java · level 2 check

a little library.

Time to build a real app: a little library. The librarian types commands, one per line. The library keeps its books in a list, lends them out, takes them back and saves everything to a file, so nothing is lost when the program ends.

The commands are `add`, `lend`, `back`, `list`, `save`, `load` and `quit`, in any capitals. `add` asks for a title and an author, while `lend` and `back` ask for a title. `list` shows every book sorted by title, with a count. `save` writes `library.csv`, one book per line, and `load` reads it back, dropping any changes made since.

Each `Book` keeps its fields private and its status in an enum. Asking for a book that is not there throws your own checked `NoSuchBookException`, which the app catches and explains kindly. Fill in each `____` with what you learned in every unit, then run it and try your own commands.

Your program

import java.io.*;
import java.nio.file.*;
import java.util.*;
public class Main {
    enum Status { IN, OUT }

static class NoSuchBookException extends ____ {
        NoSuchBookException(String title) { super("We have no book called " + title + "."); }
    }

static class Book {
        private final String title;
        private final String author;
        private Status status = Status.IN;

Book(String title, String author) {
            this.title = title;
            this.author = author;
        }

String getTitle() { return title; }
        Status getStatus() { return status; }
        void setStatus(Status status) { this.status = status; }
        String toLine() { return title + "," + author + "," + status; }

public String ____() {
            return title + " by " + author + " (" + status.name().toLowerCase() + ")";
        }
    }

static class ByTitle implements Comparator<Book> {
        public int compare(Book a, Book b) { return a.getTitle().____(b.getTitle()); }
    }

static class Library {
        static final Path FILE = Path.of("library.csv");
        private ArrayList<Book> books = new ArrayList<>();

int size() { return books.size(); }

void add(String title, String author) {
            books.____(new Book(title, author));
        }

Book find(String title) throws NoSuchBookException {
            for (Book book : books) {
                if (book.getTitle().toLowerCase().equals(title.toLowerCase())) {
                    return book;
                }
            }
            throw ____ NoSuchBookException(title);
        }

String lend(String title) throws NoSuchBookException {
            Book book = find(title);
            if (book.getStatus() == Status.OUT) {
                return book.getTitle() + " is already out.";
            }
            book.setStatus(Status.OUT);
            return "Enjoy " + book.getTitle() + "!";
        }

String takeBack(String title) throws NoSuchBookException {
            Book book = find(title);
            book.setStatus(Status.IN);
            return "Thanks for bringing back " + book.getTitle() + ".";
        }

String report() {
            books.sort(new ____());
            StringBuilder text = new StringBuilder();
            int out = 0;
            for (Book book : books) {
                text.append(book).append("\n");
                if (book.getStatus() == Status.OUT) {
                    out++;
                }
            }
            text.append("Books: ").append(books.size()).append(", out: ").append(out);
            return text.toString();
        }

void save() throws IOException {
            try (BufferedWriter file = Files.newBufferedWriter(FILE)) {
                for (Book book : books) {
                    file.write(book.toLine());
                    file.____();
                }
            }
        }

void load() throws IOException {
            ArrayList<Book> loaded = new ArrayList<>();
            for (String line : Files.readAllLines(FILE)) {
                String[] parts = line.____(",");
                Book book = new Book(parts[0], parts[1]);
                book.setStatus(Status.____(parts[2]));
                loaded.add(book);
            }
            books = loaded;
        }
    }

static String ask(Scanner keyboard, String question) {
        System.out.print(question + " ");
        return keyboard.nextLine().trim();
    }

public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        Library library = new Library();
        while (true) {
            String command = ask(keyboard, "Command?").toLowerCase();
            if (command.equals("quit")) {
                System.out.println("Goodbye!");
                break;
            }
            try {
                if (command.equals("add")) {
                    String title = ask(keyboard, "Title?");
                    String author = ask(keyboard, "Author?");
                    library.add(title, author);
                    System.out.println("Added " + title + ".");
                } else if (command.equals("lend")) {
                    System.out.println(library.lend(ask(keyboard, "Title?")));
                } else if (command.equals("back")) {
                    System.out.println(library.takeBack(ask(keyboard, "Title?")));
                } else if (command.equals("list")) {
                    System.out.println(library.report());
                } else if (command.equals("save")) {
                    library.save();
                    System.out.println("Saved " + library.size() + " to library.csv.");
                } else if (command.equals("load")) {
                    library.load();
                    System.out.println("Loaded " + library.size() + " from library.csv.");
                } else {
                    System.out.println("Try add, lend, back, list, save, load or quit.");
                }
            } catch (NoSuchBookException e) {
                System.out.println(e.getMessage());
            } catch (NoSuchFileException e) {
                System.out.println("Nothing has been saved yet.");
            } catch (IOException e) {
                System.out.println("File problem: " + e.getMessage());
            }
        }
    }
}

Fill in every ____ then run the checks.

1 It lists books sorted by titleuses: collections
2 It lends books out and takes them backuses: objects
3 It explains kindly when a book is missinguses: exceptions
4 It saves to a file and loads it backuses: files
5 It copes when nothing has been saved yetuses: exceptions
6 It understands commands in any capitalsuses: text

Take as long as you like and try as many times as you want.

a little library.

level up.

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