Writing a full library system in plain Java, no frameworks

The goal was not simply to build a library application.

The module was designed to make us practise object oriented design, SOLID, DRY and design patterns without relying on frameworks to make the architectural decisions for us.

The system needed to manage books, DVDs, physical copies, members, borrowing, returns, overdue fines and search.

That created questions frameworks normally help answer.

Where should borrowing rules live? How should different media types behave? How do you calculate fines reliably? How do you test date-dependent behaviour when “today” changes every time the program runs?

The real challenge was keeping those rules flexible without turning the application into a collection of conditionals.

I separated titles from physical copies, so one media item can have several independently borrowable holdings.

Repository interfaces isolate persistence from the application logic, while LoanRule and FinePolicy strategies keep borrowing and fine rules replaceable.

One small abstraction became particularly useful.

Instead of calling the system date directly, overdue calculations use a ClockProvider. Production receives the real date, while tests can inject a fixed one.

That makes fines and due-date behaviour deterministic.

The project also includes catalogue search by title, creator and year.

Instead of adding JUnit, I wrote a small zero-dependency test runner with separate unit and functional suites covering validation, loan behaviour, search, holding states and complete borrowing workflows.