Errors like that are not normal, and definitely indicates a bug (ie not a compile error or anything - not something you should have to debug). So some more information is needed like versions, how you are running it, where the example came from etc (as there are a few versions floating around).
<br><br><div><span class="gmail_quote">On 5/25/07, <b class="gmail_sendername">Ming Fang</b> <<a href="mailto:mingfang@mac.com">mingfang@mac.com</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hi,<br>I'm getting many of these errors when running the Sudoku Example<br><br>java.lang.NullPointerException<br> at org.drools.reteoo.ReteTuple.get(ReteTuple.java:71)<br> at org.drools.common.InstanceNotEqualsConstraint
<br>$InstanceNotEqualsConstraintContextEntry.updateFromTuple<br>(InstanceNotEqualsConstraint.java:112)<br> at org.drools.common.TripleBetaConstraints.updateFromTuple<br>(TripleBetaConstraints.java:146)<br> at
org.drools.reteoo.NotNode.assertTuple(NotNode.java:108)<br> at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple<br>(SingleTupleSinkAdapter.java:29)<br> at org.drools.reteoo.NotNode.assertTuple(NotNode.java
:121)<br> at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple<br>(SingleTupleSinkAdapter.java:29)<br> at org.drools.reteoo.NotNode.assertTuple(NotNode.java:121)<br> at<br>org.drools.reteoo.CompositeTupleSinkAdapter.createAndPropagateAssertTupl
<br>e(CompositeTupleSinkAdapter.java:73)<br> at org.drools.reteoo.LeftInputAdapterNode.assertObject<br>(LeftInputAdapterNode.java:144)<br> at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject
<br>(CompositeObjectSinkAdapter.java:317)<br> at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:<br>183)<br> at org.drools.reteoo.Rete.assertObject(Rete.java:121)<br> at org.drools.reteoo.ReteooRuleBase.assertObject
(ReteooRuleBase.java:<br>201)<br> at org.drools.reteoo.ReteooWorkingMemory.doAssertObject<br>(ReteooWorkingMemory.java:70)<br> at org.drools.common.AbstractWorkingMemory.assertObject<br>(AbstractWorkingMemory.java
:735)<br> at org.drools.base.DefaultKnowledgeHelper.assertObject<br>(DefaultKnowledgeHelper.java:72)<br> at org.drools.base.DefaultKnowledgeHelper.assertObject<br>(DefaultKnowledgeHelper.java:66)<br> at
<br>org.drools.examples.sudoku.Rule_Rule_1__If_a_field_has_the_value_0__it_i<br>s_empty__remove_it_and_insert_the_PossibleValues_0.consequence<br>(Rule_Rule_1__If_a_field_has_the_value_0__it_is_empty__remove_it_and_ins<br>
ert_the_PossibleValues_0.java:8)<br> at<br>org.drools.examples.sudoku.Rule_Rule_1__If_a_field_has_the_value_0__it_i<br>s_empty__remove_it_and_insert_the_PossibleValues_0ConsequenceInvoker.eva<br>luate<br>(Rule_Rule_1__If_a_field_has_the_value_0__it_is_empty__remove_it_and_ins
<br>ert_the_PossibleValues_0ConsequenceInvoker.java:22)<br> at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:<br>497)<br> at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java
:461)<br> at org.drools.common.AbstractWorkingMemory.fireAllRules<br>(AbstractWorkingMemory.java:372)<br> at org.drools.common.AbstractWorkingMemory.fireAllRules<br>(AbstractWorkingMemory.java:353)<br> at
org.drools.examples.sudoku.SudokuExample.testWithInput<br>(SudokuExample.java:52)<br> at org.drools.examples.sudoku.SudokuExample.main(SudokuExample.java:94)<br><br><br>Besides the fact that these errors are naturally bad,
<br>this points out the issue of Drools being very difficult debug when<br>bad things happen.<br>Can somebody give some tips of Drool debugging techniques?<br><br>Thanks,<br>--ming<br>_______________________________________________
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</a><br></blockquote></div><br>