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

Question

I wanted to iterate over an ArrayList and remove the elements, but I am getting an error: "java.util.ConcurrentModificationException".

My code:

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);
    }
}

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

Answer 1

There are two options to solve this:

• Add a new list of values you want to remove to the list within the loop, then call: originalList.removeAll(valuesToRemove).
• Use the remove() method on the iterator.

An example of the second option:

List<String> list = new ArrayList<String>();

...

for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {

    String value = iterator.next();

    if (value.length() > 5) {

        iterator.remove();

    }

}

I hope this helps you.