FileDirectory

Hi Ron,

I get an instance of UnixFileDirectory.  And there are only 6 methods for UnixFileDirectory, none of them doing what I want.

Sincerely,

Joe.

Hi Ron,

> Excellent.  Remember that you should also look at the super classes for methods.  UnixFileDirectory is the instance of “FileDirectory default” but it’s also a subclass of FileDirectory so the methods you can call include UnixFileDirectory and all the methods on the class hierarchy including the methods on FileDirectory.  Please see the method categories on FileDirectory that I mentioned.

Okay, thanks.  I tried to do something with that, but I got an error.  I think I was trying to use a class method on the instance.


> The question I asked was a bit different.  When you ask for the #entries of the instance “FileDirectory default” what class is each file?

It is an Array of DirectoryEntry objects.

> By the way, based on the question you asked there is a method on FileDirectory to determine if the file exists before you read it (it’s in the method categories I pointed out to you).  What is that method?

fileExists:

I see what is happening.  I thought the on: method changes the path, like cding a directory.  But it gives a new instance.

       
dir := FileDirectory default.
dir on: '/Users/jja/Desktop’ ==> UnixFileDirectory on '/Users/jja/Desktop'
dir fileExists: 'categories.CSV’ ==>false
dir pathName ==>'/Users/jja/Desktop/Squeak-4.3-All-in-One.app/Contents/Resources'

dir := FileDirectory on: '/Users/jja/Desktop’ ==> UnixFileDirectory on '/Users/jja/Desktop'
dir pathName ==> '/Users/jja/Desktop'
dir fileExists: 'categories.CSV’ ==> true


So on: is the equivalent of new, in that it gives a new instance.

[Ron Teitelbaum]

Exactly right.  Notice the on: method on the class side, then look at #on: on the instance side.  This is one of those “ah ha” moments.  Classes create instances so when you see something like: “Class on:” you should automatically notice that the method is being sent to the class side since you are sending the message to the class directly.  This can get confusing if there is a method that results in something like myClass := Class, or when a method calls “self class on:”.  But again when it’s done the variable holds the class itself and you are still sending messages to a class when you use “myClass on:”.  So when you are talking directly to a class the method is on the class side.  Class side method (unless they are like the method above returning a class) usually return instances of the class, so method are now instance side methods.  Returning instances is one of the main uses of class side methods.  Lookup factory methods, abstract factory, and prototype, for a good example of software patterns that creates instances from the class side. Understanding polymorphism is now very important since all methods on the hierarchy are callable (on both the class and instance side).  Even methods that are overridden on a subclass can still be called using “super method”.

This is a fundamental piece of learning Smalltalk and not simple to understand, but it is also one of those things that is really easy to learn, and once you do you really don’t think about it anymore.  It just seems natural.  (and when it’s missing in other languages you go yuck!  Why is this so difficult!).  It also means that people that understand it think that it’s simple and natural and may not spend enough time talking about it. J

One more question for you.  I mentioned returning a newly created instance or a specific class from a class side method.  Can you name another reason why you would write a method on the class side?  Why would it be a good idea to put a method on the class side instead of the instance side?  (a hint for you, I’m thinking of something where nothing is returned. (of course in Smalltalk if nothing is returned you get back self, what I mean is that nothing useful is returned)) Bonus points for 2 or more answers with or without returning something J.

All the best,

Ron Teitelbaum

Thank you,

Sincerely,

Joe.

View this message in context: Re: FileDirectory
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> On May 26, 2016, at 8:29 AM, Ron Teitelbaum [via Smalltalk] <[hidden email]> wrote:
>
> One more question for you.  I mentioned returning a newly created instance or a specific class from a class side method.  Can you name another reason why you would write a method on the class side?  

When you want the class as a whole to do something.  When there is only one instance of an object and other is not desirable or logical.

For example, one compiler: two is not practical.  One instance of the number Pi.  One instance of Nil. Why would you need another?  

[Ron Teitelbaum] if there is one instance you would still write methods on the instance side.  But yes having a class side method that provides your single instance makes sense.

> Why would it be a good idea to put a method on the class side instead of the instance side?  (a hint for you, I’m thinking of something where nothing is returned. (of course in Smalltalk if nothing is returned you get back self, what I mean is that nothing useful is returned)) Bonus points for 2 or more answers with or without returning
> something J.

You would put a method on the class side when the method applies to all instances of the class.  For example, Window closeAllWindows.  Or Process stopAllProcesses.   Or Smalltalk saveImage.

I don’t think this is what you had in mind, though.

[Ron Teitelbaum] This was exactly what I had in mind.  When you have a method that does something to all instances or is needed to find a specific instance it makes sense to implement that as a class side method.

Extra credit: Why would you use a class variable?  We know that an instance variable is used to store data within the context of an instance.  Why would you want to store data in the context of a class?

All the best,

Ron Teitelbaum

Sincerely,

Joe.

View this message in context: Re: FileDirectory
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> Extra credit: Why would you use a class variable?  We know that an instance variable is used to store data within the context of an instance.  Why would you want to store data in the context of a class?

Window closeAllWindows needs to have a list of all the windows, and sends each one the close message.   Or if a instance of a class needed to communicate with other members of its class.  For example, a class of helicopter bots that fly together as a group.  They would need talk to each other to avoid collisions.

[Ron Teitelbaum] Exactly!  Well done.  Any dynamic information that is related to all or a collection of instances should be held on a class variable.  You could of course create a framework by adding another object whose job it is to manage all these instances.  Then your class variable becomes an instance variable on your manager class.  It could also be used for a class with just one instance.  Create it and store it so that it’s reused when asked for a second time.  You will see that a lot,

^Instance ifNil: [Instance := self new. Instance].

Nice job!

Ron

Sincerely,

Joe.

View this message in context: Re: FileDirectory
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I saw a lot of code in FileDirectory for urls.  What’s that all about?  Can I use FileDirectory to browse the web?  If not, is there another web browser object?

Sincerely,

Joe.

View this message in context: Re: FileDirectory
Sent from the Squeak - Beginners mailing list archive at Nabble.com.

View this message in context: Re: FileDirectory
Sent from the Squeak - Beginners mailing list archive at Nabble.com.