2Fguava%2Fsrc%2Fcom%2Fgoogle%2Fcommon%2Fbase