What code is required to continuously loop though a HashMap?

Question

Currently my code is causing intermittent ConcurrentModificationException errors, probably because of the way I am looping through 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);
}
// updateInfo call
ViewManager.getInstance().updateInfo(summary());
}

How does one continuously loop though a HashMap and avoid a ConcurrentModificationException error?

Answer 1

You cannot modify a map while looping through it. You either have to make a copy of the map, use a ConcurrentHashMap, or use an iterator. If it is in a multi threaded environment, then you can do the modification in a synchronized block.

Another option is to use an iterator.

I rewrote your for for loop with an iterator below:

for(Iterator<Map.Entry<String, Entity>> iterator = entities.entrySet().iterator(); iterator.hasNext(); ){
    Map.Entry<String, Entity> entry = iterator.next();
    String key = entry.getKey();
    Entity item = entry.getValue();
    if (item.isDestroyed()){
        //Notice using an iterator to remove 
        iterator.remove();
        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);

}

I am not sure about the part of continuously looping a HashMap. A hash map has a finite set of keys, so you loop through the key set generally. If you for some reason want to loop continuously then you need to first tell us the reason behind it and also the terminal condition of that continuous loop (it cant really be forever, can it?).