Java: removing two objects simultaniously from different ArrayLists

Question

I have objects Bullet that I add to two ArrayLists at once, the lists are briefly described below. After certain operations are done I wish to remove a bullet from both lists. Is this approach correct? I keep on getting an error: java.util.ConcurrentModificationException

Alternatively, can you think of a better solution than ArrayList for the purpose of handling objects in this manner?

    //there are ArrayList<Bullet> bullets and ArrayList<Updatable> updatable, in the class

    public void removeBullet(Bullet bullet) {

    for (ListIterator<Bullet> bulletIterator = bullets.listIterator(); bulletIterator.hasNext();) {

        Bullet tempBullet = bulletIterator.next();

        if (tempBullet.equals(bullet)) {

            for (ListIterator<Updatable> updatableIterator = updatable.listIterator(); updatableIterator.hasNext();) {

                Updatable tempUpdatable = updatableIterator.next();
                if (tempUpdatable.equals(bullet)) {

                    updatableIterator.remove();
                    bulletIterator.remove();
                    return;

                }
            }
        }
    }

}

EDIT: The source of problem was that I used an iterator on one of the lists, at exact same time in a different place, hence the error. This code worked fine for the updatable list.

u can use comparator classes too.

Manikant Gautam
– Manikant Gautam

2016-03-25 16:46:01 +00:00
Commented Mar 25, 2016 at 16:46 — Manikant Gautam
– Manikant Gautam, Commented Mar 25, 2016 at 16:46

NAMS · Accepted Answer · 2016-03-25 16:58:58Z

3

A ConcurrentModificationException happens because you are trying to remove a bullet from an Iterator which you are also simultaneously iterating through in a for loop; java doesn't like when you do that and will throw the exception.

To solve this, you would have to iterate through both Iterators and remove them separately, or, as rdonuk stated, simply use the ArrayList remove() method, which will not throw any exceptions if you try to remove something that isn't in the ArrayList; it will return true if the object was removed, or false otherwise, so you don't even have to check if the object you want to remove is contained in the ArrayList in the first place.

answered Mar 25, 2016 at 16:58

NAMS

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Comments

rdonuk · Accepted Answer · 2016-03-25 17:01:55Z

2

Just use ArrayList remove method.

bullets.remove(bullet);

and

updatable.remove(bullet);

Edit:

remove method of iterator which used by ArrayList:

    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

As you see it is already use ArrayList.remove() method.

edited Mar 25, 2016 at 17:01

answered Mar 25, 2016 at 16:48

rdonuk

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2 Comments

Zerg Over a year ago

is it safe to remove arraylist elements trough lists.remove instead of using iterators?

Darth Android Over a year ago

@Zerg If the element only exists once in the array, then there is no difference.

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