Why does ArrayList not throw ConcurrentModificationException when modified from multiple threads?

Question

ConcurrentModificationException : This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.<

Answer 1

I don't think 'concurrent' means thread-related in this case, or at least it doesn't necessarily mean that. ConcurrentModificationExceptions usually arise from modifying a collection while in the process of iterating over it.

List list = new ArrayList();
for(String s : list)
{
     //modifying list results in ConcurrentModificationException
     list.add("don't do this");     

}

Note that the Iterator<> class has a few methods that can circumvent this:

for(Iterator it = list.iterator(); it.hasNext())
{
     //no ConcurrentModificationException
     it.remove(); 
}