Observer Pattern: News Board

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : Notification lines.
 * Approach: A subject with a list of observers.
 * Time    : O(commands * subscribers)
 * Space   : O(subscribers)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: An observer is anything that wants to be told about a message.
        interface Subscriber {
            void notify(String message);
        }

        // Step 2: The subject (the board) keeps its observers and tells all of them about each message.
        static class NewsBoard {
            private final java.util.LinkedHashMap<String, Subscriber> subscribers = new java.util.LinkedHashMap<>();

            void subscribe(String name, Subscriber subscriber) {
                subscribers.putIfAbsent(name, subscriber);
            }

            void unsubscribe(String name) {
                subscribers.remove(name);
            }

            int publish(String message) {
                for (Subscriber subscriber : subscribers.values()) {
                    subscriber.notify(message);
                }
                return subscribers.size();
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        NewsBoard board = new NewsBoard();
        java.util.List<String> output = new java.util.ArrayList<>();

        for (String line : lines) {
            String[] parts = line.split(" ", 2);

            if (parts[0].equals("subscribe")) {
                // Step 3: Each subscriber writes its own line when it is notified.
                String name = parts[1];
                board.subscribe(name, message -> output.add(name + " got: " + message));
            } else if (parts[0].equals("unsubscribe")) {
                board.unsubscribe(parts[1]);
            } else {
                // Step 4: Publishing tells all current subscribers.
                if (board.publish(parts[1]) == 0) {
                    output.add("No subscribers");
                }
            }
        }
        return output.toArray(new String[0]);
    }

    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)));
    }
}