LinkedList的Iterator()方法理解

LinkedList有两种迭代器实现，均使用内部变量next进行数据迭代：
1. LinkedList.iterator()迭代方法很好体现了java实现的继承回调机制

LinkedList本身没有iterator()方法，使用其抽象父类AbstractSequentialList<E>的iterator()方法，其中调用的listIterator()方法又是继承自上一级AbstractList<E>父类；最后在listIterator()方法中调用了listIterator(0)方法，而此方法又被LinkedList本身实现，进而回调到该方法上。

public ListIterator<E> listIterator(int index) {  // 继承回调该方法
    checkPositionIndex(index);
    return new ListItr(index);
}

private class ListItr implements ListIterator<E> {
    private Node<E> lastReturned;  // 使用node节点查找并缓存数据
    private Node<E> next;
    private int nextIndex;  // 是否遍历结束
    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() {  // 支持前后两种遍历方法
        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() {  // 不带索引值的移除
        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) {  // 添加和重置元素值的功能
        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();
    }
}

　

2. LinkedList的descendingIterator()方法，是对第一种迭代类的倒序封装，且只支持原列表的倒序遍历

public Iterator<E> descendingIterator() {
    return new DescendingIterator();
}

private class DescendingIterator implements Iterator<E> {
    private final ListItr itr = new ListItr(size());
    public boolean hasNext() {
        return itr.hasPrevious();
    }
    public E next() {
        return itr.previous();
    }
    public void remove() {
        itr.remove();
    }
}