[rules-users] Truth Maintenance, equals, and hashCode

[Mark Proctor]

ringsah at comcast.net wrote:
> I have a question about logical assertions and truth maintenance. 
> First of all, the Drools manual states:
>  
>
>
>         2.5.8.2. Important note: Equality for Java objects
>
> It is important to note that for Truth Maintenance (and logical 
> assertions) to work at all, your Fact objects (which may be Javabeans) 
> override equals and hashCode methods (from java.lang.Object) 
> correctly. As the truth maintenance system needs to know when 2 
> different physical objects are equal in value, BOTH equals and 
> hashCode must be overridden correctly, as per the Java standard.
>
> Two objects are equal if and only if their equals methods return true 
> for each other and if their hashCode methods return the same values. 
> See the Java API for more details (but do keep in mind you MUST 
> override both equals and hashCode).
>
> Does this mean that in order to use logical assertions that I /have 
> /to override equals and hashCode?
>
TMS will work if you don't override logical/hashcode but it just means 
it will always created a new logical relationship relationship. The nice 
thing about impl equals and hashcode is it means if you try and logical 
assert an object which is equal, it'll ignore it.
>
> What if I have a class that I would like to use as fact in a logical 
> assertions, but for this class equality should be the same as 
> identity. In other words, I don't want to override equals and 
> hashCode, because the versions of these methods inherited from Object 
> have the correct behavior for my application. Will this class still be 
> okay to use as facts in logical assertions?
>
It will work ok, but you need to make sure you understand the role of 
equals/hashcode to stop fact duplication - it's pretty much like the 
difference between an IdentityMap and HashMap.
>
> Thanks,
> -Hans
>
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