CODEWITHSUNDEEP
java
Mulgard
Mulgard

Reputation: 10619

HashMap java.util.ConcurrentModificationException

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;

Upvotes: 1

Views: 2790

Answers (1)

Philipp
Philipp

Reputation: 69783

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.

Upvotes: 2

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