jit

The book's code is implemented in Ruby, and the source is here, according to the author, around 6000 lines (haven't checked myself). https://github.com/jcoglan/jit If you'd really like to make a Java equivalent of any specific algorithm you can start even without a book. I guess that anybody not versed in Ruby would first have to understand a lot of Ruby details first. The author mentions other efforts relatively similar to his, in other languages, like http://gitlet.maryrosecook.com/docs/gitlet.html which is node JavaScript.

if you go through the files in the repo they are very easy to read (I would recommend starting with the test/ folder): https://github.com/jcoglan/jit obviously the lib folder requires a lot of git-specific knowledge but I didn't see any use of features that don't have exact or very similar equivalent in more popular programming languages. I think the most "complex" might be things like Struct.new(*Inputs::ATTRS) where it's creating a new anonymous class from a list of attributes which becomes the class instance attributes (nothing crazy :) )

the linked repository is amazing, I don't think I ever saw such nice code and such neat organization: https://github.com/jcoglan/jit (everything uses standard ruby libraries or default gems like minitest) I'm trying it out and doesn't work very well on macOS (git add has issue with the lockfile, but all tests pass). I might use it as it's a very easily customizable implementation of git.