为什么我使用ListIterator一直报错

package list;

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class Demo2 {
public static void main(String[] args){
List<String> al=new ArrayList<String>();
al.add("ni");
al.add("hao");
al.add("guo");
al.add("da");
al.add("xia");
ListIterator<String> li=al.listIterator();
while(li.hasNext()){
if((String) li.next()=="hao"){
System.out.println(al);
al.add(2,"ma");
}
}

}

}

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at list.Demo2.main(Demo2.java:17)
[ni, hao, guo, da, xia]

相关了解……