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# Iterator

## 介绍

定义：迭代器模式提供一种方法顺序访问一个集合对象中的各个元素，而不暴露该对象的内部表示。

适用场景：

* 访问一个集合对象的内容而无序暴露它的内部表示。
* 为遍历不同的集合提供一个统一个接口。

优点：

* 分离了集合对象的遍历行为。
* 数组、列表遍历比较简单，但是树、图遍历复杂且遍历方式多样（中序、前序、后序；BFS、DFS），若由客户端遍历，则易错且成本高。
* 每个迭代器独享游标信息，所以可以创建多个迭代器，同时遍历而不相互影响。
* 迭代器提供了抽象接口，当客户端切换新的遍历算法时，只需要获取不同的迭代器实现类。如 DFS 改成 BFS。

缺点：类的个数成对增加，每增加一种集合，就需要增加响应的迭代器实现。

## 类图

![](/files/-Lgpoolrfwxs6YJyViWb)

## 源码

JDK 的 ArrayList 实现了 Iterator 接口。

![](/files/-Lgpq73W65MgD4qqvAna)

下面代码说明：

* 迭代过程中不能直接通过 List 的方法增加或删除元素。
* 可以通过迭代器的 remove 方法删除元素。
* 迭代器每次访问 next 方法，只能调用一次 remove 方法，连续调用两次会报错。
* 若两个迭代器同时在用，其中一个调用 remove 方法后，另一个再调用 next 或 remove 方法都会报错。

```java
public class ArrayList<E> {
  transient Object[] elementData;
  private int size;

  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;

    Itr() {}

    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();
      }
    }
  }
}
```


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