Java , Removing object from ArrayList

Question

I have ClassA which has a static ArrayList of Objects

public static ArrayList<Meteorit> meteorits = new ArrayList<Meteorit>();

Now I want to remove an object from this list like this

ClassA.meteorits.remove(this);

This is written in Meteorit class . But it throws exception when I want to use the objects in the ArrayList .

Exception in thread "LWJGL Application" java.util.ConcurrentModificationException

I used Iterator to remove objects from ArrayList but now I dont have an idea how to use it in this case.

possible duplicate of Remove elements from collection while iterating — John
– John, Commented May 23, 2015 at 8:31
Why is the ArrayList static? I suggest to do it exactly like how observer design pattern did. First, make a method inside ClassA call it public removeMetorit(Meteorit meteorit) and use it from wherever exept the Meteorit class. — MChaker
– MChaker, Commented May 23, 2015 at 8:39
Also, you are calling the remove method from inside the object to be removed and this is illegal. — MChaker
– MChaker, Commented May 23, 2015 at 8:42
I want the meteorite to remove itself from the list, I don't think it's illegal. — Vahe Muradyan
– Vahe Muradyan, Commented May 23, 2015 at 9:42

Krzysztof Cichocki · Accepted Answer · 2019-11-06 19:49:01Z

3

It is because some thread is actually viewing this list in a for each loop, maybe you are trying to remove elements of this list in the body of for-each? You can't remove elements in for-each, but you can in iterator loops:

You can use iterator instead of for each to remove and view the elements of the list like this:

public static ArrayList<Meteorit> meteorits = new ArrayList<Meteorit>();

Iterator<Meteorit> itr = meteorits.iterator();

while (itr.hasNext()) {
       if (itr.next()==this) {
          itr.remove();
       }
}

edited Nov 6, 2019 at 19:49

answered May 23, 2015 at 8:44

Krzysztof Cichocki

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Comments

Brett Walker · Accepted Answer · 2015-05-23 08:30:05Z

1

When using an iterator; you need to use the iterator to remove items from a list using this:

iterator.remove();

which from the Java Docs says:

Removes from the underlying collection the last element returned by this iterator.

Removing a item from the list by any other means will result in the ConcurrentModificationException you are seeing.

answered May 23, 2015 at 8:30

Brett Walker

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2 Comments

Vahe Muradyan Over a year ago

I know this , but I need the way how to use it in this case.

Brett Walker Over a year ago

If you could supply more snippets of your code in your question in order to an appropriate answer; otherwise it is hard to apply this knowledge to your situation.

Community · Accepted Answer · 2017-05-23 12:15:41Z

1

Basically, you need to use iterator to avoid such concurrent modification:

List<String> list = new ArrayList<>();
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        iterator.remove();
    }
}

For more details, please check out this post:

Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop

edited May 23, 2017 at 12:15

CommunityBot

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answered May 23, 2015 at 9:49

Hoa Nguyen

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Comments

Alaa Abuzaghleh · Accepted Answer · 2015-05-23 08:30:06Z

0

Iterator<Integer> itr = yourlist.iterator();

// remove all even numbers
while (itr.hasNext()) {
       itr.remove();
}

this must work for you, other way to work around this issue is to use CopyOnWriteArrayList hopefully it help.

answered May 23, 2015 at 8:30

Alaa Abuzaghleh

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