Library Management System

Ownership and transitions

Title identity and physical copy identity are distinct. A loan owns one copy and member; duplicate-title validation must run before capacity failure so an exhausted title does not hide a member's duplicate request. Release checks ownership before changing availability. A small scan costs O(copies); maps keep direct loan access cheap.

Java boundaries

Use the supplied nested records as immutable value carriers. A record is only shallowly immutable: copy every retained mutable input and every outward list/map, including lists inside records. Absence is null; validation failures use IllegalArgumentException, missing-key purchase errors use NoSuchElementException, and invalid transaction boundaries use IllegalStateException. Java primitive types already exclude booleans from numeric inputs. Numeric values use long where signatures say long and all computations must fit signed 64-bit values. Calls in these service exercises are serialized.

A title can have several physical copies, while one member may hold only one active copy of that exact title:

import java.util.*;

class Example {
    public static void main(String[] args) {
        record Loan(String copyId, String title, String memberId) {}
        List<Loan> loans = List.of(new Loan("copy-1", "Title", "member"));
        boolean alreadyHeld = loans.stream().anyMatch(loan ->
            loan.memberId().equals("member") && loan.title().equals("Title"));
        assert alreadyHeld;
        assert !loans.get(0).copyId().equals("copy-2");
    }
}

Checking only copy identity would allow duplicate-title loans for one member. Checking only title identity would incorrectly prevent a different member from borrowing another copy.

Design boundary

Loan ownership is authoritative; outward records cannot change it. Validate duplicate member/title before checking free capacity so exhaustion does not alter error precedence. Due dates, reservations, and catalog editing introduce new policies and are outside this contract.