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

there are many ways to achieve this - often i use this approach

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

public class ObservableArrayList extends ArrayList {

    private @interface MethodId {
         private static final int REMOVE = 2;
         private static final int ADD = 1;
    }

    public interface ListObserver {
        void onElementAdded(E element);
        void onElementRemoved(E element);
    }

    public ObservableArrayList(int capacity) {
        super(capacity);
        ensureObserver();
    }

    public ObservableArrayList() {
        ensureObserver();
    }

    public ObservableArrayList(Collection collection) {
        super(collection);
        ensureObserver();
    }

    private List>> _listObserverWeakRefList;

    public void addObserver(ListObserver observer) {
         _listObserverWeakRefList.add(new WeakReference>   (observer));
    }

    private void ensureObserver() {
       if (_listObserverWeakRefList == null) {
            _listObserverWeakRefList = new ArrayList<>();
       }
    }

    @Override
    public boolean add(E object) {
        super.add(object);
        callObservable(MethodId.ADD, object);
        return true;
    }

    @Override
    public boolean remove(Object object) {
        boolean removed = super.remove(object);
        if (removed) callObservable(MethodId.REMOVE, object);
        return removed;
    }

    private void callObservable(@MethodId int methodId, Object element) {
         for (WeakReference> observerRef :  _listObserverWeakRefList) {
             ListObserver observer = observerRef.get();
             if (observer != null) {
                 switch (methodId) {
                    case MethodId.ADD:
                        observer.onElementAdded((E) element);
                        break;
                    case MethodId.REMOVE:
                        observer.onElementRemoved((E) element);
                        break;
                } 
            }
        }
    }

}