There are plenty of good code around. Study Linux Kernel if you want to get really high!! Basically, depends on where your interest lies .. find a good open source project in your area of interest and figure out how it works ...
My point is that as an apprentice you should study the masters. Try and duplicate their work through practice.
Here are a few projects with good Java code: Spring Framework (check mostly around the spring-core stuff), Apache DS, Google Web Toolkit APIs, Google Guava, Google Guice (pretty much almost all Google Java open source projects have similar high-quality).