问题描述

我正在阅读有关 ConcurrentModificationException 以及如何避免它的信息.找到一篇文章.该文章中的第一个清单具有类似于以下的代码，这显然会导致异常:

I was reading about ConcurrentModificationException and how to avoid it. Found an article. The first listing in that article had code similar to the following, which would apparently cause the exception:

List<String> myList = new ArrayList<String>();
myList.add("January");
myList.add("February");
myList.add("March");

Iterator<String> it = myList.iterator();
while(it.hasNext())
{
    String item = it.next();
    if("February".equals(item))
    {
        myList.remove(item);
    }
}

for (String item : myList)
{
    System.out.println(item);
}

接着用各种建议解释了如何解决问题.

Then it went on to explain how to solve the problem with various suggestions.

当我试图重现它时，我没有得到异常！为什么我没有收到异常?

When I tried to reproduce it, I didn't get the exception! Why am I not getting the exception?

推荐答案

根据Java API docs Iterator.hasNext 不会抛出 ConcurrentModificationException.

According to the Java API docs Iterator.hasNext does not throw a ConcurrentModificationException.

检查 "January" 和 "February" 后，您从列表中删除一个元素.调用 it.hasNext() 不会抛出 ConcurrentModificationException 而是返回 false.因此，您的代码干净地退出.然而，最后一个字符串永远不会被检查.如果您将 "April" 添加到列表中，则会按预期获得异常.

After checking "January" and "February" you remove one element from the list. Calling it.hasNext() does not throw a ConcurrentModificationException but returns false. Thus your code exits cleanly. The last String however is never checked. If you add "April" to the list you get the Exception as expected.

import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;

public class Main {
        public static void main(String args[]) {

                List<String> myList = new ArrayList<String>();
                myList.add("January");
                myList.add("February");
                myList.add("March");
                myList.add("April");

                Iterator<String> it = myList.iterator();
                while(it.hasNext())
                {
                    String item = it.next();
                    System.out.println("Checking: " + item);
                    if("February".equals(item))
                    {
                        myList.remove(item);
                    }
                }

                for (String item : myList)
                {
                    System.out.println(item);
                }

        }
}

http://ideone.com/VKhHWN

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