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).

On 5/25/07, Ming Fang <mingfang@mac.com> wrote:

Hi,
I'm getting many of these errors when running the Sudoku Example

java.lang.NullPointerException
        at org.drools.reteoo.ReteTuple.get(ReteTuple.java:71)
        at org.drools.common.InstanceNotEqualsConstraint
$InstanceNotEqualsConstraintContextEntry.updateFromTuple
(InstanceNotEqualsConstraint.java:112)
        at org.drools.common.TripleBetaConstraints.updateFromTuple
(TripleBetaConstraints.java:146)
        at org.drools.reteoo.NotNode.assertTuple(NotNode.java:108)
        at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple
(SingleTupleSinkAdapter.java:29)
        at org.drools.reteoo.NotNode.assertTuple(NotNode.java :121)
        at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple
(SingleTupleSinkAdapter.java:29)
        at org.drools.reteoo.NotNode.assertTuple(NotNode.java:121)
        at
org.drools.reteoo.CompositeTupleSinkAdapter.createAndPropagateAssertTupl
e(CompositeTupleSinkAdapter.java:73)
        at org.drools.reteoo.LeftInputAdapterNode.assertObject
(LeftInputAdapterNode.java:144)
        at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject
(CompositeObjectSinkAdapter.java:317)
        at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:
183)
        at org.drools.reteoo.Rete.assertObject(Rete.java:121)
        at org.drools.reteoo.ReteooRuleBase.assertObject (ReteooRuleBase.java:
201)
        at org.drools.reteoo.ReteooWorkingMemory.doAssertObject
(ReteooWorkingMemory.java:70)
        at org.drools.common.AbstractWorkingMemory.assertObject
(AbstractWorkingMemory.java :735)
        at org.drools.base.DefaultKnowledgeHelper.assertObject
(DefaultKnowledgeHelper.java:72)
        at org.drools.base.DefaultKnowledgeHelper.assertObject
(DefaultKnowledgeHelper.java:66)
        at
org.drools.examples.sudoku.Rule_Rule_1__If_a_field_has_the_value_0__it_i
s_empty__remove_it_and_insert_the_PossibleValues_0.consequence
(Rule_Rule_1__If_a_field_has_the_value_0__it_is_empty__remove_it_and_ins
ert_the_PossibleValues_0.java:8)
        at
org.drools.examples.sudoku.Rule_Rule_1__If_a_field_has_the_value_0__it_i
s_empty__remove_it_and_insert_the_PossibleValues_0ConsequenceInvoker.eva
luate
(Rule_Rule_1__If_a_field_has_the_value_0__it_is_empty__remove_it_and_ins
ert_the_PossibleValues_0ConsequenceInvoker.java:22)
        at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:
497)
        at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java :461)
        at org.drools.common.AbstractWorkingMemory.fireAllRules
(AbstractWorkingMemory.java:372)
        at org.drools.common.AbstractWorkingMemory.fireAllRules
(AbstractWorkingMemory.java:353)
        at org.drools.examples.sudoku.SudokuExample.testWithInput
(SudokuExample.java:52)
        at org.drools.examples.sudoku.SudokuExample.main(SudokuExample.java:94)


Besides the fact that these errors are naturally bad,
this points out the issue of Drools being very difficult debug when
bad things happen.
Can somebody give some tips of Drool debugging techniques?

Thanks,
--ming
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