Hi, I am trying to accomplish the following, a part time employee (eg 50%) needs to do only half the shifts a full time employee needs to do. I have created a rule, that 1. count number of shifts of certain shiftType 2. count number of shifts of a certain shiftType per employee 3. count the total number of "arbeidsbreuken" per employee and within a certain shiftType 4. do something with these numbers so that an part time employee only gets half the shifts assigned that a full time employee gets The complete rule is: rule "arbeidsbreuk" when //System.out.println("arbeidsbreuk, Drools!"); ShiftAssignment($employee : employee, $shiftType : shiftType) //count aantal shifts $assignmentTotal : Number() from accumulate( $assignment : ShiftAssignment(shiftType == $shiftType), count($assignment) ) //count aantal shifts per medewerker $assignmentTotalEmployee : Number() from accumulate( $assignmentEmployee : ShiftAssignment(employee == $employee, shiftType == $shiftType), count($assignmentEmployee) ) //count arbeidsbreuken van alle medewerkers $arbeidsbreukTotal : Number() from accumulate( //Employee($breuk : arbeidsbreuk), ShiftAssignment(employee == $employee, shiftType == $shiftType), sum($employee.getArbeidsbreuk()) ) $assignmentTotalEmployee.intValue()) then System.out.println("Arbeidsbreuk drools: " + $employee.getArbeidsbreuk() + " - " + $assignmentTotal.intValue() + " - " + $assignmentTotalEmployee.intValue() + " - " + $arbeidsbreukTotal); scoreHolder.addSoftConstraintMatch(kcontext, -(Math.abs(($employee.getArbeidsbreuk() * $assignmentTotal.intValue()) - $assignmentTotalEmployee.intValue()) * (Math.abs(($employee.getArbeidsbreuk() * $assignmentTotal.intValue()) - $assignmentTotalEmployee.intValue()))) ); end I get the following error: Exception in thread "main" org.drools.core.RuntimeDroolsException: java.lang.NullPointerException at org.drools.core.rule.Accumulate.accumulate(Accumulate.java:182) at org.drools.core.phreak.PhreakAccumulateNode.addMatch(PhreakAccumulateNode.java:756) at org.drools.core.phreak.PhreakAccumulateNode.doLeftInserts(PhreakAccumulateNode.java:164) at org.drools.core.phreak.PhreakAccumulateNode.doNode(PhreakAccumulateNode.java:81) at org.drools.core.phreak.RuleNetworkEvaluator.switchOnDoBetaNode(RuleNetworkEvaluator.java:524) at org.drools.core.phreak.RuleNetworkEvaluator.evalBetaNode(RuleNetworkEvaluator.java:494) at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:277) at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:161) at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:116) at org.drools.core.phreak.RuleExecutor.reEvaluateNetwork(RuleExecutor.java:205) at org.drools.core.phreak.RuleExecutor.evaluateNetworkAndFire(RuleExecutor.java:65) at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:936) at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1183) at org.drools.core.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:935) at org.drools.core.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:909) at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:233) at org.optaplanner.core.impl.score.director.drools.DroolsScoreDirector.calculateScore(DroolsScoreDirector.java:98) at org.optaplanner.core.impl.solver.scope.DefaultSolverScope.calculateScore(DefaultSolverScope.java:101) at org.optaplanner.core.impl.bestsolution.BestSolutionRecaller.solvingStarted(BestSolutionRecaller.java:58) at org.optaplanner.core.impl.solver.DefaultSolver.solvingStarted(DefaultSolver.java:177) at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:154) at be.ocmwturnhout.permanenties.Main.main(Main.java:495) Caused by: java.lang.NullPointerException at be.ocmwturnhout.permanenties.solver.Rule_arbeidsbreuk654888368.accumulateExpression2(Rule_arbeidsbreuk654888368.java:23) at be.ocmwturnhout.permanenties.solver.Rule_arbeidsbreuk654888368AccumulateExpression2Invoker.evaluate(Rule_arbeidsbreuk654888368AccumulateExpression2Invoker.java:25) at org.drools.core.base.accumulators.JavaAccumulatorFunctionExecutor.accumulate(JavaAccumulatorFunctionExecutor.java:107) at org.drools.core.rule.Accumulate.accumulate(Accumulate.java:173) ... 21 more Any idea how I can solve this? Thanks. Kind regards, Nick -- View this message in context: http://drools.46999.n3.nabble.com/Optaplanner-rules-NullPointerException-... Sent from the Drools: User forum mailing list archive at Nabble.com. _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users