Objects and classes

I'm REALLY new to Smalltalk. In a previous thread, I created a file  
Hello.st:

Object subclass: #Hello
        instanceVariableNames: ''
        classVariableNames: ''
        poolDictionaries: ''
        category: ''!


"Create a method"
!Hello methodsFor: 'speaking'!
publish
     Transcript show: 'Greetings. '!

! !

To load it, I do
FileStream fileIn: 'Hello.st' !

BUT, if I want to "run" Hello, I have to do
Hello new publish

My question is: why did I have to `new` Hello, whereas I don't have to  
`new` FileStream? Presumably for Hello I have to create an object by  
instantiating a class, whereas with a class like FileStream I don't  
have to. What's going on? Are there methods which only belong to  
classes and not objects, and if so, how do I define them?

Sorry if the question seems really dumb.


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Mark Carter <alt.mcarter <at> googlemail.com> writes:


Yes you guessed right, there are class methods. You can create them like this:

!Hello class methodsFor: 'speaking'!

Notice that I changed "Hello" into "Hello class". I am also a new Smalltalker so
maybe I'm wrong but as far as I know "class" is a message which returns the
class of an object.

So the code below is the new hello.st and works with both "Hello new publish" or
just "Hello publish".

Object subclass: #Hello
        instanceVariableNames: ''
        classVariableNames: ''
        poolDictionaries: ''
        category: ''!

"Create a class method"
!Hello class methodsFor: 'speaking'!
publish
    Transcript show: 'Greetings. '!

! !

"Create a method"
!Hello methodsFor: 'speaking'!
publish
    Transcript show: 'Greetings. '!

! !

Hello publish!



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On Tue, 27 May 2008 21:24:44 +0100
Mark Carter <[hidden email]> wrote:

In Smalltalk, everything is an object. You make objects do something by
sending them a message for which they (or their ancestors) have an
implementing method.

"Hello new publish" asks the class object referenced by Hello to create
a new instance, which is then asked to publish something. That's the
"normal" way of doing things.

But you can define class side methods (often as convenient
abbreviations) for creating new instances, which "hide" the call to new
within their implementation, your fileIn above is one such example.
It's implementation might very well look like:

fileIn: aPath
        ^self new fileIn: aPath

Just guessing, but it's a common pattern.

An interesting aspect of "everything is an object" is the following:

If everything is an object, even classes, then where does the class
object Hello come from? You made it by asking the Object
class object to create a subclass:

Object subclass: #Hello
        instanceVariableNames: ''
        classVariableNames: ''
        poolDictionaries: ''
        category: ''!

This "class definition" is not weird syntax, it's "just another
message", sent to Object when you accepted/evaluated the class
definition message.

s.


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> But you can define class side methods (often as convenient
> abbreviations) for creating new instances, which "hide" the call to new
> within their implementation, your fileIn above is one such example.
> It's implementation might very well look like:
>
> fileIn: aPath
> ^self new fileIn: aPath
>
> Just guessing, but it's a common pattern.

Yes, in this case it's another common usage of class methods, i.e.
"create an object, do something, discard it".  fileIn: is implemented as


    ^(self open: aPath mode: self read)
         fileIn;
         close

(more or less).

Paolo


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