Book.java

public class Book {

    private String bookId;
    private String title;
    private String author;
    private boolean isAvailable;

    // Full constructor
    public Book(String bookId,
                String title,
                String author,
                boolean isAvailable) {

        this.bookId = bookId;
        this.title = title;
        this.author = author;
        this.isAvailable = isAvailable;
    }

    // Simplified constructor
    public Book(String bookId,
                String title,
                String author) {

        this.bookId = bookId;
        this.title = title;
        this.author = author;
        this.isAvailable = true;
    }

    // Getters
    public String getBookId() {
        return bookId;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    public boolean isAvailable() {
        return isAvailable;
    }

    // Setters
    public void setBookId(String bookId) {
        this.bookId = bookId;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public void setAvailable(boolean available) {
        isAvailable = available;
    }

    // Borrow book
    public void borrowBook() {

        if (isAvailable) {
            isAvailable = false;
            System.out.println(title + " is now borrowed.");
        }
        else {
            System.out.println(title + " is already borrowed.");
        }

    }

    // Return book
    public void returnBook() {

        if (!isAvailable) {
            isAvailable = true;
            System.out.println(title + " was returned.");
        }
        else {
            System.out.println(title + " is already available.");
        }

    }

}