为什么一个循环会抛出 ConcurrentModificationException，而另一个不会?

问题描述

我在编写旅行推销员程序时遇到了这个问题.对于内部循环，我尝试了

I've run into this while writing a Traveling Salesman program. For an inner loop, I tried a

for(Point x:ArrayList<Point>) {
// modify the iterator
}

但是当向该列表添加另一个点时会引发 ConcurrentModicationException.

but when adding another point to that list resulted in a ConcurrentModicationException being thrown.

但是，当我将循环更改为

However, when I changed the loop to

for(int x=0; x<ArrayList<Point>.size(); x++) {
// modify the array
}

循环运行良好，没有抛出异常.

the loop ran fine without throwing an exception.

都是for循环，为什么一个会抛出异常而另一个不会呢?

Both a for loops, so why does one throw an exception while the other does not?

推荐答案

正如其他人解释的那样，迭代器检测到对底层集合的修改，这是一件好事，因为它可能会导致意外行为.

As others explained, the iterator detects modifications to the underlying collection, and that is a good thing since it is likely to cause unexpected behaviour.

想象一下这个修改集合的无迭代器代码:

Imagine this iterator-free code which modifies the collection:

for (int x = 0; list.size(); x++)
{
  obj = list.get(x);
  if (obj.isExpired())
  {
    list.remove(obj);
    // Oops! list.get(x) now points to some other object so if I 
    // increase x again before checking that object I will have 
    // skipped one item in the list
  }
}

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