## 迭代器（Iterator）

### Intent

提供一种顺序访问聚合对象元素的方法，并且不暴露聚合对象的内部表示。

### Class Diagram

- Aggregate 是聚合类，其中 createIterator() 方法可以产生一个 Iterator；
- Iterator 主要定义了 hasNext() 和 next() 方法；
- Client 组合了 Aggregate，为了迭代遍历 Aggregate，也需要组合 Iterator。

<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/image-20201102012333804.png" alt="image-20191130164425351" style="zoom: 67%;" /> </div><br>

### Implementation

```java
public interface Aggregate {
    Iterator createIterator();
}
```

```java
public class ConcreteAggregate implements Aggregate {

    private Integer[] items;

    public ConcreteAggregate() {
        items = new Integer[10];
        for (int i = 0; i < items.length; i++) {
            items[i] = i;
        }
    }

    @Override
    public Iterator createIterator() {
        return new ConcreteIterator<Integer>(items);
    }
}
```

```java
public interface Iterator<Item> {

    Item next();

    boolean hasNext();
}
```

```java
public class ConcreteIterator<Item> implements Iterator {

    private Item[] items;
    private int position = 0;

    public ConcreteIterator(Item[] items) {
        this.items = items;
    }

    @Override
    public Object next() {
        return items[position++];
    }

    @Override
    public boolean hasNext() {
        return position < items.length;
    }
}
```

```java
public class Client {

    public static void main(String[] args) {
        Aggregate aggregate = new ConcreteAggregate();
        Iterator<Integer> iterator = aggregate.createIterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}
```

### JDK

- [java.util.Iterator](http://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html)
- [java.util.Enumeration](http://docs.oracle.com/javase/8/docs/api/java/util/Enumeration.html)