How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList

java arraylist foreach
anon picture anon · Nov 12, 2011 · Viewed 353.2k times · Source

I have an ArrayList that I want to iterate over. While iterating over it I have to remove elements at the same time. Obviously this throws a java.util.ConcurrentModificationException.

What is the best practice to handle this problem? Should I clone the list first?

I remove the elements not in the loop itself but another part of the code.

My code looks like this:

public class Test() {
    private ArrayList<A> abc = new ArrayList<A>();

    public void doStuff() {
        for (A a : abc) 
        a.doSomething();
    }

    public void removeA(A a) {
        abc.remove(a);
    }
}

a.doSomething might call Test.removeA();

Answer

Jon Skeet picture Jon Skeet · Nov 12, 2011

Two options:

  • Create a list of values you wish to remove, adding to that list within the loop, then call originalList.removeAll(valuesToRemove) at the end
  • Use the remove() method on the iterator itself. Note that this means you can't use the enhanced for loop.

As an example of the second option, removing any strings with a length greater than 5 from a list:

List<String> list = new ArrayList<String>();
...
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
    String value = iterator.next();
    if (value.length() > 5) {
        iterator.remove();
    }
}

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