Weak hashmap - 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.

// 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(); 

} 

 

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:

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(); 

}

}