ConcurrentModificationException for ArrayList

Question

I have the following piece of code:

private String toString(List<DrugStrength> aDrugStrengthList) {
    StringBuilder str = new StringBuilder();
        for (DrugStrength aDrugStrength : aDrugStrengthList) {
            if (!aDrugStrength.isValidDrugDescription()) {
                aDrugStrengthList.remove(aDrugStrength);
            }
        }
        str.append(aDrugStrengthList);
        if (str.indexOf("]") != -1) {
            str.insert(str.lastIndexOf("]"), "\n          " );
        }
    return str.toString();
}

When I try to run it, I get ConcurrentModificationException, can anyone explain why it happens, even if the code is running in same thread? And how could I avoid it?

Answer 1

You can't remove from list if you're browsing it with "for each" loop. You can use Iterator. Replace:

for (DrugStrength aDrugStrength : aDrugStrengthList) {
    if (!aDrugStrength.isValidDrugDescription()) {
        aDrugStrengthList.remove(aDrugStrength);
    }
}

With:

for (Iterator<DrugStrength> it = aDrugStrengthList.iterator(); it.hasNext(); ) {
    DrugStrength aDrugStrength = it.next();
    if (!aDrugStrength.isValidDrugDescription()) {
        it.remove();
    }
}