Design Patterns: questions with answers

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import java.util.*;
import java.util.function.Consumer;

public class Main {
    static class EventBus {
        private final List<Consumer<String>> listeners = new ArrayList<>();
        Runnable subscribe(Consumer<String> l) {
            listeners.add(l);
            return () -> listeners.remove(l);
        }
        void publish(String e) { for (Consumer<String> l : List.copyOf(listeners)) l.accept(e); }
    }

    public static void main(String[] args) {
        EventBus bus = new EventBus();
        StringBuilder out = new StringBuilder();
        Runnable stopA = bus.subscribe(e -> out.append("A:").append(e).append(' '));
        bus.subscribe(e -> out.append("B:").append(e).append(' '));
        bus.publish("login");
        stopA.run();
        bus.publish("logout");
        System.out.println(out.toString().trim());
    }
}

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public class Main {
    enum IdGenerator {
        INSTANCE;
        private long next = 1000;
        synchronized long nextId() { return next++; }
    }

    public static void main(String[] args) throws Exception {
        Thread t = new Thread(() -> { for (int i = 0; i < 500; i++) IdGenerator.INSTANCE.nextId(); });
        t.start();
        for (int i = 0; i < 500; i++) IdGenerator.INSTANCE.nextId();
        t.join();
        System.out.println(IdGenerator.INSTANCE.nextId() + " " + (IdGenerator.valueOf("INSTANCE") == IdGenerator.INSTANCE));
    }
}