Thanks for a good, simple and clear article. I recently did a short course in Java, using Jgrasp on both PC and Mac, which really helped understanding what was happening in this article. It was fun to work in the Terminal instead of jGrasp, using javac and pico as text editor. I had a problem compiling at first because I tried compiling from inside the NineSquares folder, as others have done. As is pointed out in the author's responses you have to compile outside of the folder NineSquares for it to work.
A number of users have complained about things not compiling, or not understanding what the problem was. If one reads the javac compiler error messages its generally quite clear where one should look. It very often comes down to one's own obscure spelling mistakes: my best was "Adaptor" instead of "Adapter".
I haven't found the Java documentation referred to in the article yet either, but as it was written about 3 years ago that's not surprising. Things change. The Developer Tools Java docs are easy to find though, and in any case Java documentation is available online in various places.