I am reading a book about java spring framework and there is a code list of a method code that is not explained adequately from the author. Where can I ask for code explanation?
On Stack Overflow, assuming the question is of sufficient quality.
FYI "Please explain this: [code dump]" is not of sufficient quality. You should be explaining specifically what you do and don't understand about that code snippet, provide some context (how was it used in this book, as it will give an indication of what you're supposed to be learning from it) as well as indicate what research you've done on your own to try to figure out what that code snippet does (for example, if you don't understand a method or a keyword, have you looked up the documentation for that method/keyword?).
I think that would be an OK topic on StackOverflow if (and only if) you do it right.
Do not post a block of code and say "what's going on here?".
Instead, post small chunks, with your best understanding of what is going on, and ask a specific question about the part that you don't understand.
If you have a clear question about how a certain code works with enough information to make the question answerable this is a fine fit for StackOverflow.