[rules-dev] Dealing with null pointer exceptions thrown by parsing rules

[Greg Barton]

David, please don't take this to be picking on you, but the rule you wrote wasn't quite correct. :)

when
       $f : Fact()
       Detail(fact == $f, name == "END_SET", value == true)
       Detail(fact == $f, name == "SET", value = 1)
       Detail(fact == $f, name == "PLAYER1_SCORE", $score1 : value)
       Detail(fact == $f, name == "PLAYER2_SCORE', value < $score1)
then
...
end

Match the Details to their parent Fact.  I just want to make sure the newb doesn't get a combinatorial explosion and run screaming. :)

And, not picking on both of you, but this should be on rules-users and not rules-dev. 

--- On Tue, 2/24/09, David Sinclair <dsinclair at chariotsolutions.com> wrote:

> From: David Sinclair <dsinclair at chariotsolutions.com>
> Subject: Re: [rules-dev] Dealing with null pointer exceptions thrown by  parsing rules
> To: "Rules Dev List" <rules-dev at lists.jboss.org>
> Date: Tuesday, February 24, 2009, 12:13 PM
> Alex, please don't take this to be picking on you; this
> is more a post to
> everyone whom reads these forums.
> 
> You are not writing procedural, object-oriented, or
> functional code. You are
> writing rules to look for patterns of objects. If
> everything is done in an
> eval you are losing the majority of the benefits of using a
> rule engine, e.g
> node sharing, indexing, etc. Flesh out your object model.
> Assert all of your
> objects into Working Memory. This will allow you to reason
> over anything
> without using evals.
> 
> OK -- rant off --
> 
> Alex, you may want to try and refactor your model to be
> more "rules
> friendly". That way your could write rules more like
> the following.
> 
> when
>        $f : Fact()
>        Detail(name == "END_SET", value == true)
>        Detail(name == "SET", value = 1)
>        Detail(name == "PLAYER1_SCORE", $score1 :
> value)
>        Detail(name == "PLAYER2_SCORE', value <
> $score1)
> 
> On Tue, Feb 24, 2009 at 12:39 PM, Zevenbergen, Alex <
> azevenbergen at paddypower.com> wrote:
> 
> >  Thanks for replying.
> >
> >
> >
> > rule "Player 1 wins first set "
> >
> >     when
> >
> >       $f : Fact()
> >
> > eval
> ($f.getDetails().get("END_SET").toString() ==
> "true" &&
> Integer.valueOf($f.getDetails().get("SET").toString())
> > == 1 &&
> Integer.valueOf($f.getDetails().get("PLAYER1_SCORE").toString())
> >
> >
> Integer.valueOf($f.getDetails().get("PLAYER2_SCORE").toString()))
> >
> >     then
> >
> >       //then settle selection;
> >
> > end
> >
> >
> >
> >
> >
> > this rules runs perfectly as long as the hashmap
> contained in the ‘fact’
> > object has all the required keys and their values are
> not null. For the time
> > being I have just changed any null values to a value
> of ‘DEFAULT’ but it
> > would be preferable to be able to look for
> f.getDetails().get("END_SET") (for
> > example) knowing that the engine that sent it might
> not be in an end_set
> > state so may not have added that key.
> >
> >
> >
> > That example is very basic but as I create more rules
> for more sports it
> > could become very cumbersome to have to ensure that
> every key referenced in
> > the rules is in the hashmap each time!
> >
> >
> >
> > Alex
> >  ------------------------------
> >
> > *From:* rules-dev-bounces at lists.jboss.org [mailto:
> > rules-dev-bounces at lists.jboss.org] *On Behalf Of
> *David Sinclair
> > *Sent:* 24 February 2009 17:27
> > *To:* Rules Dev List
> > *Subject:* Re: [rules-dev] Dealing with null pointer
> exceptions thrown by
> > parsing rules
> >
> >
> >
> > I understand what you are saying Alex, but could you
> post an example rule
> > to see exactly how you are doing it? It may be that
> you could simply rewrite
> > the rule so you don't get the NPE.
> >
> > On Tue, Feb 24, 2009 at 4:24 AM, Zevenbergen, Alex
> <
> > azevenbergen at paddypower.com> wrote:
> >
> > Hi all,
> >
> >
> >
> > I have started writing my rules packages and so have
> had a decent amount of
> > success. However all my rules are based on value pairs
> from a hashmap object
> > that is contained within my fact object. This approach
> works fine if the
> > parameter the rule is looking for is in the map and
> has a value.
> >
> >
> >
> > But I will need to be able to pass null values to the
> rules (and expect
> > them to just not fire any rule that looks for that
> param), however this
> > always throws a null pointer exception.
> >
> >
> >
> > So my question is: Is there any mechanism to deal with
> this in drools.
> >
> >
> >
> > Simply setting all nulls to a default value isn’t
> preferable in this
> > situation as the app is going to be used by several
> different sources and
> > has to be able to take in many different types of
> info.
> >
> >
> >
> > Thanking you,
> >
> >
> >
> > Alex
> >
> >
> >
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