Why is this code throwing a ConcurrentModificationException?

Question

Below is the code, I am getting a ConcurrentModificationException in the subiter.next() call even though I am not modifying the underlying collection and its running as a single thread.

Tree tree=partition.getTreeofThisPartition();
Set<DzExpressionHostTupel> oldSubtupels=tree.getSubscribers();
Iterator<DzExpressionHostTupel> subiter=oldSubtupels.iterator();           
while (subiter.hasNext()){
    DzExpressionHostTupel subtupel=subiter.next();
    tree.removeSubscriber(subtupel);
}

possible duplicate of http://stackoverflow.com/questions/1655362/concurrentmodificationexception-despite-using-synchronized — NageN, Aug 26 '16 at 12:40
you must use the `subiter.remove()` method instead of `tree.removeSubscriber(subtupel)`. — NageN, Aug 26 '16 at 12:44
I need to remove the element from the existing tree where it was added before, iterator.remove() will not do that. — Himanshu Sarmah, Aug 26 '16 at 12:48
"I am not modifying the underlying collection" - you probably are. Can you show us your definition of Tree.getSubscribers()? — Klitos Kyriacou, Aug 26 '16 at 12:51
@KlitosKyriacou ya, None of this is standard library code, but Tree.getSubscribers() likely returns the listener collection itself. — ebyrob, Aug 26 '16 at 12:59

score 5 · Answer 1 · edited Jun 20 '20 at 09:12

If you read https://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html, it says:

For example, it is not generally permissible for one thread to modify a Collection while another thread is iterating over it. In general, the results of the iteration are undefined under these circumstances. Some Iterator implementations (including those of all the general purpose collection implementations provided by the JRE) may choose to throw this exception if this behavior is detected. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

(emphasis added).

I'm guessing tree.removeSubscriber(subtupel); is modifying its subscribers set.

He does not have two threads, but he is interleaving modification the data structure during the iteration. — GC_, Aug 26 '16 at 12:47

Why is this code throwing a ConcurrentModificationException?

1 Answers1