Smalltalk and Object Orientation: an Introduction - Free

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22. Testing Object Oriented Systems 22.1 Introduction Testing object oriented systems is a very important issue as more and more organizations are starting to develop Smalltalk (as well as C++ and Java ) based applications. Many such organizations have been forced to come up with their ow n solutions for assuring the quality of their product. However, little attention has been focused onto this subject at Smalltalk -centered conferences or in Smalltalk literature (see the workshop at OOPSLA-95 for a notable exception). There is a particular scarcity of literature on “how-to” test Smalltalk systems as well as tools to support such testing. For organizations just starting to use Smalltalk for major projects this is a very worrying situation. Object oriented techniques do not (and cannot) guaran tee correct programs. They may well help to produce a better system architecture and an object oriented language may promote a suitable coding style, but these features do not stop a programmer making mistakes. Although this should be self evident, for a l ong time there was a feeling that object oriented systems required less testing than systems constructed with traditional procedural languages, was prevalent (for example see the book describing the OMT method by Rumbaugh et al). Where the testing of object oriented systems has been considered it is often the user interface of the system which has actually been tested in a principled or systematic manner. These tests usually concentrate on overall system functionality, u sability issues (such as the ability of the user to use the system or the speed of response) and stress or exception testing. Stress or exception testing relate to attempting to break the operation of the system by inputting unacceptable data or crashing p art of the system (e.g. an associated relational database system) to ensure that the system can recover from such catastrophic failures. However, this chapter aims to show you that if anything, object oriented systems require more testing, not less testin g than traditional programming languages. In the remainder of this chapter we shall consider what effects inheritance, encapsulation, polymorphism and dynamic typing have on testing as well as approaches to method and class (unit) testing, object integration and system testing. 22.2 Why is testing object oriented systems hard? 22.2.1 An example To illustrate the point consider the following Smalltalk. Let us assume that we have defined a method returnSymbol which returns a symbol based on the current state of an object . Let us also assume that we have defined another method passesTheTest which returns true or false based on the state of its object. We could then write the following method: myMethod: aCollection and: anObject | aVariable | aVariable := anObject returnSymbol. aCollection do: [:item | (item passesTheTest) ifTrue: [item perform: aVariable]]. In this case it is very difficult to determine statically what is going to happen in this method. This is because until the method is executed, we do not k now the content (let alone the type (class) of the objects) in aCollection. We cannot determine therefore which objects must respond to the passesTheTest message. Nor can we determine which objects will return true from this message and thus which objects will cause the ifTrue block to be evaluated. In this example, the situation is
made worse, because the symbol held in aVariable (which is the name of the message to send to the item) is determined by anObject which is an object passed into this method. We therefore do not know: 1. What the contents of aCollection will be 2. Which classes must respond to the message passesTheTest 3. Which instances are likely to result in the ifTrue block being evaluated (this is context dependent). 4. What the message to be sent within the ifTrue block will be (this is content dependent). 5. Which objects must respond to the message we don’t yet know. It could be argued that this is true, but irrelevant. However, the point is that a number of the above issues would be resolved by the co mpiler at compile time in other languages such as Ada or Pascal. These static compile time checks can identify what type of object will be held in the aCollection, if the type of object receiving the message will understand that message etc. Where as in S malltalk, it is left up to the developer to ensure that no problems are likely to occur. As you can see from this very simple (although slightly contrived) example traditional static test generation techniques, although of use in object oriented language s such as Smalltalk, do not provide the whole answer. In the example above, the problems are due to the dynamic typing of Smalltalk and the late binding and polymorphism of object oriented languages. 22.2.2 What makes it hard? Part of the problem for object orien tation is that space of possibilities is so much greater. For example, not only are we concerned about objects sending messages requesting that other objects run specified methods correctly, we are also concerned with which class of object will receive the message and which class of object sent the message as this will therefore have an effect on which method will be used. These issues are related to the polymorphic nature of object oriented languages and the use of abstraction and inheritance as a basic system construction tool. In addition encapsulation leads to a fundamental problem of observability; as the only way to observe the state of an object is through its operations. This is fine for black box testing, but makes white box testing extremely diffic ult. This problem is compounded as traditional control flow analysis techniques are not directly applicable as there is no definite sequential order in which methods will be invoked. That is, another object may request that a method is executed at a point which cannot be statically determined (see example in introduction). Indeed, as the state of the object may affect what the method does, it may not even be possible to determine how the method will behave. Of course this does introduce the issue of “what is correct behavior”, however we will leave the issue of “correctness” until a later section. In traditional programming languages the basic unit of test is usually taken to be procedure or function. However, it should be seen from this brief discussion t hat, the object oriented equivalent i.e. the method, cannot be taken as the basic unit of test. It is affected by the state of the object, possibly by the state of the class (if the class has class variables), may interact in unforeseen ways with other methods and may rely on methods defined elsewhere in the class’s superclass hierarchy. Inheritance also affects testing and introduces the following question “after a change, to what extent should the code currently in the system be re -tested?”. In a traditio nal programming language it is usually quite straight forward to identify what parts of the system code are affected by some change. In an object oriented system it is far less obvious. The implication of the above is that in object oriented programs, suc h as those implemented in Smalltalk, the basic unit of test must be considered to be the class. That is not to say that care is not given to exercising individual methods within a class, merely that the individual methods should not be treated in isolation. 22.2.3 Why a lack of emphasis? Part of the reason for the lack of Smalltalk testing literature or tools is probably due to the background of Smalltalk. In the past it was often used as a single (or at most a few) developer’s language. It was often considered to be a personal development environment or a good prototyping tool. However few 183
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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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Page 137 and 138: 17. The Unified Modeling Language 1
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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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