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POJava HashMap cannot find key from within ListDataListener event
 

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    <p>Can someone explain why the HashMap acts like it does in this example:<br> Simple test that checks a hashmap for a key. Once in the constructor and once in ListDataListener intervallAdded method.</p> <pre><code>import java.util.HashMap; import java.util.List; import java.util.Map; import javax.swing.event.ListDataEvent; import javax.swing.event.ListDataListener; import com.jgoodies.common.collect.ArrayListModel; public class Test1 { private final Listener listener = new Listener(); private final Map&lt;List&lt;?&gt;, Object&gt; parentByCollection = new HashMap&lt;List&lt;?&gt;, Object&gt;(); public Test1(){ ArrayListModel&lt;Object&gt; list = new ArrayListModel&lt;Object&gt;(); list.addListDataListener(listener); parentByCollection.put(list, new Integer(10)); // Test containsKey locally System.out.println("Item exists (locally):" + parentByCollection.containsKey(list)); // Test containsKey via ListDataListener list.add(new Integer(20)); } /** * @param args */ public static void main(String[] args) { new Test1(); } public class Listener implements ListDataListener{ @Override public void intervalAdded(ListDataEvent e) { List&lt;?&gt; itemSource = (List&lt;?&gt;)e.getSource(); System.out.println("Item exists (listener):" + parentByCollection.containsKey(itemSource)); } @Override public void intervalRemoved(ListDataEvent e) { } @Override public void contentsChanged(ListDataEvent e) { } } } </code></pre> <p>Why does the hashmap return false from the event but true from constructor when using containsKey? Is there some java-generics "magic" I don't know about here? </p> <p>Edit:</p> <p>Just found that ArrayList's (which ArrayListModel extends) hashCode method assembles its hashcode from all its elements. Which means the hashCode changes with the items in the list. So storing a ArrayList in a HashMap is not a good idea.</p> <p>How can I solve this? Store the collections in a holder/container object instead?</p>
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    1. USMarcus
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    1. COPlease provide hashCode() method implementation for `com.jgoodies.common.collect.ArrayListModel`.
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      1. USStan Kurilin
    2. COYou can find sources here http://www.jgoodies.com/downloads/libraries.html ArrayListModel extends ArrayList and doesn't override hashCode so I guess the ArrayList's hashCode is the one that is used. But I can't see how that is relevant, the first call to containsKey returns true.
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