trying to avoid Concurrent Modification Exception

Question

I am trying to avoid the Concurrent Modification Exception. I loop over an entities HashMap to tick() each entity. Player entity moves around screen with WASD input, and to test addNewEntity() method I call GameState.addNewEntity(new Explosion(true, this.x, this.y)); when player moves right. Several explosions appear over player and then I get an error. Player class extends Creature which extends Entity. Explosion extends Entity class.

This code is from my GameState class:

private static HashMap<Integer, Entity> entities = new HashMap<Integer, Entity>();
private static HashMap<Integer, Entity> newEntities = new HashMap<Integer, Entity>();

player = new Player(game, true, 100, 100);
entities.put(0, player);

My tick() method:

public void tick() {
    // add new elements from newEntities to entities, deleting from newEntities
    for (Entry<Integer, Entity> entry : newEntities.entrySet()) { 
        entities.put(entities.size(), entry.getValue());
        newEntities.remove(entry.getValue());
    }

    System.out.println(entities.size());

    // tick the updated entities HashMap
    for (Entry<Integer, Entity> entry : entities.entrySet()) { 
        entry.getValue().tick();

        // remove dead entities
        if (!entry.getValue().isAlive) {
             entities.remove(entry.getKey());
        }
    }
}

my addNewEntity method:

public static void addNewEntity(Entity e) {
    // add a new element to newEntities. 
    newEntities.put(newEntities.size(), e);
}

my render method:

public void render(Graphics g) {
    for (Entry<Integer, Entity> entry : entities.entrySet()) { 
        entry.getValue().render(g);
    }
}

I tried to avoid the Concurrent Modification Exception by adding to the newEntities HashMap instead of adding to the entities HashMap. In my tick method, I loop over the newEntities HashMap and add each item that exsists there to entities, deleting each one in turn. Only after that do I loop over the entities HashMap.

How is it possible that I am Modifying the Map while Iterating over it?

Eran · Accepted Answer · 2016-07-14 06:02:31Z

1

Removing an entry from a Map directly while iterating over it with the enhanced for loop is not allowed. Instead, use an explicit iterator to iterate, and the iterator's remove() method.

Iterator<Map.Entry<Integer, Entity>> iter = newEntities.entrySet().iterator();
while (iter.hasNext()) { 
    Map.Entry<Integer, Entity> entry = iter.next();
    entities.put(entities.size(), entry.getValue());
    iter.remove();
}

Do the same for the second loop.

answered Jul 14, 2016 at 6:02

Eran

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1 Comment

Fabced Over a year ago

Thanks Eran. Worked like a charm.

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