I wish to have a rule that says, if I get one event, but not another one after a period, then do something. But I also want to only do this once.. So, I wrote the rules: (using the eclipse sample project) declare Message @role( event ) end rule "Hello World" when $m : Message( status == Message.HELLO ) not ( Message( status== Message.GOODBYE, this after [0s,1m] $m)) ; then System.out.println( "Hello" ); end rule "GoodBye" when $one : Message( status == Message.HELLO ) $two : Message( status == Message.HELLO ) then System.out.println( "Two hellos, remove one" ); retract($two); end But, when I run; Message message = new Message(); message.setMessage("Hello World"); message.setStatus(Message.HELLO); ksession.insert(message); Message message2 = new Message(); message2.setMessage("Hello World"); message2.setStatus(Message.HELLO); ksession.insert(message2); ksession.fireAllRules(); I get: Two hellos, remove one java.lang.NullPointerException at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:970) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:885) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1086) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:660) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:627) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:183) at com.sample.DroolsTest.main(DroolsTest.java:38) What am i doing wrong? -- View this message in context: http://old.nabble.com/Null-pointer-when-retracting-Event-which-has-an-%27... Sent from the drools - user mailing list archive at Nabble.com. _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users