Smalltalk and Object Orientation: an Introduction - Free

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20.3 Design phase The aim of the analysis phase is to identify what needs to be done and not how it is done. The design phase takes the models produced by the analysis and considers how the requirements can be achieved. It is broken down into two sub-phases. The first, called the system design, breaks the overall system down into components called subsystems, while the second, called the object design, moves the models produced towards the implementation. 20.3.1 System design phase The subsystem architecture provides the context within which the more detailed design decisions, made during the object design, are performed. The subsystem decomposition defines an architecture which can be used as the basis by which the detailed design can be partitioned among a number of des igners, thus allowing different designers to work independently on different subsystems. The steps used to generate this architecture are: 1. Organizing the system into subsystems. 2. Identifying concurrency inherent in the problem. 3. Allocating subsystems to processors and tasks. 4. Choosing an approach for management of data stores. 5. Handling access to global resources. 6. Choosing the implementation of control in software. 7. Handling boundary conditions. 8. Setting trade offs between competing priorities. Of course not all these steps will be important for all applications. For example, a batch oriented, purely serial process, probably cannot have much concurrency imposed on it. 20.3.1.1 Breaking the system into subsystems/packages Most systems will be comprised of a number of subs ystems, for example, a payroll system might possess a file subsystem, a calculation subsystem and a printing subsystem. A subsystem is not an object nor a function, but a package of classes, associations, operations, events and constraints that are interrelated and that have a reasonably well -defined and (hopefully) small interface with other subsystems. The package notation in the UML can be used to represent subsystems. A subsystem (or package) is usually characterized by the common (or associated) set of services that it provides. For example, the file package would provide a set of services to do with creating, deleting, opening, reading and writing files. The use case model may be useful in identifying such common services. Each package therefore provid es a well defined interface to the remainder of the system which allows other packages to use its facilities. Such a specified interface also allows the internals of the package to be defined independently of the rest of the system (i.e. it encapsulates th e package). In addition, there should be little or no interactions between objects within the package and objects in another package (except via the specified interfaces). Packages can be hierarchical and may be involved in client-server or peer to peer relationships with other packages. Client -server relationships are easiest to implement and maintain as one package responds to requests from another package and returns results. In peer to peer relationships both packages must be capable of responding to r equests from the other. This can result in unforeseen circularities. 20.3.1.2 Identifying concurrency Concurrency can be very important for improving the efficiency of a system. However, to take full advantage of concurrency the system must be designed around the c oncurrency inherent in the application. This can be done by examining the dynamic model for objects which receive events at the same time or perform any state transitions (and associated actions) without interacting. Such transitions 166
are therefore concurrent and can be placed in separate execution threads without effecting the operation of the system. 20.3.1.3 Allocating subsystems to processors and tasks Each concurrent package should be allocated to an independent process or processor. The system designer must therefore: • estimate performance needs and the resources needed to satisfy them, • choose hardware or software implementations for packages, • allocate packages to processors to satisfy performance needs and minimize inter processor communication, • determine the connectivity of the physical units that implements the packages. A deployment diagram can be used to illustrate the results of this step. 20.3.1.4 Management of data stores The designer must identify appropriate data stores for both internal and external data. Thi s involves identifying the complexity of the data, the size of the data, the type of access to the data (single or multiple users), access times and portability. Having identified these issues decisions can be made about whether data can be held in internal memory, on secondary storage devices, whether it should be held in flat files, relational or object database system. 20.3.1.5 Handling access to global resources The system designer must identify what global resources are required and how access to them can be controlled. Global resources include processors, disk drives, disk space, workstations as well as files, classes and databases. 20.3.1.6 Choosing the implementation of control in software The choice of the internal control mechanism used by the system will be mediate d by the facilities provided by the implementation language. For example, Ada supports concurrent tasks whereas VisualBasic does not. Smalltalk and Java support light weight processes and therefore can be said to mimic concurrent systems. The choices available are: • procedure oriented systems. Such a system represents a procedure calling mechanism in which the flow of control is passed from one procedure / method to another when the first calls the second. • event driven systems . This is the appr oach taken by the dynamic model of the analysis phase. Essentially operations are triggered off by events which are received by objects. Many window based interfaces operate in this manner. • concurrent systems. In which the system exists in several processe s which execute at the same time. Some synchronization between the processes may take place at certain times, but for the majority of the time they are completely separate. 20.3.1.7 Handling boundary conditions There are three primary boundary situations which the designer should consider. These are: initialization, termination and failure. Initialization involves setting the system into an appropriate, clean, steady state. Termination involves ensuring the that the system shuts down in an appropriate manner. Failure involves dealing with unplanned termination of the system cleanly. 167
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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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Page 117 and 118: 1. The ability to save the current
Page 119 and 120: 14.3 The class Date Class Date, a s
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Page 123 and 124: Breakpoints may be placed in a me t
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Page 127 and 128: Figure 15.4: Creating a project To
Page 129 and 130: Part Three Object Oriented Design 1
Page 131 and 132: 16. Object Oriented Analysis and De
Page 133 and 134: devolved amongst the objects). This
Page 135 and 136: driving force behind the whole of t
Page 137 and 138: 17. The Unified Modeling Language 1
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Page 143 and 144: Micro Computer 1.. * Monitor System
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Page 147 and 148: 18. UML: Dynamic Modeling And Deplo
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Page 163 and 164: 20.2.1.3 Prepare sequence and colla
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Page 171 and 172: 21. Frameworks and Patterns for Obj
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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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