Question

I get ConcurrentModificationException in the following piece of code

when i running the code it went fine but all of a sudden it throws a exception, i guess its due to the modification of the list, but i’m not sure how to fix it

if (myRulesIncr!=null)
{
    Iterator itReceivedRules = myRulesIncr.iterator();
    while (itReceivedRules.hasNext())
    {
      RuleModel currentReceived = (RuleModel) itReceivedRules.next();
      if (receivedRulesExisting!=null)
      {
        Iterator itReceivedRulesExisting = receivedRulesExisting.iterator();
        while (itReceivedRulesExisting.hasNext())
        {
            RuleModel currentExisting = (RuleModel) itReceivedRulesExisting.next();

            if(currentExisting.getRuleId().equals(currentReceived.getRuleId()))
            {
                //TODO:replace the rule else add it.
                if(currentReceived.getStatus()!="D")
                { 
                    //replace the existing rule with the new one
                    receivedRulesExisting.remove(currentExisting);
                    receivedRulesExisting.add(currentReceived);
                } 
                else
                {
                    receivedRulesExisting.remove(currentExisting); 
                }
            }
            else
            {
                //Add the new rule to the existing rules
                receivedRulesExisting.add(currentReceived);
            }
        }
      }
    }
}

Please help me out in this .

Answer 1

ConcurrentModificationException is thrown when the collection being iterated is modified externally, i.e. not via the iterator. So you need to use Iterator.remove() to avoid this exception. Moreover, instead of adding directly to the collection while iterating through it, store the items to be added in a separate collection, then add them afterwards:

  List<RuleModel> toBeAdded = new ArrayList<RuleModel>();

  if(currentReceived.getStatus()!="D")
  { 
      //replace the existing rule with the new one
      itReceivedRulesExisting.remove();
      toBeAdded.add(currentReceived);
  } 
  else
  {
      itReceivedRulesExisting.remove(); 
  }

  ...
  // after the loop terminated:
  receivedRulesExisting.addAll(toBeAdded);

Note also that I used a generic collection – it is advisable to do so, to ensure type safety and get rid of downcasts like so:

Collection<RuleModel> myRulesIncr = ...
...
Iterator<RuleModel> itReceivedRules = myRulesIncr.iterator();
...
RuleModel currentReceived = itReceivedRules.next();