1º Set the planning variable nullable in ShiftAssignment.java: @PlanningVariable(nullable = true , valueRangeProviderRefs = { "employeeRange" }, strengthComparatorClass = EmployeeStrengthComparator.class) public Employee getEmployee() { return employee; } 2º Modify the EmployeeStrengthComparator class with this: @Override public int compare(Employee a, Employee b) { // El IF primero es para poder hacer nullable la planningVariable employee if (a == null || b == null ) return 0; else // TODO refactor to DifficultyWeightFactory and use getContract().getContractLineList() // to sum maximumValue and minimumValue etc return new CompareToBuilder() .append(b.getWeekendLength(), a.getWeekendLength()) // Descending .append(a.getId(), b.getId()) .toComparison(); } 3º Modify the ShiftAssignmentPillarPartSwapMoveFactory class, on line 67 with this: try { // Introducimos este bloque en el try - catch , para ignorar la NPE. if (assignmentSequenceList.isEmpty()) { AssignmentSequence assignmentSequence = new AssignmentSequence(employee, shiftAssignment); assignmentSequenceList.add(assignmentSequence); } else { AssignmentSequence lastAssignmentSequence = assignmentSequenceList // getLast() .get(assignmentSequenceList.size() - 1); if (lastAssignmentSequence.belongsHere(shiftAssignment)) { lastAssignmentSequence.add(shiftAssignment); } else { AssignmentSequence assignmentSequence = new AssignmentSequence(employee, shiftAssignment); assignmentSequenceList.add(assignmentSequence); } } } catch (NullPointerException ignorada) { } 4º Run the nurserostering example and try to resolve, Long05.xml, for example and you will get the NPE exception: Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: Solving failed. at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.done(SolverAndPersistenceFrame.java:319) at javax.swing.SwingWorker$5.run(SwingWorker.java:737) at javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.run(SwingWorker.java:832) at sun.swing.AccumulativeRunnable.run(AccumulativeRunnable.java:112) at javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.actionPerformed(SwingWorker.java:842) at javax.swing.Timer.fireActionPerformed(Timer.java:312) at javax.swing.Timer$DoPostEvent.run(Timer.java:244) at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733) at java.awt.EventQueue.access$200(EventQueue.java:103) at java.awt.EventQueue$3.run(EventQueue.java:694) at java.awt.EventQueue$3.run(EventQueue.java:692) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) at java.awt.EventQueue.dispatchEvent(EventQueue.java:703) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138) at java.awt.EventDispatchThread.run(EventDispatchThread.java:91) Caused by: java.lang.NullPointerException at org.drools.core.phreak.RuleNetworkEvaluator.deleteChildLeftTuple(RuleNetworkEvaluator.java:778) at org.drools.core.phreak.RuleNetworkEvaluator.unlinkAndDeleteChildLeftTuple(RuleNetworkEvaluator.java:771) at org.drools.core.phreak.PhreakNotNode.doRightUpdates(PhreakNotNode.java:355) at org.drools.core.phreak.PhreakNotNode.doNode(PhreakNotNode.java:73) at org.drools.core.phreak.RuleNetworkEvaluator.switchOnDoBetaNode(RuleNetworkEvaluator.java:555) at org.drools.core.phreak.RuleNetworkEvaluator.evalBetaNode(RuleNetworkEvaluator.java:536) at org.drools.core.phreak.RuleNetworkEvaluator.evalNode(RuleNetworkEvaluator.java:372) at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:332) at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:166) at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:123) at org.drools.core.phreak.RuleExecutor.reEvaluateNetwork(RuleExecutor.java:194) at org.drools.core.phreak.RuleExecutor.evaluateNetworkAndFire(RuleExecutor.java:73) at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1003) at org.drools.core.common.DefaultAgenda.fireLoop(DefaultAgenda.java:1346) at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1284) at org.drools.core.impl.StatefulKnowledgeSessionImpl.internalFireAllRules(StatefulKnowledgeSessionImpl.java:1303) at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1293) at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1274) at org.optaplanner.core.impl.score.director.drools.DroolsScoreDirector.calculateScore(DroolsScoreDirector.java:88) at org.optaplanner.core.impl.solver.scope.DefaultSolverScope.calculateScore(DefaultSolverScope.java:116) at org.optaplanner.core.impl.phase.scope.AbstractPhaseScope.calculateScore(AbstractPhaseScope.java:124) at org.optaplanner.core.impl.localsearch.decider.LocalSearchDecider.processMove(LocalSearchDecider.java:161) at org.optaplanner.core.impl.localsearch.decider.LocalSearchDecider.doMove(LocalSearchDecider.java:149) at org.optaplanner.core.impl.localsearch.decider.LocalSearchDecider.decideNextStep(LocalSearchDecider.java:121) at org.optaplanner.core.impl.localsearch.DefaultLocalSearchPhase.solve(DefaultLocalSearchPhase.java:72) at org.optaplanner.core.impl.solver.DefaultSolver.runPhases(DefaultSolver.java:214) at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:176) at org.optaplanner.examples.common.business.SolutionBusiness.solve(SolutionBusiness.java:316) at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.doInBackground(SolverAndPersistenceFrame.java:307) at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.doInBackground(SolverAndPersistenceFrame.java:297) at javax.swing.SwingWorker$1.call(SwingWorker.java:296) at java.util.concurrent.FutureTask.run(FutureTask.java:262) at javax.swing.SwingWorker.run(SwingWorker.java:335) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at java.lang. Thread .run( Thread .java:744)