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POAvoiding an unchecked cast for cast to a collection of a generic interface in Java for an event publisher
 

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    <p>I'm trying to create a lightweight, thread-safe in-app publish/subscribe mechanism for an Android app that I'm building. My basic approach is to keep track of a list of <code>IEventSubscriber&lt;T&gt;</code> for each event type T and then be able to publish events to subscribing objects by passing along a payload of type T.</p> <p>I use generic method parameters to (I think) ensure that subscriptions are created in a type safe way. Thus, I'm pretty sure that when I obtain the list of subscribers from my subscription map when it comes time to publish an event that I'm OK casting it to a list of <code>IEventSubscriber&lt;T&gt;</code>, however, this generates the unchecked cast warning.</p> <p>My questions:</p> <ol> <li>Is the unchecked cast actually safe here?</li> <li>How can I actually check to see if the items in the subscriber list implement <code>IEventSubscriber&lt;T&gt;</code>?</li> <li>Presuming that (2) involves some nasty reflection, what would you do here?</li> </ol> <p>Code (Java 1.6):</p> <pre><code>import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import java.util.concurrent.CopyOnWriteArraySet; public class EventManager { private ConcurrentMap&lt;Class, CopyOnWriteArraySet&lt;IEventSubscriber&gt;&gt; subscriptions = new ConcurrentHashMap&lt;Class, CopyOnWriteArraySet&lt;IEventSubscriber&gt;&gt;(); public &lt;T&gt; boolean subscribe(IEventSubscriber&lt;T&gt; subscriber, Class&lt;T&gt; eventClass) { CopyOnWriteArraySet&lt;IEventSubscriber&gt; existingSubscribers = subscriptions. putIfAbsent(eventClass, new CopyOnWriteArraySet&lt;IEventSubscriber&gt;()); return existingSubscribers.add(subscriber); } public &lt;T&gt; boolean removeSubscription(IEventSubscriber&lt;T&gt; subscriber, Class&lt;T&gt; eventClass) { CopyOnWriteArraySet&lt;IEventSubscriber&gt; existingSubscribers = subscriptions.get(eventClass); return existingSubscribers == null || !existingSubscribers.remove(subscriber); } public &lt;T&gt; void publish(T message, Class&lt;T&gt; eventClass) { @SuppressWarnings("unchecked") CopyOnWriteArraySet&lt;IEventSubscriber&lt;T&gt;&gt; existingSubscribers = (CopyOnWriteArraySet&lt;IEventSubscriber&lt;T&gt;&gt;) subscriptions.get(eventClass); if (existingSubscribers != null) { for (IEventSubscriber&lt;T&gt; subscriber: existingSubscribers) { subscriber.trigger(message); } } } } </code></pre>
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    1. COJava employs both compile time and runtime type checking. For generics it relies fairly heavily on compile time checking to assure that you do not assign the wrong type of generic instance, etc. But if you should happen to do this, the runtime will still detect the error eventually, though perhaps not until you're actually attempting to use the generic instance.
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