cursorablelinkedlist.java

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        if(null != after) {
            after.setPrev(elt);
        } else {
            _head.setPrev(elt);
        }
        broadcastListableInserted(elt);
        return elt;
    }

    /**
     * Removes the given 
     * {@link org.apache.commons.collections.CursorableLinkedList.Listable} 
     * from my list.
     */
    protected void removeListable(Listable elt) {
        _modCount++;
        _size--;
        if(_head.next() == elt) {
            _head.setNext(elt.next());
        }
        if(null != elt.next()) {
            elt.next().setPrev(elt.prev());
        }
        if(_head.prev() == elt) {
            _head.setPrev(elt.prev());
        }
        if(null != elt.prev()) {
            elt.prev().setNext(elt.next());
        }
        broadcastListableRemoved(elt);
    }

    /**
     * Returns the 
     * {@link org.apache.commons.collections.CursorableLinkedList.Listable} 
     * at the specified index.
     *
     * @throws IndexOutOfBoundsException if index is less than zero or
     *         greater than or equal to the size of this list.
     */
    protected Listable getListableAt(int index) {
        if(index < 0 || index >= _size) {
            throw new IndexOutOfBoundsException(String.valueOf(index) + " < 0 or " + String.valueOf(index) + " >= " + _size);
        }
        if(index <=_size/2) {
            Listable elt = _head.next();
            for(int i = 0; i < index; i++) {
                elt = elt.next();
            }
            return elt;
        } else {
            Listable elt = _head.prev();
            for(int i = (_size-1); i > index; i--) {
                elt = elt.prev();
            }
            return elt;
        }
    }

    /**
     * Registers a {@link CursorableLinkedList.Cursor} to be notified
     * of changes to this list.
     */
    protected void registerCursor(Cursor cur) {
        // We take this opportunity to clean the _cursors list
        // of WeakReference objects to garbage-collected cursors.
        for (Iterator it = _cursors.iterator(); it.hasNext(); ) {
            WeakReference ref = (WeakReference) it.next();
            if (ref.get() == null) {
                it.remove();
            }
        }
        
        _cursors.add( new WeakReference(cur) );
    }

    /**
     * Removes a {@link CursorableLinkedList.Cursor} from
     * the set of cursors to be notified of changes to this list.
     */
    protected void unregisterCursor(Cursor cur) {
        for (Iterator it = _cursors.iterator(); it.hasNext(); ) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                // some other unrelated cursor object has been 
                // garbage-collected; let's take the opportunity to
                // clean up the cursors list anyway..
                it.remove();
                
            } else if (cursor == cur) {
                ref.clear();
                it.remove();
                break;
            }
        }
    }

    /**
     * Informs all of my registered cursors that they are now
     * invalid.
     */
    protected void invalidateCursors() {
        Iterator it = _cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor != null) {
                // cursor is null if object has been garbage-collected
                cursor.invalidate();
                ref.clear();
            }
            it.remove();
        }
    }

    /**
     * Informs all of my registered cursors that the specified
     * element was changed.
     * @see #set(int,java.lang.Object)
     */
    protected void broadcastListableChanged(Listable elt) {
        Iterator it = _cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove(); // clean up list
            } else {
                cursor.listableChanged(elt);
            }
        }
    }

    /**
     * Informs all of my registered cursors that the specified
     * element was just removed from my list.
     */
    protected void broadcastListableRemoved(Listable elt) {
        Iterator it = _cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove(); // clean up list
            } else {
                cursor.listableRemoved(elt);
            }
        }
    }

    /**
     * Informs all of my registered cursors that the specified
     * element was just added to my list.
     */
    protected void broadcastListableInserted(Listable elt) {
        Iterator it = _cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove();  // clean up list
            } else {
                cursor.listableInserted(elt);
            }
        }
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeInt(_size);
        Listable cur = _head.next();
        while (cur != null) {
            out.writeObject(cur.value());
            cur = cur.next();
        }
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        _size = 0;
        _modCount = 0;
        _cursors = new ArrayList();
        _head = new Listable(null,null,null);
        int size = in.readInt();
        for (int i=0;i<size;i++) {
            this.add(in.readObject());
        }
    }

    //--- protected attributes ---------------------------------------

    /** The number of elements in me. */
    protected transient int _size = 0;

    /**
     * A sentry node.
     * <p>
     * <tt>_head.next()</tt> points to the first element in the list,
     * <tt>_head.prev()</tt> to the last. Note that it is possible for
     * <tt>_head.next().prev()</tt> and <tt>_head.prev().next()</tt> to be
     * non-null, as when I am a sublist for some larger list.
     * Use <tt>== _head.next()</tt> and <tt>== _head.prev()</tt> to determine
     * if a given 
     * {@link org.apache.commons.collections.CursorableLinkedList.Listable} 
     * is the first or last element in the list.
     */
    protected transient Listable _head = new Listable(null,null,null);

    /** Tracks the number of structural modifications to me. */
    protected transient int _modCount = 0;

    /**
     * A list of the currently {@link CursorableLinkedList.Cursor}s currently
     * open in this list.
     */
    protected transient List _cursors = new ArrayList();

    //--- inner classes ----------------------------------------------

    static class Listable implements Serializable {
        private Listable _prev = null;
        private Listable _next = null;
        private Object _val = null;

        Listable(Listable prev, Listable next, Object val) {
            _prev = prev;
            _next = next;
            _val = val;
        }

        Listable next() {
            return _next;
        }

        Listable prev() {
            return _prev;
        }

        Object value() {
            return _val;
        }

        void setNext(Listable next) {
            _next = next;
        }

        void setPrev(Listable prev) {
            _prev = prev;
        }

        Object setValue(Object val) {
            Object temp = _val;
            _val = val;
            return temp;
        }
    }

    class ListIter implements ListIterator {
        Listable _cur = null;
        Listable _lastReturned = null;
        int _expectedModCount = _modCount;
        int _nextIndex = 0;

        ListIter(int index) {
            if(index == 0) {
                _cur = new Listable(null,_head.next(),null);
                _nextIndex = 0;
            } else if(index == _size) {
                _cur = new Listable(_head.prev(),null,null);
                _nextIndex = _size;
            } else {
                Listable temp = getListableAt(index);
                _cur = new Listable(temp.prev(),temp,null);
                _nextIndex = index;
            }
        }

        public Object previous() {
            checkForComod();
            if(!hasPrevious()) {
                throw new NoSuchElementException();
            } else {
                Object ret = _cur.prev().value();
                _lastReturned = _cur.prev();
                _cur.setNext(_cur.prev());
                _cur.setPrev(_cur.prev().prev());
                _nextIndex--;
                return ret;
            }
        }

        public boolean hasNext() {
            checkForComod();
            return(null != _cur.next() && _cur.prev() != _head.prev());
        }

        public Object next() {
            checkForComod();
            if(!hasNext()) {
                throw new NoSuchElementException();
            } else {
                Object ret = _cur.next().value();
                _lastReturned = _cur.next();
                _cur.setPrev(_cur.next());
                _cur.setNext(_cur.next().next());
                _nextIndex++;
                return ret;
            }
        }

        public int previousIndex() {
            checkForComod();
            if(!hasPrevious()) {
                return -1;
            }
            return _nextIndex-1;
        }

        public boolean hasPrevious() {
            checkForComod();
            return(null != _cur.prev() && _cur.next() != _head.next());
        }

        public void set(Object o) {
            checkForComod();
            try {
                _lastReturned.setValue(o);
            } catch(NullPointerException e) {
                throw new IllegalStateException();
            }
        }

        public int nextIndex() {
            checkForComod();
            if(!hasNext()) {
                return size();
            }
            return _nextIndex;
        }

        public void remove() {
            checkForComod();
            if(null == _lastReturned) {
                throw new IllegalStateException();
            } else {
                _cur.setNext(_lastReturned == _head.prev() ? null : _lastReturned.next());
                _cur.setPrev(_lastReturned == _head.next() ? null : _lastReturned.prev());
                removeListable(_lastReturned);
                _lastReturned = null;
                _nextIndex--;
                _expectedModCount++;
            }
        }

        public void add(Object o) {
            checkForComod();
            _cur.setPrev(insertListable(_cur.prev(),_cur.next(),o));
            _lastReturned = null;
            _nextIndex++;
            _expectedModCount++;
        }

        protected void checkForComod() {
            if(_expectedModCount != _modCount) {
                throw new ConcurrentModificationException();