第20章《迭代器模式》

The returned iterator is fail-fast.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator iterator() {
        return new Itr();
    }
/**
 * An optimized version of AbstractList.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;

    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();
        }
    }

    @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 &amp;&amp; modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}</code></pre>


 iterator() {
    return new Itr();
}
// An optimized version of AbstractList.Itr

//这是一个私有类,对抽象List的迭代器做了进一步优化.

private class Itr implements Iterator {

int cursor;       // 返回元素的索引

int lastRet = -1; // 上一次返回元素的索引.如果上一次没有返回值,则值为-1.

//用于记录当前迭代过程中,ArrayList结构修改次数.初始化为创建迭代器时候ArrayList的结构修改次数.

int expectedModCount = modCount; 
public boolean hasNext() {
    return cursor != size;
}

@SuppressWarnings("unchecked")
public E next() {
    checkForComodification(); //在返回下一个元素时,先对ArrayList结构是否做了修改做合法性检查
    int i = cursor;//i表示返回元素的数组下标
    if (i >= size)
        throw new NoSuchElementException();
    //为什么这里要声明一个elementData数组的引用呢?
    //查阅资料说:因为这里要使用elementData两次,而内部类访问外部类时,是通过保存了外部类的指针而实现的.
    //为了避免两次指针书写上的重复,所以这里声明了一个引用.
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
        throw new ConcurrentModificationException();
    cursor = i + 1; //访问完一个元素后,游标cursor要+1
    //注意:这里既对lastRet赋值,又返回了索引为i的数组值.这种写法在jdk源码中很常见.
    return (E) elementData[lastRet = i]; 
}

//迭代器的删除方式是:根据上次访问元素的下标,删除上次访问的元素.所以对集合ArrayList在遍历过程中同时删除元素的操作,//直接调用ArrayList的remove方法会出现错误,而通过迭代器删除元素却是可以的.要注意这一点.
public void remove() {
    if (lastRet < 0)  //因为通过lastRet来删除元素,所以必须对它的合法性进行合法检查
        throw new IllegalStateException();
    checkForComodification();//

    try {
        ArrayList.this.remove(lastRet);//调用外部类方法remove(),上次上次访问的元素.
        cursor = lastRet;//更新迭代器游标:更改为上次访问元素的数组下标
        //remove()操作作为下一个操作的前一个操作,因为没有访问任何元素,所以把上次访问元素的索引lastRet置为-1
        lastRet = -1;
        expectedModCount = modCount; //因为此时ArrayList结构发生更改,所以迭代器记录结构更改的值也要同步更新.
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}

//函数式接口的参数表明:这是jdk8加入的新方法
//功能:通过迭代器直接操作迭代器里面包含的所有数据.
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
    Objects.requireNonNull(consumer);
    final int size = ArrayList.this.size;//size值不可更改,就是当前ArrayList包含了几个元素
    int i = cursor;//开始索引为游标值
    if (i >= size) {
        return;
    }
    //这里和上面不一样地方是:添加了final,故elementData里面的值可以更改,但是大小不能再改.
    final Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length) {
        throw new ConcurrentModificationException();
    }
    //执行消费者函数接口方法
    while (i != size &amp;&amp; modCount == expectedModCount) {
        consumer.accept((E) elementData[i++]);
    }
    //这里对游标cursor,lastRet的更改只有一次,是为了减少因为迭代操作导致的java堆写拥塞
    // update once at end of iteration to reduce heap write traffic
    cursor = i;
    lastRet = i - 1;
    checkForComodification();
}
//这是一个final方法,用于检测当前迭代过程中ArrayList结构是否发生变化.因为本身ArrayList不是线程安全的.
final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}
}




import java.util.ArrayList;

import java.util.Iterator;

/**


@Author: cxh


@CreateTime: 18/1/24 21:23


@ProjectName: JavaBaseTest

*/

public class Client {

public static void main(String[] args) {

ArrayList list=new ArrayList<>();

list.add(1);

list.add(2);

list.add(3);

//生成迭代器并访问

Iterator iterator=list.iterator();

while (iterator.hasNext()){

System.out.print(iterator.next()+",");

}

System.out.println();

System.out.println("-----");
 //jdk8新增方法,forEachRemaining
 iterator=list.iterator();
 iterator.forEachRemaining(a-> System.out.print((a+1)+","));
}

}



输出：



                        
（编辑：李大同）
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