插入 ArrayList 时出现 java.util.ConcurrentModificationException

java.util.ConcurrentModificationException while inserting in ArrayList

提问by user2681668

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());
        }
    }
}

The particular piece of code throws an error:

特定的代码段引发错误：

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)

回答by Aniket Thakur

You are modifying the Collection and then trying to use the same iterator.

您正在修改集合，然后尝试使用相同的迭代器。

1. Get the Collection iterator again

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

2. or Use 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());
   
   }
   

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());
   
   }
   

回答by Subhrajyoti Majumder

It happens due to array list is modified after creation of Iterator.

这是由于创建Iterator.

The iterators returned by this ArrayList's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

此 ArrayList 的 iterator 和 listIterator 方法返回的迭代器是快速失败的：如果在创建迭代器后的任何时间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，面对并发修改，迭代器快速而干净地失败，而不是冒着在未来不确定的时间出现任意、非确定性行为的风险。

Documentation

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 

What you should do here create new iterator object after modification.

你应该在这里做的是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());

回答by criszhao

You add object to list, after the iterator is instanced. This will change the value of modCount in inner class AbstractList$Itr.class. next() method of iterator will call checkForComodification() method, which throws a ConcurrentModificationException. And this is so called fail-fast.

在实例化迭代器后，将对象添加到列表中。这将改变内部类 AbstractList$Itr.class 中 modCount 的值。迭代器的 next() 方法将调用 checkForComodification() 方法，该方法抛出 ConcurrentModificationException。这就是所谓的快速失败。

 //add in abstractList
 public void add(int index, E element) {
    if (index<0 || index>size)
        throw new IndexOutOfBoundsException();
    checkForComodification();
    l.add(index+offset, element);
    expectedModCount = l.modCount;
    size++;
    modCount++;  //modCount changed
}

In AbstractList$Itr

在 AbstractList$Itr 中

int expectedModCount;

public E next() {
        checkForComodification(); // cause ConcurrentModificationException
    try {
    E next = get(cursor);
    lastRet = cursor++;
    return next;
    } catch (IndexOutOfBoundsException e) {
    checkForComodification();
    throw new NoSuchElementException();
    }
}

 private void checkForComodification() {
    if (l.modCount != expectedModCount)  //modCount not equals to itr.expectedModCount
        throw new ConcurrentModificationException();
}

redo this code after your add:

添加后重做此代码：

al.add(2, "inserted");
lir = al.iterator();