Hi, I'm new here. I'm reading a book "The Art and Science of Java" and I'v encountered one exercise that I can't figure out how to make it... Where can I ask for help?

Beginner's forum is generally a good place to begin. Breaking down a problem into sub problems that are easier to tackle can be a hard skill to learn. Look at part of the issue, and write a solution to it, then look for another small issue you can write something for. Keep picking away at the problem till you have it all solved and working. After awhile you'll start to get better at gauging what needs done and how to actually do it.