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);
}

Answer 1

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.