为什么我在更新集合元素时没有得到并发修改异常？

Question

我刚读到，如果我们在调用迭代器方法后添加、删除或更新集合

我理解为什么添加和删除集合元素会导致并发修改异常，但为什么更新会导致并发修改？毕竟我们在更新元素时不会在结构上改变任何东西。

例如下面的代码来自 arraylist 实现

public E set(int index, E element) {
    rangeCheck(index);

    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

我们不更新实际用于检查并发修改的变量"modcount"。

我也尝试了我的自定义代码：

public static void main(String[] args) {
    ArrayList l= new ArrayList();
    l.add("string");
    l.add(3);
    Iterator it=l.iterator();
    Object o=it.next();
    l.set(0, "element");
    l.remove(o);
    //l.add(7);
    //it.next();

    System.out.println(it.next());
    System.out.println(l.get(0));
    int i;
    System.out.println(j+" "+j);
}

这也不会抛出并发修改异常。

我可以知道为什么吗？

Answer 1

why updating should cause concurrent modification?

如果"updating"是指调用set方法，则不会引起并发修改。在 List 中设置元素的值不是结构修改，因此在迭代期间执行时不会导致 ConcurrentModificationException。

引自 ArrayList Javadoc：

A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification ...

The iterators returned by this class'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.

为什么我在更新集合元素时没有得到并发修改异常？

Why im not getting concurrent modification exception during update of collection element?

java

concurrentmodification