I have been using Drools for a project and I have run into one snag that I thought would be possible. I have a list of objects that I need to collect a single property value from each object. I would think accumulate would work, but I keep getting an IncompatibleClassChangeError. Here is an example of the rule condition: $b : BaseObject() ContainingObject($myList : subList) from $b.containingObject $titleList : ArrayList() from accumulate( m : MyObject() from $myList, init( ArrayList tempList = new ArrayList(); ), action( tempList.add(m.getTitle()); ), result( tempList ) ) The basic idea is that the Base Object contains another object (ContainingObject) that contains a list. This list needs to be iterated over to get the title from each object in the list. I will then use the $titleList in the action part of the rule. Can somebody point me in the right direction? Thanks. Rob Diana