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ahucoderahucoder
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Merge pull request #2 from ahucoder/dev
The Observer Pattern
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11 files changed

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‎pom.xml‎

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<artifactId>spring-boot-starter-web</artifactId>
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<version>3.4.5</version>
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</dependency>
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<dependency>
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<groupId>com.google.guava</groupId>
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<artifactId>guava</artifactId>
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<version>33.4.0-jre</version>
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</dependency>
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</dependencies>
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</project>

‎src/main/java/observer/App.java‎

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package observer;
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import observer.event.WeatherUpdateEvent;
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import observer.eventbus.EventBus;
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import observer.listener.User;
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import observer.publisher.WeatherStationPublisher;
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public class App {
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public static void main(String[] args) throws InterruptedException {
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EventBus eventBus = new EventBus();
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WeatherStationPublisher weatherStation = new WeatherStationPublisher(eventBus);
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// create users
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User user1 = new User("Tom", System.out::println);
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User user2 = new User("Jerry", System.out::println);
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// subscribe to weather update events
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eventBus.subscribe(WeatherUpdateEvent.class, user1);
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eventBus.subscribe(WeatherUpdateEvent.class, user2);
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// activate the weather station
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weatherStation.start();
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System.out.println("The weather broadcast system has been activated");
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// stop after running for a period of time
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Thread.sleep(6100);
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eventBus.unsubscribe(WeatherUpdateEvent.class, user1);
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System.out.println("user1 has been unsubscribed");
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Thread.sleep(6100);
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weatherStation.stop();
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System.out.println("The weather broadcast system has been turned off");
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}
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}

‎src/main/java/observer/event/BaseEvent.java‎

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package observer.event;
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import java.text.SimpleDateFormat;
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import java.util.Date;
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import java.util.Objects;
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public abstract class BaseEvent<T> implements Event<T> {
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private final long timestamp;
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private final T source;
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private static final SimpleDateFormat DATE_FORMAT =
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new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
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protected BaseEvent(T source) {
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this.timestamp = System.currentTimeMillis();
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this.source = Objects.requireNonNull(source);
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}
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@Override
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public long timestamp() {
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return timestamp;
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}
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@Override
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public T source() {
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return source;
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}
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public String formattedTimestamp() {
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return DATE_FORMAT.format(new Date(timestamp));
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}
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}

‎src/main/java/observer/event/Event.java‎

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package observer.event;
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public interface Event<T> {
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long timestamp();
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T source();
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}

‎src/main/java/observer/event/WeatherUpdateEvent.java‎

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package observer.event;
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public class WeatherUpdateEvent extends BaseEvent<String> {
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public WeatherUpdateEvent(String source) {
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super(source);
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}
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}

‎src/main/java/observer/eventbus/EventBus.java‎

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package observer.eventbus;
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import com.google.common.collect.Lists;
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import com.google.common.collect.Maps;
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import observer.event.Event;
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import observer.listener.EventListener;
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import java.util.List;
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import java.util.Map;
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/**
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* Event bus, responsible for event publishing and subscription
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*/
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public class EventBus {
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private final Map<Class<?>, List<EventListener<?>>> listenerMap = Maps.newConcurrentMap();
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/**
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* subscribe event
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*
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* @param eventClass event type
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* @param eventListener event listener
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* @param <E> event generics
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*/
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public <E extends Event<?>> void subscribe(Class<E> eventClass, EventListener<E> eventListener) {
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listenerMap.computeIfAbsent(eventClass, v -> Lists.newCopyOnWriteArrayList()).add(eventListener);
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}
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/**
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* cancel
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*
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* @param eventClass event type
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* @param eventListener event listener
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* @param <E> event generics
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*/
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public <E extends Event<?>> void unsubscribe(Class<E> eventClass, EventListener<E> eventListener) {
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List<EventListener<?>> eventListeners = listenerMap.get(eventClass);
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if (eventListeners != null) {
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eventListeners.remove(eventListener);
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}
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}
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/**
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* publish event
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*
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* @param event event
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* @param <E> event generics
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*/
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@SuppressWarnings("unchecked")
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public <E extends Event<?>> void publish(E event) {
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List<EventListener<?>> eventListeners = listenerMap.get(event.getClass());
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if (eventListeners != null) {
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eventListeners.forEach(eventListener -> ((EventListener<E>) eventListener).onEvent(event));
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}
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}
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}

‎src/main/java/observer/listener/EventListener.java‎

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package observer.listener;
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import observer.event.Event;
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@FunctionalInterface
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public interface EventListener<T extends Event<?>> {
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void onEvent(T event);
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}

‎src/main/java/observer/listener/User.java‎

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package observer.listener;
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import observer.event.WeatherUpdateEvent;
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import java.util.function.Consumer;
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public class User implements EventListener<WeatherUpdateEvent> {
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private final String name;
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private final Consumer<String> consumer;
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public User(String name, Consumer<String> consumer) {
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this.name = name;
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this.consumer = consumer;
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}
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@Override
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public void onEvent(WeatherUpdateEvent event) {
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String message = String.format("[%s] received weather update event: %s, current time is %s", name, event.source(), event.formattedTimestamp());
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consumer.accept(message);
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}
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}

‎src/main/java/observer/publisher/Publisher.java‎

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package observer.publisher;
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import observer.event.Event;
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import observer.eventbus.EventBus;
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import java.util.Objects;
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public abstract class Publisher<E extends Event<?>> {
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private final EventBus eventBus;
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private final int intervalMillis;
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private volatile boolean running;
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private Thread workerThread;
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public Publisher(EventBus eventBus, int intervalMillis) {
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this.eventBus = Objects.requireNonNull(eventBus);
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this.intervalMillis = intervalMillis;
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}
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protected abstract E getEventInfo();
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public final void start() {
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if (running) {
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return;
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}
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running = true;
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workerThread = new Thread(() -> {
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try {
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while (running) {
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E event = getEventInfo();
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if (event != null) {
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eventBus.publish(event);
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Thread.sleep(intervalMillis);
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}
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}
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} catch (InterruptedException e) {
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Thread.currentThread().interrupt();
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}
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});
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workerThread.start();
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}
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public final void stop() {
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running = false;
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if (workerThread != null) {
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workerThread.interrupt();
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}
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}
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public final boolean isRunning() {
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return running;
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}
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}

‎src/main/java/observer/publisher/WeatherStationPublisher.java‎

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package observer.publisher;
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import observer.event.WeatherUpdateEvent;
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import observer.eventbus.EventBus;
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import java.util.Random;
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public class WeatherStationPublisher extends Publisher<WeatherUpdateEvent> {
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private final Random random = new Random();
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public WeatherStationPublisher(EventBus eventBus) {
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super(eventBus, 3000);
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}
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@Override
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protected WeatherUpdateEvent getEventInfo() {
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String info = random.nextBoolean() ? "Sunny Day" : "Rain Day";
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return new WeatherUpdateEvent(info);
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}
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}

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