For my assignment I have to develop small library system. I'm doing ok, I'have developped borrower and book classes and they work just fine... I was wondering if anyone could just suggest how to implement copies for the book. (Sometimes the most popular books will have more copies)I'm not asking for code, any suggestion will do.... Do I need new class named Copy? I know that I need an integer value that will count how many copies are added.

That class would probably be Library itself. A Library has a collection of Book objects. A Book is either "checked out" or "available". The Book collection could easily have more than one copy of a Book object, each with it's own state. When a Borrower checks out a Book object, the state would be changed to "checked out" and perhaps set properties such as "dateDue" and a reference to the current Borrower.