JAVA - Removing non iterator element from iterating list

Question

I have a problem while going through this iterator loop, in which i go through each element of this.env, but within this list, wanting to remove a different element of said list. When I try to remove any item of said iterating list, I receive this error: java.util.ConcurrentModificationException, and as far as I understand this is caused due to modifying the iterating list without using iterator.remove().

Code:

public void envActions(IOHandler ioHandler, PlayerClass player){
    Iterator<WorldElement> worldElementIterator = this.env.iterator();
    while(worldElementIterator.hasNext()){
        WorldElement worldElement = worldElementIterator.next();
    //for(WorldElement worldElement:this.env){
        if(worldElement instanceof EntityClass){
            EntityClass entity=(EntityClass) worldElement;
            if(entity.nature.contains("hostile")){
                MonsterClass mEntity=(MonsterClass) entity;
                if(!(mEntity.attacks.size()*(Math.random()+0.25)>=mEntity.attacks.size())){
                    Double followerNum = (Math.random()*player.followers.size());
                    Integer followerNumInt=followerNum.intValue();
                    if(followerNumInt<2){
                        PlayerClass target=player;
                        Double attacknumD=mEntity.attacks.size()*Math.random();
                        Integer attacknum= attacknumD.intValue();
                        Integer playerarmor=player.getArmorValue();
                        int enemydamage=mEntity.attacks.get(attacknum).getDamage()*(1-(playerarmor/1000));
                        target.health=target.health-enemydamage;
                        ioHandler.printToConsole("\nThe "+mEntity.name+" attacked you with "+mEntity.attacks.get(attacknum).getAttack()+" and did "+mEntity.attacks.get(attacknum).getDamage()+" damage! you have "+player.health+" health left!");
                    } else {
                        FriendlyCreatureClass target=player.followers.get(followerNumInt);
                        Double attacknumD=mEntity.attacks.size()*Math.random();
                        Integer attacknum= attacknumD.intValue();
                        int enemydamage=mEntity.attacks.get(attacknum).getDamage();
                        target.health=target.health-enemydamage;
                        if(!target.isAlive()){
                            ioHandler.printToConsole("\nThe " + mEntity.name + " attacked " + target.name + " with " + mEntity.attacks.get(attacknum).getAttack() + " and did " + mEntity.attacks.get(attacknum).getDamage() + " damage! " + target.name+" died! Farewell "+target.name+".");
                            target.died(ioHandler, this, player, true);
                            //>>>> THIS IS WHERE I WOUlD LIKE TO REMOVE 'target' FROM THE env LIST <<<<
                        } else {
                            ioHandler.printToConsole("\nThe "+mEntity.name+" attacked "+target.name+" with "+mEntity.attacks.get(attacknum).getAttack()+" and did "+mEntity.attacks.get(attacknum).getDamage()+" damage! "+target.name+" has "+target.health+" health left!");
                        }
                    }
                }

Please have mercy with my coding skills, since I am only a beginner in java / Android, though any suggestions are greatly appreciated!

Thanks in advance!

Wouldn't that remove the iterator itself? I want to remove a custom element in the list... — TheFloppyToast
– TheFloppyToast, Commented Mar 30, 2016 at 22:04
No I answered below for you. It removes the most recently accessed element, in your case worldElement is removed from the collection. The iterator isn't removed, it's the element. — Lucas Crawford
– Lucas Crawford, Commented Mar 30, 2016 at 22:08
You are trying to remove an element (target) that is different from the Iterator's current element (worldElement). This is quite an unusual situation. How do these elements relate to each other in the list? E.g. is target always to the left of worldElement? — Paul Boddington
– Paul Boddington, Commented Mar 30, 2016 at 22:10
@PaulBoddington yes exactly that's my problem, and no the target is being randomly selected... — TheFloppyToast
– TheFloppyToast, Commented Mar 30, 2016 at 22:12

amahfouz · Accepted Answer · 2016-03-30 22:10:17Z

1

If you are NOT using Iterator for whatever reason, you can collect the elements to be deleted in a Set when you are iterating over the collection, and after the loop you iterate over the Set and delete the elements from the original collection.

Note that this is only workable for small-sized collections.

HashSet toDelete = new HashSet();

for (...) {
    if (...)
       toDelete.add(item);
}
// end for

foreach (item in toDelete) {
    // delete from original collection
}

Otherwise, you can just call remove() on the iterator.

edited Mar 30, 2016 at 22:10

answered Mar 30, 2016 at 22:05

amahfouz

2,4122 gold badges19 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

TheFloppyToast Over a year ago

But if i don't remove the elements that need to be removed after every loop round, it will trigger the loop again and execute the actions even though it should not even exist?

Lucas Crawford Over a year ago

He shouldn't do this, it makes no sense. If he is already using an Iterator, just utilize the correct functionality

Louis Wasserman Over a year ago

@LucasCrawford Iterator.remove() won't work if you're not removing the element that was just visited in the iteration.

Louis Wasserman Over a year ago

@TheFloppyToast then you should also check if the element is in toDelete and skip it in the iteration process. This really is probably the only way you're going to be able to solve your problem.

TheFloppyToast Over a year ago

@LouisWasserman yep that worked! if you are curious, I posted the working code snippet below

TheFloppyToast · Accepted Answer · 2016-03-30 22:29:46Z

I think it works now with adding a toDelete list, checking if the iterating item is already in the list, if it is, then skip it, and afterwards deleting it.

I haven't thoroughly checked for bugs, but for now it's fine. Working code:

public void envActions(IOHandler ioHandler, PlayerClass player){
    List<WorldElement> toDelete=new ArrayList<>();
    Iterator<WorldElement> worldElementIterator = this.env.iterator();
    while(worldElementIterator.hasNext()){
        WorldElement worldElement = worldElementIterator.next();
    //for(WorldElement worldElement:this.env){
        if(worldElement instanceof EntityClass && !toDelete.contains(worldElement)){
            EntityClass entity=(EntityClass) worldElement;
            if(entity.nature.contains("hostile")){
                MonsterClass mEntity=(MonsterClass) entity;
                if(!(mEntity.attacks.size()*(Math.random()+0.25)>=mEntity.attacks.size())){
                    Double followerNum = (Math.random()*player.followers.size());
                    Integer followerNumInt=followerNum.intValue();
                    if(followerNumInt<2){
                        PlayerClass target=player;
                        Double attacknumD=mEntity.attacks.size()*Math.random();
                        Integer attacknum= attacknumD.intValue();
                        Integer playerarmor=player.getArmorValue();
                        int enemydamage=mEntity.attacks.get(attacknum).getDamage()*(1-(playerarmor/1000));
                        target.health=target.health-enemydamage;
                        ioHandler.printToConsole("\nThe "+mEntity.name+" attacked you with "+mEntity.attacks.get(attacknum).getAttack()+" and did "+mEntity.attacks.get(attacknum).getDamage()+" damage! you have "+player.health+" health left!");
                    } else {
                        FriendlyCreatureClass target=player.followers.get(followerNumInt);
                        Double attacknumD=mEntity.attacks.size()*Math.random();
                        Integer attacknum= attacknumD.intValue();
                        int enemydamage=mEntity.attacks.get(attacknum).getDamage();
                        target.health=target.health-enemydamage;
                        if(!target.isAlive()){
                            ioHandler.printToConsole("\nThe " + mEntity.name + " attacked " + target.name + " with " + mEntity.attacks.get(attacknum).getAttack() + " and did " + mEntity.attacks.get(attacknum).getDamage() + " damage! " + target.name + " died! Farewell " + target.name + ".");
                            target.died(ioHandler, this, player, false);
                            toDelete.add(target);
                        } else {
                            ioHandler.printToConsole("\nThe "+mEntity.name+" attacked "+target.name+" with "+mEntity.attacks.get(attacknum).getAttack()+" and did "+mEntity.attacks.get(attacknum).getDamage()+" damage! "+target.name+" has "+target.health+" health left!");
                        }
                    }
                }
    for(WorldElement worldElement:toDelete){
        this.env.remove(worldElement);
    }
    return;

Thanks for the fast responses!

Collectives™ on Stack Overflow

JAVA - Removing non iterator element from iterating list

2 Answers 2

5 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related