Iterator iterator = list.iterator();
      while(iterator.hasNext()){
        String string = iterator.next();
        //do something
      }

  public interface Iterator<E> {
  //判断容器内是否还有可供访问的元素
  boolean hasNext();
  //返回迭代器刚越过的元素的引用，返回值是 E
  E next();
  //删除迭代器刚越过的元素
  default void remove() {
    throw new UnsupportedOperationException("remove");
  }
}

private class Itr implements Iterator<E> {
  //....
}

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

  private class Itr implements Iterator<E> {

    protected int limit = ArrayList.this.size;

    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 < limit;
    }

    @SuppressWarnings("unchecked")
    public E next() {
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
      int i = cursor;
      if (i >= limit)
        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();
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();

      try {
        ArrayList.this.remove(lastRet);
        cursor = lastRet;
        lastRet = -1;
        expectedModCount = modCount;
        limit--;
      } 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;

      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
    }
  }

    protected int limit = ArrayList.this.size;

    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 < limit;
  }

    public E next() {
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
      int i = cursor;
      if (i >= limit)
        throw new NoSuchElementException();
      Object[] elementData = ArrayList.this.elementData;
      if (i >= elementData.length)
        throw new ConcurrentModificationException();
      cursor = i + 1;
      return (E) elementData[lastRet = i];
    }

 private abstract class HashIterator<E> implements Iterator<E> {
    HashMapEntry<K,V> next;    // next entry to return
    int expectedModCount;  // For fast-fail
    int index;       // current slot
    HashMapEntry<K,V> current;   // current entry

    HashIterator() {
      expectedModCount = modCount;
      if (size > 0) { // advance to first entry
        HashMapEntry[] t = table;
        while (index < t.length && (next = t[index++]) == null)
          ;
      }
    }

    public final boolean hasNext() {
      return next != null;
    }

    final Entry<K,V> nextEntry() {
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
      HashMapEntry<K,V> e = next;
      if (e == null)
        throw new NoSuchElementException();

      if ((next = e.next) == null) {
        HashMapEntry[] t = table;
        while (index < t.length && (next = t[index++]) == null)
          ;
      }
      current = e;
      return e;
    }

    public void remove() {
      if (current == null)
        throw new IllegalStateException();
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
      Object k = current.key;
      current = null;
      HashMap.this.removeEntryForKey(k);
      expectedModCount = modCount;
    }
  }

    HashMapEntry<K,V> next;    // next entry to return
    int expectedModCount;  // For fast-fail
    int index;       // current slot
    HashMapEntry<K,V> current;   // current entry

    public final boolean hasNext() {
      return next != null;
    }

    final Entry<K,V> nextEntry() {
      if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
      HashMapEntry<K,V> e = next;
      if (e == null)
        throw new NoSuchElementException();
      // 一个Entry就是一个单向链表
      // 若该Entry的下一个节点不为空，就将next指向下一个节点;
      // 否则，将next指向下一个链表(也是下一个Entry)的不为null的节点。
      if ((next = e.next) == null) {
        HashMapEntry[] t = table;
        while (index < t.length && (next = t[index++]) == null)
          ;
      }
      current = e;
      return e;
    }