What is Low-Level Design?

// Book.java
public class Book {
    private String title;
    private String author;
    private String ISBN;
    private boolean isAvailable;

    public Book(String title, String author, String ISBN) {
        this.title = title;
        this.author = author;
        this.ISBN = ISBN;
        this.isAvailable = true;
    }

    public boolean isAvailable() { return isAvailable; }
    public void markBorrowed() { this.isAvailable = false; }
    public void markReturned() { this.isAvailable = true; }
    public String getISBN() { return ISBN; }
}

// Member.java
public class Member {
    private String name;
    private String memberId;
    private List<Book> borrowedBooks;

    public Member(String name, String memberId) {
        this.name = name;
        this.memberId = memberId;
        this.borrowedBooks = new ArrayList<>();
    }

    public void borrowBook(Book book) {
        if (book.isAvailable()) {
            book.markBorrowed();
            borrowedBooks.add(book);
        }
    }

    public void returnBook(Book book) {
        book.markReturned();
        borrowedBooks.remove(book);
    }
}