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

[Zevenbergen, Alex]

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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