设计模式--迭代器模式

1 概述

由于迭代器模式实在太常见了，所以自己就简单写一下就好。

1.1 定义
迭代器模式(Iterator Design)提供一种方法顺序访问容器中的各个元素，而又不暴露其内部具体的实现。

1.2 应用
JDK中的java.util包所有Collection集合都实现了Iterable接口，该接口有一个方法返回Iterator对象。
JDK中foreach语法内部就是迭代器原理。

2 详解

JDK中Iterable接口，该接口会返回一个Iterator对象。

 1 public interface Iterable<T> {
 2     Iterator<T> iterator();
 3 }
 4 
 5 public interface Iterator<E> {
 6     boolean hasNext();
 7     
 8     E next();
 9     
10     default void remove() {
11         throw new UnsupportedOperationException("remove");
12     }
13     
14     default void forEachRemaining(Consumer<? super E> action) {
15         Objects.requireNonNull(action);
16         while (hasNext())
17             action.accept(next());
18     }
19 }

JDK中Collection接口

1 public interface Collection<E> extends Iterable<E> {
2     Iterator<E> iterator();
3 }

JDK中ArrayList类

 1 public class ArrayList<E> extends AbstractList<E>
 2         implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
 3     
 4     @Override 
 5     public Iterator<E> iterator() {
 6         return new Itr();
 7     }
 8     
 9     private class Itr implements Iterator<E> {
10         int cursor;       // index of next element to return
11         int lastRet = -1; // index of last element returned; -1 if no such
12         int expectedModCount = modCount;
13 
14         public boolean hasNext() {
15             return cursor != size;
16         }
17 
18         @SuppressWarnings("unchecked")
19         public E next() {
20             checkForComodification();
21             int i = cursor;
22             if (i >= size)
23                 throw new NoSuchElementException();
24             Object[] elementData = ArrayList.this.elementData;
25             if (i >= elementData.length)
26                 throw new ConcurrentModificationException();
27             cursor = i + 1;
28             return (E) elementData[lastRet = i];
29         }
30 
31         public void remove() {
32             if (lastRet < 0)
33                 throw new IllegalStateException();
34             checkForComodification();
35 
36             try {
37                 ArrayList.this.remove(lastRet);
38                 cursor = lastRet;
39                 lastRet = -1;
40                 expectedModCount = modCount;
41             } catch (IndexOutOfBoundsException ex) {
42                 throw new ConcurrentModificationException();
43             }
44         }
45 
46         @Override
47         @SuppressWarnings("unchecked")
48         public void forEachRemaining(Consumer<? super E> consumer) {
49             Objects.requireNonNull(consumer);
50             final int size = ArrayList.this.size;
51             int i = cursor;
52             if (i >= size) {
53                 return;
54             }
55             final Object[] elementData = ArrayList.this.elementData;
56             if (i >= elementData.length) {
57                 throw new ConcurrentModificationException();
58             }
59             while (i != size && modCount == expectedModCount) {
60                 consumer.accept((E) elementData[i++]);
61             }
62             // update once at end of iteration to reduce heap write traffic
63             cursor = i;
64             lastRet = i - 1;
65             checkForComodification();
66         }
67 
68         final void checkForComodification() {
69             if (modCount != expectedModCount)
70                 throw new ConcurrentModificationException();
71         }
72     }
73 
74 }