java的Iterator源码浅析

在java的集合中，List接口继承Collection接口，AbstractList类实现了List接口，在AbstractList中的内部类Itr实现了Iterator接口

ArrayList实现List接口并继承AbstractList类，结构图如下：（图片出自网络）

Iterator接口源码：

public interface Iterator<E> {
    boolean hasNext();    
    E next();
 
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
   
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

AbstractList的内部类Itr实现了Iterator接口，如下所示：

  private class Itr implements Iterator<E> {
        /**元素的下标
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;

        /**上一个元素的下标。如果元素已被删除就设置为-1
         * Index of element returned by most recent call to next or
         * previous.  Reset to -1 if this element is deleted by a call
         * to remove.
         */
        int lastRet = -1;

        /**允许修改的次数，违规操作会抛异常
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        int expectedModCount = modCount;
       /*检查是否还有下一个元素*/
        public boolean hasNext() {
            return cursor != size();
        }
      /*光标下移，并且返回当前的元素*/
        public E next() {
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }
       /*移除元素*/
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }

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

ArrayList中的iterator()方法：

    public Iterator<E> iterator() {
        return new Itr();
    }

ArrayList中的内部类Itr源码功能类似于AbstractList的内部类Itr。

阅读了Iterator的源码，再回头看Iterator遍历List的过程，理解就会深刻很多。

        List<String> list=new ArrayList<String>();
        list.add("apple"); list.add("banana"); list.add("watermelon");
        for (Iterator<String> iterator=list.iterator();iterator.hasNext();) {
            System.out.println( iterator.next());
       }