迭代器

之前对迭代器的理解一直存在误区
ListIterator
首先迭代器都是实现了Iterator()接口的，这里面定义了一些方法规范，我们的集合框架都要实现Iterable接口，他里面定义了一个返回迭代器的方法，也就是说，所有的集合框架都要有迭代器让外界拿到用来对集合进行遍历

迭代器没有当前元素一说，他只是一个游标（cursor），这个游标总在元素之间

每次调用next()方法游标向后移动一位，跨过一个元素，并返回跨过的元素

对于ListIterator来说，成员变量next就理解为游标

private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;
        ....
}

当调用next()的时候，游标会向后移动一位，跨过一个元素，而这个跨过的元素会作为返回值返回

public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

调用previous()会向前移动一位，返回的还是刚刚越过的元素，所以我先调用next()，越过一个元素，在调用previous()返回到刚刚位置，迭代器拿到的都是同一个（他们越过的元素）

 public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

而对于所以的remove()方法来说，都是要先调用next()方法（previous()），再调用remove()方法，因为remove()方法是对上一个返回的元素进行操作，而再调用remove()后会把lastReturned置为null

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++;
        }

ListIterator

private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

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

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

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

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

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

        public void remove() {
            //remove操作的都是lastReturned上一个返回的
            checkForComodification();
            //必须先迭代才能remove
            if(lastReturned == null){
                throw new IllegalStateException();
            }
            //接下来又要分两种情况
            //①向前迭代后删除 这个时候lastReturned == next，我们需要把next重新赋值
            //②向后迭代后
            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if(lastReturned == next){
                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++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }

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

    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;
        }
    }

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标签：Node,lastReturned,迭代,private,next,nextIndex,public
来源： https://blog.csdn.net/weixin_43907800/article/details/104056017