javasystem-error

Removing an object is causing freezes

[For clarification, 'monsters' is a list of NPCs that when shot by an object 'bullet' are removed from the list. ]

Error at start:

W/System.err: java.util.ConcurrentModificationException
W/System.err:     at java.util.ArrayList$Itr.next(ArrayList.java:831)
W/System.err:     at com.companyname.product.GameplayScene.update(GameplayScene.java:113)
//...

I believe this means that it is trying to perform an action on something in the list that isn't there. So my question is could I please get some help identifying the point where an object is being called that isn't there, as I can't seem to find it. I know which class it is in as the first line references it for each error.

        for (Bullet bullet : player.getList())
            if (npcManager.bulletCollide(bullet)) { //checks for a collision between all npcs and all bullets
                 player.getList().remove(bullet);//npc is removed in method above

        }

       if (!gameOver) {
        npcManager.update(); //updates movement of npcs
        if (npcManager.playerCollideNPC(player)) {
            //end game
        }
    }

    for (RectNPC NPC : npcManager.getList())
        if (obstacleManager.NPCCollide(NPC)) { //checks collision between npcs and other objects
            //
        }

        int i = 0;
        //checks collisions between NPCs themselves
        while (i < (Constants.NUMBER_ENEMIES - 1)) {
            if (npcManager.getList().size() <= 1)
                return;
            if (Rect.intersects(npcManager.getList().get(i).getRectangle(), npcManager.getList().get(i + 1).getRectangle()))
                npcManager.getList().get(i).setRectangle(200);
            i += 1;
        }
    }

(Above) I imagine the issue will be with calling something in the update (shown above) where the object is no longer available, is there an issue with any of how I am removing the NPCs?

 public boolean bulletCollide(Bullet bullet) {
    for(RectNPC npc : monsters) {
        if(npc.bulletCollideNPC(bullet)) {
            monsters.remove(npc);
            monsters.add(0, new RectNPC(new Rect(100, 100, 200, 200), Color.rgb(255, 0, 0), 25));
            return true;
        }
    }
    return false;
}

(Above) Code where I am removing the NPC, I've included incase it is helpful - however it does do what it's meant to so I don't believe this is the problem.

My main question is, is there anything here that would be causing this error (the game stopping for a few frames and giving the error at the top) - or is there something/somewhere else in specific I should be looking at? Also I know some of my syntax isn't great, I apologise it's my first Java project.

Current Iterators:

for(Iterator<RectNPC> iterator = monsters.iterator(); iterator.hasNext();) {
        if(iterator.next().bulletCollideNPC(bullet)) {
            iterator.remove();
            monsters.add(0, new RectNPC(new Rect(100, 100, 200, 200), Color.rgb(255, 0, 0), 25));
            return true;
        }
    }

and 

for (Iterator<Bullet> iterator = player.getList().iterator(); iterator.hasNext(); ) {
            if (npcManager.bulletCollide(iterator.next())) {
                iterator.remove();
                //
            }
        }
Solution

  • In java, you cannot do for( T variable : collection ) { collection.remove( variable ); } In other words, you cannot be removing items from a collection which is being iterated with a for loop. It will result in a ConcurrentModificationException. Google for "java fail-fast iterator" for more information about why and how.

    You have two options: