I'm reading a book which mentions that there are some tools which can assist code reading and understanding complex systems, and most developers are not aware of them, but the book does not name any of them. Can you please recommend some? I'm particularly interested in tools which are preferably free and support Java. Also if IntelliJ IDEA has some features or plugins which can help, please mention them.
Take a look at Doxygen ...
... and then write to the book's author and ask them to
- name those tools for you
- update the next edition (and seriously consider not buying any more of their books)