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());
}