CopyOnWriteArrayList.java

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 * 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.AbstractList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.RandomAccess;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

/**
 * A thread-safe variant of {@link java.util.ArrayList} in which all mutative
 * operations ({@code add}, {@code set}, and so on) are implemented by
 * making a fresh copy of the underlying array. <p>
 * 
 * ArrayList 线程安全的一种变体。更新操作会在底层上创建一个新的数组来进行。
 *
 * <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 {@code ConcurrentModificationException}.
 * The iterator will not reflect additions, removals, or changes to
 * the list since the iterator was created.  Element-changing
 * operations on iterators themselves ({@code remove}, {@code set}, and
 * {@code add}) are not supported. These methods throw
 * {@code UnsupportedOperationException}. <p>
 * 
 * 通常情况下，消耗比较大，但是在读比写多的情况下，效率比较高。
 * 迭代操作时会创建一个快照，在快照上进行迭代。
 * 迭代时，不允许对快照进行修改，即使是迭代器的修改方法也不行。
 *
 * <p>All elements are permitted, including {@code null}. <p>
 * 
 * null 元素也是被允许的。
 *
 * <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>
 * 
 * 跟其它的并发集合一样，在一个线程中将对象放到 CopyOnWriteArrayList 中，在 happen-before 之后，可以在另外一个线程中对这个元素进行访问或者移除。
 *
 * <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 */
    final transient ReentrantLock lock = new ReentrantLock();

    /**The array, accessed only via getArray/setArray. */
    private transient volatile 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.
     * 创建一个长度为0空的数组
     */
    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;
        if (c.getClass() == CopyOnWriteArrayList.class) {
            elements = ((CopyOnWriteArrayList<?>)c).getArray();
        } else {
            elements = c.toArray();
            // c.toArray might (incorrectly) not return Object[] (see 6260652)

            /*
             * 调用的是Arrays.ArrayList.toArray()
             * Object[] array = Arrays.asList("A").toArray();
             * 因为array的实际类型为String[].class
             * System.out.println(array.getClass() == Object[].class); // false
             */
            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 {@code true} if this list contains no elements.
     *
     * @return {@code true} if this list contains no elements
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * Tests 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 {@code true} if this list contains the specified element.
     * More formally, returns {@code true} if and only if this list contains
     * at least one element {@code e} 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 {@code true} 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 {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index {@code i} 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 {@code index} or later in the list;
     *         {@code -1} 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 {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index {@code i} 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 {@code index} 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 {
            @SuppressWarnings("unchecked")
            CopyOnWriteArrayList<E> clone =
                (CopyOnWriteArrayList<E>) super.clone();
            clone.resetLock();
            return clone;
        } 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
     * {@code null}.  (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 {@code x} is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of {@code String}:
     *
     *  <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @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];
    }

    /**
     * <p>
     * 直接通过下标去定位数组中的数值，因为array是volatile的，增删改之后，其它线程都会感知到，
     * 而增删改操作都是先获取锁，再copy一个新数组进行操作，
     * 所以对于读操作来说，直接读就行，甚至连CAS都不需要。
     * 这样就可以保证读写之间不会有锁冲突，对于读多写少的情况是极好的。
     * </p>
     *
     * {@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);

            // 如果值不一样则复制一个新数组进行更新操作（相等就没必要操作了）
            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
                // 虽然没有必要操作了，但我还是要操作一下（不解😅）
                setArray(elements);
            }
            return oldValue;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Appends the specified element to the end of this list. <p>
     * 
     * 在列表后面添加元素
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        // 加独占锁，其他线程会被阻塞
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            // 将当前数组的内容拷贝到新数组
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            // 将元素添加数组的最后
            newElements[len] = e;
            
            // 将新数组设置为当前数组
            setArray(newElements);
            
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     *
     * 与{@link java.util.ArrayList#add(int, Object)}操作不同。
     * <br>
     * <br>
     * <p>
     *  1. 新建一个比旧数组长度大1的新数组 <br>
     *  2. 将旧数组index前面的数组复制到新数组index的前面 <br>
     *  3. 将旧数组index后面的数组复制到新数组index的后面 <br>
     *  4. 然后将新数组index处的值赋值为element
     * </p>
     * <br>
     * 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) { // 表示将元素添加到最后的位置，则直接先扩容再赋值就行了
                newElements = Arrays.copyOf(elements, len + 1);
            } else {
                // 新建数组
                newElements = new Object[len + 1];
                // 复制index前面的部分
                System.arraycopy(elements, 0, newElements, 0, index);
                // 复制index后面的部分
                System.arraycopy(elements, index, newElements, index + 1, numMoved);
            }

            // 给指定位置赋值
            newElements[index] = element;
            setArray(newElements);
        } finally {
            lock.unlock();
        }
    }

    /**
     * <p>
     * 复制元素时与{@link java.util.ArrayList#remove(int)}不同
     * </p>
     *
     * 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) {
        // 加独占锁
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            // 获取当前索引处的元素
            E oldValue = get(elements, index);

            int numMoved = len - index - 1;
            
            // 表示删除的是最后一个元素
            if (numMoved == 0) {
                setArray(Arrays.copyOf(elements, len - 1));
            } else { // 删除其它位置的元素
                Object[] newElements = new Object[len - 1];
                // 先将要删除位置前面的元素复制到新数组
                System.arraycopy(elements, 0, newElements, 0, index);
                // 然后将要删除位置后面的元素复制到新数组
                System.arraycopy(elements, index + 1, newElements, index, numMoved);
                // 设置新数组为当前数组
                setArray(newElements);
            }
            // 返回删除的元素
            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
     * {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns {@code true} 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 {@code true} if this list contained the specified element
     */
    public boolean remove(Object o) {
        Object[] snapshot = getArray();
        // 找到o当前的位置
        int index = indexOf(o, snapshot, 0, snapshot.length);
        // 由于没有加锁，所以可能snapshot被改动过
        return (index < 0) ? false : remove(o, snapshot, index);
    }

    /**
     * A version of remove(Object) using the strong hint that given
     * recent snapshot contains o at the given index.
     */
    private boolean remove(Object o, Object[] snapshot, int index) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            /*
             * 因为获取snapshot时并没有加锁，所有有可能这个时候会被修改过
             * 如果其它线程修改过snapshot，则需要重新定位index的值
             */
            if (snapshot != current) findIndex: {

                int prefix = Math.min(index, len);

                for (int i = 0; i < prefix; i++) {
                    /*
                     * 如果元素存在，snapshot变动过，而且元素位置被移动过，重新定位元素位置
                     * 如果current与snapshot中当前位置的元素不相等，但是o却跟current当前位置的元素相等，
                     * 那么表示元素的位置被移动到了i处
                     */
                    if (current[i] != snapshot[i] && eq(o, current[i])) {
                        index = i;
                        break findIndex;
                    }
                }
                // index为数组最后一位的元素，但是这个元素已经被其它线程删除掉了
                if (index >= len) {
                    return false;
                }
                // 如果元素存在，不管snapshot怎么变动，但是元素位置没有变动过，则直接跳出
                if (current[index] == o) {
                    break findIndex;
                }
                // 该元素不存在，或者该元素不是数组末尾的元素，但是被删除掉了，所以以上的操作都找不到元素的位置
                index = indexOf(o, current, index, len);
                if (index < 0) {
                    return false;
                }
            }

            Object[] newElements = new Object[len - 1];
            System.arraycopy(current, 0, newElements, 0, index);
            System.arraycopy(current, index + 1, newElements, index, len - index - 1);

            setArray(newElements);

            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Removes from this list all of the elements whose index is between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by {@code (toIndex - fromIndex)} elements.
     * (If {@code toIndex==fromIndex}, 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})
     */
    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();
        }
    }

    /**
     * Appends the element, if not present.
     *
     * @param e element to be added to this list, if absent
     * @return {@code true} if the element was added
     */
    public boolean addIfAbsent(E e) {
        Object[] snapshot = getArray();
        return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
            addIfAbsent(e, snapshot);
    }

    /**
     * A version of addIfAbsent using the strong hint that given
     * recent snapshot does not contain e.
     */
    private boolean addIfAbsent(E e, Object[] snapshot) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            if (snapshot != current) {
                // Optimize for lost race to another addXXX operation
                int common = Math.min(snapshot.length, len);
                for (int i = 0; i < common; i++)
                    if (current[i] != snapshot[i] && eq(e, current[i]))
                        return false;
                if (indexOf(e, current, common, len) >= 0)
                        return false;
            }
            Object[] newElements = Arrays.copyOf(current, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns {@code true} if this list contains all of the elements of the
     * specified collection.
     *
     * @param c collection to be checked for containment in this list
     * @return {@code true} 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 {@code true} 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) {
        if (c == null) throw new NullPointerException();
        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 {@code true} 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) {
        if (c == null) throw new NullPointerException();
        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;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            int added = 0;
            // uniquify and compact elements in cs
            for (int i = 0; i < cs.length; ++i) {
                Object e = cs[i];
                if (indexOf(e, elements, 0, len) < 0 &&
                    indexOf(e, cs, 0, added) < 0)
                    cs[added++] = e;
            }
            if (added > 0) {
                Object[] newElements = Arrays.copyOf(elements, len + added);
                System.arraycopy(cs, 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 {@code true} 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.getClass() == CopyOnWriteArrayList.class) ?
            ((CopyOnWriteArrayList<?>)c).getArray() : c.toArray();
        if (cs.length == 0)
            return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len == 0 && cs.getClass() == Object[].class)
                setArray(cs);
            else {
                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 {@code true} 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();
        }
    }

    public void forEach(Consumer<? super E> action) {
        if (action == null) throw new NullPointerException();
        Object[] elements = getArray();
        int len = elements.length;
        for (int i = 0; i < len; ++i) {
            @SuppressWarnings("unchecked") E e = (E) elements[i];
            action.accept(e);
        }
    }

    public boolean removeIf(Predicate<? super E> filter) {
        if (filter == null) throw new NullPointerException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len != 0) {
                int newlen = 0;
                Object[] temp = new Object[len];
                for (int i = 0; i < len; ++i) {
                    @SuppressWarnings("unchecked") E e = (E) elements[i];
                    if (!filter.test(e))
                        temp[newlen++] = e;
                }
                if (newlen != len) {
                    setArray(Arrays.copyOf(temp, newlen));
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    public void replaceAll(UnaryOperator<E> operator) {
        if (operator == null) throw new NullPointerException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len);
            for (int i = 0; i < len; ++i) {
                @SuppressWarnings("unchecked") E e = (E) elements[i];
                newElements[i] = operator.apply(e);
            }
            setArray(newElements);
        } finally {
            lock.unlock();
        }
    }

    public void sort(Comparator<? super E> c) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            Object[] newElements = Arrays.copyOf(elements, elements.length);
            @SuppressWarnings("unchecked") E[] es = (E[])newElements;
            Arrays.sort(es, c);
            setArray(newElements);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Saves this list to a stream (that is, serializes it).
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @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.
     */
    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 this list from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    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 ({@code "[]"}).  Adjacent elements are separated by
     * the characters {@code ", "} (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
     * {@code remove} method. <p>
     * 
     * 返回一个迭代器。返回的是 list 的一个快照。没有进行同步。不支持 remove。
     * 
     * @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
     * {@code remove}, {@code set} or {@code add} 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
     * {@code remove}, {@code set} or {@code add} methods.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator<E> listIterator(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);
    }

    /**
     * Returns a {@link Spliterator} over the elements in this list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE},
     * {@link Spliterator#ORDERED}, {@link Spliterator#SIZED}, and
     * {@link Spliterator#SUBSIZED}.
     *
     * <p>The spliterator provides a snapshot of the state of the list
     * when the spliterator was constructed. No synchronization is needed while
     * operating on the spliterator.
     *
     * @return a {@code Spliterator} over the elements in this list
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator
            (getArray(), Spliterator.IMMUTABLE | Spliterator.ORDERED);
    }

    static final class COWIterator<E> implements ListIterator<E> {
        /** 
         * Snapshot of the array 
         * 
         * 数组的快照
         */
        private final Object[] snapshot;
        
        /** 
         * Index of element to be returned by subsequent call to next.  
         * 
         * 下一个元素的索引
         */
        private int cursor;

        private COWIterator(Object[] elements, int initialCursor) {
            cursor = initialCursor;
            snapshot = elements;
        }

        public boolean hasNext() {
            return cursor < snapshot.length;
        }

        public boolean hasPrevious() {
            return cursor > 0;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (! hasNext())
                throw new NoSuchElementException();
            return (E) snapshot[cursor++];
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            if (! hasPrevious())
                throw new NoSuchElementException();
            return (E) snapshot[--cursor];
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code remove}
         *         is not supported by this iterator.
         */
        public void remove() {
            throw new UnsupportedOperationException();
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code set}
         *         is not supported by this iterator.
         */
        public void set(E e) {
            throw new UnsupportedOperationException();
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code add}
         *         is not supported by this iterator.
         */
        public void add(E e) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            Object[] elements = snapshot;
            final int size = elements.length;
            for (int i = cursor; i < size; i++) {
                @SuppressWarnings("unchecked") E e = (E) elements[i];
                action.accept(e);
            }
            cursor = size;
        }
    }

    /**
     * Returns a view of the portion of this list between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * The returned list is backed by this list, so changes in the
     * returned list are reflected in this list.
     *
     * <p>The semantics of the list returned by this method become
     * undefined if the backing list (i.e., this list) is modified in
     * any way other than via the returned list.
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a view of the specified range within this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public List<E> subList(int fromIndex, int toIndex) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (fromIndex < 0 || toIndex > len || fromIndex > toIndex)
                throw new IndexOutOfBoundsException();
            return new COWSubList<E>(this, fromIndex, toIndex);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Sublist for CopyOnWriteArrayList.
     * This class extends AbstractList merely for convenience, to
     * avoid having to define addAll, etc. This doesn't hurt, but
     * is wasteful.  This class does not need or use modCount
     * mechanics in AbstractList, but does need to check for
     * concurrent modification using similar mechanics.  On each
     * operation, the array that we expect the backing list to use
     * is checked and updated.  Since we do this for all of the
     * base operations invoked by those defined in AbstractList,
     * all is well.  While inefficient, this is not worth
     * improving.  The kinds of list operations inherited from
     * AbstractList are already so slow on COW sublists that
     * adding a bit more space/time doesn't seem even noticeable.
     */
    private static class COWSubList<E>
        extends AbstractList<E>
        implements RandomAccess
    {
        private final CopyOnWriteArrayList<E> l;
        private final int offset;
        private int size;
        private Object[] expectedArray;

        // only call this holding l's lock
        COWSubList(CopyOnWriteArrayList<E> list,
                   int fromIndex, int toIndex) {
            l = list;
            expectedArray = l.getArray();
            offset = fromIndex;
            size = toIndex - fromIndex;
        }

        // only call this holding l's lock
        private void checkForComodification() {
            if (l.getArray() != expectedArray)
                throw new ConcurrentModificationException();
        }

        // only call this holding l's lock
        private void rangeCheck(int index) {
            if (index < 0 || index >= size)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ",Size: "+size);
        }

        public E set(int index, E element) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                rangeCheck(index);
                checkForComodification();
                E x = l.set(index+offset, element);
                expectedArray = l.getArray();
                return x;
            } finally {
                lock.unlock();
            }
        }

        public E get(int index) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                rangeCheck(index);
                checkForComodification();
                return l.get(index+offset);
            } finally {
                lock.unlock();
            }
        }

        public int size() {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                return size;
            } finally {
                lock.unlock();
            }
        }

        public void add(int index, E element) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                if (index < 0 || index > size)
                    throw new IndexOutOfBoundsException();
                l.add(index+offset, element);
                expectedArray = l.getArray();
                size++;
            } finally {
                lock.unlock();
            }
        }

        public void clear() {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                l.removeRange(offset, offset+size);
                expectedArray = l.getArray();
                size = 0;
            } finally {
                lock.unlock();
            }
        }

        public E remove(int index) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                rangeCheck(index);
                checkForComodification();
                E result = l.remove(index+offset);
                expectedArray = l.getArray();
                size--;
                return result;
            } finally {
                lock.unlock();
            }
        }

        public boolean remove(Object o) {
            int index = indexOf(o);
            if (index == -1)
                return false;
            remove(index);
            return true;
        }

        public Iterator<E> iterator() {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                return new COWSubListIterator<E>(l, 0, offset, size);
            } finally {
                lock.unlock();
            }
        }

        public ListIterator<E> listIterator(int index) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                if (index < 0 || index > size)
                    throw new IndexOutOfBoundsException("Index: "+index+
                                                        ", Size: "+size);
                return new COWSubListIterator<E>(l, index, offset, size);
            } finally {
                lock.unlock();
            }
        }

        public List<E> subList(int fromIndex, int toIndex) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                checkForComodification();
                if (fromIndex < 0 || toIndex > size || fromIndex > toIndex)
                    throw new IndexOutOfBoundsException();
                return new COWSubList<E>(l, fromIndex + offset,
                                         toIndex + offset);
            } finally {
                lock.unlock();
            }
        }

        public void forEach(Consumer<? super E> action) {
            if (action == null) throw new NullPointerException();
            int lo = offset;
            int hi = offset + size;
            Object[] a = expectedArray;
            if (l.getArray() != a)
                throw new ConcurrentModificationException();
            if (lo < 0 || hi > a.length)
                throw new IndexOutOfBoundsException();
            for (int i = lo; i < hi; ++i) {
                @SuppressWarnings("unchecked") E e = (E) a[i];
                action.accept(e);
            }
        }

        public void replaceAll(UnaryOperator<E> operator) {
            if (operator == null) throw new NullPointerException();
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                int lo = offset;
                int hi = offset + size;
                Object[] elements = expectedArray;
                if (l.getArray() != elements)
                    throw new ConcurrentModificationException();
                int len = elements.length;
                if (lo < 0 || hi > len)
                    throw new IndexOutOfBoundsException();
                Object[] newElements = Arrays.copyOf(elements, len);
                for (int i = lo; i < hi; ++i) {
                    @SuppressWarnings("unchecked") E e = (E) elements[i];
                    newElements[i] = operator.apply(e);
                }
                l.setArray(expectedArray = newElements);
            } finally {
                lock.unlock();
            }
        }

        public void sort(Comparator<? super E> c) {
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                int lo = offset;
                int hi = offset + size;
                Object[] elements = expectedArray;
                if (l.getArray() != elements)
                    throw new ConcurrentModificationException();
                int len = elements.length;
                if (lo < 0 || hi > len)
                    throw new IndexOutOfBoundsException();
                Object[] newElements = Arrays.copyOf(elements, len);
                @SuppressWarnings("unchecked") E[] es = (E[])newElements;
                Arrays.sort(es, lo, hi, c);
                l.setArray(expectedArray = newElements);
            } finally {
                lock.unlock();
            }
        }

        public boolean removeAll(Collection<?> c) {
            if (c == null) throw new NullPointerException();
            boolean removed = false;
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                int n = size;
                if (n > 0) {
                    int lo = offset;
                    int hi = offset + n;
                    Object[] elements = expectedArray;
                    if (l.getArray() != elements)
                        throw new ConcurrentModificationException();
                    int len = elements.length;
                    if (lo < 0 || hi > len)
                        throw new IndexOutOfBoundsException();
                    int newSize = 0;
                    Object[] temp = new Object[n];
                    for (int i = lo; i < hi; ++i) {
                        Object element = elements[i];
                        if (!c.contains(element))
                            temp[newSize++] = element;
                    }
                    if (newSize != n) {
                        Object[] newElements = new Object[len - n + newSize];
                        System.arraycopy(elements, 0, newElements, 0, lo);
                        System.arraycopy(temp, 0, newElements, lo, newSize);
                        System.arraycopy(elements, hi, newElements,
                                         lo + newSize, len - hi);
                        size = newSize;
                        removed = true;
                        l.setArray(expectedArray = newElements);
                    }
                }
            } finally {
                lock.unlock();
            }
            return removed;
        }

        public boolean retainAll(Collection<?> c) {
            if (c == null) throw new NullPointerException();
            boolean removed = false;
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                int n = size;
                if (n > 0) {
                    int lo = offset;
                    int hi = offset + n;
                    Object[] elements = expectedArray;
                    if (l.getArray() != elements)
                        throw new ConcurrentModificationException();
                    int len = elements.length;
                    if (lo < 0 || hi > len)
                        throw new IndexOutOfBoundsException();
                    int newSize = 0;
                    Object[] temp = new Object[n];
                    for (int i = lo; i < hi; ++i) {
                        Object element = elements[i];
                        if (c.contains(element))
                            temp[newSize++] = element;
                    }
                    if (newSize != n) {
                        Object[] newElements = new Object[len - n + newSize];
                        System.arraycopy(elements, 0, newElements, 0, lo);
                        System.arraycopy(temp, 0, newElements, lo, newSize);
                        System.arraycopy(elements, hi, newElements,
                                         lo + newSize, len - hi);
                        size = newSize;
                        removed = true;
                        l.setArray(expectedArray = newElements);
                    }
                }
            } finally {
                lock.unlock();
            }
            return removed;
        }

        public boolean removeIf(Predicate<? super E> filter) {
            if (filter == null) throw new NullPointerException();
            boolean removed = false;
            final ReentrantLock lock = l.lock;
            lock.lock();
            try {
                int n = size;
                if (n > 0) {
                    int lo = offset;
                    int hi = offset + n;
                    Object[] elements = expectedArray;
                    if (l.getArray() != elements)
                        throw new ConcurrentModificationException();
                    int len = elements.length;
                    if (lo < 0 || hi > len)
                        throw new IndexOutOfBoundsException();
                    int newSize = 0;
                    Object[] temp = new Object[n];
                    for (int i = lo; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) elements[i];
                        if (!filter.test(e))
                            temp[newSize++] = e;
                    }
                    if (newSize != n) {
                        Object[] newElements = new Object[len - n + newSize];
                        System.arraycopy(elements, 0, newElements, 0, lo);
                        System.arraycopy(temp, 0, newElements, lo, newSize);
                        System.arraycopy(elements, hi, newElements,
                                         lo + newSize, len - hi);
                        size = newSize;
                        removed = true;
                        l.setArray(expectedArray = newElements);
                    }
                }
            } finally {
                lock.unlock();
            }
            return removed;
        }

        public Spliterator<E> spliterator() {
            int lo = offset;
            int hi = offset + size;
            Object[] a = expectedArray;
            if (l.getArray() != a)
                throw new ConcurrentModificationException();
            if (lo < 0 || hi > a.length)
                throw new IndexOutOfBoundsException();
            return Spliterators.spliterator
                (a, lo, hi, Spliterator.IMMUTABLE | Spliterator.ORDERED);
        }

    }

    private static class COWSubListIterator<E> implements ListIterator<E> {
        private final ListIterator<E> it;
        private final int offset;
        private final int size;

        COWSubListIterator(List<E> l, int index, int offset, int size) {
            this.offset = offset;
            this.size = size;
            it = l.listIterator(index+offset);
        }

        public boolean hasNext() {
            return nextIndex() < size;
        }

        public E next() {
            if (hasNext())
                return it.next();
            else
                throw new NoSuchElementException();
        }

        public boolean hasPrevious() {
            return previousIndex() >= 0;
        }

        public E previous() {
            if (hasPrevious())
                return it.previous();
            else
                throw new NoSuchElementException();
        }

        public int nextIndex() {
            return it.nextIndex() - offset;
        }

        public int previousIndex() {
            return it.previousIndex() - offset;
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

        public void set(E e) {
            throw new UnsupportedOperationException();
        }

        public void add(E e) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            int s = size;
            ListIterator<E> i = it;
            while (nextIndex() < s) {
                action.accept(i.next());
            }
        }
    }

    // Support for resetting lock while deserializing
    private void resetLock() {
        UNSAFE.putObjectVolatile(this, lockOffset, new ReentrantLock());
    }
    private static final sun.misc.Unsafe UNSAFE;
    private static final long lockOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = CopyOnWriteArrayList.class;
            lockOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("lock"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}