[rules-users] sequencing by business logic/ classcastexception in consequence

Hi, I'm new to jBoss Rules and I'm hitting a few snags. I appreciate your insight. Among other things I want to use business rules to compute the right sequence of actions that conforms to legal business rules. The only way I could think of to implement this using rules is to use rewriting production rules similar to: input: ACCAAAC rule: CA -> AC My first question is: what other ways are there to compute a sequence that conforms to sequencing business rules? My second question concerns the nullpointerexceptions and classcastexceptions I'm encountering in my rules. I've got a sequence of C's who are either B's or A's. I.e. C is the common superclass. This superclass has the methods: public C getPrevious(); public C getNext(); Actually it's in dutch so it's public class Straf extends InsluitingsTitel {} and public class PreventieveInsluitingsTitel extends InsluitingsTitel {} and public abstract class InsluitingsTitel{ public C getPrevious(); public C getNext(); } The rule is as follows: rule "WSV 68.1" when // any PreventieveInsluitingsTitel should precede all Straf secondPosition: Straf ( firstPosition: previous ) thirdPosition: PreventieveInsluitingsTitel(previous==secondPosition, fourthPosition:next) then System.out.println( "switching wsv 68.1 "+secondPosition+" "+thirdPosition); thirdPosition.switchWithPrevious(); if(firstPosition!=null){ modify(firstPosition); } modify( secondPosition); modify( thirdPosition); if(fourthPosition!=null){ //in the next statement a classcastexception is thrown modify(fourthPosition); } end results in a classcastexception. The sequence is at that moment { Straf, Straf, PreventieveInsluitingsTitel, Straf } and the middle two items are matched by the LHS secondPosition and thirdPosition. Here's the stacktrace. java.lang.ClassCastException: org.dji.domein.insgro.Straf at org.drools.base.org.dji.domein.insgro.PreventieveInsluitingsTitel$getPrevious.getValue(Unknown Source) at org.drools.base.ClassFieldExtractor.getValue(Unknown Source) at org.drools.rule.Declaration.getValue(Unknown Source) at com.sample.Rule_WSV_68_1_0ConsequenceInvoker.evaluate (Rule_WSV_68_1_0ConsequenceInvoker.java:22) at org.drools.common.DefaultAgenda.fireActivation(Unknown Source) at org.drools.common.DefaultAgenda.fireNextItem(Unknown Source) at org.drools.common.AbstractWorkingMemory.fireAllRules(Unknown Source) at org.drools.common.AbstractWorkingMemory.fireAllRules(Unknown Source) at org.dji.domein.insgro.VolgordeTest.main(VolgordeTest.java:32) org.drools.spi.ConsequenceException: java.lang.ClassCastException: org.dji.domein.insgro.Straf at org.drools.common.DefaultAgenda.fireActivation(Unknown Source) at org.drools.common.DefaultAgenda.fireNextItem(Unknown Source) at org.drools.common.AbstractWorkingMemory.fireAllRules(Unknown Source) at org.drools.common.AbstractWorkingMemory.fireAllRules(Unknown Source) at org.dji.domein.insgro.VolgordeTest.main(VolgordeTest.java:32) Caused by: java.lang.ClassCastException: org.dji.domein.insgro.Straf at org.drools.base.org.dji.domein.insgro.PreventieveInsluitingsTitel$getNext.getValue(Unknown Source) at org.drools.base.ClassFieldExtractor.getValue(Unknown Source) at org.drools.rule.Declaration.getValue(Unknown Source) at com.sample.Rule_WSV_68_1_0ConsequenceInvoker.evaluate (Rule_WSV_68_1_0ConsequenceInvoker.java:22) ... 5 more Thanks in advance for your insight. Regards, -- Joost de Vries