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<TITLE>Automatically Removing an Unreferenced Element from a Hash Table
(Java Developers Almanac Example)
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<h3>
e357.
Automatically Removing an Unreferenced Element from a Hash Table</h3>
When a key is added to a map, the map will prevent the key from being
garbage-collected. However, a <font color="#0066ff"><i>weak map</i></font> will automatically remove
a key if the key is not being referenced by any other object. An
example where this type of map might be useful is a registry where a
registrant is automatically removed after it is garbage-collected.
<pre>
// Create the weak map
Map weakMap = new WeakHashMap();
// Add a key to the weak map
weakMap.put(<font color="#0066ff"><i>keyObject</i></font>, <font color="#0066ff"><i>valueObject</i></font>);
// Get all keys that are still being referenced
Iterator it = weakMap.keySet().iterator();
while (it.hasNext()) {
// Get key
Object key = it.next();
}
</pre>
The weak map does not automatically release the value if it is no
longer used. To enable automatically release of the value, the value
must be wrapped in a <code>WeakReference</code> object:
<pre>
WeakReference weakValue = new WeakReference(<font color="#0066ff"><i>valueObject</i></font>);
weakMap.put(<font color="#0066ff"><i>keyObject</i></font>, weakValue);
// Get all keys that are still being referenced and check whether
// or not the value has been garbage-collected
it = weakMap.keySet().iterator();
while (it.hasNext()) {
// Get key
Object key = it.next();
weakValue = (WeakReference)weakMap.get(key);
if (weakValue == null) {
// Value has been garbage-collected
} else {
// Get value
valueObject = weakValue.get();
}
}
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Related Examples
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e355. <a class="eglink" href="coll_HashTables.html?l=rel">
Creating a Hash Table
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e356. <a class="eglink" href="coll_MapOrder.html?l=rel">
Creating a Map That Retains Order-of-Insertion
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See also:
<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Arrays">
Arrays
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Bits">
Bits
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Collections">
Collections
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Dates">
Dates
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Lists">
Lists
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Property%20Files">
Property Files
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Sets">
Sets
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Sorted%20Collections">
Sorted Collections
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Time">
Time
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<a class="eglink" href="/egs/java.util/pkg.html?l=rel#Timers">
Timers
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