Example of event sourcing in java

EventSource.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;


/**
 * Main entry point
 */
class EventSource {
    public static void
    main(String args[]) {
        Service.startRepeatingEvent();

        EventStore eventStore = new EventStore();
        Client clientA = new Client(eventStore);

        System.out.println("\nAccumlating events...");
        try { Thread.sleep(3000L); } catch (Exception e) { }
        System.out.println("\nFinal state: "+ clientA.state);

        System.out.println("\nEvent store contents...");
        for (Object event : eventStore.events) {
            System.out.println((Map) event);
        }

        EventStore newStore = new EventStore();
        Client clientB = new Client(newStore);

        System.out.println("\nReplaying events on new instance...");
        eventStore.replay(clientA, clientB);
        System.out.println("\nFinal state: "+ clientB.state);
    }
}


/**
 * Handles events
 */
interface EventHandler {
    void handle(Object event, Object data);
}


/**
 * Can be subscribed to
 */
interface Subscribable {
    void subscribe(EventHandler subscriber, Object publisher, Object event);
}


/**
 * Example of a client object
 */
class Client implements EventHandler {
    EventStore eventStore;
    String     state = "";

    Client(EventStore es) {
        eventStore = es;
        eventStore.subscribe(this, new Service(), Service.Event.TIMER);
    }

    @Override public void
    handle(Object event, Object data) {
        state = event +": "+ data;
        System.out.println(state);
    }
}


/**
 * All (state changing) events routed through an event store instance
 */
class EventStore implements EventHandler, Subscribable {
    List events        = new ArrayList();
    Map  subscriptions = new HashMap();

    void
    addSubscriber(EventHandler subscriber, Object event) {
        Set subscribers = (Set) subscriptions.getOrDefault(event, new HashSet());
        subscribers.add(subscriber);
        subscriptions.put(event, subscribers);
    }

    @Override public void
    subscribe(EventHandler subscriber,
              Object       publisher,
              Object       event)
    {
        addSubscriber(subscriber, event);
        ((Subscribable) publisher).subscribe(this, publisher, event);
        System.out.println("\nSubscribe to class: "+ publisher);
    }

    @Override public void
    handle(Object event, Object data) {
        /* Save event */
        Map map = new HashMap();
        map.put("event", event);
        map.put("data", data);
        events.add(map);

        /* Notify subscribers */
        Set subscribers = (Set) subscriptions.getOrDefault(event, new HashSet());
        for (Object subscriber : subscribers) {
            ((EventHandler) subscriber).handle(event, data);
        }
    }

    void
    replay(EventHandler oldClient, EventHandler newClient) {
        Set emptySet = new HashSet();
        for (Object event : events) {
            Set subscribers = (Set) subscriptions.getOrDefault(
                    ((Map) event).get("event"), emptySet);
            if (subscribers.contains(oldClient)) {
                newClient.handle(
                        ((Map) event).get("event"), ((Map) event).get("data"));
            }
        }
    }
}


/**
 * An example of a service
 */
class Service extends java.util.TimerTask implements Subscribable {
    static enum Event { TIMER }
    static Set  subscribers = new HashSet();

    static void
    startRepeatingEvent() {
        new java.util.Timer(true).scheduleAtFixedRate(new Service(), 0L, 1000L);
    }

    /* Publish repeating event */
    @Override public void
    run() {
        for (Object subscriber : subscribers) {
            ((EventHandler) subscriber).handle(
                    Event.TIMER, java.time.LocalDateTime.now());
        }
    }

    @Override public void
    subscribe(EventHandler subscriber,
              Object       publisher,
              Object       event)
    {
        subscribers.add(subscriber);
    }
}