Library Management System

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One answer 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 keeps its own state. borrower is null when the book is on the shelf.
        static class Book {
            private String borrower = null;

            boolean isAvailable() {
                return borrower == null;
            }

            void lendTo(String user) {
                borrower = user;
            }

            void giveBack() {
                borrower = null;
            }

            String getBorrower() {
                return borrower;
            }
        }

        // Step 2: The Library keeps all books in a map from the title to the Book object.
        static class Library {
            private final java.util.Map<String, Book> books = new java.util.HashMap<>();

            String process(String[] parts) {
                String command = parts[0];
                Book book = books.get(parts[1]);

                if (command.equals("add")) {
                    if (book != null) {
                        return "exists";
                    }
                    books.put(parts[1], new Book());
                    return "added";
                }

                // Step 3: All other commands need an existing book.
                if (book == null) {
                    return "not found";
                }
                if (command.equals("borrow")) {
                    if (!book.isAvailable()) {
                        return "already borrowed";
                    }
                    book.lendTo(parts[2]);
                    return "borrowed";
                }
                if (command.equals("return")) {
                    if (book.isAvailable()) {
                        return "not borrowed";
                    }
                    book.giveBack();
                    return "returned";
                }
                return book.isAvailable() ? "available" : "borrowed by " + book.getBorrower();
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        Library library = new Library();
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            // Step 4: Send every command line to the library and keep its answer.
            output[i] = library.process(lines[i].split(" "));
        }
        return output;
    }

    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)));
    }
}