Smalltalk and Object Orientation: an Introduction - Free

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A capitalization convention is used consistently throughout the standard image. Most Smallta programmers adhere to this standard which it is therefore advisable to follow: lk • Private variables (instance variables, temporaries) start with an initial lower-case letter. • Shared variables (class variables, global variables, pool variables) start with an initial upper-case letter. Message selectors should also start with a lower -case letter. For both variable names and message selectors, if the name is a combination of two or more words, the convention is to capitalize the first letter of the second word onwards. For example: displayTotalPay returnStudentName 34.2.3 Pseudo variable names A pseudo variable name refers to an object which cannot be changed; these include: • nil. A value used when no other value is appropriate, such as un -initialized variables. nil is the sole instance of class Undefined Object. • true. Represents truth. The sole instance of class True. • false. Represents falsehood. The sole instance of the class False. True and False are subclasses of Boolean which implements Boolean algebra and control structures. In addition there are two pseudo variables whose value changes depending upon the context. They are therefore not constants, however the programmer has no ability to change their value. self. This refers to the receiver of a message i tself. It literally means “the object within which the method is executing”. It is used to send a message to the object requesting the another method defined on the object is executed. super. Also refers to the message receiver, but the method search star ts in the superclass of the class in which super is used. This is often used if the functionality of a method is to be extended rather than overwritten. For example: myMethod: anObject new code before super. super myMethod. new code after super. There is also a pseudo message, which just returns the receiver of the message as the result of evaluating the message. This message is yourself: yourself. A message which returns the receiv er of the message. It can be used in any situation, however its most common use is in a cascade of messages, where it is necessary to return the original receiver of the cascaded messages. 34.2.4 Declaring a variable To declare a temporary variable, place the variable to be declared between two bars, e.g. | x y z | indicates that the variables x, y and z are temporary variables. If you reference an undeclared variable the system will assume you wish to declare a new global variable and do so. Global variables should, by convention, always start with a capital letter. They can be deleted by looking at the Smalltalk system dictionary and deleting the appropriate entry. 286
34.3 Classes and methods 34.3.1 Defining a class The structure of a class definition is: Where NameOfSuperclass subclass: #NameOfClass instanceVariableNames: 'instVarName1 instVarName2' classVariableNames: 'ClassVarName1 ClassVarName2' poolDictionaries: '' category: 'Visual Organiser' NameOfSuperclass is the parent class of the class to be defined. NameOfClass is the name of the class being defined. (Note that at this stage it does not exist and must therefore be preceded by a #). instanceVariableNames is used to list the instance variable to be used with this class. classVariableNames is used to define the class variable to be used with this class. poolDictionaries defines the pool variables to be used with this class. category defines the category in which this class will be placed. Note that this definition may not define all the variables associated wi th this class, for example, it may have inherited instance variables from its parent. 34.3.2 Methods The structure of a method is: Where messagePattern: argument1 additionalPattern: argument2 .... "comment" | temporaries | statements messagePattern and additionalPattern: represents the name of the method. Notice that the method name is divided up amongst the arguments and that a part of the message name which precedes an argument has a trailing colon ‘:’. arguments the names of arguments in the message pattern are accessible within the method. “comment” is a comment describing the operation performed by the method and any other useful information. | temporaries | is used to define variables which are local to the method. They must be declared at the beginning of the method (just after the message pattern) and are initially nil. statements represents any legal set of Smalltalk statements. These statements are used to implement the behaviour of the method. 34.3.3 Message expressions Message expressions describe messages to receivers. The value of the expression is determined by the method it invokes. For example, the following is a message expression: newStatus := thisPerson marries: thatPerson This expression is made up of an assignment and a message exp ression. In turn the message expression is made up of a receiver, a message selector and an argument: • thisPerson is the receiver 287
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Smalltalk and Object Orientation: A
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Chapter 7: Smalltalk Constructs Thi
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The user interface construction fac
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Preface 1. INTRODUCTION TO OBJECT O
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12.5 A CLASS SIDE METHOD...........
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27.3 VISUALWORKS WINDOW PAINTING TO
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Part One Introduction to Object Ori
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interfaces between different parts
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een to ease the transition from one
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encapsulation. Attempting to introd
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is a controversial subject which is
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will become second nature to many o
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or return something. It has since b
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defining the behavior of different
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In time most developers start to ac
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2.4.3.4 Smalltalk environments are
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3. Constructing an Object Oriented
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Wash wipe switch Wiper motor Relay
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Point 2 above is more complicated.
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services working? working? working?
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Part Two The Smalltalk Language 42
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other systems. As the browsers, ins
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VisualAge (a version of Smalltalk w
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4.5.2 The VisualWorks Launcher and
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Class categories. These are group s
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5. A Little Smalltalk 5.1 Introduct
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Notifiers can be displayed under a
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These concepts of messages, receive
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Select the “Debug” option. This
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6. Smalltalk Building Blocks 6.1 In
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6.3.1 Class definitions In Smalltal
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Object Class1 Class 2 method search
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“Now calculate y “ y := x * 23.
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7. Smalltalk Constructs 7.1 Introdu
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They are more efficient for storage
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7.5.3 true, false and nil variables
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anArray at: 1 max: 10. This will tr
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Figure 8.1: Defining a new class 8.
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this convention at the moment, howe
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9. Control and Iteration 9.1 Introd
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9.3.3 Block temporary variables Blo
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The brackets round the 1 to: 10 are
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OrderedCollection class (respective
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holding other objects. Typically, c
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| temp | temp := Set new. temp add:
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Next we will define a message proto
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11. Further Collection Classes 11.1
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for (i = 0; i
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x inspect. This has some similariti
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11.9.3 The reject: message The reje
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• asSortedCollection. This create
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Figure 12.2: The Organizer class de
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12.6 The private-updating protocol
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do: [:item | Transcript show: item
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13. Streams and Files 13.1 Introduc
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Notice that in this example we used
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to reconstruct the data into object
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1. The ability to save the current
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14.3 The class Date Class Date, a s
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aCharacter isAlphabetic. aCharacter
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Breakpoints may be placed in a me t
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If the file does not exist you will
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Figure 15.4: Creating a project To
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Part Three Object Oriented Design 1
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16. Object Oriented Analysis and De
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devolved amongst the objects). This
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driving force behind the whole of t
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17. The Unified Modeling Language 1
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Deployment diagrams . Deployment di
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Window abstract +size : Area = (10,
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Micro Computer 1.. * Monitor System
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connectedTo * Lamp * Weight Sensor
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18. UML: Dynamic Modeling And Deplo
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edisplay() Controller window A Wind
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18.2.3.1 A start point A start poin
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etween nodes and their stereotype.
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OMT is a prescriptive method, it pr
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confirming the users’ needs, thei
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19.3.2.2 Prepare a data dictionary
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car, truck and bus under a supercla
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20.2.1.3 Prepare sequence and colla
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20.2.2.5 Specify optimization crite
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are therefore concurrent and can be
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20.3.2.3 Design optimization The an
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21. Frameworks and Patterns for Obj
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As the framework takes shape you sh
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about a set of classes, than to att
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instanceVariableNames: '' classVari
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21.5.5 Running the editor You have
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Part Four Testing and Style 181
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made worse, because the symbol held
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22.3.2 Incremental testing Incremen
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control the execution of the method
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22.7.2 The Perform: message The per
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From the scenario that a person mig
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23.4 Self testing An approach, sugg
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24. Smalltalk Style Guidelines 24.1
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24.2.4 Adding variables to system c
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should use a dictionary to hold the
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• A method should only send messa
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Table 24.2: Class message categorie
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25. The Perform and Dependency Mech
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• Inheritance (class to class rel
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You can find the methods which impl
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These messages are sent automatical
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26. The Model-View-Controller Archi
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and wit hdrawals and requests for t
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Figure 26.5: Part of the Controller
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model 4. update 1. balance (view) 0
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displayed. The second component add
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27. Graphical User Interface Constr
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27.3.1 Palette The Palette tool pro
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uild a running user interface from
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names) and the actions for buttons
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Edit button, then the user interfac
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27.11 Visual reuse The aim of visua
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