java 迭代器初始指向_java 迭代器的深入分析

遇到一个问题,怎么对一个集合实现双重遍历的过程中进行元素的筛选删除?

刚开始的思路是使用迭代器之后再在中间使用for-each语句,但是经查阅资料发现

如果要对JAVA集合进行遍历删除时必须用迭代器

比如这段代码

参考http://blog.51cto.com/guojuanjun/1348450

public static void main(String args[]) {

Listfamous = new ArrayList();

famous.add("liudehua");

famous.add("madehua");

famous.add("liushishi");

famous.add("tangwei");

for (String s : famous) {

if (s.equals("madehua")) {

famous.remove(s);

}

}

}

查阅这篇博客可以知道,java在for-each语句中其实用的时迭代器操作,ArrayList与Iterator混合使用时会导致各自的状态出现不一样，最终出现异常。看一下AbstractList中的源码

private class Itr implements Iterator {

/**

* Index of element to be returned by subsequent call to next.

*/

int cursor = 0;

/**

* Index of element returned by most recent call to next or

* previous. Reset to -1 if this element is deleted by a call

* to remove.

*/

int lastRet = -1;

/**

* The modCount value that the iterator believes that the backing

* List should have. If this expectation is violated, the iterator

* has detected concurrent modification.

*/

int expectedModCount = modCount;

public boolean hasNext() {

return cursor != size();

}

public E next() {

checkForComodification();

try {

int i = cursor;

E next = get(i);

lastRet = i;

cursor = i + 1;

return next;

} catch (IndexOutOfBoundsException e) {

checkForComodification();

throw new NoSuchElementException();

}

}

public void remove() {

if (lastRet < 0)

throw new IllegalStateException();

checkForComodification();

try {

AbstractList.this.remove(lastRet);

if (lastRet < cursor)

cursor--;

lastRet = -1;

expectedModCount = modCount;

} catch (IndexOutOfBoundsException e) {

throw new ConcurrentModificationException();

}

}

final void checkForComodification() {

if (modCount != expectedModCount)

throw new ConcurrentModificationException();

}

}

在这里,元素下标从0开始,cursor初始指向第一个元素,只要cursor小于表的长度,hasnext返回true.

每调用一次next方法,cursor加1,lastret指向上一次返回的元素下标

可以看到这里通过checkForComodification进行检查,如果是这个迭代器以外的操作进行remove

会报错

最后不可以声明两个迭代器,也会抛出上述错误

package adf.sample.util;

import java.util.ArrayList;

import java.util.Iterator;

public class Test {

public static void main(String args[])

{

ArrayList list = new ArrayList();

list.add(5);

list.add(3);

list.add(2);

Iterator it = list.iterator();

Iterator it2 = list.iterator();

System.out.println(it.next());

it.remove();

it2.remove();

System.out.println(it2.next());

}

}

“`

5

Exception in thread “main” java.lang.IllegalStateException

at java.util.ArrayList$Itr.remove(ArrayList.java:872)

at Test.main(Test.java:15)