Nothing taught me as much about API design and testing as Google's Guava. I know there are lots of
minor gripes with that library, but is it so incredibly consistent. It is also amazing that no one mentioned the Java Collections (ArrayList, LinkedList, HashSet, HashMap, TreeSet, TreeMap). They are some of the highest quality
generic collections that exist anywhere. (For C++, I would say the same about an STL implementation by a high quality project like VCC/GCC/Clang.) For "Java high honors", try to understand how concurrent collections work, e.g., ConcurrentHashMap!
For highly specialised containers, try: (1) https://labs.carrotsearch.com/hppc.html, (2) https://github.com/OpenHFT/Chronicle-Queue, and (3) https://github.com/LMAX-Exchange/disruptor