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Why is the HashMap iterator intermittently failing?

There are times when I start my program and it runs fine and then other times when it spits out this error:

Exception in thread "Thread-3" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
	at java.util.HashMap$EntryIterator.next(HashMap.java:847)
	at java.util.HashMap$EntryIterator.next(HashMap.java:845)
	at domain.EntityManager$UpdateLoop.updateContent(EntityManager.java:130)
	at domain.EntityManager$UpdateLoop.run(EntityManager.java:111)
	at java.lang.Thread.run(Thread.java:722)

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What could be causing this? This is the only code that is iterating the HashMap:

            for (Map.Entry<String, Entity> entry : entities.entrySet()) {
                String key = entry.getKey();
                Entity item = entry.getValue();
                if (item.isDestroyed()){
                    entities.remove(key);
                    ViewManager.getInstance().removeItem(key);
                    //INSTRUCT THE ENTITY TO PERFORM IT'S DESTROYED BEHAVIOR item.Destroyed()                    
                } else {
                    item.update(1);
                    ConsoleItem ci = new ConsoleItemImpl(item.getIdentifier(), item.getLocation(), ColorStringConverter.getInstance().StringToColor(item.getSide()), item.getAngle(), item.getShape(), item.toString(), item.isDestroyed(), item.isDamaged());
                    ViewManager.getInstance().updateItem(ci);                    
                }
                item.update(1);
            }

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And then there are a couple methods earlier that populate the HashMap:

    public void initialize(String side, int ships, int charges, double speed) throws InvalidDataException {
        for (int i=0; i < ships; i++) {                                   
            cs = new CargoShip(side, charges, speed);       
            entities.put(cs.getIdentifier(), cs);            
        }
    }
    
    public void initializeClouds(int clouds, int duration, double size) throws InvalidDataException {
        for (int i=0; i < clouds; i++) {                                   
            dc = new DebrisCloud(duration, size);       
            entities.put(dc.getIdentifier(), dc);            
        }        
    }

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ASKER CERTIFIED SOLUTION

for_yan

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for_yan

T think this has relevant information over how you iterate the HashMap:
http://stackoverflow.com/questions/1066589/java-iterate-through-hashmap

for_yan

In particular read this remark in the above link:
One caveat: if you want to remove items mid-iteration, you'll need to do so via an Iterator (see karim79's answer). However, changing item values is OK (see Map.Entry).

Eindoofus

ASKER

Thanks :). How could I change the following code to work with that synchronized feature?

private HashMap<String, Entity> entities = new HashMap<String, Entity>();

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And also what import statements do I need to use for it to work?

for_yan

I think their suggestion is to create the hashMap in this way:

Map m = Collections.synchronizedMap(new HashMap(...));

In this stackoverflow link they suggest to change the way you iterate and use
explicit iterator, as you are removing the entries, just as they mentioned

for_yan

Maybe another way is to use old-style Hashtable ?

I would also think about removing entries outside of iteration loop - I'd mark them, say make a list of them and remove directly
outside of iteration process.
Well, no one suggests it, but it kind of seems safer to me - not very well understand how it happens iteration with simulatneous removal
of entries over which iteration is going.

But I would rather be trying all these options starting with the simplest - say change the way of iteration to explicit iterator
as those folks in stackoverflow suggest.