/*

 * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group.  Adapted and released, under explicit permission,

 * from JDK ArrayList.java which carries the following copyright:

 *

 * Copyright 1997 by Sun Microsystems, Inc.,

 * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.

 * All rights reserved.

 */

package java.util.concurrent;

import java.util.*;

import java.util.concurrent.locks.*;

/**

 * A thread-safe variant of {@link java.util.ArrayList} in which all mutative

 * operations (<tt>add</tt>, <tt>set</tt>, and so on) are implemented by

 * making a fresh copy of the underlying array.

 *

 * <p> This is ordinarily too costly, but may be <em>more</em> efficient

 * than alternatives when traversal operations vastly outnumber

 * mutations, and is useful when you cannot or don't want to

 * synchronize traversals, yet need to preclude interference among

 * concurrent threads.  The "snapshot" style iterator method uses a

 * reference to the state of the array at the point that the iterator

 * was created. This array never changes during the lifetime of the

 * iterator, so interference is impossible and the iterator is

 * guaranteed not to throw <tt>ConcurrentModificationException</tt>.

 * The iterator will not reflect additions, removals, or changes to

 * the list since the iterator was created.  Element-changing

 * operations on iterators themselves (<tt>remove</tt>, <tt>set</tt>, and

 * <tt>add</tt>) are not supported. These methods throw

 * <tt>UnsupportedOperationException</tt>.

 *

 * <p>All elements are permitted, including <tt>null</tt>.

 *

 * <p>Memory consistency effects: As with other concurrent

 * collections, actions in a thread prior to placing an object into a

 * {@code CopyOnWriteArrayList}

 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

 * actions subsequent to the access or removal of that element from

 * the {@code CopyOnWriteArrayList} in another thread.

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @since 1.5

 * @author Doug Lea

 * @param <E> the type of elements held in this collection

 */

public class CopyOnWriteArrayList<E>

    implements List<E>, RandomAccess, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = 8673264195747942595L;

    /** The lock protecting all mutators */

    transient ReentrantLock lock = new ReentrantLock();

    /** The array, accessed only via getArray/setArray. */

    private volatile transient Object[] array;

    /**

     * Gets the array.  Non-private so as to also be accessible

     * from CopyOnWriteArraySet class.

     */

    final Object[] getArray() {

        return array;

    }

    /**

     * Sets the array.

     */

    final void setArray(Object[] a) {

        array = a;

    }

    /**

     * Creates an empty list.

     */

    public CopyOnWriteArrayList() {

        setArray(new Object[0]);

    }

    /**

     * Creates a list containing the elements of the specified

     * collection, in the order they are returned by the collection's

     * iterator.

     *

     * @param c the collection of initially held elements

     * @throws NullPointerException if the specified collection is null

     */

    public CopyOnWriteArrayList(Collection<? extends E> c) {

        Object[] elements = c.toArray();

        // c.toArray might (incorrectly) not return Object[] (see 6260652)

        if (elements.getClass() != Object[].class)

            elements = Arrays.copyOf(elements, elements.length, Object[].class);

        setArray(elements);

    }

    /**

     * Creates a list holding a copy of the given array.

     *

     * @param toCopyIn the array (a copy of this array is used as the

     *        internal array)

     * @throws NullPointerException if the specified array is null

     */

    public CopyOnWriteArrayList(E[] toCopyIn) {

        setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));

    }

    /**

     * Returns the number of elements in this list.

     *

     * @return the number of elements in this list

     */

    public int size() {

        return getArray().length;

    }

    /**

     * Returns <tt>true</tt> if this list contains no elements.

     *

     * @return <tt>true</tt> if this list contains no elements

     */

    public boolean isEmpty() {

        return size() == 0;

    }

    /**

     * Test for equality, coping with nulls.

     */

    private static boolean eq(Object o1, Object o2) {

        return (o1 == null ? o2 == null : o1.equals(o2));

    }

    /**

     * static version of indexOf, to allow repeated calls without

     * needing to re-acquire array each time.

     * @param o element to search for

     * @param elements the array

     * @param index first index to search

     * @param fence one past last index to search

     * @return index of element, or -1 if absent

     */

    private static int indexOf(Object o, Object[] elements,

                               int index, int fence) {

        if (o == null) {

            for (int i = index; i < fence; i++)

                if (elements[i] == null)

                    return i;

        } else {

            for (int i = index; i < fence; i++)

                if (o.equals(elements[i]))

                    return i;

        }

        return -1;

    }

    /**

     * static version of lastIndexOf.

     * @param o element to search for

     * @param elements the array

     * @param index first index to search

     * @return index of element, or -1 if absent

     */

    private static int lastIndexOf(Object o, Object[] elements, int index) {

        if (o == null) {

            for (int i = index; i >= 0; i--)

                if (elements[i] == null)

                    return i;

        } else {

            for (int i = index; i >= 0; i--)

                if (o.equals(elements[i]))

                    return i;

        }

        return -1;

    }

    /**

     * 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) {

        Object[] elements = getArray();

        return indexOf(o, elements, 0, elements.length) >= 0;

    }

    /**

     * {@inheritDoc}

     */

    public int indexOf(Object o) {

        Object[] elements = getArray();

        return indexOf(o, elements, 0, elements.length);

    }

    /**

     * Returns the index of the first occurrence of the specified element in

     * this list, searching forwards from <tt>index</tt>, or returns -1 if

     * the element is not found.

     * More formally, returns the lowest index <tt>i</tt> such that

     * <tt>(i&nbsp;&gt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,

     * or -1 if there is no such index.

     *

     * @param e element to search for

     * @param index index to start searching from

     * @return the index of the first occurrence of the element in

     *         this list at position <tt>index</tt> or later in the list;

     *         <tt>-1</tt> if the element is not found.

     * @throws IndexOutOfBoundsException if the specified index is negative

     */

    public int indexOf(E e, int index) {

        Object[] elements = getArray();

        return indexOf(e, elements, index, elements.length);

    }

    /**

     * {@inheritDoc}

     */

    public int lastIndexOf(Object o) {

        Object[] elements = getArray();

        return lastIndexOf(o, elements, elements.length - 1);

    }

    /**

     * Returns the index of the last occurrence of the specified element in

     * this list, searching backwards from <tt>index</tt>, or returns -1 if

     * the element is not found.

     * More formally, returns the highest index <tt>i</tt> such that

     * <tt>(i&nbsp;&lt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,

     * or -1 if there is no such index.

     *

     * @param e element to search for

     * @param index index to start searching backwards from

     * @return the index of the last occurrence of the element at position

     *         less than or equal to <tt>index</tt> in this list;

     *         -1 if the element is not found.

     * @throws IndexOutOfBoundsException if the specified index is greater

     *         than or equal to the current size of this list

     */

    public int lastIndexOf(E e, int index) {

        Object[] elements = getArray();

        return lastIndexOf(e, elements, index);

    }

    /**

     * Returns a shallow copy of this list.  (The elements themselves

     * are not copied.)

     *

     * @return a clone of this list

     */

    public Object clone() {

        try {

            CopyOnWriteArrayList c = (CopyOnWriteArrayList)(super.clone());

            c.resetLock();

            return c;

        } catch (CloneNotSupportedException e) {

            // this shouldn't happen, since we are Cloneable

            throw new InternalError();

        }

    }

    /**

     * Returns an array containing all of the elements in this list

     * in proper sequence (from first to last element).

     *

     * <p>The returned array will be "safe" in that no references to it are

     * maintained by this list.  (In other words, this method must allocate

     * a new array).  The caller is thus free to modify the returned array.

     *

     * <p>This method acts as bridge between array-based and collection-based

     * APIs.

     *

     * @return an array containing all the elements in this list

     */

    public Object[] toArray() {

        Object[] elements = getArray();

        return Arrays.copyOf(elements, elements.length);

    }

    /**

     * Returns an array containing all of the elements in this list in

     * proper sequence (from first to last element); the runtime type of

     * the returned array is that of the specified array.  If the list fits

     * in the specified array, it is returned therein.  Otherwise, a new

     * array is allocated with the runtime type of the specified array and

     * the size of this list.

     *

     * <p>If this list fits in the specified array with room to spare

     * (i.e., the array has more elements than this list), the element in

     * the array immediately following the end of the list is set to

     * <tt>null</tt>.  (This is useful in determining the length of this

     * list <i>only</i> if the caller knows that this list does not contain

     * any null elements.)

     *

     * <p>Like the {@link #toArray()} method, this method acts as bridge between

     * array-based and collection-based APIs.  Further, this method allows

     * precise control over the runtime type of the output array, and may,

     * under certain circumstances, be used to save allocation costs.

     *

     * <p>Suppose <tt>x</tt> is a list known to contain only strings.

     * The following code can be used to dump the list into a newly

     * allocated array of <tt>String</tt>:

     *

     * <pre>

     *     String[] y = x.toArray(new String[0]);</pre>

     *

     * Note that <tt>toArray(new Object[0])</tt> is identical in function to

     * <tt>toArray()</tt>.

     *

     * @param a the array into which the elements of the 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 all the elements in this list

     * @throws ArrayStoreException if the runtime type of the specified array

     *         is not a supertype of the runtime type of every element in

     *         this list

     * @throws NullPointerException if the specified array is null

     */

    @SuppressWarnings("unchecked")

    public <T> T[] toArray(T a[]) {

        Object[] elements = getArray();

        int len = elements.length;

        if (a.length < len)

            return (T[]) Arrays.copyOf(elements, len, a.getClass());

        else {

            System.arraycopy(elements, 0, a, 0, len);

            if (a.length > len)

                a[len] = null;

            return a;

        }

    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")

    private E get(Object[] a, int index) {

        return (E) a[index];

    }

    /**

     * {@inheritDoc}

     *

     * @throws IndexOutOfBoundsException {@inheritDoc}

     */

    public E get(int index) {

        return get(getArray(), index);

    }

    /**

     * Replaces the element at the specified position in this list with the

     * specified element.

     *

     * @throws IndexOutOfBoundsException {@inheritDoc}

     */

    public E set(int index, E element) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            E oldValue = get(elements, index);

            if (oldValue != element) {

                int len = elements.length;

                Object[] newElements = Arrays.copyOf(elements, len);

                newElements[index] = element;

                setArray(newElements);

            } else {

                // Not quite a no-op; ensures volatile write semantics

                setArray(elements);

            }

            return oldValue;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Appends the specified element to the end of this list.

     *

     * @param e element to be appended to this list

     * @return <tt>true</tt> (as specified by {@link Collection#add})

     */

    public boolean add(E e) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            Object[] newElements = Arrays.copyOf(elements, len + 1);

            newElements[len] = e;

            setArray(newElements);

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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).

     *

     * @throws IndexOutOfBoundsException {@inheritDoc}

     */

    public void add(int index, E element) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (index > len || index < 0)

                throw new IndexOutOfBoundsException("Index: "+index+

                                                    ", Size: "+len);

            Object[] newElements;

            int numMoved = len - index;

            if (numMoved == 0)

                newElements = Arrays.copyOf(elements, len + 1);

            else {

                newElements = new Object[len + 1];

                System.arraycopy(elements, 0, newElements, 0, index);

                System.arraycopy(elements, index, newElements, index + 1,

                                 numMoved);

            }

            newElements[index] = element;

            setArray(newElements);

        } finally {

            lock.unlock();

        }

    }

    /**

     * Removes the element at the specified position in this list.

     * Shifts any subsequent elements to the left (subtracts one from their

     * indices).  Returns the element that was removed from the list.

     *

     * @throws IndexOutOfBoundsException {@inheritDoc}

     */

    public E remove(int index) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            E oldValue = get(elements, index);

            int numMoved = len - index - 1;

            if (numMoved == 0)

                setArray(Arrays.copyOf(elements, len - 1));

            else {

                Object[] newElements = new Object[len - 1];

                System.arraycopy(elements, 0, newElements, 0, index);

                System.arraycopy(elements, index + 1, newElements, index,

                                 numMoved);

                setArray(newElements);

            }

            return oldValue;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Removes the first occurrence of the specified element from this list,

     * if it is present.  If this list does not contain the element, it is

     * unchanged.  More formally, removes the element with the lowest index

     * <tt>i</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>

     * (if such an element exists).  Returns <tt>true</tt> if this list

     * contained the specified element (or equivalently, if this list

     * changed as a result of the call).

     *

     * @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) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (len != 0) {

                // Copy while searching for element to remove

                // This wins in the normal case of element being present

                int newlen = len - 1;

                Object[] newElements = new Object[newlen];

                for (int i = 0; i < newlen; ++i) {

                    if (eq(o, elements[i])) {

                        // found one;  copy remaining and exit

                        for (int k = i + 1; k < len; ++k)

                            newElements[k-1] = elements[k];

                        setArray(newElements);

                        return true;

                    } else

                        newElements[i] = elements[i];

                }

                // special handling for last cell

                if (eq(o, elements[newlen])) {

                    setArray(newElements);

                    return true;

                }

            }

            return false;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Removes from this list all of the elements whose index is between

     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.

     * Shifts any succeeding elements to the left (reduces their index).

     * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.

     * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)

     *

     * @param fromIndex index of first element to be removed

     * @param toIndex index after last element to be removed

     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of range

     *         ({@code{fromIndex < 0 || toIndex > size() || toIndex < fromIndex})

     */

    private void removeRange(int fromIndex, int toIndex) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (fromIndex < 0 || toIndex > len || toIndex < fromIndex)

                throw new IndexOutOfBoundsException();

            int newlen = len - (toIndex - fromIndex);

            int numMoved = len - toIndex;

            if (numMoved == 0)

                setArray(Arrays.copyOf(elements, newlen));

            else {

                Object[] newElements = new Object[newlen];

                System.arraycopy(elements, 0, newElements, 0, fromIndex);

                System.arraycopy(elements, toIndex, newElements,

                                 fromIndex, numMoved);

                setArray(newElements);

            }

        } finally {

            lock.unlock();

        }

    }

    /**

     * Append the element if not present.

     *

     * @param e element to be added to this list, if absent

     * @return <tt>true</tt> if the element was added

     */

    public boolean addIfAbsent(E e) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            // Copy while checking if already present.

            // This wins in the most common case where it is not present

            Object[] elements = getArray();

            int len = elements.length;

            Object[] newElements = new Object[len + 1];

            for (int i = 0; i < len; ++i) {

                if (eq(e, elements[i]))

                    return false; // exit, throwing away copy

                else

                    newElements[i] = elements[i];

            }

            newElements[len] = e;

            setArray(newElements);

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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

     * @throws NullPointerException if the specified collection is null

     * @see #contains(Object)

     */

    public boolean containsAll(Collection<?> c) {

        Object[] elements = getArray();

        int len = elements.length;

        for (Object e : c) {

            if (indexOf(e, elements, 0, len) < 0)

                return false;

        }

        return true;

    }

    /**

     * Removes from this list all of its elements that are contained in

     * the specified collection. This is a particularly expensive operation

     * in this class because of the need for an internal temporary array.

     *

     * @param c collection containing elements to be removed from this list

     * @return <tt>true</tt> if this list changed as a result of the call

     * @throws ClassCastException if the class of an element of this list

     *         is incompatible with the specified collection

     *         (<a href="../Collection.html#optional-restrictions">optional</a>)

     * @throws NullPointerException if this list contains a null element and the

     *         specified collection does not permit null elements

     *         (<a href="../Collection.html#optional-restrictions">optional</a>),

     *         or if the specified collection is null

     * @see #remove(Object)

     */

    public boolean removeAll(Collection<?> c) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (len != 0) {

                // temp array holds those elements we know we want to keep

                int newlen = 0;

                Object[] temp = new Object[len];

                for (int i = 0; i < len; ++i) {

                    Object element = elements[i];

                    if (!c.contains(element))

                        temp[newlen++] = element;

                }

                if (newlen != len) {

                    setArray(Arrays.copyOf(temp, newlen));

                    return true;

                }

            }

            return false;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Retains only the elements in this list that are contained in the

     * specified collection.  In other words, removes from this list all of

     * its elements that are not contained in the specified collection.

     *

     * @param c collection containing elements to be retained in this list

     * @return <tt>true</tt> if this list changed as a result of the call

     * @throws ClassCastException if the class of an element of this list

     *         is incompatible with the specified collection

     *         (<a href="../Collection.html#optional-restrictions">optional</a>)

     * @throws NullPointerException if this list contains a null element and the

     *         specified collection does not permit null elements

     *         (<a href="../Collection.html#optional-restrictions">optional</a>),

     *         or if the specified collection is null

     * @see #remove(Object)

     */

    public boolean retainAll(Collection<?> c) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (len != 0) {

                // temp array holds those elements we know we want to keep

                int newlen = 0;

                Object[] temp = new Object[len];

                for (int i = 0; i < len; ++i) {

                    Object element = elements[i];

                    if (c.contains(element))

                        temp[newlen++] = element;

                }

                if (newlen != len) {

                    setArray(Arrays.copyOf(temp, newlen));

                    return true;

                }

            }

            return false;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Appends all of the elements in the specified collection that

     * are not already contained in this list, to the end of

     * this list, in the order that they are returned by the

     * specified collection's iterator.

     *

     * @param c collection containing elements to be added to this list

     * @return the number of elements added

     * @throws NullPointerException if the specified collection is null

     * @see #addIfAbsent(Object)

     */

    public int addAllAbsent(Collection<? extends E> c) {

        Object[] cs = c.toArray();

        if (cs.length == 0)

            return 0;

        Object[] uniq = new Object[cs.length];

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            int added = 0;

            for (int i = 0; i < cs.length; ++i) { // scan for duplicates

                Object e = cs[i];

                if (indexOf(e, elements, 0, len) < 0 &&

                    indexOf(e, uniq, 0, added) < 0)

                    uniq[added++] = e;

            }

            if (added > 0) {

                Object[] newElements = Arrays.copyOf(elements, len + added);

                System.arraycopy(uniq, 0, newElements, len, added);

                setArray(newElements);

            }

            return added;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Removes all of the elements from this list.

     * The list will be empty after this call returns.

     */

    public void clear() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            setArray(new Object[0]);

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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.

     *

     * @param c collection containing elements to be added to this list

     * @return <tt>true</tt> if this list changed as a result of the call

     * @throws NullPointerException if the specified collection is null

     * @see #add(Object)

     */

    public boolean addAll(Collection<? extends E> c) {

        Object[] cs = c.toArray();

        if (cs.length == 0)

            return false;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            Object[] newElements = Arrays.copyOf(elements, len + cs.length);

            System.arraycopy(cs, 0, newElements, len, cs.length);

            setArray(newElements);

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Inserts all of the elements in the specified collection into this

     * list, starting 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.

     *

     * @param index index at which to insert the first element

     *        from the specified collection

     * @param c collection containing elements to be added to this list

     * @return <tt>true</tt> if this list changed as a result of the call

     * @throws IndexOutOfBoundsException {@inheritDoc}

     * @throws NullPointerException if the specified collection is null

     * @see #add(int,Object)

     */

    public boolean addAll(int index, Collection<? extends E> c) {

        Object[] cs = c.toArray();

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            Object[] elements = getArray();

            int len = elements.length;

            if (index > len || index < 0)

                throw new IndexOutOfBoundsException("Index: "+index+

                                                    ", Size: "+len);

            if (cs.length == 0)

                return false;

            int numMoved = len - index;

            Object[] newElements;

            if (numMoved == 0)

                newElements = Arrays.copyOf(elements, len + cs.length);

            else {

                newElements = new Object[len + cs.length];

                System.arraycopy(elements, 0, newElements, 0, index);

                System.arraycopy(elements, index,

                                 newElements, index + cs.length,

                                 numMoved);

            }

            System.arraycopy(cs, 0, newElements, index, cs.length);

            setArray(newElements);

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Saves the state of the list to a stream (that is, serializes it).

     *

     * @serialData The length of the array backing the list is emitted

     *               (int), followed by all of its elements (each an Object)

     *               in the proper order.

     * @param s the stream

     */

    private void writeObject(java.io.ObjectOutputStream s)

        throws java.io.IOException{

        s.defaultWriteObject();

        Object[] elements = getArray();

        // Write out array length

        s.writeInt(elements.length);

        // Write out all elements in the proper order.

        for (Object element : elements)

            s.writeObject(element);

    }

    /**

     * Reconstitutes the list from a stream (that is, deserializes it).

     *

     * @param s the stream

     */

    private void readObject(java.io.ObjectInputStream s)

        throws java.io.IOException, ClassNotFoundException {

        s.defaultReadObject();

        // bind to new lock

        resetLock();

        // Read in array length and allocate array

        int len = s.readInt();

        Object[] elements = new Object[len];

        // Read in all elements in the proper order.

        for (int i = 0; i < len; i++)

            elements[i] = s.readObject();

        setArray(elements);

    }

    /**

     * Returns a string representation of this list.  The string

     * representation consists of the string representations of the list's

     * elements in the order they are returned by its iterator, enclosed in

     * square brackets (<tt>"[]"</tt>).  Adjacent elements are separated by

     * the characters <tt>", "</tt> (comma and space).  Elements are

     * converted to strings as by {@link String#valueOf(Object)}.

     *

     * @return a string representation of this list

     */

    public String toString() {

        return Arrays.toString(getArray());

    }

    /**

     * Compares the specified object with this list for equality.

     * Returns {@code true} if the specified object is the same object

     * as this object, or if it is also a {@link List} and the sequence

     * of elements returned by an {@linkplain List#iterator() iterator}

     * over the specified list is the same as the sequence returned by

     * an iterator over this list.  The two sequences are considered to

     * be the same if they have the same length and corresponding

     * elements at the same position in the sequence are <em>equal</em>.

     * Two elements {@code e1} and {@code e2} are considered

     * <em>equal</em> if {@code (e1==null ? e2==null : e1.equals(e2))}.

     *

     * @param o the object to be compared for equality with this list

     * @return {@code true} if the specified object is equal to this list

     */

    public boolean equals(Object o) {

        if (o == this)

            return true;

        if (!(o instanceof List))

            return false;

        List<?> list = (List<?>)(o);

        Iterator<?> it = list.iterator();

        Object[] elements = getArray();

        int len = elements.length;

        for (int i = 0; i < len; ++i)

            if (!it.hasNext() || !eq(elements[i], it.next()))

                return false;

        if (it.hasNext())

            return false;

        return true;

    }

    /**

     * Returns the hash code value for this list.

     *

     * <p>This implementation uses the definition in {@link List#hashCode}.

     *

     * @return the hash code value for this list

     */

    public int hashCode() {

        int hashCode = 1;

        Object[] elements = getArray();

        int len = elements.length;

        for (int i = 0; i < len; ++i) {

            Object obj = elements[i];

            hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());

        }

        return hashCode;

    }

    /**

     * Returns an iterator over the elements in this list in proper sequence.

     *

     * <p>The returned iterator provides a snapshot of the state of the list

     * when the iterator was constructed. No synchronization is needed while

     * traversing the iterator. The iterator does <em>NOT</em> support the

     * <tt>remove</tt> method.

     *

     * @return an iterator over the elements in this list in proper sequence

     */

    public Iterator<E> iterator() {

        return new COWIterator<E>(getArray(), 0);

    }

    /**

     * {@inheritDoc}

     *

     * <p>The returned iterator provides a snapshot of the state of the list

     * when the iterator was constructed. No synchronization is needed while

     * traversing the iterator. The iterator does <em>NOT</em> support the

     * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.

     */

    public ListIterator<E> listIterator() {

        return new COWIterator<E>(getArray(), 0);

    }

    /**

     * {@inheritDoc}

     *

     * <p>The returned iterator provides a snapshot of the state of the list

     * when the iterator was constructed. No synchronization is needed while

     * traversing the iterator. The iterator does <em>NOT</em> support the

     * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.

     *

     * @throws IndexOutOfBoundsException {@inheritDoc}

     */

    public ListIterator<E> listIterator(final int index) {

        Object[] elements = getArray();

        int len = elements.length;

        if (index<0 || index>len)

            throw new IndexOutOfBoundsException("Index: "+index);

        return new COWIterator<E>(elements, index);

    }

    private static class COWIterator<E> implements ListIterator<E> {

        /** Snapshot of the array */

        private final Object[] snapshot;