Smalltalk and Object Orientation: an Introduction - Free

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13.4.2 Reading from BOSS You can either load the whole contents of a BOSS file, read individual objects from a BOSS file (or if you know the position of the object you are interested in) access a single object from any position in the file. This last technique is much faster than reading n - 1 objects to get to the nth object, but it does assume that you know that the object you are interested in is the nth object. The procedure to follow is presented below. 1. First create a data stream to be used to read the BOSS file. This is typically a chieved by send a readStream message to a file name which represents the name of the BOSS file. 2. Now create a BOSS object by sending an onOld: creation message with the newly created data stream as the parameter. You can optimize this process if you know t hat you are not going to write to the BOSS file. To do this you can use the onOldNoScan: message (rather than the onOld: message). 3. Obtain the objects in the BOSS file using the appropriate access message (e.g. contents or next). 4. Once you have finished reading from the boss file you should close it. The next message listed in item 3 above will return a single object which can then be saved into an appropriate collection / data structure etc. The contents message will return an array of objects which can then be manipulated in the same way as any other array of objects. For example: | valuesFileStream bosFileStream| valuesFileStream := bosFileName asFilename readStream. bosFileStream := BinaryObjectStorage onNew: valuesFileStream. aBag := Bag new. [[bosFileStream atEnd]whileFalse: [aBag add: bosFileStream next]] valueNowOrOnUnwindDo: [valuesFileStream close]. This example illustrates two things, first it illustrates the equivalent of a while not end of file (EOF) style construct for BOSS files. This cons truct is the atEnd message. This message is used to test whether the end of the BOSS file has been reached. It is therefore possible to use it in a while loop which says “read while not end of file”. The other thing this illustrates is the practical use of a whileFalse: loop. Many other languages would force a construct such as not (fileName EOF) onto the programmer. The whileFalse: construct is far more elegant. Finally, the position: message is used to position the BOSS file pointer so that a single objec t can be read from a specific point in the BOSS file. However, a record must have been maintained of what the position is (note the third object stored is unlikely to have been stored at position 3). This message could be used to create a file based versio n of a collection class. We shall look at this in more detail in the next section and so will leave a detailed discussion of this message until then. 13.4.3 Warnings about BOSS files You should only use BOSS files to store data objects and never for interface objects. This is because the objects you BOSS out will lose their reference to onscreen objects. You should also avoid BOSSing out any objects associated with the execution machinery of VisualWorks. Finally, you should take care never to BOSS out a circular references as this can confuse the BOSS system. 13.5 Using files with the Financial Manager This application builds on that carried out in the last chapter. The aim of that FinancialManager application was to keep a record of deposits an d withdrawals from a current account. Associated with this, it kept an up to date balance as well as allowing statements to be printed. A problem with that version of the application was that there was no way of permanently storing the account information. As soon as the FinancialManager object was destroyed (or reference to it lost) then all the information associated with it was also lost. The aim of this exercise is to extend the existing application by adding the following features: 116
1. The ability to save the current statement to a file. 2. The ability to load an existing statement from a file. This means that you should provide two new interfaces to your application, one to load a file and one to save a file. These files will contain a formatted version of the information held in the statement. The format of the text file might be: newline. For example: deposit 24.00 withdraw 13.00 Note you should try to use a BOSS file as this will make accessing the contents of the file much e asier. Other points to note include: • Check to see that the file exists before loading it. • Define a save: method which will save the current statement to the specified file. • Define a load: method which will load a statement into the statement collection. If you are successful then you should be able to evaluate the following in a Workspace and get the same result from both FinancialManager objects. | aFinancialManager anotherFinancialManager| aFinancialManager := FinancialManager new. aFinancialManager deposit: 25.00. aFinancialManager withdraw: 12.00. aFinancialManager withdraw: 8.00. Transcript show: 'The current balance is ' , aFinancialManager balance printString. Transcript cr. aFinanacialManager statement. aFinanacialManager save: 'account1'. anotherFinancialManager := FinancialManager new. anotherFinancialManager load: 'account1'. Transcript show: 'The current balance is ' , anotherFinancialManager balance printString. Transcript cr. anotherFinancialManager statement. 13.6 Summary In this chapter you have encountered streams and their use in file input and output. Many simple Smalltalk applications never need to worry about file access, however, if you are ever going to do something where information needs to be stored or shared between images, th en you are going to need to interact with the host file system. You have now seen the basic facilities available, the Binary Object Streaming Service (BOSS) approach to storing and accessing information in files is particularly useful. You should now spend some time exploring the stream and file facilities available in your Smalltalk system. 117
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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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Page 67 and 68: “Now calculate y “ y := x * 23.
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Page 89 and 90: holding other objects. Typically, c
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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
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Page 137 and 138: 17. The Unified Modeling Language 1
Page 139 and 140: Deployment diagrams . Deployment di
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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 155 and 156: OMT is a prescriptive method, it pr
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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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