Why does one loop throw a ConcurrentModificationException, while the other doesn't?

问题

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
}

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

However, when I changed the loop to

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

the loop ran fine without throwing an exception.

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

回答1:

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.

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
  }
}

回答2:

The first example uses an iterator, the second does not. It is the iterator that checks for concurrent modification.

回答3:

the first code is using an iterator so modifying the collection is not allowed. The second code you are accessing each object with x.get(i), so not using an iterator, modifications thus are allowed

回答4:

You cannot modify a List while you are iterating over it which you are doing in the first example. In the second you simply have a regular for loop.

回答5:

If you run the code and observe you find that first iteration of the loop works fine but the second throws ConcurrentModicationException

if is because next() method checks if the number of the elements did not change.

For nice explanation see http://javaadami.blogspot.com/2007/09/enhanced-for-loop-and.html

来源：https://stackoverflow.com/questions/2397321/why-does-one-loop-throw-a-concurrentmodificationexception-while-the-other-doesn

标签

java

iterator

concurrentmodification