In README.md there is this line:

There is also an excellent tutorial here : https://www.howtographql.com/graphql-java/0-introduction/

I started following the tutorial and then noticed that it was a bit out of date. Then I saw this comment howtographql/howtographql#100 (comment) in which the author of the tutorial says

And now I think I must rewrite the tutorial as the landscape changed too much in the meantime.

So maybe README.md should be changed to say that it's a "slightly out-of-date" tutorial, or something like that.