Removing duplicate objects in a Java ArrayList causes concurrent modification error [duplicate]

Question

Possible Duplicate:
Concurrent Modification Exception : adding to an ArrayList

I'm attempting to remove duplicate values from an arraylist with this method:

 public void hasDuplicates(List<Artifact> p_cars) {
    final List<String> usedNames = new ArrayList<String>();
    for (Artifact car : p_cars) {
        final String name = car.getObjectId();

        if (usedNames.contains(name)) {
            p_cars.remove(car);

        }

        usedNames.add(name);
    }

}

How can I remove these duplicate values without concurrent modifying the arraylist?

I'm doing this on an arraylist that populates a listview if that helps with the context.

Thanks!

Side note: the method should be named "removeDuplicates", and it should use a Set<String> to track the used names. — JB Nizet
– JB Nizet, Commented May 17, 2012 at 17:36
What @JB said... or usedNames.add(name) should be in an else { } block. — Dilum Ranatunga
– Dilum Ranatunga, Commented May 17, 2012 at 17:41
@Dilum: putting it in an else block would still lead to O(n) lookups rather than O(1) with a HashSet. A Set is the natural choice for unique values. — JB Nizet
– JB Nizet, Commented May 17, 2012 at 17:44

Pablo · Accepted Answer · 2012-05-17 17:50:00Z

6

You can't modify a collection while you are iterating over it. The only exception is using the remove method of the iterator:

 public void hasDuplicates(List<Artifact> p_cars) {
    final List<String> usedNames = new ArrayList<String>();
    Iterator<Artifact> it = p_cars.iterator();
    while (it.hasNext()) {
        Artifact car = it.next();
        final String name = car.getObjectId();

        if (usedNames.contains(name)) {
            it.remove();

        } else {
            usedNames.add(name);
        }
    }

}

edited May 17, 2012 at 17:50

answered May 17, 2012 at 17:39

Pablo

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1 Comment

Dilum Ranatunga Over a year ago

This is the correct way to modify a collection you are iterating over. However, for this usecase, we need to know just how big p_cars is, and how many duplicates are expected.

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