Hi Dennis,

I don't think I've seen you on this list so welcome! I think your question is a very good one. I read the other responses you received and they are very good. I thought I'd give you a practical example of what is called a factory pattern to explain why this is so important and how it works.

A factory pattern is basically a pattern that decides what class to build and gives it back to you. In other words you send it off to the factory method and it builds an instance of the class that you need.

Ok so now an example:

Person class>>newForAge: anAge "Instantiate and return to the sender the class that is appropriate for anAge" aClass := Person subclasses detect: [:aClass | aClass ageRange includes: anAge] ifNone: [self error: 'Age Range for: ', anAge asString, ' is not defined in subclasses of person]. ^aClass createWithAge: anAge

Ok so that is the factory. It says go through my subclasses and find one that matches anAge then create an instance of that class with that age.

To make this work we need classes and ageRanges so we create subclasses of Person

Person subclass: SeniorPerson Person subclass: AdultPerson Person subclass: MinorPerson

Now age ranges

SeniorPerson class > ageRange ^NumericRange from: 65 to: 999

AdultPerson class > ageRange ^NumericRange from: 21 to: 65

MinorPerson class > ageRange ^NumericRange from: 0 to 21

(Ok so there is no NumericRange, I looked but didn't find one but you can build it yourself. You just need to implement #includes: aNumber in it)

Ok so now here comes the part that answers your question.

Say there is code that you want run on all of these classes, do you implement the code on each subclass? Is there some way to write the code on the superclass so that it can be in just one place?

The answer is yes! You can put the code on the superclass.

Person createWithAge: anAge ^self new age: anAge; yourself

This might look strange but look at what it is doing. It creates an instance and sets the age, which since you would define the value age on the superclass Person and not on each subclass this makes sense.

Now how can we use this method to create a subclass?

Like this:

Senior class > createWithAge: anAge "Return to the sender an instance of Senior with anAge set" ^super createWithAge: anAge

Now look closely. What this says is run the method createWithImage: anAge of the superclass for me (Senior class). When the method executes self on person = Senior class. Try debugging the method and see.

You could use this to further specialize the object.

Senior class > createWithAge: anAge "return to the sender an instance of the receiver" | aSenior | aSenior := super createWithAge: anAge. "set Senior specific values" aSenior seniorDiscount: (self seniorDiscountForAge: anAge). ^aSenior

So you can see super really means run a superclass method on the class or instance that calls it. The self of the method that is called is the instance or class that called it.

I hope that helped, if not please say so,

Ron Teitelbaum President / Principal Software Engineer US Medical Record Specialists Ron@USMedRec.com

________________________________________ From: dennis petrocelli Sent: Friday, October 13, 2006 9:57 AM

Greetings smalltalkers,   I'm a newbie trying to get my head around inheritance.  I'm puzzled by the following example.  It seems to do what I mean, but not what I type ;)  This is a good thing, but I'd get further if I understood the semantics a bit better.   from http://www.cosc.canterbury.ac.nz/wolfgang.kreutzer/cosc205/smalltalk1.html   Class: Monster Superclass: Object Category: Sesame Street Instance variables: colour tummy   initialization   initialize    self colour: #green. self tummy: Bag new   "There is only a single class method and no class variables."   creation new    ^ super new initialize   What I don't understand is this:  Monster new returns an instance of Monster, what you'd want it to. But Monster new executes "^ super new" which to my mind should return an instance of Object, not Monster.  Just to confuse myself further, I typed:   Monster superclass new   and got an Object.  It seems to me that "super new" and "superclass new" should do the same thing.   Any points to clarify my confusion would be greatly appreciated.   Thanks, Dennis Petrocelli

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