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

Question

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

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


        

Answer 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
  }
}