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POJava: implementation of notification provider vs. hashCode-driven Map
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  1. POJava: implementation of notification provider vs. hashCode-driven Map
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    copied!<p>I have implemented abstract generic provider for notification bunch of generic listeners <code>E</code>, descendants have to override <code>notifyListener(E)</code> with specific notification code. For backing list of listeners I choose <code>WeakHashMap&lt;K,V&gt;</code>. Listeners must be held as weak references:</p> <pre><code>abstract public class NotificationProvider&lt;E&gt; { private Map&lt;E, Object&gt; listeners = new WeakHashMap&lt;E, Object&gt;(); public addListener(E listener) { listeners.put(listener, null); } public void notifyListeners() { for (E listener: listeners.keySet()) notifyListener(listener); } abstract protected void notifyListener(E listener); } </code></pre> <p>Typical use:</p> <pre><code> NotificationProvider&lt;MyListener&gt; provider; provider = new NotificationProvider&lt;MyListener&gt;() { @Override protected void notifyListener(MyListener listener) { listener.myNotification(); } } provider.addListener(myListener1); provider.addListener(myListener2); provider.notifyListeners(); </code></pre> <p>Everything works well, but when I need <code>AbstractList</code> descendant class as listener, backing <code>WeakHashMap</code> accepts only one listener instance! It's clear -- methods <code>hashCode()</code> and <code>equals()</code> on listeners return same value for all of instances (empty lists), so <code>WeakHashMap.put</code> only replace previously added listener.</p> <pre><code> public class MyList extends AbstractList&lt;MyItem&gt; { // some implementation } NotificationProvider&lt;MyList&gt; provider; provider = new NotificationProvider&lt;MyList&gt;() { @Override protected void notifyListener(MyList listener) { // some implementation } } MyList list1 = new MyList(); MyList list2 = new MyList(); provider.addListener(list1); provider.addListener(list2); provider.notifyListeners(); // only list2 instance is notified </code></pre> <p>What is the best solution?</p> <ol> <li><p>use another non-hashCode backing collection -- but <code>WeakHashMap</code> is so sweet for me, because automatically managing weak references for me</p></li> <li><p>use non-generic listener, for example abstract class with simple <code>equals() { return (this == object); }</code> implementation -- but this is not so flexible</p></li> <li><p>use some wrapper for listeners with simple equals() -- but this wrapper cannot be transparent to <code>addListener(E)</code> caller due to weak references</p></li> </ol> <p>Another ideas?</p>
 

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