2011-08-12 02:28:47 [SEVERE] java.util.ConcurrentModificationException

Anyone have any idea what this means 2011-08-12 02:28:47 [SEVERE] java.util.ConcurrentModificationException?

 

http://download.oracle.com/javase/1.4.2/docs/api/java/util/ConcurrentModificationException.html

 

To simplify it a bit more. This error occurs when you try to remove elements from a collection you are currently looping in. For example, this causes the error:

Code:

ArrayList<String> items = new ArrayList<String>();
items.add("test");
items.add("bukkit");
items.add("add");
items.add("remove");
items.add("post");
for (String item : items) {
    if (item.equalsIgnoreCase("remove")) {
        items.remove(item);
    }
}

Why? Because you removed an item in the 'items' array you currently loop in. The underlying java code can't perform a 'next' because the collection has changed. Two ways to fix this:


- Clone the array/list and loop through that:

Code:

for (String item : items.toArray(new String[0])) {
    items.remove(item);
}

- Store the items to remove in a temporary list and loop through that list afterwards

Code:

ArrayList<String> removelist = new ArrayList<String>();
for (String item : items) {
    removelist.add(item);
}
for (String item : removelist) {
    items.remove(item);
}

There are more ways. The same can happen if you would change a variable in a separate ASync thread.