Library Lending with Classes

/*
 * Problem : Run the library.
 * Input   : Command lines.
 * Output  : One answer line per command.
 * Approach: Book and Library classes
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: A Book knows its own state: who has it (null means it is on the shelf).
        static class Book {
            final String title;
            String holder;

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

            // Step 2: The book guards its own rules.
            boolean lend(String user) {
                if (holder != null) {
                    return false;
                }
                holder = user;
                return true;
            }

            boolean giveBack() {
                if (holder == null) {
                    return false;
                }
                holder = null;
                return true;
            }
        }

        // Step 3: The Library owns the books and answers the commands.
        static class Library {
            private final java.util.Map<String, Book> books = new java.util.HashMap<>();

            String run(String[] part) {
                if (part[0].equals("add")) {
                    if (books.containsKey(part[1])) {
                        return "exists";
                    }
                    books.put(part[1], new Book(part[1]));
                    return "added";
                }
                Book book = books.get(part[1]);
                if (book == null) {
                    return "no such book";
                }
                if (part[0].equals("borrow")) {
                    return book.lend(part[2]) ? "borrowed" : "not available";
                }
                if (part[0].equals("return")) {
                    return book.giveBack() ? "returned" : "not borrowed";
                }
                return book.holder == null ? "shelf" : book.holder;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        Library library = new Library();
        String[] out = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            out[i] = library.run(lines[i].split(" "));
        }
        return out;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}