Is there a Map implementation with listeners for Java?

Question

I would like a Map implementation in which i could add listeners for put() events.

Is there anything like that in the standard or any 3rd party libraries?

Answer 1

I'm not aware of any standard or 3rd party, but it is easy, just create a class which wraps another Map and implements the Map interface:

public class MapListener<K, V> implements Map<K, V> {

    private final Map<K, V> delegatee;

    public MapListener(Map<K, V> delegatee) {
        this.delegatee = delegatee;
    }

    // implement all Map methods, with callbacks you need.

}

Answer 2

Season to taste. This is representative, not normative. Of course it has issues.

public class ListenerMap extends HashMap {

    public static final String PROP_PUT = "put";
    private PropertyChangeSupport propertySupport;

    public ListenerMap() {
        super();
        propertySupport = new PropertyChangeSupport(this);
    }

    public String getSampleProperty() {
        return sampleProperty;
    }

    @Override
    public Object put(Object k, Object v) {
        Object old = super.put(k, v);
        propertySupport.firePropertyChange(PROP_PUT, old, v);
        return old;
    }

        public void addPropertyChangeListener(PropertyChangeListener listener) {
        propertySupport.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        propertySupport.removePropertyChangeListener(listener);
    }
}