Good way to have a Collection Listener ?

Question

Is there a better way to have a listener on a java collection than wrap it in a class implementing the observer pattern ?

Answer 1

Well, if you don't actually need a java.util.Collection or List instance, you could use a DefaultListModel. I'm not aware of any "real" Collection implementations with builtin listener/observer support.

Answer 2

Apache Events.

"Commons-Events provides additional classes for firing and handling events. It focusses on the Java Collections Framework, providing decorators to other collections that fire events."

Answer 3

You should check out Glazed Lists

It contains observable List classes, which fire events whenever elements are added, removed, replaced, etc

Answer 4

You can using the ForwardingSet, ForwardingList, etc., from Google Collections http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/ForwardingCollection.html to decorate a particular instance with the desired behavior.

Here's my own implementation that just uses plain JDK APIs:

// create an abstract class that implements this interface with blank implementations
// that way, annonymous subclasses can observe only the events they care about
public interface CollectionObserver<E> {

    public void beforeAdd(E o);

    public void afterAdd(E o);

    // other events to be observed ...

}

// this method would go in a utility class
public static <E> Collection<E> observedCollection(
    final Collection<E> collection, final CollectionObserver<E> observer) {
        return new Collection<E>() {
            public boolean add(final E o) {
             observer.beforeAdd(o);
             boolean result = collection.add(o);
             observer.afterAdd(o);
             return result;
            }

            // ... generate rest of delegate methods in Eclipse

    };
    }