The Design of Smalltalk

1
Introduction to Smalltalk

• The Design of Smalltalk
• Understanding Code

2
Documentation

• Smalltalk with Style guidelines for writing
  code that complies with the common idioms of
  Smalltalk (on-line)
• Visualworks
• Application Developers Guide
• Basic Libraries
• GUI Developers Guide

3
Sample Code

• factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial

4
Sample Code

• factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• The Method Name

5
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• A Comment

6
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• A String

7
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Referring to the object itself

8
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Periods separate statements (can skip on last
  line)

9
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Return the value from the method

10
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Blocks

11
The Design of Smalltalk

• Smalltalk is really simple
• It was meant to be
• In its simplicity lies its greatness
• We aim to make simple things simple and complex
  things possible. Alan Kay
• Foundations of Smalltalk
• Everything is an object
• Class-based inheritance
• All computation is done through message passing
• Modeled on English (e.g., periods end statements)

12
Everything is an Object

• There are no exceptions to the object rule.
• Numbers, Booleans, etc.
• nil is an object (it is the only instance of
  ProtoObject, the root of the class inheritance
  tree)
• Even every class is an object
• Blocks and methods are objects too

13
Class-Based Inheritance

• Smalltalk is the Canonical Class-Based
  Object-Oriented Language
• Every object is an instance of some class
• All classes (except ProtoObject) have a parent
  class
• Parent is superclass
• Child is subclass
• Subclasses inherit behavior and structure from
  parent class

14
Class Hierarchy SmallInteger

• Magnitude
• Character
• Date
• Number
• Float
• Fraction
• Integer
• SmallInteger
• LargePositiveInteger
• LargeNegativeInteger
• Time

15
Lets Make a Class
16
Message Passing

• All computation is triggered through message
  sends (sending a message to an object)
• Messages trigger methods
• Almost all of Smalltalk is ltreceiverObjectgt
  ltmessagegt
• 5 factorial
• 5 is the receiver object, factorial is the
  message
• 5 3
• 5 is the receiver object, is the message, 3 is
  an argument
• How do we get from the message to the method?

17
Message Passing

• Answer Through the Class Hierarchy
• Class determines behavior.
• Check the class of the receiver object for the
  method. If not found, check the parent. Go
  through the class hierarchy until found or return
  a does not understand error.
• 5 factorial
• 5 is a SmallInteger. Its parent (Integer)
  implements factorial.
• 5 3
• 5 is a SmallInteger, which implements

18
Message Passing

• There can be more than one method implementing
  the same message
• Which method gets executed is based on the class
  of the receiver
• Methods in Object (like printString) can be sent
  to any object
• The decision is made at runtime
• This is late-binding
• More than one kind of object can respond to the
  same message in its own way
• 5 3 and 5.1 3.2 are very different methods
• This is polymorphism

19
Message Passing

• All computation is triggered through message
  passing
• Even control structures are handled as message
  sends

20
Control Structures

• test ifTrue Execute if test is true
• test ifTrue Execute if test is true
• ifFalseExecute if test is false
• 1 to 10 do i Iterate for i equals 1 to
  10
• test whileTrue Repeat this until test is
  false

21
Control Structures

• test ifTrue Execute if test is true
• 1 to 10 do i Iterate for i equals 1 to
  10
• test whileTrue Repeat this until test is
  false
• Receiver
• Message (ifTrue ifTrueifFalse todo
  whileTrue)
• Argument(s)

22
Control Structures

• test ifTrue Execute if test is true
• 1 to 10 do i Iterate for i equals 1 to
  10
• test whileTrue Repeat this until test is
  false
• Remember, everything is an object. Even blocks
  are objects
• Boolean, False, and True implement ifTrue
• Number implements todo
• BlockContext implements whileTrue

23
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Receiver Message Argument

24
Compiler and Blocks

• Compiler is always available (and is of course an
  object)
• Compiler evaluate 3 4 returns 7
• Blocks Code elements that are objects
• aBlock Transcript show foo. blocks can
  be assigned to variables
• aBlock value. do it
• Blocks can take arguments
• asArgumentBlock x x1.
• anArgumentBlock value 5. returns 6

25
3 timesRepeat Transcipt showfoo

• (Integer) timesRepeat aBlock
• Evaluate the argument, aBlock, the number of
  times represented by the receiver.
• count
• count 1.
• count lt self test for while
• whileTrue
• aBlock value. evaluate block
• count count 1 increment count

26
Message Passing

• Message Names are Objects (Symbols)
• factorial todo whileTrue
• You can use them like any other object (save them
  to a variable, etc.)
• The following evaluate the same
• 5 factorial
• 5 perform factorial
• 5 perform perform with factorial

27
Kinds of Operators(Ways to Send Messages)

• Binary
• - / // quotient \\ remainder or mod
  do not short-circuit
• Unary
• abs not sin positive asUppercase
• Keywords with
• aDictionary at aKey put aValue
• todo ifTrueifFalse
• Order of precedence (not algebraic)
• (First to Last) Parenthesis, Unary, Binary,
  Keyword
• First come First served

28
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Unary Operators

29
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Binary Operators

30
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Keyword Selectors

31
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• Figure out the precedent

32
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• 1st Parenthesis

33
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• 2nd Unary (Receiver)

34
Sample Code

• (Integer) factorial
• Returns the factorial of this integer.
• 5 factorial evaluates to 120.
• self lt 0 ifTrue self error Not defined.
• self 0 ifTrue 1.
• self (self - 1) factorial
• 3rd Binary (Receiver) (Argument)

See Application Developers Guide, Chapter 4
35
Class-Based Inheritance

• Instances and classes do not respond to the same
  messages (Java and Smalltalk differ here)
• In Java, instances will respond to class (static)
  methods.
• In Smalltalk, they are separate. The class
  responds to a message with class methods. The
  instances respond to a message with instance
  methods.
• In general, the Smalltalk separation is better
  for good object-oriented programming style. A
  class and its instance arent the same type of
  object therefore, they shouldnt share methods.
  Consider
• rover Dog new.
• rover name Rover.
• brody rover new. Does that make sense?
• brody rover class new. Thats better.

36
Class-Based Inheritance

• Classes are instances too
• Each class has its own metaclass
• The class is an instance of that metaclass
• Because of this, all class methods are actually
  instance methods
• Again, Smalltalk is simple There is only one
  kind of method (instance methods)

37
Class-Based Inheritance

• Accessing an Object
• All instance variables are private
• You cant just reach in and manipulate an object.
  Good OO Programming Ask. Dont Touch.
• You can use simple accessors and mutators when
  appropriate to simulate public variables
• All methods are public
• If a object implements a message, you can send
  that message to the object and it will try to do
  it.
• This can lead to errors, but Smalltalk deals with
  errors fairly well.
• You will occasionally see comments or category
  names warning that a message is really private.

38
Modifying Classes and Methods

• Trust the Programmer
• In Smalltalk, you can modify any class or method.
• You can create new methods on any class.
• So, if you need to add a factorial capability,
  add it as a method to the Integer class. Thats
  where it belongs. Consider the following
• FactorialProcessor factorialFor 5 not good OO
  code Make the object do the work.
• 5 factorial
• 5.0 factorial returns an error.

39
Creating a New ClassUse the SystemBrowser

• Smalltalk defineClass NameOfClass
• superclass NameOfSuperclass
• indexedType none
• private false
• instanceVariableNames instVarName1
  instVarName2
• classVariableNames classVarName1
• imports
• category categoryName

See Application Developers Guide, Chapter 5.
40
Creating a New ClassUse the SystemBrowser

• NOTES
• Namespaces help prevent name pollution.
  Everything is always accessible in Smalltalk, so
  traditionally, there was only one Point class.
  If I wanted to create a Point class, I would need
  to prefix it with the class name, like
  SudokuPoint. Now we can use namespaces to
  achieve a cleaner naming. It is similar to a
  package in Java.
• Packages are ways to organize code in smalltalk,
  but have no effect on name visibility. The
  default package is Smalltalk.
• Superclass of your object. Core.Object is the
  default. Same idea as in Java everything is
  subclass of Object.
• Private if true, this class is only visible or
  usable by other classes in its namespace.
• IndexedType always use none. See Chapter 5 of
  App Dev Guide for more info

41
Variables

• foo bar Local Variables
• foo 1. Assignment
• Variables Any word starting with a letter
• Start with lowercase, unless it is global
• You dont declare variable type
• All variables have a type (class of the object
  they point to)
• Variables start as nil (type ProtoObject)

42
Variables

• Variables point to objects
• Objects with no references are garbage collected
• You can have multiple variables point to the same
  object
• a (1 2 3). An Array with 1, 2, and 3
• b a.
• a at 2 put 75.
• b Returns (1 75 3)
• Making copies (shallow) copy vs. deepCopy
• Checking for sameness
• equality objects have the same value
• equivalence same object

43
CollectionsChoosing a Collection Class

• Array (fixed size, ordered)
• OrderedCollection (like Array, but can grow in
  size)
• SortedCollection (OrderedCollection, but sorted
  according to block)
• Dictionary (hash table)
• Set (unordered, no redundancies)
• Bag (count the number of pieces)

See Basic Libraries, Chapter 1.
44
Collections

• Adding and removing elements
• add addAll remove removeAll
• Testing
• isEmpty includes occurrencesOf
• Enumerating
• squeak do char Transcript show char
  printString
• squeak select char char isVowel uea
• (4 1 6) reject num num even (1)
• (4 1 6) select num num odd (1)
• (4 1 6) collect num num factorial (24 1
  720)
• (4 1 6) detect num num even 4

45
Dictionaries

• Has Key/Value pairs (like a hash table)
• a Dictionary new.
• a at orange put orange.
• a at apple put red.
• a at 5 put 678.
• a keys. Set(orange 5 apple)
• a values. OrderedCollection(678 orange red)
• a at orange. orange
• a keysAndValuesDo key value enumerating

46
Odds and Ends

• Characters
• Examples 1 a .
• String is a collection of Characters
• String concatenation (comma)
• peanut butter, and , jelly peanut butter
  and jelly
• Cascade (semicolon)
• Sending multiple messages to the same object
• Dog new
• name Rover
• breed Mutt
• owner me.
• Point
• 2 _at_ 3 (or Point x 2 y 3)