ALINK="#FF0000">

LINUX GAZETTE

 [ Prev ][ Table of Contents ][ Front Page ][ Talkback ][ FAQ ][ Next ]

"Linux Gazette...making Linux just a little more fun!"

Objects, Classes and Other Things

By Jason Steffler

Abstract

    For those who haven't read the previous articles, be sure to read the statement of purpose first.  This month, we're going to discuss objects as well as classes, messages and encapsulation.  For those looking to read the whole series locally or info about upcoming articles, you can check the MST page.  For those looking for further information on learning Squeak, here are some good resources.
    I also need to cover another item before we get into this article, and it's important enough to put at the top as opposed to the Q&A section.  I had a number of people ask me how I knew what code to type, and where they can find what objects Smalltalk has.  I plan on getting to this in article 4.  I'm holding off on discussing this to simplify the presentation and concentrate on fundamental concepts first.  I've often thought that the message isn't the medium, but rather the volume of the medium.  This has a number of connotations; in this context I don't want to present too much too fast and overwhelm the folks who are coming in with no programming experience at all.
    As a side note, I find it humourous and sad to see technical books that are selling by the pound these days.  You see things like: '1000 pages of <technology X> for only $19.95!', and the Core Java 2 Fundamentals book that is 742 pages!  It's often been said that the syntax of Smalltalk is so simple that you can put it on a postcard, as there are 2 operators, 5 reserved words1, and 9 reserved characters.  For those interested I'll add an interlude: all I learned about Smalltalk syntax I learned from a postcard.

Quote of the day

"Smalltalk is a wonderful language to work with - in fact, it's hard to imagine a serious programming language being more fun than Smalltalk.  Certainly, I've had more fun programming in Smalltalk than any other language I've worked with; so much fun that at times it's seemed incredible that I've also been paid to enjoy myself."
        -Alec Sharp, "Smalltalk by Example", pXIX

A first look at objects

    Last month, we left off with describing an object as anything you can think of that is a noun.  We implicitly extended this concept by talking about actions that objects can do when asked.  Let's extend this concept explicitly now by describing actions objects can do when asked as verbs.  For example, you could consider a Person as an object.  You could ask the Person object to do things like:
  1. Person, would you please sing?
  2. Person, would you please sing Mary Had A Little Lamb?
  3. Person, would you please sing Mary Had A Little Lamb, and do it loudly.
  4. Person, what is your height?
    Notice action 1 was just an activity with no constraints on it.  We don't tell the person what to sing, or how fast, or how loud, etc.  In our 2nd request, we specify the song to sing and in the 3rd request we also specify that it should be sung loudly.  Action 4 shows that we can not only ask the person to do something, but also ask them something about themselves.  That's right, objects have properties just like a real world thing would (we'll come back to this).  Assuming we had a Person object (we don't, at least not yet), the corresponding Smalltalk code would look like:

   (Person new) sing.
   (Person new) sing: 'MaryHadALittleLamb'.
   (Person new) sing: 'MaryHadALittleLamb' andDoIt: 'loudly'.
   (Person new) sing: 'MaryHadALittleLamb' andDoIt: 'quietly'.
   (Person new) whatIsMyHeight.

    Pretty easy stuff eh?2  Notice how the Smalltalk code is very readable and is very similar to how I initially wrote the questions in English.  Each one of these requests would be what we Smalltalkers call a message that the Person responds to, and the method in which they respond is determined by what we Smalltalkers call a method.  Again, pretty easy and intuitive stuff.
    Note on the last message, I switched the perspective around to whatIsMyHeight as opposed to whatIsYourHeight.  We could easily have made a method called whatIsYourHeight, but it's common practice to name methods from the perspective of the object3.
    Now, you'll notice that each request has (Person new) in it; you'd be correct in assuming we're asking a 5 different people to do something - we're asking a new Person to do something each time.  What if we want to ask the same person to do everything?  There's a few ways we could do this, one of them is:

    | aPerson |
    aPerson := (Person new).
    aPerson sing.
    aPerson sing: 'MaryHadALittleLamb'.
    aPerson sing: 'MaryHadALittleLamb' andDoIt: 'loudly'.
    aPerson sing: 'MaryHadALittleLamb' andDoIt: 'quietly'.
    aPerson whatIsMyHeight.

    The first line is declaring a temporary variable.  Hmm, this is the first traditional computer term that we've used so far in our discussion, not too bad.  Since we don't have a name for the person, we'll just call the person aPerson.  Much better than 'x', 'y', or 'i' that you often see in other programming languages.  Not that you couldn't call a variable x in Smalltalk, it's just that you're encouranged to name things descriptively.  The common convention is to run your words together with capitalizing each successive word (IMHO, this includes acronyms too).  For example, you could ask the Person to runToTheDmv.  So in the above code snippet, we're creating a new person and assigning (:=) that person to a temporary variable called aPerson.  Then we're asking aPerson to perform their various methods by sending them messages.
    So the question naturally arises, what is 'Person'?  Thinking in terms of nouns, a Person is a specific class or subset of nouns.  Well, in Smalltalk Person is an object too, but it's a special kind of object that is called a class.  You can think of a class as a blueprint object for making related objects.  When we ask a class to make a new instance of an object, it's called instantiating an object.  Now, coming back to the properties of an object, they are stored in what are called instance variables of the object4.  When we were asking aPerson for their height, they probably responded with what they had stored in their instance variable (we don't know for sure, as we don't know how the person determines their height).
    Revisting our conception of what an object is, we can now refine it:  an object is a grouping of messages and data that its messages can operate on.  This brings us to our next subject:  Encapsulation.

Encapsu-what?

    Encapsulation is a fancy term to describe the grouping of messages and data within something we call an object, such that other objects can't see the data and can only get access to it via messages.  The reason for the emphasis on this topic is that this is a big difference from the way that procedural programming traditionally viewed programs.  Traditionally, the data and the methods for changing the data were two very separate beasties.  Often, when these are in two different parts of your program, they get out of synch and it's very hard to maintain the functions that manipulate the data when the structure of the data changes or vice versa.  This is one of the problems that OO programming tries to address, by keeping the data and the methods for changing the data close, it's easier to keep them in synch.  In fact, if you change how the data is stored in an object, or the method by which you change that data, any other objects are none the wiser.  This is a Good Thing, as when you make changes, you make them in one spot, as opposed to many spots.
    So, though we could guess at what aPerson's height is, we don't really know until we ask them whatIsYourHeight.  Now, the person could respond by remembering the last time he walked past a height marker in the local Quick-E-Mart.  After a number of times of asking their height, they realize that maybe they should give a better answer, so they change the method of their respone by checking their height against a measuring tape.  To us, we had no idea that how they determined their answer changed, and that's good as we really don't care how they determine it, we only care about the answer.

Responsibilities

    A Very Key thing in OO programming, is considering the responsibilities that an object should have.  Just like a real world person, aPerson object also has responsibilities.  In our example, aPerson is rather lucky, as they only have the responsibility of singing or answering their height.  They don't have the changeTheStinkyBaby responsibility.
    Figuring out the appropriate responsibilities for objects is one of the key things in OO programming.  If you don't have appropriate responsibilites, you run into problems like object bloat.  This is when an object does too many things and is 'spread too thin'.  A jack of all trades does everything pretty poorly.  On the other hand though, you need to strike a good balance, as a specialist that is too specialized does nothing very well, and it takes a huge number of specialists to do anything.
    Did I mention figuring out the appropriate responsibilities for objects is one of the key things in OO programming?

Putting it all together

    I've been holding off running any code thus far, as I wanted you to concentrate on the concepts.  In the same vein, I'm going to hold off describing how to make a Person class to concentrate on other concepts first.  We'll get to stepping through creating a Person class in the article after next, as I want to cover inheritence, polymorphism and abstract classes first.  In the meantime though, I've included the source code below, if you're curious and want to peek or if you want to compare the code against other languages you know.
    To load the code, we need to file it in to Squeak.  If you're reading this remotely, you need to first download the code from here, rename it to remove the ".txt" extension, save it where ever you want.  Now open the file list (Menus>open...>file list), find your downloaded file (Person-Article2.st), left click on the file in the upper right corner to select, then middle click>fileIn. For the read-along folks, the file browser looks like:

    Now, you can go back to the above code and execute it.  If I was really motivated/sadistic, for the singing parts I could have actually recorded myself singing the songs and have the commands play them.  However, I took the obvious shortcut and just opened windows with the song text in them.
    To exeute code, highlight the code, middle click>do it  (or you can hit Alt-d).  Try doing one line at a time, or multiple lines at a time.  You'll note with the second example that uses a temporary variable, you'll have to highlight multiple lines to get that temporary variable included in execution.  Try commenting out parts with  "double quotes" - double quotes, are the Smalltalk comment indicator.  ie:  "This is a comment"  You'll notice that when we comment out code and re-execute that we didn't need to recompile, which is nice... compiling is so passé and time consuming.
    For the read-along folk, when you execute, this is what you'll see:

(Person new) sing.

(Person new) sing: 'MaryHadALittleLamb'.

(Person new) sing: 'MaryHadALittleLamb' andDoIt: 'loudly'.

(Person new) sing: 'MaryHadALittleLamb' andDoIt: 'quietly'.

(Person new) whatIsMyHeight.

 

Looking forward

    The next article will cover inheritence, polymorphism, and abstract classes as well as introducing the collection classes. Note:  this time around, the sweet squeak is going to do some explaining, so be sure to read that section.

All I learned about Smalltalk syntax I learned from a postcard

This is an aside really, if you're coming into Smalltalk cold and are confused by this table, don't worry about it as we'll be covering this as we go along.  This is here for the curious or the folks who have other programming experience to compare.

A Sweet Squeak

    This section typically won't explore/explain code or example, but this time we'll make an exception.  This time, we're going to play with numbers, as that's a common thing for introductory programming articles/books to do and is an easy way to compare languages.  People with some programming experience will appreciate this more, people with no programming experience will wonder why all languages can't do this.
    Let's start of with factorials.  For those not familiar with a factorial, it's most easily described by examples:
        1 factorial = 1
        2 factorial = 2*1
        3 factorial = 3*2*1
        4 factorial = 4*3*2*1
    When you do the below snippet, you won't see anything happen.  That's because the below code doesn't open any windows to report back it's results.  To see it's results, you can do to things instead of doing it: "Try printing this, you'll see the answer '120' printed in the workspace"
   5 factorial.

"Now print this, and you'll see a very large number as the result, since it's 1067 digits long, I'm not going to paste it in here.  Note, this takes 5.9 seconds to run on my P200, which is pretty respectable performance.
    Also note the size of the numbers you can work with - you don't have the usual predefined fixed limits such as an int that has the range from -2,147,483,648 to 2,147,483,647."
    1000 factorial

"If you want to and have the time, just for grins try 10000 factorial (I didn't have the patience to run this on my machine, even in another thread)"

"For the curious, no I didn't count the number of digits returned from 1000 factorial, since the message factorial returns a LargeInteger, we can just ask that LargeInteger what size it is."
    (1000 factorial) size

"If you want to check that the correct numbers are actually being computed, try this and it should give you an answer of 1000"
    1000 factorial // 999 factorial

"Looking for what kind of precision you can get?  Try:"
    123443/5432
"The interesting thing you'll note is that it returns a Fraction!  No rounding off to the first 5 decimal places by default.  Instead of printing it, try inspecting this guy, you'll see a Fraction object, with a numerator and denominator just as you'd expect:"

"Of course, you can use floats too, in which case you do get a rounding off - to 14 places give or take depending on the flavour of Smalltalk you're using.  Try this, and you'll get the answer: 22.72426793416332"
    123443.45/5432.23

"Finally, for those curious about how long things take, to time something in Smalltalk you can print this, which will print out the milliseconds it took to run.  These measurements are not even meant to be toy benchmarks, but are just presented for interest."
    Time millisecondsToRun: [100 factorial]
    Time millisecondsToRun: [1000 factorial]
"On my P200, the above lines took:
    0.020 seconds
    5.967 seconds"

    People with some programming experience will notice that we didn't have to fuss with what types of numbers we're working with, (integers, large integers, floats, large floats), or type mismatches, or predefined size limitations, or wrapping primitive types in objects then unwrapping them or any other of this type of nonsense ;-).  We just naturally typed in what we wanted to do without having to do any jumping through hoops for the sake of the computer.  This comes from the power of P&P:  Pure objects and Polymorphism (which we'll discuss next time).

Questions and Answers

These are the answers for questions on previous articles that I could work through in my limited time available.  I picked out the ones I thought most appropriate for the series.  If you want a faster response, or I didn't get to your question, try posting your question to the comp.lang.smalltalk newsgroup, or the Swiki.

Q: Can you show how your examples can be done in Java?
I'll try and answer this without getting on a soapbox (language questions are often equivalent to religous questions).  There's three parts to this answer:

  1. Over the years, I've programmed in a decent number of languages/environments6.  I've been programming in Java off and on since '95, and like any language it has its pros and cons.  However, I don't find programming in Java very fun.  On my day job I'll work in Java, C, C++, etc as needed since they're just tools to get a job done, but for my hobby projects I use Smalltalk.
  2. I find Java is just redoing a lot of stuff that Smalltalk already had (garbage collection, virtual machine, JIT VM, write once run anywhere) with the baggage of trying to be similar to C/C++ (primitive types, large amount of syntax, encouraging functional programming, strong typing).  I don't want to play around with old news for hobby projects, I like to play with new and nifty stuff.
  3. I really like meta programming, which I just can't do in Java or Windoze.
Q: What is a good beginner's Smalltalk book?
A: This really depends on what your motivations are.  For Squeak, I don't know of any good beginner's books as all the material I've seen on it has been free online resources (even better than a book IMHO) Be sure to check out The Squeak FAQ (This is also a Wiki, so the cool thing is that you can post your own questions to a living document).
    Personally, I've found many beginner Smalltalk books to be written at too simple of a level.  If pressed, I'd have to say my favourite introductory Smalltalk book is Smalltalk by Example, by Alec Sharp, ISBN 0-07-913036-4.  It's geared towards beginner->intermediate topics for VisualWorks Smalltalk.  If you want you can get the NonCommercial version of VisualWorks to play in, though many of his examples should work in Squeak.

Q: What is a skin?
A: A skin is an installable look-n-feel or theme.  In squeak you can install a Windoze look-n-feel, MacOS Aqua look-n-feel, etc.  (not sure how many skins are out there or what state they're in).  I remember VisualWorks Smalltalk having the skins concept back in '94 (wasn't called a skin back then)- it's one of the things about Smalltalk that first caught my eye.  At the time I had just spent a year doing a very painful port of OpenWindows to Motif for parts of a C based application, then I stolled past a coworker's desk and they showed me how they could switch the look-n-feel of their Smalltalk application from Windoze to Motif to MacOS with a click of the mouse.  Talk about a productivity boost!

Q: Can you have Smalltalk run in web browsers?
A: You certainly can, in fact I thought about setting up a Squeaklet that people could execute the snippets from this series in from the comfort of their web browsers... yeah, you can have a development environment in your web browser, not just runtime code.  However, it was just one more thing for me to do in my limited time and I decided to forgo it for now.  This is a possible future topic.  BTW - most flavours of Smalltalk have some mechanism to run thin clients in a web browser.

Q: Where is the 'main' function?
A: Smalltalk doesn't have a 'main' function, this can be confusing to Smalltalk newbies as so many other languages have this notion.  Conceptually, Smalltalk is an always running set of live objects which is why there is no 'main' function - if your enviroment is always running, having a 'main' function is nonsensical as you're not starting or ending anywhere.  When you want to start an application you've written, you merely ask it to open up its window that it uses as a starting point.  When you deliver an application, you merely open up your application's starting window and package your environment (this is a simplification here).
    Realistically though, you have to have some starting point as you need to shut down your computer sometimes.  Well, Smalltalk does what is called saving an image.  It's called an image because what you're saving is really a snapshot in time of your environment.  When you start it up again, everything is exactly where you left it.  To do this, Smalltalk has some bootstrap code to get itself going again, which could technically be considered a 'main' function.  However, the point is that you do not have a 'main' function when writing an application.

Article Glossary

This is a glossary of terms that I've used for the first time in this series, or a term that I want to refine.  If you don't see a term defined here, try the ongoing glossary in the local location: [LL].
Class
        (def 1-simple)  You can think of a class as a blueprint object for making objects.
Encapsulation
        The grouping of messages and data within something we call an object, such that other objects can't see the data and can only get access to it via messages.
File it in
      The act of loading Smalltalk code into Squeak.
Instance Variable
        (def 1-simple)  The place where objects store their properties/characteristics
Instantiate
        {In-stan-shee-ate} When we ask a class to make a new instance of an object, we say that we're instantiating that object.
Message
        (def 1-simple)  A request you can ask of an object.
Method
        (def 1-simple)  Determines how an object will respond to a message.  The method in which an object responds is determined by a method.
Object
        (def 2) an object is a grouping of messages and data that its messages can operate on.
Object bloat
        (def1 - simple) When an object does too many things and is 'spread too thin'.  A jack of all trades does everything pretty poorly.
Responsibility
        The things that an object can/should do.
Toy benchmark
        A benchmark is a method of measuring the performance of something, and a toy benchmark is a trivial benchmark that doesn't give a good refection of performance as it's too simple or too narrow.
Variable
        (def 1-simple)  A holding reference for something, for example, a holding space for an object.  It gives you a handle to refer to that something that it is holding on to for you.

Footnotes

[1] As Eric Clayberg once pointed out in comp.lang.smalltalk, technically speaking Smalltalk has no reserved words, since you can create methods using these reserved names (though you sure wouldn't want to!)  Though I agree on this technicality, I include these words, as for practical purposes they are reserved.
[2] Does it show I'm a Canuck?
[3] Actually, in practice we'd probably just name the method height, with the whatIsYour or whatIsMy height implied.
[4] Properties of objects can be stored in other places too, but I'm not going into that now, this is a very common place to store things.
[5] See [1]
[6] Smalltalk (VisualWorks, VSE, VisualAge, Squeak), Java (VisualAge, J++), C, C++, Tcl/Tk/Expect, sh/ksh/csh scripting, Turbo Pascal, Fortran. / Solaris, SunOS, HP-UX, Linux, Windows NT, 95, & 3.1, Mac OS

Statement of purpose

    When I wrote the first Making Smalltalk with the Penguin article back in March of 2000 [LL], my target audience was experienced programmers who didn't have much exposure to OO programming or to Smalltalk.  The article's intent was to give an overview of my favourite programming language on my favourite operating system.  Since then, I've had a fair amount of email asking introductory type questions about Smalltalk and OO programming.  So I thought I'd try my hand at a small series.
    The target audience for this series are people new to OO or new to programming altogether.  The intent is to not only introduce OO programming, but to also spread the fun of Smalltalking.  Why do this format/effort when there's lots of good reference material out there?  Two reasons really:  1) Tutorials are great, but can be static and dated pretty quickly.  2) An ongoing series tends to be more engaging and digestible.
    To help address the second reason above, my intent is to keep the articles concise so they can be digested in under an hour.  Hopefully, as newbies follow along, they can refer back to the original article and make more sense of it.  I plan on having a touch of advanced stuff once in a while to add flavour and as before, the articles are going to be written for read-along or code-along people.
    Something new I'm going to try is to make the ongoing series viewable in a contiguous fashion and downloadable in one chunk for people who want to browse the series locally.  To do this, click on TOC grapic to at the top of the article.  The articles also have 2 sets of links:  one set for www links, another set for local links, indicated as: [LL] for downloaded local reading.

Why Smalltalk?

    I believe Smalltalk is the best environment to learn OO programming in because: In particular, I'm going to use Squeak as the playing vehicle.  You'll notice this is a different flavour of Smalltalk than I used in my first article.  I've never used Squeak before, so this'll be a learning experience for me too.  The reasons for this are:

Person Sample Smalltalk Code

This is a sample of what the Smalltalk code looks like for the curious or for people who want to compare with known languages.  For people who are confused by below code, don't worry, as we'll be stepping through how you create it and what it means in a future article.

"This is a Class definition"

Object subclass: #Person
 instanceVariableNames: ''
 classVariableNames: ''
 poolDictionaries: ''
 category: 'MakingSmalltalk-Article2'


"My Characteristics is a category of methods for the class (similar to an interface in Java (but it's not enforced))"

Person methodsFor: 'My Characteristics'
"The 1 method in the My Characteristics category"
whatIsMyHeight
 "Actually, in practice we'd probably just name this method 'height', with the 'whatIsMy' implied.
 Simple example to show how a query about my characteristic can be done.  Ah-ha - notice that the height is not being returned via an instance variable as we guessed at above, but is in fact hardcoded... A BAD PRACTICE TO DO, but is fine for this example to keep things simple, and wanted to show how to do a ' in a string"

 (Workspace new contents: 'My height is 5'' 7"') openLabel: 'This is my height'! !

"This is the singing category, it has 6 methods"
Person methodsFor: 'Singing'
"And the methods for singing - method 1 of 6"
maryHadALittleLambLyrics

 ^'Mary had a little lamb, little lamb, little lamb, Mary had a little lamb whose fleece was white as snow.'

"singing method 2 of 6, we use the 'my' prefix convention to indicate a private method"
mySing: someLyrics inManner: anAdjective withTitle: aTitle
 "Using simple logic here for illustrative purposes - if the adjective is not 'loudly' or 'quietly' just ignore how we're being asked to sing"

 | tmpLyrics |
 anAdjective = 'loudly'
  ifTrue: [tmpLyrics := someLyrics asUppercase].
 anAdjective = 'quietly'
  ifTrue: [tmpLyrics := someLyrics asLowercase].
 self mySing: tmpLyrics withTitle: aTitle

"singing method 3 of 6"
mySing: someLyrics withTitle: aTitle

 (Workspace new contents: someLyrics) openLabel: aTitle

"singing method 4 of 6"
sing

 self mySing: 'Do be do be doooooo.' withTitle: 'A bad impression of Sinatra'

"singing method 5 of 6"
sing: aSong

 aSong = 'MaryHadALittleLamb'
  ifTrue: [self mySing: self maryHadALittleLambLyrics withTitle: 'Mary had a little lamb']
  ifFalse: [self sing].

"singing method 6 of 6"
sing: aSong andDoIt: anAdjective

 aSong = 'MaryHadALittleLamb'
  ifTrue: [self mySing: self maryHadALittleLambLyrics inManner: anAdjective withTitle: 'Mary had a little lamb']
  ifFalse: [self sing].

Series Table of Contents

Previous Articles

The image on the right links to Steffler's site.

Copyright © 2000, Jason Steffler.
Copying license http://www.linuxgazette.net/copying.html
Published in Issue 60 of Linux Gazette, December 2000

[ Prev ][ Table of Contents ][ Front Page ][ Talkback ][ FAQ ][ Next ]