Question

How to avoid ConcurrentModificationException when using ArrayList on Android?

Can someone give the basic rule of thumb to avoid it?

Answer 1

By not adding/removing the elements when iterating over your arraylist.

This code would throw the ConcurrentModificationException as you are modifying the list while iterating.

List<String> al = new ArrayList<String>();
al.add("2");
for(String s: al) {
al.remove("2");
}

This code wouldn’t throw the ConcurrentModificationException.

 List<String> al = new Arraylist<String>();
 Iterator<String> itr = al.iterator();
 while(itr.hasNext()){
   itr.next();
   itr.remove();
 }    

You could also use some of the class’s from java.util.Concurrent package such as java.util.concurrent.CopyOnWriteArrayList which wouldn’t throw ConcurrentModificationException when you modify the list while iterating:

  CopyOnWriteArrayList<String> al = new CopyOnWriteArrayList<String>();
  al.add("2");
  Iterator<String> itr = al.iterator();
  while(itr.hasNext()){
  itr.next();
  al.add("3");
 }