Java 迭代器 nextIndex 指针

Question

你能解释一下迭代器结果的行为吗:

    ArrayList< String > list = new ArrayList< >(
            Arrays.asList( new String[] { "a", "b", "c", "d" } ) );
    int i = 0;
    ListIterator< String > iterator = list.listIterator();
    while( iterator.hasNext() ) {
        if( ++i == 3 ) {
            System.out.println(
                    iterator.previous() + iterator.nextIndex() );
        }
        System.out.println( iterator.next() + iterator.nextIndex() );
    }

输出为:a1 b2 b1 b2 c3 d4为什么第三个输出是“b1”而不是“a1”？我弄清楚了结构

0 1 2 3 元素索引

a b c d 元素值

Answer 1

请参阅 ListIterator 的 Javadoc :

A ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next().

和ListIterator.previous() :

Returns the previous element in the list and moves the cursor position backwards... (Note that alternating calls to next and previous will return the same element repeatedly.)

您在 b 和 c 之间调用 previous()，它返回前一个元素 (b )并将光标设置回一个位置(在 a 和 b 之间)，以便 nextIndex() 现在返回 1 >.