The document discusses class initialization in object-oriented programming. It explains that class initialization is used to initialize class variables and instance variables when a class is loaded. The class initialization method, typically named "initialize", is automatically called by the system and can be used to set up default values for properties that are shared across all class instances. The document provides an example Date class that defines various class variables for storing things like month names and days in a month, and initializes these values in its Date class>>initialize method.

1. Stéphane Ducasse 1
Stéphane Ducasse
stephane.ducasse@inria.fr
http://stephane.ducasse.free.fr/
Elements of Design -
Class initialization
Class initialization

2. S.Ducasse 2
Class Methods - Class InstanceVariables
Classes (Packet class) represents class (Packet).
Class instance variables are instance variables of class
They should represent the state of class: number of
created instances, number of messages sent,
superclasses, subclasses....
Class methods represent class behavior: instance
creation, class initialization, counting the number of
instances....
If you weaken the second point: class state and
behavior can be used to define common properties
shared by all the instances

3. S.Ducasse 3
Class Initialization
How do we know that all the class behavior has been
loaded?
At the end !
Automatically called by the system at load time or
explicitly by the programmer.
Used to initialize a classVariable, a pool dictionary or
class instance variables.
‘Classname initialize’ at the end of the saved files in
Squeak
In postLoadAction: inVW

4. S.Ducasse 4
Example of class initialization
Magnitude subclass: #Date
instanceVariableNames: 'day year'
classVariableNames:
'DaysInMonth FirstDayOfMonth MonthNames
SecondsInDay WeekDayNames’

5. S.Ducasse 5
Date class>>initialize
Date class>>initialize
"Initialize class variables representing the names of the months and
days and the number of seconds, days in each month, and first day of
each month. "
MonthNames := #(January February March April May
June July August September October November December ).
SecondsInDay := 24 * 60 * 60.
DaysInMonth := #(31 28 31 30 31 30 31 31 30 31 30 31 ).
FirstDayOfMonth := #(1 32 60 91 121 152 182 213 244 274 305 335 ).
WeekDayNames := #(MondayTuesday WednesdayThursday Friday
Saturday Sunday )

6. S.Ducasse 5
Date class>>initialize
Date class>>initialize
"Initialize class variables representing the names of the months and
days and the number of seconds, days in each month, and first day of
each month. "
MonthNames := #(January February March April May
June July August September October November December ).
SecondsInDay := 24 * 60 * 60.
DaysInMonth := #(31 28 31 30 31 30 31 31 30 31 30 31 ).
FirstDayOfMonth := #(1 32 60 91 121 152 182 213 244 274 305 335 ).
WeekDayNames := #(MondayTuesday WednesdayThursday Friday
Saturday Sunday )