Automatically Removing an Unreferenced Element from a Hash Table
When a key is added to a map, the map will prevent the key from being
garbage-collected. However, a weak map 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.
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 WeakReference object:
// Create the weak map
Map weakMap = new WeakHashMap();
// Add a key to the weak map
weakMap.put(keyObject, valueObject);
// Get all keys that are still being referenced
Iterator it = weakMap.keySet().iterator();
while (it.hasNext()) {
// Get key
Object key = it.next();
}
WeakReference weakValue = new WeakReference(valueObject);
weakMap.put(keyObject, 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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