問題描述

我有以下代碼，我希望它會拋出 ConcurrentModificationException，但它運行成功.為什么會這樣?

I have the below code and I would expect it to throw a ConcurrentModificationException, but it runs successfully. Why does this happen?

public void fun(){
    List <Integer>lis = new ArrayList<Integer>();
    lis.add(1);
    lis.add(2);

    for(Integer st:lis){
        lis.remove(1);
        System.out.println(lis.size());
    }
}

public static void main(String[] args) {
    test t = new test();
    t.fun();
}

推薦答案

List 上的remove(int) 方法刪除指定位置的元素.在開始循環之前，您的列表如下所示:

The remove(int) method on List removes the element at the specified position. Before you start your loop, your list looks like this:

[1, 2]

然后你在列表上啟動一個迭代器:

Then you start an iterator on the list:

[1, 2]
 ^

您的 for 循環然后刪除 位置 1 的元素，即數字 2:

Your for loop then removes the element at position 1, which is the number 2:

[1]
 ^

迭代器在下一個隱含的 hasNext() 調用中返回 false，循環終止.

The iterator, on the next implied hasNext() call, returns false, and the loop terminates.

如果您向列表中添加更多元素，您將收到 ConcurrentModificationException.然后隱式 next() 會拋出.

You will get a ConcurrentModificationException if you add more elements to the list. Then the implicit next() will throw.

請注意，來自 JCF 的 ArrayList 的 Javadoc:

As a note, from the Javadoc for ArrayList from the JCF:

請注意，無法保證迭代器的快速失敗行為，因為一般來說，在存在不同步的并發修改的情況下無法做出任何硬保證.快速失敗的迭代器會盡最大努力拋出 ConcurrentModificationException.因此，編寫一個依賴此異常來確保其正確性的程序是錯誤的:迭代器的快速失敗行為應該只用于檢測錯誤.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

這可能實際上是Oracle ArrayList 迭代器實現中的一個錯誤；hasNext() 不檢查修改:

This is probably actually a bug in the Oracle ArrayList iterator implementation; hasNext() does not check for modification:

public boolean hasNext() {
    return cursor != size;
}

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