public Iterator<E> iterator() {
    return new Itr();
}
private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

    public boolean hasNext() {
        return
 
 cursor != size;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw
 
 new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1
 
;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

    @Override
    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> consumer) {
        Objects.requireNonNull(consumer);
        final int size = ArrayList.this.size;
        int i = cursor;
        if (i >= size) {
            return;
        }
        final Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length) {
            throw new ConcurrentModificationException();
        }
        while (i != size && modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

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

iterator()方法新建並返回一個ArrayList的迭代器，該迭代器是個內部類，擴充套件了Iterator< E >介面。迭代器分別使用兩個int型變數cursor和lastRet分別指向下一個返回的元素和最近返回的元素。
迭代器中用得最多的方法是hasNext()和next()。hasNext()將cursor與ArrayList的size進行比較，若相等說明無下一個元素，則返回false，否則返回true；next()首先比較cursor與size，若cursor>=size則丟擲異常，否則返回cursor所指向的元素並將cursor賦給lastRet，然後cursor++。
remove方法則刪除lastRet所指向的元素，即上一個返回的元素，並將lastRet賦給cursor，同時lastRet = -1。