java的Iterator源碼淺析

在java的集合中，List接口繼承Collection接口，AbstractList類實現了List接口，在AbstractList中的內部類Itr實現了Iterator接口

ArrayList實現List接口並繼承AbstractList類，結構圖如下：（圖片出自網絡）

Iterator接口源碼：

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的內部類Itr實現了Iterator接口，如下所示：

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

        /**上一個元素的下標。如果元素已被刪除就設置為-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;

        /**允許修改的次數，違規操作會拋異常
         * 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;
       /*檢查是否還有下一個元素*/
        public boolean hasNext() {
            return cursor != size();
        }
      /*光標下移，並且返回當前的元素*/
        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中的內部類Itr源碼功能類似於AbstractList的內部類Itr。

閱讀了Iterator的源碼，再回頭看Iterator遍歷List的過程，理解就會深刻很多。

        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());
 }