I haven’t written Java in my day-to-day for quite some time now, but the first thing that came to mind for a good open source Java codebase is Guava by Google:

I haven’t written Java in my day-to-day for quite some time now, but the first thing that came to mind for a good open source Java codebase is Guava by Google: https://github.com/google/guava https://github.com/google/guava It’s a collection of core libraries for Java that I believe should have a lot of useful patterns/concepts to learn from. I also highly recommend Effective Java by Joshua Bloch (https://www.amazon.com/Effective-Java-Joshua-Bloch/dp/0134685997/ https://www.amazon.com/Effective-Java-Joshua-Bloch/dp/013468...). An incredible text for any kind of programmer, and even better if you’re specifically interested in Java. Good luck!