How to Avoid ConcurrentModificationException When Adding to an ArrayList During Iteration?

Concurrent Modification Exception: Adding to an ArrayList

Problem

An application encounters a ConcurrentModificationException when modifying an ArrayList (mElements) within an OnTouchEvent method. The code iterates over mElements using an Iterator, and upon specific touch events, attempts to add a new element to the list. This occurs within a synchronized(mElements) block.

Exception

java.util.ConcurrentModificationException

Cause

The ConcurrentModificationException occurs because the ArrayList is being modified (by adding elements) while it is being traversed using an Iterator. Iterators expect the underlying collection to remain unchanged during iteration. However, adding elements to the list during iteration violates this expectation.

Solution

To resolve the exception, it is necessary to add the new elements to the list after the iteration has completed. This can be achieved by using an additional List to hold the new elements and adding them to the main mElements list after the iteration.

List<Element> thingsToBeAdd = new ArrayList<Element>();
for (Iterator<Element> it = mElements.iterator(); it.hasNext(); ) {
    Element element = it.next();
    // ...
    if (element.cFlag) {
        thingsToBeAdd.add(new Element("crack", getResources(), (int) touchX, (int) touchY));
        element.cFlag = false;
    }
}
mElements.addAll(thingsToBeAdd);

Additional Option

As an alternative to the above solution, consider using the enhanced for-each loop syntax, which avoids the need for an Iterator and handles the synchronization internally:

for (Element element : mElements) {
    // ...
    if (element.cFlag) {
        thingsToBeAdd.add(new Element("crack", getResources(), (int) touchX, (int) touchY));
        element.cFlag = false;
    }
}
mElements.addAll(thingsToBeAdd);

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