HashMap java.util.ConcurrentModificationException

Question

i have the following code:

public List<String> processMap(Map<String, String> aMap) {
    Cloner cloner = new Cloner();

    Map<String, String> tempMap = cloner.deepClone(aMap);

    while(!tempMap.isEmpty()) {
        Iterator<Entry<String, String>> iterator = tempMap.entrySet().iterator();

        while(iterator.hasNext()) {
            Entry<String, String> entry = iterator.next(); // !!!
        }
    }

    return null;
}

To deep copy the map i use this library: Cloner

I marked the line where i unfortunately get a ava.util.ConcurrentModificationException with '!!!'

Can you please tell me why i get this exception?

The complete code:

    Cloner cloner = new Cloner();

    Map<String, FreebaseType> tempFreebaseTypes = new HashMap<String, FreebaseType>();
    Map<String, FreebaseType> freebaseTypesCopy = cloner.deepClone(freebaseTypes);

    while(!freebaseTypesCopy.isEmpty()) {
        Iterator<Entry<String, FreebaseType>> iterator = freebaseTypesCopy.entrySet().iterator();

        while(iterator.hasNext()) {
            Entry<String, FreebaseType> entry = iterator.next();

            if(tempFreebaseTypes.containsKey(entry.getValue().getSuperType()) || entry.getValue().getSuperType() == null) {
                tempFreebaseTypes.put(entry.getValue().getType(), entry.getValue());

                freebaseTypesCopy.remove(entry.getKey());
            } 
        }
    }

    List<FreebaseType> sortedFreebaseTypes = new ArrayList<FreebaseType>();
    Iterator<Entry<String, FreebaseType>> iterator = tempFreebaseTypes.entrySet().iterator();

    while(iterator.hasNext()) {
        Entry<String, FreebaseType> entry = iterator.next();

        sortedFreebaseTypes.add(entry.getValue());
    }

    return sortedFreebaseTypes;

not sure . some thing like . iterator.remove(obj) in for loop — Srinath Ganesh
– Srinath Ganesh, Commented Apr 23, 2014 at 15:18
I would guess that Cloner clones parts of the implementation of the map which aren't supposed to be cloned which results in a corrupted Map. — Philipp
– Philipp, Commented Apr 23, 2014 at 15:19
What else is in your while loop? Can you come up with a short but complete program demonstrating the problem? — Jon Skeet
– Jon Skeet, Commented Apr 23, 2014 at 15:19
When it's a map of <String, String> why do you use an external library anyway? You can copy the content of one map into another with newMap.putAll(originalMap) — Philipp
– Philipp, Commented Apr 23, 2014 at 15:22

Philipp · Accepted Answer · 2014-04-23 15:33:19Z

2

While you iterate a map with an iterator, you are not allowed to modify the map directly, or you get a ConcurrentModificationException. The only way to modify the map is indirectly through the iterators remove() method, which removes the entry the iterator is currently pointing to from the map.

Replace the line

freebaseTypesCopy.remove(entry.getKey());

with

iterator.remove();

and the code should work.

edited Apr 23, 2014 at 15:33

answered Apr 23, 2014 at 15:26

Philipp

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6 Comments

Mulgard Over a year ago

So how can i actually add/remove elements in my List, Map or whatever while im iterating it? I need to do it.

Philipp Over a year ago

As I wrote, you can remove the current element through the iterators remove-method. But you can not add any elements to the currently iterated collection and neither can you remove elements the iterator is not currently pointing to.

Philipp Over a year ago

The documentation says: "void remove(): Removes from the underlying collection the last element returned by this iterator." What else do you think this method could do?

Philipp Over a year ago

@Mulgard When your program runs in an infinite loop, I would rather suspect a bug in the if-condition before the remove which is never satisfied for some elements, so they won't ever get deleted. That would mean that while(!freebaseTypesCopy.isEmpty()) will never terminate. Did you trace your code with the debugger?

Philipp Over a year ago

@Mulgard Why do you have the remove in the if-condition in the first place? Either you want to remove all elements from freebaseTypesCopy, then putting the remove into the if-condition would be pointless, or you don't want to remove all elements, then the while(!freebaseTypesCopy.isEmpty()) loop would be counter-productive.

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