Concurrent Modification Exception : adding to an ArrayList

Question

The problem occurs at

Element element = it.next();

And this code which contains that line, is inside of an OnTouchEvent

Answer 1

The accepted solution (to create a copy of the collection) usually works well.

However, if the Element contains another collection this does not make a deep copy!

Example:

class Element {
   List kids;

   getKids() {
      return kids;
   }
}

Now when you create a copy of the List of Elements:

for (Element element : new ArrayList(elements)) { ... }

You can still get a ConcurrentModificationException if you iterate over element.getKids() and, parally, alter the kids of that element.

Looking back it's obvious, but I ended up in this thread so maybe this hint helps others, too:

class Element {
   List kids;

   getKids() {
      // Return a copy of the child collection
      return new ArrayList(kids);
   }
}