Library.java

import java.util.ArrayList;
import java.util.List;

public class Library {

    // The core data structure to hold all Book objects
    private List<Book> books;

    // Constructor initializes the ArrayList
    public Library() {
        this.books = new ArrayList<>();
    }

    // --- Core Feature 1: Adding a Book ---

    public void addBook(Book book) {
        // Simple check to ensure ISBN is unique before adding
        for (Book existingBook : books) {
            if (existingBook.getIsbn().equals(book.getIsbn())) {
                System.out.println("Error: Book with ISBN " + book.getIsbn() + " already exists.");
                return;
            }
        }
        books.add(book);
        System.out.println("Book added successfully: " + book.getTitle());
    }

    // --- Core Feature 2: Displaying All Books ---

    public void displayAllBooks() {
        if (books.isEmpty()) {
            System.out.println("\nThe library catalog is empty. Please add some books first.");
            return;
        }
        System.out.println("\n--- Library Catalog (" + books.size() + " Books) ---");
        for (Book book : books) {
            // The custom toString() method from the Book class is automatically used here
            System.out.println(book);
        }
        System.out.println("------------------------------------");
    }

    // --- Core Feature 3: Searching for a Book ---

    /**
     * Searches the library for books matching the query in title or ISBN.
     * @param query The search term (case-insensitive).
     * @return A list of matching Book objects.
     */
    public List<Book> searchBook(String query) {
        List<Book> foundBooks = new ArrayList<>();
        String lowerCaseQuery = query.toLowerCase();

        for (Book book : books) {
            // Check if title or ISBN contains the query
            if (book.getTitle().toLowerCase().contains(lowerCaseQuery) ||
                book.getIsbn().contains(lowerCaseQuery)) {
                foundBooks.add(book);
            }
        }
        return foundBooks;
    }

    // --- Core Feature 4: Issuing a Book ---

    public boolean issueBook(String isbn) {
        for (Book book : books) {
            if (book.getIsbn().equals(isbn)) {
                if (!book.isIssued()) {
                    book.setIsIssued(true); // Update status via the setter
                    return true; // Success
                } else {
                    System.out.println("Error: The book " + book.getTitle() + " is already issued.");
                    return false;
                }
            }
        }
        System.out.println("Error: Book with ISBN " + isbn + " not found.");
        return false; // Book not found
    }

    // --- Core Feature 5: Returning a Book ---

    public boolean returnBook(String isbn) {
        for (Book book : books) {
            if (book.getIsbn().equals(isbn)) {
                if (book.isIssued()) {
                    book.setIsIssued(false); // Update status via the setter
                    return true; // Success
                } else {
                    System.out.println("Error: The book " + book.getTitle() + " was not issued.");
                    return false;
                }
            }
        }
        System.out.println("Error: Book with ISBN " + isbn + " not found.");
        return false; // Book not found
    }
}