Publisher.java

interface Publisher {
	void publish(Message m);
}

PublisherImpl.java

class PublisherImpl implements Publisher {
	private Map<Topic, Subscriber[]> subscribers;

	public Publisher(Map<Topic, Subscriber[]> subscribers) {
		this.subscribers = subscribers;
	}

	public void publish(Message m) {
		topic = new Topic(m.getType());
		subscribers.getOrDefault(topic, {}).forEach(subscriber -> subscriber.handle(m));
	}
}

UseCase.java

// use case
publisher = new PublisherImpl(someSubscribers);
publisher.publish(new SomethingCoolHappened());