Smalltalk and Object Orientation: an Introduction - Free

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30.3 VisualWorks memory management Maximum Memory OldSpace PermSpace Object Memory LargeSpace NewSpace Object Engine memory Object Engine 0 Virtual Machine Figure 30.1: Memory organization in VisualWorks VisualWorks provides automatic memory management. Essentially it does so by allocating a portion of memory as and when required. Then, when memory is short it searches through memory looking for areas which are no longer referenced. These areas of memory are then freed up (deallocated) so that they can be re -allocated. This process is often referred to as “garbage collection”. A second process, invoked with garbage collection is memory compaction. This involves moving all the allocated memory blocks together so that free memory is contiguous rather than fragmented. The Smalltalk memory is divided up as shown in Figure 30.1. As can be seen from this figure there is an area of memory allocated to the Virtual Machine (the object engine). This area, below the black line, holds both the virtual machine and the memory it uses. This area is outside the scope of this book and will therefore not be considered in greater detail. The regions in the Object Memory are used to hold the objects created by the system and by user programs. It is useful to distinguish between parts of this memory, not least because the VisualLauncher offers the user two different types of garbage collection, one called “Garbage Collection” and one called “Global Garbage Collection”. The options relate to which parts of memory are processed. 30.3.1 NewSpace and OldSpace NewSpace is fixed in size (the default for NewSpace is about 200 K) and is used to hold newly created objects. When the amount of memory used in NewSpace crosses the scavenge threshold , the “generation scavenger” is called. This is a program which frees up memory from objects wh ich are no longer referenced. This memory can then be used again. If an object has survived for long enough it may get moved in OldSpace. References to objects in OldSpace are not quite as fast as those to objects in NewSpace, however, OldSpace is not fixe d in size and can grow as long as there is memory available. The scavenger does not operate in OldSpace as it is optimized for NewSpace handling objects which are rapidly created and then de-referenced. 30.3.2 LargeSpace This is a special area of memory for very large byte objects (typically larger than 1 KB) e.g. bitmaps. This saves the need to move large areas of memory around because of small frequently used areas of memory being repeatedly allocated and deallocated. Objects in LargeSpace are only freed up or moved when LargeSpace is compacted as part of a “compacting garbage collection” or when LargeSpace has filled up. 258
30.3.3 PermSpace This is used to house objects which are rarely eligible for garbage collection. Examples of such objects are the system classes, tools and compilers. None of the garbage collections attempt to process this area of memory except the “Global garbage collector”. This is initiated by the user from the VisualLauncher. Again this is a performance “tweak” and means that most of the time items such as a class do not have to be checked to see if the memory they occupy should be reclaimed nor must PermSpace be compacted. 30.4 Garbage collection and memory compaction There are a number of different ways in which garbage collection and memory compaction can occur. The first is the “Generation Scavenger”. This is called whenever NewSpace exceeds the scavenge threshold. It only processes objects in NewSpace (although it does place objects in OldSpace). Another garbage collector is the incremental garbage collector (or IGC). This is called from Smalltalk code (as opposed to the scavenger which is called automatically by the Virtual Machine). The IGC reclaims objects in OldSpace, NewSpace and LargeSpace. It is called the incremental garbage collector because it works in an incremental manner to avoid disturbing the executing VisualWorks environment. The two remaining garbage collectors are the “Compacting Garbage Collector” and the “Global Garbage Collector”. These two garbage collect ors are under user control and are initiated from the VisualLauncher. The compacting garbage collector is run when the user selects the “Garbage Collection” option on the special menu on the Launcher. The global garbage collector is run when the user select the “Global Garbage Collection” option on the same menu (and processes all of the memory available). Finally, there is a data compactor which runs on the OldSpace which does not do any garbage collection, only memory compaction. This runs considerably faster than the garbage collectors. 30.5 Placing objects in PermSpace PermSpace is an error of memory which is intended to hold objects which will never die or which will only rarely die. This means that most of the garbage collection methods do not need to worry about what is in PermSpace, thus reducing the amount of memory they must scan. Obviously some of the objects in PermSpace may die off, the global garbage collector therefore scans PermSpace as well. It is possible to move objects from OldSpace to PermSpac e to increase the performance of the garbage collector. This effect is achieved because by moving objects out of OldSpace the number of objects which must be considered is reduced. This can have a major effect on the performance of the garbage collector. When you start up your “clean” image, most of the objects (including classes) present in the system will be located in PermSpace. When you create new objects these will reside in NewSpace or OldSpace. To relocate them into PermSpace it is necessary to save the current image using the “Perm Save As ...” option from the “ File” menu option in the VisualLauncher. This writes the objects in OldSpace into PermSpace in the saved image. It does not affect where objects are located in the current (executing) image, only in the saved image. It is therefore necessary to exit the image and start up the newly saved image to have an executing image, in which the objects are stored in PermSpace. There will however be a number of objects in PermSpace which are transient (i.e . likely to die in the short term). These objects will have been placed into PermSpace, because all objects, whether or not you wanted them to, will have been saved into PermSpace. Example of such objects may included windows, objects with which you were w orking etc. To remove these from the image (thus improving the performance of the garbage collector) perform a global garbage collection. Once you have done this save the image in the normal manner (i.e. “ Save as...”). Having started up the newly saved ima ge you could increase the time taken to start the image by performing one last save of the image. This is necessary because the global garbage collector compacts the objects in PermSpace. This means that when the image is started up, the system must reloca te these objects. If you make one more image, then the objects will already have been relocated and the startup time will be minimized. 259
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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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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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Page 273 and 274: ehavior, but from the point of view
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Page 292 and 293: [Coleman et al 1994] D. Coleman, P.
Page 294 and 295: OOPSLA/ECOOP’90, Joint Conference
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