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.

Answer 1

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();
       }
}

Answer 2

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.

Answer 3

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

Answer 4

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.