Smalltalk and Object Orientation: an Introduction - Free

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will become second nature to many once they have worked with object oriented systems for a while. The key thing is to try things out as you go along and if possible have someone around who understands a bit about object orientation - they can often illuminate and simplify an otherwise gloomy network of tunnels. 1.12 Further reading There are of course a great many books available on object orientation some of the best known include [Booch 1994; Budd 1991; Wirfs-Brock et al 1990; Cox and Novobilski 1991]. An excellent book aimed at managers and senior programmers who want to learn how to apply object oriented technology successfully to their projects is [Booch 1996]. Another good book in a similar style is [Yourdon 1994]. Other books which may be of interest to those attempting to convince themselves or others that object technology can actually work are [Harmon and Taylor 1993], [Love 1993] and [Meyer and Nerson 1993]. Other places to find useful r eferences are the Journal of Object Oriented Programming, Pub. SIGS Publications, ISSN 0896-8438 and the OOPSLA conferences. These are a set of world wide conferences on Object Oriented Programming: Systems, Languages and Applications (hence OOPSLA). They are held every year, references for some recent ones are listed at the back of this book. There is also a European Conference on Object Oriented Programming called ECOOP. Some of these conference proceedings are also listed at the back of this book. For further reading on the software crisis and approaches aimed at solving it see [Brooks 1987] and [Cox 1990]. For a discussion of the nature of scientific discovery, refinement and revolution see [Kuhn 1962]. 24
2. Elements of Object Orientation 2.1 Introduction This chapter is intended to reinforce what you have already learnt in Chapter 1. It concisely defines the terminology introduced in the last chapter and attempts to clarify issues associated with hierarchies. It also discusses some of the perceived strengths an d weaknesses of the object oriented approach. Some guidance on the approach to take in learning about objects is also offered. 2.2 Terminology In Chapter 1 a number of terms were introduced during the discussion of object orientation recap on those terms and introduce a number of new ones. . Here we Class. A class is a definition for a combination of data and procedures which operate on those procedures. Instances of other classes can only access that data or those proc edures via specified interfaces. A class acts as a template when creating new instances. That is, a class does not hold any data, the data is held in the instance. However, the class specifies what data will be held. This will be considered in more detail in Part 2 of this book. The relationship between a class, its superclass and any subclasses is illustrated in Figure 2.1. Superclass Class Subclass 1 Subclass 2 Subclass n Figure 2.1: The relationship between Class, Superclass and Subclass Subclass. A subclass is a class which inherits from another class. For example, in the last chapter, Undergraduate-Student was a subclass of Student. Subclasses are of course classes in their own right. The term subclass merely indicates what is inherited by what. Any class can have any number of subclasses. Superclass. A superclass is the parent of a class. It is the class from which the current class inherits. For example, in the last chapter, Student class was the superclass of Undergraduate- Student. In Smalltalk a class can only have one superclass. Instance / Object. An instance is an example of a class. All instances of a class possess t he same data variables but have their own data in these data variables. Each instance of a class will also respond to the same set of requests. Instance variable. This is the special name given to the data which is held by an obje ct. The “state” of an object at any particular moment relates to the current values held by its instance variables. (In Smalltalk there are also class variable s but a discussion of these will be left until later). Figure 2.2 illustrates a definition for a class in pseudo code. This definition includes some instance variable definitions fuel, mileage and name. Method. Method is the name given to a procedure defined within an object. T he name stems from its use in early versions of Smalltalk where it was a method used to get an object to do something 25
Page 1 and 2: Smalltalk and Object Orientation: A
Page 3 and 4: Chapter 7: Smalltalk Constructs Thi
Page 5 and 6: The user interface construction fac
Page 7 and 8: Preface 1. INTRODUCTION TO OBJECT O
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Page 13: Part One Introduction to Object Ori
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Page 45 and 46: other systems. As the browsers, ins
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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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Figure 27.17: Defining the parent c
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important, as these facilities hand
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28.3 The AddressBook class 28.3.1 T
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The aspect methods can then be simp
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the name is in the address book, it
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29.2 The custom view widget 29.2.1
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DrawingView model SmallDraw drawing
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The action of adding a new graphic
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The next method we shall consider i
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In the updating protocol we need to
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The displaying protocol contains th
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30. Memory Management and Garbage C
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30.3.3 PermSpace This is used to ho
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31. Concurrency in Smalltalk 31.1 I
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The Semaphore class provides facili
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Once the process wakes up it can de
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31.4.3 Acknowledgments Acknowledgme
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“There are two kinds of objects i
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Object Object class Collection Coll
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ehavior, but from the point of view
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32.5.5 The whole enchilada Behaviou
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33. The Future for Object Technolog
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solutions are needed which suit the
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scripting and query language to use
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34. Appendix: The Smalltalk Languag
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A capitalization convention is used
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• marries: is the message selecto
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Array OrderedCollection SortedColle
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[Coleman et al 1994] D. Coleman, P.
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OOPSLA/ECOOP’90, Joint Conference
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detect.............................
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True...............................
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