List list = Collections.synchronizedList(new LinkedList(...));

  transient int size = 0;
    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;
    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;

private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;
        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

    /**
     * Returns a list-iterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * Obeys the general contract of {@code List.listIterator(int)}.<p>
     *
     * The list-iterator is <i>fail-fast</i>: if the list is structurally
     * modified at any time after the Iterator is created, in any way except
     * through the list-iterator's own {@code remove} or {@code add}
     * methods, the list-iterator will throw a
     * {@code ConcurrentModificationException}.  Thus, in the face of
     * concurrent modification, the iterator fails quickly and cleanly, rather
     * than risking arbitrary, non-deterministic behavior at an undetermined
     * time in the future.
     *
     * @param index index of the first element to be returned from the
     *              list-iterator (by a call to {@code next})
     * @return a ListIterator of the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see List#listIterator(int)
     */
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);//也是快速失败 
    }

    final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;//l，最后结点  lp,l,null (prev, element, next)
        final Node<E> newNode = new Node<>(l, e, null);//新结点 把值e的newNode的prev指向l 
        last = newNode;//最后指针指向新加入的最后结点
        if (l == null)
            first = newNode;//为空是同一个 l=last=null
        else
            l.next = newNode;//l的next指向newNode
        size++;//数量加
        modCount++;//结构变化次数加
    }

    /**
     * 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) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;
        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }
        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }
        x.item = null;
        size--;
        modCount++;
        return element;
    }