看看它是如何new的呢。
public ListIterator<E> listIterator() { return new ListItr(0); } public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); }
又出现了一个内部类,看看它是怎样实现的。
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; } @SuppressWarnings("unchecked") 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]; } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(lastRet, e); } 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(); } } }
总体的代码不难看懂。现在总结一下,它们之间的区别。
1. iterator只能向后遍历,而ListIterator可以向前,也可以向后。
2. ListIterator可以增加元素,但iterator没有。