Online BookStore

1. Define User Interface
Create an interface that defines user-related operations.

java
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// User interface for defining user properties and actions
public interface User {
    String getUsername();
    String getPassword();
    void setUsername(String username);
    void setPassword(String password);
}
2. Implement the User Interface
Create a SimpleUser class that implements the User interface.

java
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// Implementation of the User interface
public class SimpleUser implements User {
    private String username;
    private String password;

    public SimpleUser(String username, String password) {
        this.username = username;
        this.password = password;
    }

    @Override
    public String getUsername() {
        return username;
    }

    @Override
    public String getPassword() {
        return password;
    }

    @Override
    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public void setPassword(String password) {
        this.password = password;
    }
}
3. Update the Bookstore Interface
Extend the Bookstore interface to include user management functionality.

java
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// Extended Bookstore interface with user management
public interface Bookstore {
    void addBook(Book book);
    void listBooks();
    double checkout();
    void registerUser(User user);
    User login(String username, String password);
}
4. Implement User Management in the Bookstore
Update the OnlineBookstore class to support user registration and login.

java
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import java.util.HashMap;
import java.util.Map;

// Implementation of the Bookstore interface
public class OnlineBookstore implements Bookstore {
    private List<Book> books;
    private List<Book> cart;
    private Map<String, User> users; // To store registered users
    private User currentUser; // To keep track of the currently logged-in user

    public OnlineBookstore() {
        books = new ArrayList<>();
        cart = new ArrayList<>();
        users = new HashMap<>();
        currentUser = null;
    }

    @Override
    public void addBook(Book book) {
        books.add(book);
    }

    @Override
    public void listBooks() {
        System.out.println("Available Books:");
        for (Book book : books) {
            System.out.println(book);
        }
    }

    public void addToCart(Book book) {
        if (currentUser != null) {
            cart.add(book);
        } else {
            System.out.println("Please log in to add items to the cart.");
        }
    }

    @Override
    public double checkout() {
        if (currentUser != null) {
            double total = 0;
            for (Book book : cart) {
                total += book.getPrice();
            }
            cart.clear(); // Empty the cart after checkout
            return total;
        } else {
            System.out.println("Please log in to checkout.");
            return 0;
        }
    }

    @Override
    public void registerUser(User user) {
        if (!users.containsKey(user.getUsername())) {
            users.put(user.getUsername(), user);
            System.out.println("User registered successfully.");
        } else {
            System.out.println("Username already exists.");
        }
    }

    @Override
    public User login(String username, String password) {
        User user = users.get(username);
        if (user != null && user.getPassword().equals(password)) {
            currentUser = user;
            System.out.println("Login successful.");
            return user;
        } else {
            System.out.println("Invalid username or password.");
            return null;
        }
    }
}
5. Update the Main Class
Demonstrate how to use the user management functionality.

java
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public class Main {
    public static void main(String[] args) {
        OnlineBookstore bookstore = new OnlineBookstore();

        // Add some books to the bookstore
        bookstore.addBook(new SimpleBook("Book One", "Author A", 19.99));
        bookstore.addBook(new SimpleBook("Book Two", "Author B", 29.99));
        bookstore.addBook(new SimpleBook("Book Three", "Author C", 9.99));

        // Register users
        User user1 = new SimpleUser("user1", "password1");
        bookstore.registerUser(user1);
        User user2 = new SimpleUser("user2", "password2");
        bookstore.registerUser(user2);

        // Login as a user
        User loggedInUser = bookstore.login("user1", "password1");
        if (loggedInUser != null) {
            // List available books
            bookstore.listBooks();

            // Add books to cart
            bookstore.addToCart(new SimpleBook("Book One", "Author A", 19.99));
            bookstore.addToCart(new SimpleBook("Book Three", "Author C", 9.99));

            // Checkout
            double total = bookstore.checkout();
            System.out.println("Total amount for checkout: $" + total);
        }
    }
}