問題描述
我有以下代碼引發 ConcurrentModificationException，因為我在同一個列表上使用了兩個不同的迭代器，其中一個正在修改列表.所以，第二個迭代器在讀取列表時會拋出異常，因為其他迭代器已經修改了列表.
I have the following code which throws ConcurrentModificationException because I am using two different iterators on the same list and one of them is modifying the list. So, the second iterator throws the exception when reading the list because some other iterator has modified the list.
List<Integer> list = new ArrayList<>();
populate(list);//A method that adds integers to list
ListIterator<Integer> iterator1 = list.listIterator();
ListIterator<Integer> iterator2 = list.listIterator();
while (iterator1.hasNext()) {
if(iterator1.next() < 5)
iterator1.remove();
}
while (iterator2.hasNext()){
if(iterator2.next() < 5) {
//Call handler
}
}
我的問題是 iterator2 如何知道 internally 如果 list 尚未到達某個元素，則它已被其他迭代器修改iterator1 還刪除了哪個?它如何確定其他一些 iterator 已經改變了 list?一種方法是跟蹤大小，但這不是原因，因為其他一些迭代器可以替換任何元素.
My question is how does iterator2 know internally that the list has has been modified by some other iterator if it has not reached an element which is yet removed by iterator1? How does it figure out that some other iterator has mutated the list? One way could be keep track of size but that can't be the reason since some other iterator can just replace any element.
推薦答案
回答此類問題的一個好方法是查看源代碼，例如 ArrayList 的源代碼.搜索 ConcurrentModificationException.
A good way to answer questions like this is to look at the source code, for example the source code for ArrayList. Search for ConcurrentModificationException.
你應該能夠說事情是這樣工作的:
You should be able to tell that things work rather like this:
- 集合對象有一個修改計數，該計數從零開始，并在發生添加或刪除或類似操作時增加.
- 創建迭代器對象時，我們將集合的當前修改計數存儲在迭代器中.
- 每次使用迭代器時，它都會檢查集合的 mod 計數與迭代器在創建時獲得的 mod 計數.如果這些值不同，則會引發異常.
在您的情況下，列表中 iterator1 執行的刪除操作會更改列表的結構操作計數 (modCount).當 iterator2 被要求刪除時，它會看到它最初收到的 expectedModCount 為 0，與列表的當前 mod 計數不同.
In your case, remove operations performed by iterator1 on the list change the structural operation count (modCount) of the list. When iterator2 is asked to remove, it sees its expectedModCount, which it received initially as 0, differing from the current mod count of the list.
需要注意的是，it.remove 是一個特例.當迭代器自己刪除時，它的 expectedModCount 會相應地調整，以與底層列表保持同步.
It should be noted that it.remove is a special case. When an iterator does a remove itself, its expectedModCount adjusts accordingly, to keep in sync with the underlying list.
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