Mister Nono wrote Thats why I said "/you need Lists or Maps as values of the TreeMap/". *Here is fast solution:* import Vehicle import java.util.* rule "Print grouped vehicles" when $map : Map() from accumulate ($v : Vehicle($w : wheels), init(TreeMap map=new TreeMap()), action( if(map.containsKey($w)) ((List)map.get($w)).add($v); else map.put($w, new ArrayList(Arrays.asList($v)));), result(map)) then System.out.println($map); end -- View this message in context: http://drools.46999.n3.nabble.com/To-group-some-objects-by-type-tp4022085... Sent from the Drools: User forum mailing list archive at Nabble.com.