方法顺序访问一个对象中各个元素，而又不暴露对象的内部表示。
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 && 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>
代码.
//下面这个方法,显然返回一个实现了Iterator接口的类,且这个类是ArrayList私有的,//只是为了生成ArrayList的迭代器而存在的.
public Iterator 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<a href="/tag/xiugai/" target="_blank" class="keywords">修改</a>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<a href="/tag/shanchu/" target="_blank" class="keywords">删除</a>方式是:根据上次访问元素的下标,<a href="/tag/shanchu/" target="_blank" class="keywords">删除</a>上次访问的元素.所以对集合ArrayList在遍历过程中同时<a href="/tag/shanchu/" target="_blank" class="keywords">删除</a>元素的操作,//直接<a href="/tag/diaoyong/" target="_blank" class="keywords">调用</a>ArrayList的remove<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>会出现<a href="/tag/cuowu/" target="_blank" class="keywords">错误</a>,而通过迭代器<a href="/tag/shanchu/" target="_blank" class="keywords">删除</a>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<a href="/tag/shanchu/" target="_blank" class="keywords">删除</a>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);//<a href="/tag/diaoyong/" target="_blank" class="keywords">调用</a>外部类<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>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();
}
}
//<a href="/tag/hanshu/" target="_blank" class="keywords">函数</a>式接口的参数表明:这是jdk8加入的新<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>
//<a href="/tag/gongneng/" target="_blank" class="keywords">功能</a>: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:<a href="/tag/tianjia/" target="_blank" class="keywords">添加</a>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<a href="/tag/hanshu/" target="_blank" class="keywords">函数</a>接口<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>
while (i != size && 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<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>,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<a href="/tag/fangfa/" target="_blank" class="keywords">方法</a>,forEachRemaining
iterator=list.iterator();
iterator.forEachRemaining(a-> System.out.print((a+1)+","));
}
}
输出：
