Why Does Modifying an ArrayList During a For-Each Loop Cause a ConcurrentModificationException?

ConcurrentModificationException for ArrayList

Consider the following 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 executed, this code triggers a ConcurrentModificationException. This exception occurs because modifying the ArrayList while iterating over it using a for each loop is unsafe.

To resolve this issue, replace the for each loop with an iterator loop:

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

An iterator provides a safe way to remove elements from an ArrayList while iterating over it. It maintains a cursor that keeps track of the current position in the list, ensuring that the removal of an element does not affect the iteration process.

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