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Re: Newbie Key Value-coding and Subclasses Question
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Re: Newbie Key Value-coding and Subclasses Question


  • Subject: Re: Newbie Key Value-coding and Subclasses Question
  • From: David Avendasora <email@hidden>
  • Date: Fri, 10 Feb 2006 06:57:49 +0000

Hi Arturo,

I have a reputation as being a bit ummm *strict* in my interpretation of what the proper O-O way is of doing things. It has served me well with code and less so with ppl :-)

No worries! This is my first OO programming project and being *somewhat* over the age of 14, I'm having a slow time reconciling it with my *strict* Relational Database design background. I'm trying to be OO, but sometimes I wake up in the night screaming "It's not normalized, it's not normalized!!!" Be strict. It helps me.



Arturo,

On question 1, I am currently testing for what type of Class it is by reading my PartType field using:

if (((String) valueForKeyPath("part.partType.partTypeName")).equals ("Finished Part")) {...}

and this works just fine, but my understanding was that instanceof is a better OO way of doing things - especially since the string partTypeName is a value from a DB field which could potentially change.

The problem isn't so much with doing the above or using instanceof. The problem is that once your code becomes aware of object-types then you're on a slow sure path to perdition. You might want to take a look at the book HeadsFirst Design Patterns (the typical GoF reference is too heavyweight for most developers) for reasons to avoid instanceof et al.

I have bought it and am currently about a third of the way through. I understand the write to an interface and not to the implementation concept, but I am having a hard time understanding exactly how to apply it in this situation.


All I'm doing here is copying some default data from a Part object to a Label object. Depending upon what type of Part (Raw, Intermediate or Finished) they have different subsets of information. All three have some things like partDescription but only a Finished has unitQuantity so I have to figure out what type of Part it is first to know what information I can get from this part. I suppose I could create an interface of "Labelable" and require all Part types to have the fields I'm pulling so I wouldn't have to worry about what type of Part it is, but then I'm solidifying some UI convenience as a requirement in the code.

At this point I'm sure you are going to as "Why are you copying the data? Just use the relationship to grab the information from the associated Part." The problem is that a Label can potentially use different information, I'm just copying this info across as the default. The user can change it to something else, although they usually won't.


IIRC, the proper WO way of doing things is to use single table inheritance for the above situation. Is that what you're doing?

(sheepishly) No, I'm using Vertical Inheritance, even after reading all the admonishment against it in the WO documentation. See the bit about my dreams above.


On the second question, The problem is that "Label" is not related to "Finished", but to it's superclass "Part". Once I know that the "Part" I'm dealing with is a "Finished" (subject of question 1) then how do I call a method from the "Finished" subclass using key value-coding? valueForKeyPath("part.unitQuantity") does not work because unitQuantity is not a attribute of "Part" it is an attribute of "Finished".

If your object inheritance hierarchy is correct then you should be able to do valueForKeyPath("part.unitQuantity"). Let's say you have Part as the base of your inheritance hierarchy. If Finished extends Part then you are done.

AHHG. It's working perfectly. I *swear* I tried to do this first and it didn't work.


This actually makes me feel MUCH better because what I wanted to do first was the right thing (if you ignore the whole "testing for object-type" and "vertical inheritance" things). I thought that this way just seemed right, and it would have been the best way, but when it didn't work (or so I thought), I was stumped and started trying other things. I must have had some other bug in my code.

Thanks for the help!

Dave





Dave

On Feb 9, 2006, at 4:17 PM, Arturo Perez wrote:

David Avendasora wrote:
Hi.

I have a relationship from Label to Part (toOne)

Part has three subclasses: Raw, Intermediate and Finished. All of these (Part, Raw, Intermediate and Finished) are all EOGenericRecord classes right now.

I'm running into two problems

1) How do I perform an instanceof to determine what subclass of Part takeStoredValueForKey("part") returns when run on a Label object? Do I have to generate the custom classes first for Part and it's subclasses?

2) How do I call subclass-specific methods with key value-coding when the relationship is to the superclass? I have no problem executing superclass-specific methods using valueForKeyPath ("part.partDescription"), but I want something along the lines of valueForKeyPath("finished.unitQuantity") (unitQuantity is an attribute of the Finished subclass of Part)

Well, instanceof in an OO design is generally considered bad practice. You may want to reconsider if your design requires it. If you really do need to know what something is I think you can do something like getting the EODescription/ EOClassDescription of the EOGenericRecord and one of the attributes is the name you've given it.

What's the problem you're running into with the second thing? If you have a relationship to something and you do what you've written it should just work.

-arturo









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  • Follow-Ups:
    • Re: Newbie Key Value-coding and Subclasses Question
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References: 
 >Newbie Key Value-coding and Subclasses Question (From: David Avendasora <email@hidden>)
 >Re: Newbie Key Value-coding and Subclasses Question (From: Arturo Perez <email@hidden>)
 >Re: Newbie Key Value-coding and Subclasses Question (From: David Avendasora <email@hidden>)
 >Re: Newbie Key Value-coding and Subclasses Question (From: Arturo Perez <email@hidden>)

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