Java: java.util.ConcurrentModificationException unexplainable

Question

List<ModelElement> elementList; //holds all model element
List<UUID> selectedElements;

public ModelElement getElement(UUID id)
{
    ModelElement element = null;
    for (ModelElement e : elementList){
        if (e.getId().equals(id) ){
            element = e;
            break;
        }
    }
    return element;
}

public void deleteElement(UUID id)
{
    selectedElements.remove(id);
    elementList.remove(getElement(id));
}

public void deleteElement(List<UUID> ids)
{
    for (UUID id : ids)
    {
        deleteElement(id);
    }

}

Why does this code create a java.util.ConcurrentModificationException? It works fine, if I call the single id version, but crashes if I use the List one.

We can't answer that either unless you post a complete, runnable program. You obviously have multiple threads somewhere, but this code shows nothing about what threads are using it. — Dark Falcon
– Dark Falcon, Commented May 19, 2015 at 16:06
Are you passing selectedElements into the deleteElement call? A short but complete program demonstrating the problem would really help... — Jon Skeet
– Jon Skeet, Commented May 19, 2015 at 16:07
@Jon Skeet That was the the problem. The ids is the same reference as selectedElements. — András Kovács
– András Kovács, Commented May 19, 2015 at 16:10
Right. So that means you're iterating over something, and deleting from it within the for loop - that's why it fails. — Jon Skeet
– Jon Skeet, Commented May 19, 2015 at 16:12

hitz · Accepted Answer · 2015-05-19 16:26:14Z

2

To avoid CME, try using iterator if you want to call deleteElement() with selectedElements list:

public void deleteElement(List<UUID> ids)
{
    Iterator idIter = ids.iterator();
    while(idIter.hasNext())
    {
        UUID id = (UUID)idIter.next();
        elementList.remove(getElement(id));
        idIter.remove();            
    }
}

answered May 19, 2015 at 16:26

hitz

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