What are the key design decisions in the library management system?

Direct Answer

Key decisions: separate Book (catalog) from BookItem (physical copy) for inventory tracking, use Queue for FIFO reservation fairness, apply Strategy pattern for extensible search and fine calculation, implement Observer pattern for return notifications, and define role-based permissions with Member versus Librarian classes for access control.

Deep Dive & Architecture

Identity and history drive the model

The first decision is Book versus BookCopy: one describes a title and the other tracks a barcode's state. The second is making Loan a record of the borrowing event instead of storing only a currentBorrower field. That preserves due dates and return history.

Changing rules sit behind clear policies

Search may use a query object or specialised indexes; fine calculation belongs behind a FinePolicy; notifications belong behind a channel-neutral service. These boundaries let search criteria, member rules, and message channels change without rewriting the core entities.

Consistency matters at transitions

Borrowing must claim an available copy and create its loan together. Returning must close the loan and either release or hold the copy together. Reservations need an ordered, expiring queue, and role checks should protect catalogue changes. A useful design states these invariants before adding optional patterns, because the invariants—not the number of classes—make the system correct.

Code Example

abstract class User {
    protected final String id;
    User(String id) { this.id = id; }
    abstract boolean canManageCatalog();
}
class Member extends User {
    Member(String id) { super(id); }
    boolean canManageCatalog() { return false; }   // borrow/reserve only
}
class Librarian extends User {
    Librarian(String id) { super(id); }
    boolean canManageCatalog() { return true; }     // add/remove books, waive fines
}
// callers check user.canManageCatalog() before privileged actions

Inheritance models the IS-A (Librarian IS-A User) and centralizes permission rules.

Common Mistakes & Pitfalls

• Summarising structure only — also state why each decision was made.
• No access control — a Member shouldn't delete catalog entries; model roles.
• Forgetting trade-offs — e.g. denormalizing availability counts for fast display vs consistency cost.
• Over-patterning — only highlight patterns that solved a real problem in your design.