Do you examine the source to every single C++ library you program with? The Java source code is in most cases not stuff you want to mess with in the first place. Secondly, if you want to see the code to the various Java classes, you CAN! I\'ve taken a look at some of the classes through the NetBeans IDE... As for fixing, modifying, etc. you just extend existing classes... that should cover most of the changes you\'ve mentioned unless you want to start coding in Assembly, that is...
Let\'s look at this from another point of view: Do you look at the source code to C++?
Also, comparing Java to CS makes no sense. Java is a LANGUAGE, CS is a group of libraries forming a game ENGINE.