So far, none of the mini-lessons have been too frustrating, although some of them are a bit perplexing and short on context. It reminds me a bit of the problem sets I used to do for symbolic logic, except I'm not always sure what the 'logic' is. (I suspect this is characteristic of the genre It's not as if programmers spend a lot of time talking to people who don't understand the premises of a programming language.)
In any case, it's been a nice change of pace in the usual "read, think, write" pattern of my days. I'm planning to stick with it for a while, at least. For people who want to learn more about Java the old-fashioned way: my developer friend recommends this book.