Hi, I am getting null pointer exception when i retract objects from working memory. please help me in resolving this problem.... my rule goes like this... package com.moto.nms import com.moto.nms.Event; import com.adventnet.nms.topodb.ManagedObject; import com.motorola.nmatg.nms.topology.ManagedEntity; global com.moto.nms.Consecutive con; // Consecutive is an simple class file which has two simple methods, getValue() and setValue(). rule "duplicate" salience 29 when e1 : Event( firsttime1 : time , severity1 : severity == 1 ) e2 : Event( firsttime2 : time > firsttime1 , severity == severity1 ) not Event( severity == -1 , time > firsttime1 , time < firsttime2) then System.out.println("matched the fact"); e1.setCompressionStatus("compress_"+con.getValue()); e2.setCompressionStatus("compress_"+con.getValue()); end rule "duplicate_x" salience 29 when e1 : Event(firsttime1 : time , severity == 1 ) e2 : Event(firsttime2 : time > firsttime1 , severity == 1) exists Event( severity == -1, time > firsttime1 , time < firsttime2) then System.out.println("matched 2nd fact"); con.setValue(con.getValue() +1); end exception is like this... Caught Exceptionjava.lang.NullPointerException java.lang.NullPointerException at org.drools.reteoo.beta.DefaultRightMemory.remove(Unknown Source) at org.drools.reteoo.BetaMemory.remove(Unknown Source) at org.drools.reteoo.NotNode.retractObject(Unknown Source) at org.drools.reteoo.ObjectSource.propagateRetractObject(Unknown Source) at org.drools.reteoo.RightInputAdapterNode.retractTuple(Unknown Source) at org.drools.reteoo.TupleSource.propagateRetractTuple(Unknown Source) at org.drools.reteoo.NotNode.retractTuple(Unknown Source) at org.drools.reteoo.TupleSource.propagateRetractTuple(Unknown Source) at org.drools.reteoo.JoinNode.retractTuple(Unknown Source) at org.drools.reteoo.LeftInputAdapterNode.retractObject(Unknown Source) at org.drools.reteoo.ObjectSource.propagateRetractObject(Unknown Source) at org.drools.reteoo.AlphaNode.retractObject(Unknown Source) at org.drools.reteoo.ObjectSource.propagateRetractObject(Unknown Source) at org.drools.reteoo.ObjectTypeNode.retractObject(Unknown Source) at org.drools.reteoo.Rete.retractObject(Unknown Source) at org.drools.reteoo.ReteooRuleBase.retractObject(Unknown Source) at org.drools.reteoo.ReteooWorkingMemory.doRetract(Unknown Source) at org.drools.common.AbstractWorkingMemory.retractObject(Unknown Source) at org.drools.common.AbstractWorkingMemory.retractObject(Unknown Source) at com.moto.nms.TestEvents3.retractAll(TestEvents3.java:219) at com.moto.nms.TestEvents3.readRule(TestEvents3.java:158) at com.moto.nms.TestEvents3.main(TestEvents3.java:242) TestEvents3.java class is like this.... package com.moto.nms; //related to IO.. import java.io.InputStreamReader; import java.io.Reader; import java.io.InputStream; import java.util.List; import java.util.Iterator; //related to Drools.. import org.drools.RuleBase; import org.drools.RuleBaseFactory; import org.drools.compiler.PackageBuilder; import org.drools.rule.Package; import org.drools.WorkingMemory; import org.drools.FactHandle; import org.drools.FactException; import com.moto.nms.Event; import com.moto.nms.NMSEvent; import com.moto.nms.TopoInterface1; import com.moto.nms.GenerateEvents; import com.adventnet.nms.topodb.ManagedObject; import com.moto.nms.Consecutive; public class TestEvents3 { private Reader reader; private RuleBase rulebase; private WorkingMemory wm; private List factHandles; public TestEvents3(){ } public void readRule(String readername) throws Exception{ InputStream str = null; str = TestEvents.class.getResourceAsStream(readername); if(str == null) { System.out.println("unable to read rule file"); return; } reader = new InputStreamReader(str); //read in the source PackageBuilder builder = new PackageBuilder(); //building up the rule package builder.addPackageFromDrl(reader); Package pkg = builder.getPackage(); //get the compiled package rulebase = RuleBaseFactory.newRuleBase(); rulebase.addPackage( pkg ); //adding the package to the rule base Consecutive con = new Consecutive(); wm = rulebase.newWorkingMemory(); wm.setGlobal("con", con); System.out.println("\n\n"+System.currentTimeMillis()); Event ev1 = new Event(183, 1, "port02", "LOS", System.currentTimeMillis()); //Thread.sleep(450); Event ev9 = new Event(192, 1, "port02", "LOS", System.currentTimeMillis()); //Thread.sleep(400); Event ev10 = new Event(307, 1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(500); Event ev11= new Event(306, 1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(200); Event ev12= new Event(305, 1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(250); Event ev13= new Event(304, -1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(600); Event ev14= new Event(302, 1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(200); Event ev15= new Event(301, 1, "port02", "LOS", System.currentTimeMillis()); Thread.sleep(200); Event ev16= new Event(308, 1, "port02", "LOS", System.currentTimeMillis()); wm.assertObject(ev1); wm.assertObject(ev9); wm.assertObject(ev10); wm.assertObject(ev11); wm.assertObject(ev12); wm.assertObject(ev13); wm.assertObject(ev14); wm.assertObject(ev15); wm.assertObject(ev16); System.out.println("\n\n"+System.currentTimeMillis()); System.out.println("\n\nFiring rules...\n"); wm.fireAllRules(); factHandles = wm.getObjects(); System.out.println("\n\nAll the Objects in WOrking Memory are as Follows...\n"); for(int i=0; i<factHandles.size(); i++) { System.out.println(factHandles.get(i)); } System.out.println("The CompressionStatus of the events are..."); System.out.println(ev1.getCompressionStatus()); System.out.println(ev9.getCompressionStatus()); System.out.println(ev10.getCompressionStatus()); System.out.println(ev11.getCompressionStatus()); System.out.println(ev12.getCompressionStatus()); System.out.println(ev13.getCompressionStatus()); System.out.println(ev14.getCompressionStatus()); System.out.println(ev15.getCompressionStatus()); System.out.println(ev16.getCompressionStatus()); System.out.println("The DeletionStatus of the events are..."); System.out.println(ev1.getDeletionStatus()); System.out.println(ev9.getDeletionStatus()); System.out.println(ev10.getDeletionStatus()); System.out.println(ev11.getDeletionStatus()); System.out.println(ev12.getDeletionStatus()); System.out.println(ev13.getDeletionStatus()); System.out.println(ev14.getDeletionStatus()); System.out.println(ev15.getDeletionStatus()); System.out.println(ev16.getDeletionStatus()); Thread.sleep(1000); System.out.println("Starting retracting...."); Thread.sleep(1000); retractAll(); System.out.println(System.currentTimeMillis()); } public void retractAll() { factHandles = wm.getFactHandles(); Iterator itr = factHandles.iterator(); if(factHandles == null) { System.out.println("invalid FactHandler List"); return; } System.out.println("size of fact handles..:"+factHandles.size()); while(itr.hasNext()) { try{ FactHandle fh = (FactHandle)itr.next(); if(fh == null) System.out.println("Here object is null, Cannot be retracted"); else { wm.retractObject(fh); System.out.println("Retracted object : "+ fh); } }catch (FactException e){ e.printStackTrace(); } } return; } public static void main(String[] args){ TestEvents3 te = new TestEvents3(); try{ te.readRule("/AlarmSuppression.drl"); }catch(Exception ex){ System.out.println("Caught Exception" +ex); ex.printStackTrace();} } }