Question

Possible Duplicate:
ConcurrentModificationException and a HashMap

I am getting the following exception

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.TreeMap$PrivateEntryIterator.nextEntry(Unknown Source)
at java.util.TreeMap$KeyIterator.next(Unknown Source)
at Types$AdjList.makeConnected(Types.java:281)
at Main.main(Main.java:56)

while executing the following code

public void makeConnected() {
        TreeSet<Node> exploredNodes = new TreeSet<Node>();
        TreeSet<Node> unexploredNodes = new TreeSet<Node>();
for (Node n : unexploredNodes) {
        ...
        exploredNodes.add(n);
        unexploredNodes.remove(n);
        ...
}

I am not using the iterator like in HashMap but need to use a Set that may grow or reduce based on some condition.
I will accept and give points to all answers. Look forward to how to replies on how to solve this problem of ConcurrentModificationException
Thanks,
Somnath

Answer 1

The for loop internally uses iterators and you are not removing the elements using the iterator. Hence, the problem. Use iterator’s remove method as below:

for (Iterator iterator = exploredNodes.iterator(); iterator.hasNext();) {
     Node n = (Node) iterator.next();
     unexploredNodes.add(n);         
     iterator.remove();
}