插入ArrayList时出现java.util.ConcurrentModificationException

Question

import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;

public class MyList {
    public static void main(String[] args) {
        ArrayList<String> al = new ArrayList<String>();

        al.add("S1");
        al.add("S2");
        al.add("S3");
        al.add("S4");

        Iterator<String> lir = al.iterator();

        while (lir.hasNext()) {
            System.out.println(lir.next());
        }

        al.add(2, "inserted");

        while (lir.hasNext()) {
           System.out.println(lir.next());
        }
    }
}

特定的代码会引发错误:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at collections.MyList.main(MyList.java:32)

Answer 1

它发生的原因是数组列表在创建后被修改Iterator.

此ArrayList的iterator和listIterator方法返回的迭代器是快速失败的:如果在创建迭代器之后的任何时候对列表进行结构修改,除了通过迭代器自己的remove或add方法之外,迭代器将抛出ConcurrentModificationException.因此,在并发修改的情况下,迭代器快速而干净地失败,而不是在未来的未确定时间冒任意,非确定性行为的风险.

文档

Iterator<String> lir = al.iterator(); // Iterator created

while (lir.hasNext()) 
    System.out.println(lir.next());
al.add(2, "inserted"); // List is modified here
while (lir.hasNext()) 
    System.out.println(lir.next());// Again it try to access list 

你应该在这里做什么在修改后创建新的迭代器对象.

...
al.add(2, "inserted");
lir = al.iterator();
while (lir.hasNext()) 
    System.out.println(lir.next());

Answer 2

您正在修改 Collection，然后尝试使用相同的迭代器。

1. 再次获取Collection迭代器

   al.add(2, "inserted");
   Iterator<String> lirNew = al.iterator();
   while (lirNew.hasNext()) {
   System.out.println(lirNew.next());
   }
   

2. 或使用ListIterator

   ArrayList<String> al = new ArrayList<String>();
   
   al.add("S1");
   al.add("S2");
   al.add("S3");
   al.add("S4");
   
   ListIterator<String> lir = al.listIterator();
   
   while (lir.hasNext()) {
       System.out.println(lir.next());
   
   }
   
   lir.add("insert");
   
   while (lir.hasNext()) {
       System.out.println(lir.next());
   
   }