Smalltalk and Object Orientation: an Introduction - Free

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A good discussion of incremental testing for C++ c an be found in [Harrold, McGregor and Fitzpatrick 1992]. While [Feigenbaum 1995] discusses some of the features which could be introduced into Smalltalk which would improve its potential as a development language. The Syntropy object oriented development m ethod makes explicit use of assertions. This method is documented in [Cook and Daniels 1994]. There are also many references which discuss various aspects of testing which are as appropriate for object oriented systems as they are for procedural systems. For example, [Beizer 1990], [Freedman 1991], [Myers 1979], [Pyle 1991] and [ISO 1993]. 194
24. Smalltalk Style Guidelines 24.1 Introduction Good programming style in any language helps promote the readability, understandability and clarity of your code. In many languages there are established standards to which many people adhere (sometimes without realizing it). For example, the way in which a C or Pascal program is indented is such a standard. However, style guidelines which have evolved for these proc edural languages do not cover many of the issues which are important in Smalltalk. These are issues relating to classes, instances and methods. As non-object oriented languages do not have these concepts, they do not have standards for dealing with them. Of course you should not forget all the pearls of wisdom and programming practices that you have learnt using other languages. Just as in any other language there are a number of acknowledged bad practices (which are not specific to Smalltalk), for example, the use of global variables! In this chapter (as in the whole of the book) we will assume therefore that you do not need to have the basic concepts explained, instead we will try to concentrate on those style issues which are specific to Smalltalk. The re mainder of the chapter is structured in the following manner: Section two considers style issues associated with variables including temporary variables, instance variables as well as class, class instance and pool variables. Section three provides some gu idance on the use of classes within Smalltalk. Section four considers methods, their naming, use and code placement within methods. Section five discusses how to name message protocols for both instance and class methods. 24.2 Variables 24.2.1 Naming variables In Smalltalk variable names such as t1, i, j, or temp should rarely be used. Instead variable names should either be descriptive (known as semantic variables) or should indicate the type of object which the variable will hold (known as typed variables). Which app roach should be chosen depends on both personal style and the situation in which the variables are being used. Instance and class variables, for example, tend to have semantic-oriented names, while parameters to methods often have typed names. The semantic approach has the advantage that less needs to be assumed about what the variable will be used for. Since subclasses can inherit instance and class variables, the point at which they are defined and the point at which they are used may be very remote. Thu s any contextual meaning and commentary, provided with their definition, will have been lost. Examples, of semantic variable names include: score currentWorkingMemory TotalPopulationSize The typed approach is often adopted for parameter names as it is ea sier to see what type of object is required as an argument to a method. For example: add: anObject ifAbsent: aBlock at: anIndex put: anObject Although some methods mix the use of the two, for example: at: key put: anObject . 195
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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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Page 159 and 160: 19.3.2.2 Prepare a data dictionary
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Page 163 and 164: 20.2.1.3 Prepare sequence and colla
Page 165 and 166: 20.2.2.5 Specify optimization crite
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Page 169 and 170: 20.3.2.3 Design optimization The an
Page 171 and 172: 21. Frameworks and Patterns for Obj
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Page 179 and 180: 21.5.5 Running the editor You have
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Page 189 and 190: 22.7.2 The Perform: message The per
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Page 197 and 198: 24.2.4 Adding variables to system c
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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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