Title: The Design of Smalltalk
1Introduction to Smalltalk
- The Design of Smalltalk
- Understanding Code
2Documentation
- 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
3Sample 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
4Sample 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
5Sample 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
6Sample 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
7Sample 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
8Sample 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)
9Sample 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
10Sample 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
11The 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)
12Everything 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
13Class-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
14Class Hierarchy SmallInteger
- Magnitude
- Character
- Date
- Number
- Float
- Fraction
- Integer
- SmallInteger
- LargePositiveInteger
- LargeNegativeInteger
- Time
15Lets Make a Class
16Message 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?
17Message 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
18Message 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
19Message Passing
- All computation is triggered through message
passing - Even control structures are handled as message
sends
20Control Structures
- test ifTrue Execute if test is true
- test ifTrue Execute if test is true
- ifFalse Execute if test is false
- 1 to 10 do i Iterate for i equals 1 to
10 - test whileTrue Repeat this until test is
false
21Control 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)
22Control 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
23Sample 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
24Compiler 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
253 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
26Message 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
27Kinds 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
28Sample 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
29Sample 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
30Sample 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
31Sample 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
32Sample 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
33Sample 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)
34Sample 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
35One More Time
- a string changeFrom 6 2 negated to 3 with
(a a)
a string changeFrom 6 2 to 3 with (a
a) a string changeFrom 4 to 3 with (a
a) 'a saaing'
36Class-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.
37Class-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)
38Class-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.
39Modifying 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.
40Creating 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.
41Creating 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
42Variables
- 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)
43Variables
- 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
44CollectionsChoosing 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.
45Collections
- 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
46Dictionaries
- 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
47Odds 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)