public interface Iterator<E> {
    /**
     *是否還有下一個(gè)元素
     */
    boolean hasNext();
    /**
     *下一個(gè)元素
     */
    E next();
    /**
     * 從集合中刪除最后一個(gè)返回的元素
     */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
    /**
     * 傳入一個(gè)Consumer對(duì)剩余的每個(gè)元素執(zhí)行指定的操作，直到所有元素已處理或操作引發(fā)異常
     */
    default void forEachRemaining(Consumer<? super E> action) {
        //requireNonNull 靜態(tài)方法將會(huì)在參數(shù)為null時(shí)主動(dòng)拋出NullPointerException 異常。
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

public interface ListIterator<E> extends Iterator<E> {
    
    /**
     * 是否有后繼
     */
    boolean hasNext();
    /**
     * 后繼節(jié)點(diǎn)
     */
    E next();

    /**
     * 是否有前驅(qū)
     */
    boolean hasPrevious();

    /**
     * 前驅(qū)節(jié)點(diǎn)
     */
    E previous();

    /**
     * 下一個(gè)節(jié)點(diǎn)的下標(biāo)
     */
    int nextIndex();

    /**
     * 前驅(qū)節(jié)點(diǎn)的下標(biāo)
     */
    int previousIndex();

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

    /**
     * 替換最后返回的元素
     */
    void set(E e);

    /**
     * 插入一個(gè)結(jié)點(diǎn)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;       // 當(dāng)前下標(biāo)
        int lastRet = -1; // 最后一個(gè)元素的索引，沒有返回1
        int expectedModCount = modCount;//創(chuàng)建迭代器時(shí)列表被修改的次數(shù)，防止多線程操作。快速失敗
        /**
        * 先檢查一下是否列表已經(jīng)被修改過
        * 做一些簡(jiǎn)單的越界檢查
        * 返回元素，并且記錄下返回元素的下標(biāo)給lastRet，當(dāng)前下標(biāo)+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];
        }
        /**
        * 調(diào)用ArrayListdie自身的remove方法移除元素
        * ArrayListdie自身的remove 會(huì)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();
            }
        }
        //把剩下為next遍歷的所有元素做一個(gè)遍歷消費(fèi)
        @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();
        }
        //檢查是否列表已經(jīng)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];
        }
        //根據(jù)lastRet設(shè)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> {
        //返回的節(jié)點(diǎn)
        private Node<E> lastReturned;
        //下一個(gè)節(jié)點(diǎn)
        private Node<E> next;
        //下一個(gè)節(jié)點(diǎn)索引
        private int nextIndex;
        //修改次數(shù)
        private int expectedModCount = modCount;

        ListItr(int index) {
            //根據(jù)傳進(jìn)來的數(shù)字設(shè)置next等屬性，默認(rèn)傳0
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }
        //直接調(diào)用節(jié)點(diǎn)的后繼指針
        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }
        //返回節(jié)點(diǎn)的前驅(qū)
        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中按一定規(guī)則移除大量元素時(shí)用這個(gè)方法
        * 為什么會(huì)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++;
        }
    }