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

You can using the ForwardingSet, ForwardingList, etc., from Guava 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 {

    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  Collection observedCollection(
    final Collection collection, final CollectionObserver observer) {
        return new Collection() {
            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

    };
    }