concurrentmodification

Why does the following occur? Shouldn't both work? List<String> items = data; for( String id : items ) { List<String> otherItems = otherData; // 1. addAll() //Causes ConcurrentModificationException items.addAll(otherItems); // 2. .add() //Doesn't cause exceptions for( String otherId : otherItems ) { items.add(otherId); } } Is it because add() adds to the collection Items, but addAll() creates a new collection thus modifying Items to be a different instance of List? Edit items and otherItems are of concrete type ArrayList<String> . Neither operation is proper, since it modifies the collection

Code: I have a HashMap private Map<K, V> map = new HashMap<>(); One method will put K-V pair into it by calling put(K,V) . The other method wants to extract a set of random elements from its values: int size = map.size(); // size > 0 V[] value_array = map.values().toArray(new V[size]); Random rand = new Random(); int start = rand.nextInt(size); int end = rand.nextInt(size); // return value_array[start .. end - 1] The two methods are called in two different concurrent threads . Error: I got a ConcurrentModificationException error: at java.util.HashMap$HashIterator.nextEntry(Unknown Source) at

I'd like to run tests that simulate users modifying certain data at the same time for a grails application. Are there any plug-ins / tools / mechanisms I can use to do this efficiently? They don't have to be grails specific. It should be possible to fire multiple actions in parallel. I'd prefer to run the tests on functional level (so far I'm using Selenium for other tests) to see the results from the user perspective. Of course this can be done in addition to integration testing if you'd recommend to run concurrent modification tests on integration level as well. I have used Geb (http:/

I'm creating a multithread chat in java. When user u1 sends a message to user u2 but user u2 is not connected, user u1 sends the message to the server and user u2 will receive the message once he connects to the server. The messages who are not sent are added to an ArrayList. Once a user connects, he checks if he's the recipient of a pending message. If he is, the message is sent to him and then removed from the pending messages list. This is how I do it: for(Iterator<String> itpendingmsgs = pendingmsgs.iterator(); itpendingmsgs.hasNext();) { String pendingmsg = itpendingmsgs.next(); String

This question already has an answer here: Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop 23 answers I have a method test(), in which I am trying to compare two LinkedHashMaps against each other and modify the contents of one of the maps by removing the key/value pair if it is found in both LHM's. I keep getting a ConcurrentModificationException when running this method. I understand WHY I am getting the exception (since I am trying to modify the list that is being looped over). I'm not sure how to go forth with this however. I have this