# Model ## Overview The `model` package contains the core domain objects for the library-style workflow in this codebase: books, members, and loans. These classes act as the in-memory representation of the system’s primary entities and carry just enough behavior to support borrowing and returning books. This package appears to exist as a small, self-contained data model layer. It does not depend on any other package in the indexed code, and the classes communicate directly with each other through object references rather than through a separate persistence or service abstraction. ## Key Classes ### [Book](model/Book.java:3) `Book` represents a single library item that can be borrowed. It stores identifying and descriptive metadata, plus an availability flag that reflects whether the book is currently available for checkout. #### Responsibilities - Hold the book’s identity and bibliographic details: `id`, `title`, `author`, and `isbn`. - Track whether the book is currently available. - Provide a string representation that is suitable for display in logs, lists, or console output. #### Important methods - [`Book.Book(String id, String title, String author, String isbn)`](model/Book.java:10) - Constructs a book with the supplied metadata. - Initializes `available` to `true`, so newly created books start in the borrowable state. - [`Book.getId()`](model/Book.java:18), [`Book.getTitle()`](model/Book.java:19), [`Book.getAuthor()`](model/Book.java:20), [`Book.getIsbn()`](model/Book.java:21) - Standard accessors for the book fields. - Return the raw stored values without transformation. - [`Book.isAvailable()`](model/Book.java:22) - Returns the current availability flag. - [`Book.setAvailable(boolean available)`](model/Book.java:23) - Updates the availability flag. - This is the mechanism other model objects use to mark the book as borrowed or returned. - [`Book.toString()`](model/Book.java:25) - Produces a human-readable summary in the form `[id] "title" by author (ISBN: isbn) - status`. - The status is derived from the availability flag and is rendered as `Available` or `Borrowed`. #### Design notes `Book` is intentionally lightweight. It does not enforce borrowing rules itself; instead, it exposes mutable availability so that `Loan` can coordinate state changes. ### [Member](model/Member.java:3) `Member` represents a person who can borrow books. It is a simple identity and contact record with no workflow logic of its own. #### Responsibilities - Store the member’s identity and contact details: `id`, `name`, and `email`. - Provide a concise string representation for display purposes. #### Important methods - [`Member.Member(String id, String name, String email)`](model/Member.java:8) - Creates a member record with the provided values. - [`Member.getId()`](model/Member.java:14), [`Member.getName()`](model/Member.java:15), [`Member.getEmail()`](model/Member.java:16) - Standard getters for the member fields. - [`Member.toString()`](model/Member.java:18) - Returns a formatted summary like `[id] name (email)`. #### Design notes `Member` is a pure data holder in this module. It does not participate in validation, authorization, or loan limits in the code that was provided. ### [Loan](model/Loan.java:5) `Loan` links a `Book` to a `Member` and tracks the lifecycle of that borrowing transaction. It is the only class in this package that contains business state transitions beyond simple property storage. #### Responsibilities - Represent the association between one book and one member. - Record when the loan was created and when it was returned. - Update the book’s availability when the loan is returned. - Provide a readable status string that reflects the loan lifecycle. #### Important methods - [`Loan.Loan(String id, Book book, Member member)`](model/Loan.java:13) - Creates a new loan for the supplied book and member. - Captures the current date in `borrowDate` using `LocalDate.now()`. - Marks the loan as not returned by setting `returned` to `false`. - [`Loan.returnBook()`](model/Loan.java:21) - Records the current date in `returnDate`. - Marks the loan as returned. - Calls `book.setAvailable(true)` so the associated book becomes borrowable again. - [`Loan.getId()`](model/Loan.java:27), [`Loan.getBook()`](model/Loan.java:28), [`Loan.getMember()`](model/Loan.java:29) - Accessors for the loan identity and linked entities. - [`Loan.isReturned()`](model/Loan.java:30) - Returns whether the loan has been completed. - [`Loan.toString()`](model/Loan.java:32) - Builds a summary string in the form `Loan[id] memberName -> "bookTitle" | status`. - If the loan has not been returned, the status shows the borrow date and explicitly notes that it is not returned. - If the loan has been returned, the status shows the return date. #### Design notes `Loan` is the bridge between `Book` and `Member`. Its `returnBook()` method is the key mutation point in the module because it coordinates loan state with book availability. ## How It Works A typical borrow/return flow in this module looks like this: 1. A `Book` is created with descriptive metadata and starts as available. 2. A `Member` is created to represent the borrower. 3. A `Loan` is created to associate that member with that book. 4. The `Loan` records the borrow date and begins in the active, not-returned state. 5. When the book is returned, `Loan.returnBook()` marks the loan complete and flips the book back to available. In practice, this means `Loan` owns the transaction state while `Book` owns the inventory state. The two are kept in sync manually through the `returnBook()` method. ```mermaid flowchart LR Book["Book"] --> Loan["Loan"] Member["Member"] --> Loan ``` ## Data Model The package models three entities and one relationship: - **Book**: identity plus bibliographic details, with mutable availability. - **Member**: identity plus contact details. - **Loan**: a transaction record that references one `Book` and one `Member`. The `Loan` object stores two dates internally: - `borrowDate` — set at construction time - `returnDate` — set when `returnBook()` is called The `returned` boolean is the primary state flag used to decide how the loan should be displayed and whether the return date should be shown. ## Dependencies and Integration This module is intentionally isolated. Based on the indexed source, there are no resolved package dependencies and no cross-module relationships. The only external API used directly in the code is `java.time.LocalDate` in `Loan`, which is used to capture borrow and return dates. ## Notes for Developers - `Book` availability is mutable and is updated by `Loan.returnBook()`. If you add other code paths that change borrowing state, make sure they also keep the book’s `available` flag consistent. - `Loan` does not currently prevent double-return behavior or validate whether a book was already available before construction. If stricter business rules are needed, they would need to be added explicitly. - The `toString()` methods are designed for human-readable output, not structured serialization. - The model classes use direct field access internally and provide only simple getters/setters. There is no immutability, validation layer, or persistence logic in this package.