java中LinkedList使用迭代器优化移除批量元素原理

public interface Iterator<E> {
    /**
     *是否还有下一个元素
     */
    boolean hasNext();
    /**
     *下一个元素
     */
    E next();
    /**
     * 从集合中删除最后一个返回的元素
     */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
    /**
     * 传入一个Consumer对剩余的每个元素执行指定的操作，直到所有元素已处理或操作引发异常
     */
    default void forEachRemaining(Consumer<? super E> action) {
        //requireNonNull 静态方法将会在参数为null时主动抛出NullPointerException 异常。
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

public interface ListIterator<E> extends Iterator<E> {
    
    /**
     * 是否有后继
     */
    boolean hasNext();
    /**
     * 后继节点
     */
    E next();

    /**
     * 是否有前驱
     */
    boolean hasPrevious();

    /**
     * 前驱节点
     */
    E previous();

    /**
     * 下一个节点的下标
     */
    int nextIndex();

    /**
     * 前驱节点的下标
     */
    int previousIndex();

    /**
     * 移除元素
     */
    void remove();

    /**
     * 替换最后返回的元素
     */
    void set(E e);

    /**
     * 插入一个结点到最后返回的元素之后
     */
    void add(E e);
}

public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        int cursor;       // 当前下标
        int lastRet = -1; // 最后一个元素的索引，没有返回1
        int expectedModCount = modCount;//创建迭代器时列表被修改的次数，防止多线程操作。快速失败
        /**
        * 先检查一下是否列表已经被修改过
        * 做一些简单的越界检查
        * 返回元素，并且记录下返回元素的下标给lastRet，当前下标+1，
        */
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
        /**
        * 调用ArrayListdie自身的remove方法移除元素
        * ArrayListdie自身的remove 会修改modCount的值所以需要更新迭代器的expectedModCount
        * expectedModCount = modCount;
        */
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();
            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
        //把剩下为next遍历的所有元素做一个遍历消费
        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }
        //检查是否列表已经被修改了
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
        public boolean hasPrevious() {
            return cursor != 0;
        }
        public int nextIndex() {
            return cursor;
        }
        public int previousIndex() {
            return cursor - 1;
        }
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }
        //根据lastRet设置元素
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();
            try {
                ArrayList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

private class ListItr implements ListIterator<E> {
        //返回的节点
        private Node<E> lastReturned;
        //下一个节点
        private Node<E> next;
        //下一个节点索引
        private int nextIndex;
        //修改次数
        private int expectedModCount = modCount;

        ListItr(int index) {
            //根据传进来的数字设置next等属性，默认传0
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }
        //直接调用节点的后继指针
        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }
        //返回节点的前驱
        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }
        /**
        * 最重要的方法，在LinkedList中按一定规则移除大量元素时用这个方法
        * 为什么会比list.remove效率高呢;
        */
        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }
    }