e357. automatically removing an unreferenced element from a hash table.txt

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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();
        }
    }