/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.lang.ref.WeakReference;

A doubly-linked list implementation of the List interface, supporting a ListIterator that allows concurrent modifications to the underlying list. 
 Implements all of the optional List operations, the stack/queue/dequeue operations available in LinkedList and supports a ListIterator that allows concurrent modifications to the underlying list (see cursor). 

Note that this implementation is not synchronized.
Author:
Rodney Waldhoff, Janek Bogucki, Simon Kitching
See Also:
LinkedList
Deprecated:
Use new version in list subpackage, which has been rewritten
 and now returns the cursor from the listIterator method. Will be removed in v4.0
Since:
Commons Collections 1.0
Version:
$Revision: 647116 $ $Date: 2008-04-11 13:23:08 +0200 (Fri, 11 Apr 2008) $/**
 * A doubly-linked list implementation of the {@link List} interface,
 * supporting a {@link ListIterator} that allows concurrent modifications
 * to the underlying list.
 * <p>
 * Implements all of the optional {@link List} operations, the
 * stack/queue/dequeue operations available in {@link java.util.LinkedList}
 * and supports a {@link ListIterator} that allows concurrent modifications
 * to the underlying list (see {@link #cursor}).
 * <p>
 * <b>Note that this implementation is not synchronized.</b>
 *
 * @deprecated Use new version in list subpackage, which has been rewritten
 *  and now returns the cursor from the listIterator method. Will be removed in v4.0
 * @see java.util.LinkedList
 * @since Commons Collections 1.0
 * @version $Revision: 647116 $ $Date: 2008-04-11 13:23:08 +0200 (Fri, 11 Apr 2008) $
 * 
 * @author Rodney Waldhoff
 * @author Janek Bogucki
 * @author Simon Kitching
 */
public class CursorableLinkedList implements List, Serializable {
    
Ensure serialization compatibility /** Ensure serialization compatibility */    
    private static final long serialVersionUID = 8836393098519411393L;

    //--- public methods ---------------------------------------------

    
Appends the specified element to the end of this list.
Params:
o – element to be appended to this list.
Returns:
true/**
     * Appends the specified element to the end of this list.
     *
     * @param o element to be appended to this list.
     * @return <tt>true</tt>
     */
    public boolean add(Object o) {
        insertListable(_head.prev(),null,o);
        return true;
    }

    
Inserts the specified element at the specified position in this list.
Shifts the element currently at that position (if any) and any subsequent
 elements to the right (adds one to their indices).
Params:
index – index at which the specified element is to be inserted.
element – element to be inserted.
Throws:
ClassCastException – if the class of the specified element
          prevents it from being added to this list.
IllegalArgumentException – if some aspect of the specified
            element prevents it from being added to this list.
IndexOutOfBoundsException – if the index is out of range
            (index < 0 || index > size())./**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any subsequent
     *  elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted.
     * @param element element to be inserted.
     *
     * @throws ClassCastException if the class of the specified element
     *           prevents it from being added to this list.
     * @throws IllegalArgumentException if some aspect of the specified
     *             element prevents it from being added to this list.
     * @throws IndexOutOfBoundsException if the index is out of range
     *             (index &lt; 0 || index &gt; size()).
     */
    public void add(int index, Object element) {
        if(index == _size) {
            add(element);
        } else {
            if(index < 0 || index > _size) {
                throw new IndexOutOfBoundsException(String.valueOf(index) + " < 0 or " + String.valueOf(index) + " > " + _size);
            }
            Listable succ = (isEmpty() ? null : getListableAt(index));
            Listable pred = (null == succ ? null : succ.prev());
            insertListable(pred,succ,element);
        }
    }

    
Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified Collection's Iterator. The behavior of this operation is unspecified if the specified collection is modified while the operation is in progress. (Note that this will occur if the specified collection is this list, and it's nonempty.) 
Params:
c – collection whose elements are to be added to this list.
Throws:
ClassCastException – if the class of an element in the specified
         collection prevents it from being added to this list.
IllegalArgumentException – if some aspect of an element in the
        specified collection prevents it from being added to this
        list.
Returns:
true if this list changed as a result of the call./**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * {@link Collection}'s {@link Iterator}.  The behavior of this operation is
     * unspecified if the specified collection is modified while
     * the operation is in progress.  (Note that this will occur if the
     * specified collection is this list, and it's nonempty.)
     *
     * @param c collection whose elements are to be added to this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     *
     * @throws ClassCastException if the class of an element in the specified
     *          collection prevents it from being added to this list.
     * @throws IllegalArgumentException if some aspect of an element in the
     *         specified collection prevents it from being added to this
     *         list.
     */
    public boolean addAll(Collection c) {
        if(c.isEmpty()) {
            return false;
        }
        Iterator it = c.iterator();
        while(it.hasNext()) {
            insertListable(_head.prev(),null,it.next());
        }
        return true;
    }

    
Inserts all of the elements in the specified collection into this list at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in this list in the order that they are returned by the specified Collection's Iterator. The behavior of this operation is unspecified if the specified collection is modified while the operation is in progress. (Note that this will occur if the specified collection is this list, and it's nonempty.) 
Params:
index – index at which to insert first element from the specified
               collection.
c – elements to be inserted into this list.
Throws:
ClassCastException – if the class of one of elements of the
           specified collection prevents it from being added to this
           list.
IllegalArgumentException – if some aspect of one of elements of
        the specified collection prevents it from being added to
        this list.
IndexOutOfBoundsException – if the index is out of range (index
         < 0 || index > size()).
Returns:
true if this list changed as a result of the call./**
     * Inserts all of the elements in the specified collection into this
     * list at the specified position.  Shifts the element currently at
     * that position (if any) and any subsequent elements to the right
     * (increases their indices).  The new elements will appear in this
     * list in the order that they are returned by the specified
     * {@link Collection}'s {@link Iterator}.  The behavior of this operation is
     * unspecified if the specified collection is modified while the
     * operation is in progress.  (Note that this will occur if the specified
     * collection is this list, and it's nonempty.)
     *
     * @param index index at which to insert first element from the specified
     *                collection.
     * @param c elements to be inserted into this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     *
     * @throws ClassCastException if the class of one of elements of the
     *            specified collection prevents it from being added to this
     *            list.
     * @throws IllegalArgumentException if some aspect of one of elements of
     *         the specified collection prevents it from being added to
     *         this list.
     * @throws IndexOutOfBoundsException if the index is out of range (index
     *          &lt; 0 || index &gt; size()).
     */
    public boolean addAll(int index, Collection c) {
        if(c.isEmpty()) {
            return false;
        } else if(_size == index || _size == 0) {
            return addAll(c);
        } else {
            Listable succ = getListableAt(index);
            Listable pred = (null == succ) ? null : succ.prev();
            Iterator it = c.iterator();
            while(it.hasNext()) {
                pred = insertListable(pred,succ,it.next());
            }
            return true;
        }
    }

    
Inserts the specified element at the beginning of this list. (Equivalent to add(0,o)). 
Params:
o – element to be prepended to this list.
Returns:
true/**
     * Inserts the specified element at the beginning of this list.
     * (Equivalent to {@link #add(int,java.lang.Object) <tt>add(0,o)</tt>}).
     *
     * @param o element to be prepended to this list.
     * @return <tt>true</tt>
     */
    public boolean addFirst(Object o) {
        insertListable(null,_head.next(),o);
        return true;
    }

    
Inserts the specified element at the end of this list. (Equivalent to add(Object)). 
Params:
o – element to be appended to this list.
Returns:
true/**
     * Inserts the specified element at the end of this list.
     * (Equivalent to {@link #add(java.lang.Object)}).
     *
     * @param o element to be appended to this list.
     * @return <tt>true</tt>
     */
    public boolean addLast(Object o) {
        insertListable(_head.prev(),null,o);
        return true;
    }

    
Removes all of the elements from this list.  This
list will be empty after this call returns (unless
it throws an exception).
/**
     * Removes all of the elements from this list.  This
     * list will be empty after this call returns (unless
     * it throws an exception).
     */
    public void clear() {
        /*
        // this is the quick way, but would force us
        // to break all the cursors
        _modCount++;
        _head.setNext(null);
        _head.setPrev(null);
        _size = 0;
        */
        Iterator it = iterator();
        while(it.hasNext()) {
            it.next();
            it.remove();
        }
    }

    
Returns true if this list contains the specified element.
More formally, returns true if and only if this list contains
at least one element e such that
(o==null ? e==null : o.equals(e)).
Params:
o – element whose presence in this list is to be tested.
Returns:
true if this list contains the specified element./**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested.
     * @return <tt>true</tt> if this list contains the specified element.
     */
    public boolean contains(Object o) {
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            if((null == o && null == elt.value()) || 
               (o != null && o.equals(elt.value()))) {
                return true;
            }
        }
        return false;
    }

    
Returns true if this list contains all of the elements of the
specified collection.
Params:
c – collection to be checked for containment in this list.
Returns:
true if this list contains all of the elements of the
        specified collection./**
     * Returns <tt>true</tt> if this list contains all of the elements of the
     * specified collection.
     *
     * @param c collection to be checked for containment in this list.
     * @return <tt>true</tt> if this list contains all of the elements of the
     *         specified collection.
     */
    public boolean containsAll(Collection c) {
        Iterator it = c.iterator();
        while(it.hasNext()) {
            if(!this.contains(it.next())) {
                return false;
            }
        }
        return true;
    }

    
Returns a ListIterator for iterating through the elements of this list. Unlike iterator, a cursor is not bothered by concurrent modifications to the underlying list. 
 Specifically, when elements are added to the list before or after the cursor, the cursor simply picks them up automatically. When the "current" (i.e., last returned by ListIterator.next or ListIterator.previous) element of the list is removed, the cursor automatically adjusts to the change (invalidating the last returned value--i.e., it cannot be removed). 
 Note that the returned ListIterator does not support the ListIterator.nextIndex and ListIterator.previousIndex methods (they throw UnsupportedOperationException when invoked. 

Historical Note: In previous versions of this class, the object 
returned from this method was required to be explicitly closed. This 
is no longer necessary.
See Also:
cursor(int)
listIterator
Cursor/**
     * Returns a {@link ListIterator} for iterating through the
     * elements of this list. Unlike {@link #iterator}, a cursor
     * is not bothered by concurrent modifications to the
     * underlying list.
     * <p>
     * Specifically, when elements are added to the list before or
     * after the cursor, the cursor simply picks them up automatically.
     * When the "current" (i.e., last returned by {@link ListIterator#next}
     * or {@link ListIterator#previous}) element of the list is removed,
     * the cursor automatically adjusts to the change (invalidating the
     * last returned value--i.e., it cannot be removed).
     * <p>
     * Note that the returned {@link ListIterator} does not support the
     * {@link ListIterator#nextIndex} and {@link ListIterator#previousIndex}
     * methods (they throw {@link UnsupportedOperationException} when invoked.
     * <p>
     * Historical Note: In previous versions of this class, the object 
     * returned from this method was required to be explicitly closed. This 
     * is no longer necessary.
     *
     * @see #cursor(int)
     * @see #listIterator
     * @see CursorableLinkedList.Cursor
     */
    public CursorableLinkedList.Cursor cursor() {
        return new Cursor(0);
    }

    
Returns a ListIterator for iterating through the elements of this list, initialized such that ListIterator.next will return the element at the specified index (if any) and ListIterator.previous will return the element immediately preceding it (if any). Unlike iterator, a cursor is not bothered by concurrent modifications to the underlying list. 
Throws:
IndexOutOfBoundsException – if the index is out of range (index
           < 0 || index > size()).
See Also:
cursor
listIterator(int)
Cursor/**
     * Returns a {@link ListIterator} for iterating through the
     * elements of this list, initialized such that
     * {@link ListIterator#next} will return the element at
     * the specified index (if any) and {@link ListIterator#previous}
     * will return the element immediately preceding it (if any).
     * Unlike {@link #iterator}, a cursor
     * is not bothered by concurrent modifications to the
     * underlying list.
     *
     * @see #cursor
     * @see #listIterator(int)
     * @see CursorableLinkedList.Cursor
     * @throws IndexOutOfBoundsException if the index is out of range (index
     *            &lt; 0 || index &gt; size()).
     */
    public CursorableLinkedList.Cursor cursor(int i) {
        return new Cursor(i);
    }

    
Compares the specified object with this list for equality.  Returns
true if and only if the specified object is also a list, both
lists have the same size, and all corresponding pairs of elements in
the two lists are equal.  (Two elements e1 and
e2 are equal if (e1==null ? e2==null :
e1.equals(e2)).)  In other words, two lists are defined to be
equal if they contain the same elements in the same order.  This
definition ensures that the equals method works properly across
different implementations of the List interface.
Params:
o – the object to be compared for equality with this list.
Returns:
true if the specified object is equal to this list./**
     * Compares the specified object with this list for equality.  Returns
     * <tt>true</tt> if and only if the specified object is also a list, both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal</i>.  (Two elements <tt>e1</tt> and
     * <tt>e2</tt> are <i>equal</i> if <tt>(e1==null ? e2==null :
     * e1.equals(e2))</tt>.)  In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.  This
     * definition ensures that the equals method works properly across
     * different implementations of the <tt>List</tt> interface.
     *
     * @param o the object to be compared for equality with this list.
     * @return <tt>true</tt> if the specified object is equal to this list.
     */
    public boolean equals(Object o) {
        if(o == this) {
            return true;
        } else if(!(o instanceof List)) {
            return false;
        }
        Iterator it = ((List)o).listIterator();
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            if(!it.hasNext() || (null == elt.value() ? null != it.next() : !(elt.value().equals(it.next()))) ) {
                return false;
            }
        }
        return !it.hasNext();
    }

    
Returns the element at the specified position in this list.
Params:
index – index of element to return.
Throws:
IndexOutOfBoundsException – if the index is out of range (index
          < 0 || index >= size()).
Returns:
the element at the specified position in this list./**
     * Returns the element at the specified position in this list.
     *
     * @param index index of element to return.
     * @return the element at the specified position in this list.
     *
     * @throws IndexOutOfBoundsException if the index is out of range (index
     *           &lt; 0 || index &gt;= size()).
     */
    public Object get(int index) {
        return getListableAt(index).value();
    }

    
Returns the element at the beginning of this list.
/**
     * Returns the element at the beginning of this list.
     */
    public Object getFirst() {
        try {
            return _head.next().value();
        } catch(NullPointerException e) {
            throw new NoSuchElementException();
        }
    }

    
Returns the element at the end of this list.
/**
     * Returns the element at the end of this list.
     */
    public Object getLast() {
        try {
            return _head.prev().value();
        } catch(NullPointerException e) {
            throw new NoSuchElementException();
        }
    }

    
Returns the hash code value for this list.  The hash code of a list
is defined to be the result of the following calculation:
 hashCode = 1;
 Iterator i = list.iterator();
 while (i.hasNext()) {
     Object obj = i.next();
     hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
 }

This ensures that list1.equals(list2) implies that
list1.hashCode()==list2.hashCode() for any two lists,
list1 and list2, as required by the general
contract of Object.hashCode.
See Also:
Object.hashCode()
Object.equals(Object)
equals(Object)
Returns:
the hash code value for this list./**
     * Returns the hash code value for this list.  The hash code of a list
     * is defined to be the result of the following calculation:
     * <pre>
     *  hashCode = 1;
     *  Iterator i = list.iterator();
     *  while (i.hasNext()) {
     *      Object obj = i.next();
     *      hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
     *  }
     * </pre>
     * This ensures that <tt>list1.equals(list2)</tt> implies that
     * <tt>list1.hashCode()==list2.hashCode()</tt> for any two lists,
     * <tt>list1</tt> and <tt>list2</tt>, as required by the general
     * contract of <tt>Object.hashCode</tt>.
     *
     * @return the hash code value for this list.
     * @see Object#hashCode()
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    public int hashCode() {
        int hash = 1;
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            hash = 31*hash + (null == elt.value() ? 0 : elt.value().hashCode());
        }
        return hash;
    }

    
Returns the index in this list of the first occurrence of the specified
element, or -1 if this list does not contain this element.
More formally, returns the lowest index i such that
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
Params:
o – element to search for.
Returns:
the index in this list of the first occurrence of the specified
        element, or -1 if this list does not contain this element./**
     * Returns the index in this list of the first occurrence of the specified
     * element, or -1 if this list does not contain this element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for.
     * @return the index in this list of the first occurrence of the specified
     *         element, or -1 if this list does not contain this element.
     */
    public int indexOf(Object o) {
        int ndx = 0;

        // perform the null check outside of the loop to save checking every
        // single time through the loop.
        if (null == o) {
            for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
                if (null == elt.value()) {
                    return ndx;
                }
                ndx++;
            }
        } else {

            for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
                if (o.equals(elt.value())) {
                    return ndx;
                }
                ndx++;
            }
        }
        return -1;
    }

    
Returns true if this list contains no elements.
Returns:
true if this list contains no elements./**
     * Returns <tt>true</tt> if this list contains no elements.
     * @return <tt>true</tt> if this list contains no elements.
     */
    public boolean isEmpty() {
        return(0 == _size);
    }

    
Returns a fail-fast iterator.
See Also:
iterator.iterator/**
     * Returns a fail-fast iterator.
     * @see List#iterator
     */
    public Iterator iterator() {
        return listIterator(0);
    }

    
Returns the index in this list of the last occurrence of the specified
element, or -1 if this list does not contain this element.
More formally, returns the highest index i such that
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
Params:
o – element to search for.
Returns:
the index in this list of the last occurrence of the specified
           element, or -1 if this list does not contain this element./**
     * Returns the index in this list of the last occurrence of the specified
     * element, or -1 if this list does not contain this element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for.
     * @return the index in this list of the last occurrence of the specified
     *            element, or -1 if this list does not contain this element.
     */
    public int lastIndexOf(Object o) {
        int ndx = _size-1;

        // perform the null check outside of the loop to save checking every
        // single time through the loop.
        if (null == o) {
            for(Listable elt = _head.prev(), past = null; null != elt && past != _head.next(); elt = (past = elt).prev()) {
                if (null == elt.value()) {
                    return ndx;
                }
                ndx--;
            }
        } else {
            for(Listable elt = _head.prev(), past = null; null != elt && past != _head.next(); elt = (past = elt).prev()) {
                if (o.equals(elt.value())) {
                    return ndx;
                }
                ndx--;
            }
        }
        return -1;
    }

    
Returns a fail-fast ListIterator.
See Also:
listIterator.listIterator/**
     * Returns a fail-fast ListIterator.
     * @see List#listIterator
     */
    public ListIterator listIterator() {
        return listIterator(0);
    }

    
Returns a fail-fast ListIterator.
See Also:
listIterator.listIterator(int)/**
     * Returns a fail-fast ListIterator.
     * @see List#listIterator(int)
     */
    public ListIterator listIterator(int index) {
        if(index<0 || index > _size) {
            throw new IndexOutOfBoundsException(index + " < 0 or > " + _size);
        }
        return new ListIter(index);
    }

    
Removes the first occurrence in this list of the specified element.
If this list does not contain the element, it is
unchanged.  More formally, removes the element with the lowest index i
such that (o==null ? get(i)==null : o.equals(get(i))) (if
such an element exists).
Params:
o – element to be removed from this list, if present.
Returns:
true if this list contained the specified element./**
     * Removes the first occurrence in this list of the specified element.
     * If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index i
     * such that <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> (if
     * such an element exists).
     *
     * @param o element to be removed from this list, if present.
     * @return <tt>true</tt> if this list contained the specified element.
     */
    public boolean remove(Object o) {
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            if(null == o && null == elt.value()) {
                removeListable(elt);
                return true;
            } else if(o != null && o.equals(elt.value())) {
                removeListable(elt);
                return true;
            }
        }
        return false;
    }

    
Removes the element at the specified position in this list (optional
operation).  Shifts any subsequent elements to the left (subtracts one
from their indices).  Returns the element that was removed from the
list.
Params:
index – the index of the element to removed.
Throws:
IndexOutOfBoundsException – if the index is out of range (index
           < 0 || index >= size()).
Returns:
the element previously at the specified position./**
     * Removes the element at the specified position in this list (optional
     * operation).  Shifts any subsequent elements to the left (subtracts one
     * from their indices).  Returns the element that was removed from the
     * list.
     *
     * @param index the index of the element to removed.
     * @return the element previously at the specified position.
     *
     * @throws IndexOutOfBoundsException if the index is out of range (index
     *            &lt; 0 || index &gt;= size()).
     */
    public Object remove(int index) {
        Listable elt = getListableAt(index);
        Object ret = elt.value();
        removeListable(elt);
        return ret;
    }

    
Removes from this list all the elements that are contained in the
specified collection.
Params:
c – collection that defines which elements will be removed from
         this list.
Returns:
true if this list changed as a result of the call./**
     * Removes from this list all the elements that are contained in the
     * specified collection.
     *
     * @param c collection that defines which elements will be removed from
     *          this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     */
    public boolean removeAll(Collection c) {
        if(0 == c.size() || 0 == _size) {
            return false;
        } else {
            boolean changed = false;
            Iterator it = iterator();
            while(it.hasNext()) {
                if(c.contains(it.next())) {
                    it.remove();
                    changed = true;
                }
            }
            return changed;
        }
    }

    
Removes the first element of this list, if any.
/**
     * Removes the first element of this list, if any.
     */
    public Object removeFirst() {
        if(_head.next() != null) {
            Object val = _head.next().value();
            removeListable(_head.next());
            return val;
        } else {
            throw new NoSuchElementException();
        }
    }

    
Removes the last element of this list, if any.
/**
     * Removes the last element of this list, if any.
     */
    public Object removeLast() {
        if(_head.prev() != null) {
            Object val = _head.prev().value();
            removeListable(_head.prev());
            return val;
        } else {
            throw new NoSuchElementException();
        }
    }

    
Retains only the elements in this list that are contained in the
specified collection.  In other words, removes
from this list all the elements that are not contained in the specified
collection.
Params:
c – collection that defines which elements this set will retain.
Returns:
true if this list changed as a result of the call./**
     * Retains only the elements in this list that are contained in the
     * specified collection.  In other words, removes
     * from this list all the elements that are not contained in the specified
     * collection.
     *
     * @param c collection that defines which elements this set will retain.
     *
     * @return <tt>true</tt> if this list changed as a result of the call.
     */
    public boolean retainAll(Collection c) {
        boolean changed = false;
        Iterator it = iterator();
        while(it.hasNext()) {
            if(!c.contains(it.next())) {
                it.remove();
                changed = true;
            }
        }
        return changed;
    }

    
Replaces the element at the specified position in this list with the
specified element.
Params:
index – index of element to replace.
element – element to be stored at the specified position.
Throws:
ClassCastException – if the class of the specified element
          prevents it from being added to this list.
IllegalArgumentException – if some aspect of the specified
           element prevents it from being added to this list.
IndexOutOfBoundsException – if the index is out of range
            (index < 0 || index >= size()).
Returns:
the element previously at the specified position./**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index index of element to replace.
     * @param element element to be stored at the specified position.
     * @return the element previously at the specified position.
     *
     * @throws ClassCastException if the class of the specified element
     *           prevents it from being added to this list.
     * @throws IllegalArgumentException if some aspect of the specified
     *            element prevents it from being added to this list.
     * @throws IndexOutOfBoundsException if the index is out of range
     *             (index &lt; 0 || index &gt;= size()).
     */
    public Object set(int index, Object element) {
        Listable elt = getListableAt(index);
        Object val = elt.setValue(element);
        broadcastListableChanged(elt);
        return val;
    }

    
Returns the number of elements in this list.
Returns:
the number of elements in this list./**
     * Returns the number of elements in this list.
     * @return the number of elements in this list.
     */
    public int size() {
        return _size;
    }

    
Returns an array containing all of the elements in this list in proper sequence. Obeys the general contract of the Collection.toArray method. 
Returns:
an array containing all of the elements in this list in proper
        sequence./**
     * Returns an array containing all of the elements in this list in proper
     * sequence.  Obeys the general contract of the {@link Collection#toArray} method.
     *
     * @return an array containing all of the elements in this list in proper
     *         sequence.
     */
    public Object[] toArray() {
        Object[] array = new Object[_size];
        int i = 0;
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            array[i++] = elt.value();
        }
        return array;
    }

    
Returns an array containing all of the elements in this list in proper sequence; the runtime type of the returned array is that of the specified array. Obeys the general contract of the Collection.toArray method. 
Params:
a –      the array into which the elements of this list are to
              be stored, if it is big enough; otherwise, a new array of the
              same runtime type is allocated for this purpose.
Throws:
ArrayStoreException – 
                  if the runtime type of the specified array
                  is not a supertype of the runtime type of every element in
                  this list.
Returns:
an array containing the elements of this list./**
     * Returns an array containing all of the elements in this list in proper
     * sequence; the runtime type of the returned array is that of the
     * specified array. Obeys the general contract of the
     * {@link Collection#toArray} method.
     *
     * @param a      the array into which the elements of this list are to
     *               be stored, if it is big enough; otherwise, a new array of the
     *               same runtime type is allocated for this purpose.
     * @return an array containing the elements of this list.
     * @exception ArrayStoreException
     *                   if the runtime type of the specified array
     *                   is not a supertype of the runtime type of every element in
     *                   this list.
     */
    public Object[] toArray(Object a[]) {
        if(a.length < _size) {
            a = (Object[])Array.newInstance(a.getClass().getComponentType(), _size);
        }
        int i = 0;
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            a[i++] = elt.value();
        }
        if(a.length > _size) {
            a[_size] = null; // should we null out the rest of the array also? java.util.LinkedList doesn't
        }
        return a;
    }

    
Returns a String representation of this list, suitable for debugging. 
Returns:
a String representation of this list, suitable for debugging./**
     * Returns a {@link String} representation of this list, suitable for debugging.
     * @return a {@link String} representation of this list, suitable for debugging.
     */
    public String toString() {
        StringBuffer buf = new StringBuffer();
        buf.append("[");
        for(Listable elt = _head.next(), past = null; null != elt && past != _head.prev(); elt = (past = elt).next()) {
            if(_head.next() != elt) {
                buf.append(", ");
            }
            buf.append(elt.value());
        }
        buf.append("]");
        return buf.toString();
    }

    
Returns a fail-fast sublist.
See Also:
subList.subList(int, int)/**
     * Returns a fail-fast sublist.
     * @see List#subList(int,int)
     */
    public List subList(int i, int j) {
        if(i < 0 || j > _size || i > j) {
            throw new IndexOutOfBoundsException();
        } else if(i == 0 && j == _size) {
            return this;
        } else {
            return new CursorableSubList(this,i,j);
        }
    }

    //--- protected methods ------------------------------------------

    
Inserts a new value into my
list, after the specified before element, and before the
specified after element
Returns:
the newly created Listable/**
     * Inserts a new <i>value</i> into my
     * list, after the specified <i>before</i> element, and before the
     * specified <i>after</i> element
     *
     * @return the newly created 
     * {@link org.apache.commons.collections.CursorableLinkedList.Listable}
     */
    protected Listable insertListable(Listable before, Listable after, Object value) {
        _modCount++;
        _size++;
        Listable elt = new Listable(before,after,value);
        if(null != before) {
            before.setNext(elt);
        } else {
            _head.setNext(elt);
        }

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

    
Removes the given Listable from my list. /**
     * 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 Listable at the specified index. 
Throws:
IndexOutOfBoundsException – if index is less than zero or
        greater than or equal to the size of this list./**
     * 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 Cursor to be notified of changes to this list. /**
     * 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 Cursor from the set of cursors to be notified of changes to this list. /**
     * 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.
/**
     * 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 Also:
set(int, Object)/**
     * 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.
/**
     * 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.
/**
     * 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. /** The number of elements in me. */
    protected transient int _size = 0;

    
A sentry node.

_head.next() points to the first element in the list,
_head.prev() to the last. Note that it is possible for
_head.next().prev() and _head.prev().next() to be
non-null, as when I am a sublist for some larger list.
Use == _head.next() and == _head.prev() to determine if a given Listable is the first or last element in the list. /**
     * 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. /** Tracks the number of structural modifications to me. */
    protected transient int _modCount = 0;

    
A list of the currently Cursors currently open in this list. /**
     * 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();
            }
        }
    }

    public class Cursor extends ListIter implements ListIterator {
        boolean _valid = false;

        Cursor(int index) {
            super(index);
            _valid = true;
            registerCursor(this);
        }

        public int previousIndex() {
            throw new UnsupportedOperationException();
        }

        public int nextIndex() {
            throw new UnsupportedOperationException();
        }

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

        protected void listableRemoved(Listable elt) {
            if(null == _head.prev()) {
                _cur.setNext(null);
            } else if(_cur.next() == elt) {
                _cur.setNext(elt.next());
            }
            if(null == _head.next()) {
                _cur.setPrev(null);
            } else if(_cur.prev() == elt) {
                _cur.setPrev(elt.prev());
            }
            if(_lastReturned == elt) {
                _lastReturned = null;
            }
        }

        protected void listableInserted(Listable elt) {
            if(null == _cur.next() && null == _cur.prev()) {
                _cur.setNext(elt);
            } else if(_cur.prev() == elt.prev()) {
                _cur.setNext(elt);
            }
            if(_cur.next() == elt.next()) {
                _cur.setPrev(elt);
            }
            if(_lastReturned == elt) {
                _lastReturned = null;
            }
        }

        protected void listableChanged(Listable elt) {
            if(_lastReturned == elt) {
                _lastReturned = null;
            }
        }

        protected void checkForComod() {
            if(!_valid) {
                throw new ConcurrentModificationException();
            }
        }

        protected void invalidate() {
            _valid = false;
        }

        
Mark this cursor as no longer being needed. Any resources
associated with this cursor are immediately released.
In previous versions of this class, it was mandatory to close
all cursor objects to avoid memory leaks. It is no longer
necessary to call this close method; an instance of this class
can now be treated exactly like a normal iterator.
/**
         * Mark this cursor as no longer being needed. Any resources
         * associated with this cursor are immediately released.
         * In previous versions of this class, it was mandatory to close
         * all cursor objects to avoid memory leaks. It is <i>no longer</i>
         * necessary to call this close method; an instance of this class
         * can now be treated exactly like a normal iterator.
         */
        public void close() {
            if(_valid) {
                _valid = false;
                unregisterCursor(this);
            }
        }
    }

}

Deprecated:
Use new version in list subpackage, which has been rewritten
 and now returns the cursor from the listIterator method. Will be removed in v4.0/**
 * @deprecated Use new version in list subpackage, which has been rewritten
 *  and now returns the cursor from the listIterator method. Will be removed in v4.0
 */
class CursorableSubList extends CursorableLinkedList implements List {

    //--- constructors -----------------------------------------------

    CursorableSubList(CursorableLinkedList list, int from, int to) {
        if(0 > from || list.size() < to) {
            throw new IndexOutOfBoundsException();
        } else if(from > to) {
            throw new IllegalArgumentException();
        }
        _list = list;
        if(from < list.size()) {
            _head.setNext(_list.getListableAt(from));
            _pre = (null == _head.next()) ? null : _head.next().prev();
        } else {
            _pre = _list.getListableAt(from-1);
        }
        if(from == to) {
            _head.setNext(null);
            _head.setPrev(null);
            if(to < list.size()) {
                _post = _list.getListableAt(to);
            } else {
                _post = null;
            }
        } else {
            _head.setPrev(_list.getListableAt(to-1));
            _post = _head.prev().next();
        }
        _size = to - from;
        _modCount = _list._modCount;
    }

    //--- public methods ------------------------------------------

    public void clear() {
        checkForComod();
        Iterator it = iterator();
        while(it.hasNext()) {
            it.next();
            it.remove();
        }
    }

    public Iterator iterator() {
        checkForComod();
        return super.iterator();
    }

    public int size() {
        checkForComod();
        return super.size();
    }

    public boolean isEmpty() {
        checkForComod();
        return super.isEmpty();
    }

    public Object[] toArray() {
        checkForComod();
        return super.toArray();
    }

    public Object[] toArray(Object a[]) {
        checkForComod();
        return super.toArray(a);
    }

    public boolean contains(Object o) {
        checkForComod();
        return super.contains(o);
    }

    public boolean remove(Object o) {
        checkForComod();
        return super.remove(o);
    }

    public Object removeFirst() {
        checkForComod();
        return super.removeFirst();
    }

    public Object removeLast() {
        checkForComod();
        return super.removeLast();
    }

    public boolean addAll(Collection c) {
        checkForComod();
        return super.addAll(c);
    }

    public boolean add(Object o) {
        checkForComod();
        return super.add(o);
    }

    public boolean addFirst(Object o) {
        checkForComod();
        return super.addFirst(o);
    }

    public boolean addLast(Object o) {
        checkForComod();
        return super.addLast(o);
    }

    public boolean removeAll(Collection c) {
        checkForComod();
        return super.removeAll(c);
    }

    public boolean containsAll(Collection c) {
        checkForComod();
        return super.containsAll(c);
    }

    public boolean addAll(int index, Collection c) {
        checkForComod();
        return super.addAll(index,c);
    }

    public int hashCode() {
        checkForComod();
        return super.hashCode();
    }

    public boolean retainAll(Collection c) {
        checkForComod();
        return super.retainAll(c);
    }

    public Object set(int index, Object element) {
        checkForComod();
        return super.set(index,element);
    }

    public boolean equals(Object o) {
        checkForComod();
        return super.equals(o);
    }

    public Object get(int index) {
        checkForComod();
        return super.get(index);
    }

    public Object getFirst() {
        checkForComod();
        return super.getFirst();
    }

    public Object getLast() {
        checkForComod();
        return super.getLast();
    }

    public void add(int index, Object element) {
        checkForComod();
        super.add(index,element);
    }

    public ListIterator listIterator(int index) {
        checkForComod();
        return super.listIterator(index);
    }

    public Object remove(int index) {
        checkForComod();
        return super.remove(index);
    }

    public int indexOf(Object o) {
        checkForComod();
        return super.indexOf(o);
    }

    public int lastIndexOf(Object o) {
        checkForComod();
        return super.lastIndexOf(o);
    }

    public ListIterator listIterator() {
        checkForComod();
        return super.listIterator();
    }

    public List subList(int fromIndex, int toIndex) {
        checkForComod();
        return super.subList(fromIndex,toIndex);
    }

    //--- protected methods ------------------------------------------

    
Inserts a new value into my
list, after the specified before element, and before the
specified after element
Returns:
the newly created Listable/**
     * Inserts a new <i>value</i> into my
     * list, after the specified <i>before</i> element, and before the
     * specified <i>after</i> element
     *
     * @return the newly created {@link CursorableLinkedList.Listable}
     */
    protected Listable insertListable(Listable before, Listable after, Object value) {
        _modCount++;
        _size++;
        Listable elt = _list.insertListable((null == before ? _pre : before), (null == after ? _post : after),value);
        if(null == _head.next()) {
            _head.setNext(elt);
            _head.setPrev(elt);
        }
        if(before == _head.prev()) {
            _head.setPrev(elt);
        }
        if(after == _head.next()) {
            _head.setNext(elt);
        }
        broadcastListableInserted(elt);
        return elt;
    }

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

    
Test to see if my underlying list has been modified by some other process. If it has, throws a ConcurrentModificationException, otherwise quietly returns. 
Throws:
ConcurrentModificationException – /**
     * Test to see if my underlying list has been modified
     * by some other process.  If it has, throws a
     * {@link ConcurrentModificationException}, otherwise
     * quietly returns.
     *
     * @throws ConcurrentModificationException
     */
    protected void checkForComod() throws ConcurrentModificationException {
        if(_modCount != _list._modCount) {
            throw new ConcurrentModificationException();
        }
    }

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

    
My underlying list /** My underlying list */
    protected CursorableLinkedList _list = null;

    
The element in my underlying list preceding the first element in my list. /** The element in my underlying list preceding the first element in my list. */
    protected Listable _pre = null;

    
The element in my underlying list following the last element in my list. /** The element in my underlying list following the last element in my list. */
    protected Listable _post = null;

}