# Class WeakMapChangeListener<K,V>

java.lang.Object
javafx.collections.WeakMapChangeListener<K,V>

Type Parameters:
  `K` - the key element type
  `V` - the value element type

All Implemented Interfaces:
  `WeakListener, MapChangeListener<K,V>`

public final class WeakMapChangeListener<K,V> extends Object implements MapChangeListener<K,V>, WeakListener
A `WeakMapChangeListener` can be used, if an `ObservableMap` should only maintain a weak reference to the listener. This helps to avoid memory leaks, that can occur if observers are not unregistered from observed objects after use.

`WeakMapChangeListener` are created by passing in the original `MapChangeListener`. The `WeakMapChangeListener` should then be registered to listen for changes of the observed object.

Note: You have to keep a reference to the `MapChangeListener`, that was passed in as long as it is in use, otherwise it will be garbage collected to soon.

Since:
  JavaFX 2.1
See Also:
  - `MapChangeListener`
  - `ObservableMap`
  - `WeakListener`

- ## Nested Class Summary

  ### Nested classes/interfaces declared in interface MapChangeListener

  `MapChangeListener.Change<K,V>`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `WeakMapChangeListener(MapChangeListener<K,V> listener)`
  The constructor of `WeakMapChangeListener`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `onChanged(MapChangeListener.Change<? extends K, ? extends V> change)`
  Called after a change has been made to an ObservableMap.
  `boolean`
  `wasGarbageCollected()`
  Returns `true` if the linked listener was garbage-collected.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### WeakMapChangeListener

    public WeakMapChangeListener(MapChangeListener<K,V> listener)
    The constructor of `WeakMapChangeListener`.
    Parameters:
      `listener` - The original listener that should be notified

- ## Method Details

  - ### wasGarbageCollected

    public boolean wasGarbageCollected()
    Returns `true` if the linked listener was garbage-collected. In this case, the listener can be removed from the observable.
    Specified by:
      `wasGarbageCollected` in interface `WeakListener`
    Returns:
      `true` if the linked listener was garbage-collected.

  - ### onChanged

    public void onChanged(MapChangeListener.Change<? extends K, ? extends V> change)
    Called after a change has been made to an ObservableMap. This method is called on every elementary change (put/remove) once. This means, complex changes like keySet().removeAll(Collection) or clear() may result in more than one call of onChanged method.
    Specified by:
      `onChanged` in interface `MapChangeListener<K,V>`
    Parameters:
      `change` - the change that was made