1. Using UML, Patterns, and Java Object-Oriented Software Engineering Chapter 10, Mapping Models to Code

2. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2 Overview  Object design is situated between system design and implementation. Object design is not very well understood and if not well done, leads to a bad system implementation.  In this lecture, we describe a selection of transformations to illustrate a disciplined approach to implementation to avoid system degradation. 1. Operations on the object model:  Optimizations to address performance requirements 2.Implementation of class model components:  Realization of associations  Realization of operation contracts 3.Realizing entity objects based on selected storage strategy  Mapping the class model to a storage schema

3. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 3 Characteristics of Object Design Activities  Developers perform transformations to the object model to improve its modularity and performance.  Developers transform the associations of the object model into collections of object references, because programming languages do not support the concept of association.  If the programming language does not support contracts, the developer needs to write code for detecting and handling contract violations.  Developers often revise the interface specification to accommodate new requirements from the client.  All these activities are intellectually not challenging  However, they have a repetitive and mechanical flavor that makes them error prone.

4. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 4 State of the Art of Model-based Software Engineering  The Vision  During object design we would like to implement a system that realizes the use cases specified during requirements elicitation and system design.  The Reality  Different developers usually handle contract violations differently.  Undocumented parameters are often added to the API to address a requirement change.  Additional attributes are usually added to the object model, but are not handled by the persistent data management system, possibly because of a miscommunication.  Many improvised code changes and workarounds that eventually yield to the degradation of the system.

5. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 5 Model transformations Source code space Forward engineering Refactoring Reverse engineering Model space Model transformation

6. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 6 Model Transformation Example Object design model before transformation Object design model after transformation: Advertiser +email:Address Player +email:Address LeagueOwner +email:Address PlayerAdvertiserLeagueOwner User +email:Address

7. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 7 Refactoring Example: Pull Up Field public class Player { private String email; //... } public class LeagueOwner { private String eMail; //... } public class Advertiser { private String email_address; //... } public class User { private String email; } public class Player extends User { //... } public class LeagueOwner extends User { //... } public class Advertiser extends User { //... }

8. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 8 Refactoring Example: Pull Up Constructor Body public class User { private String email; } public class Player extends User { public Player(String email) { this.email = email; } public class LeagueOwner extends User{ public LeagueOwner(String email) { this.email = email; } public class Advertiser extendsUser{ public Advertiser(String email) { this.email = email; } public class User { public User(String email) { this.email = email; } } public class Player extends User { public Player(String email) { super(email); } } public class LeagueOwner extends User { public LeagueOwner(String email) { super(email); } } public class Advertiser extends User { public Advertiser(String email) { super(email); } }

9. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 9 Forward Engineering Example public class User { private String email; public String getEmail() { return email; } public void setEmail(String value){ email = value; } public void notify(String msg) { //.... } /* Other methods omitted */ } public class LeagueOwner extends User { private int maxNumLeagues; public int getMaxNumLeagues() { return maxNumLeagues; } public void setMaxNumLeagues (int value) { maxNumLeagues = value; } /* Other methods omitted */ } User LeagueOwner +maxNumLeagues:int Object design model before transformation Source code after transformation +email:String +notify(msg:String)

10. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 10 Other Mapping Activities  Optimizing the Object Design Model  Mapping Associations  Mapping Contracts to Exceptions  Mapping Object Models to Tables

11. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 11 Collapsing an object without interesting behavior PersonSocialSecurity number:String Person SSN:String Object design model before transformation Object design model after transformation ?

12. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 12 Delaying expensive computations Object design model before transformation Object design model after transformation Image filename:String paint() data:byte[] Image filename:String RealImage data:byte[] ImageProxy filename:String image 10..1 paint() ?

13. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 13 Other Mapping Activities Optimizing the Object Design Model  Mapping Associations  Mapping Contracts to Exceptions  Mapping Object Models to Tables

14. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 14 Realization of a unidirectional, one-to-one association AccountAdvertiser 11 Object design model before transformation Source code after transformation public class Advertiser { private Account account; public Advertiser() { account = new Account(); } public Account getAccount() { return account; } } ?

15. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 15 Bidirectional one-to-one association public class Advertiser { /* The account field is initialized * in the constructor and never * modified. */ private Account account; public Advertiser() { account = new Account(this); } public Account getAccount() { return account; } } Account Advertiser 11 Object design model before transformation Source code after transformation public class Account { /* The owner field is initialized * during the constructor and * never modified. */ private Advertiser owner; public Account(owner:Advertiser) { this.owner = owner; } public Advertiser getOwner() { return owner; } }

16. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 16 Bidirectional, one-to-many association public class Advertiser { private Set accounts; public Advertiser() { accounts = new HashSet(); } public void addAccount(Account a) { accounts.add(a); a.setOwner(this); } public void removeAccount(Account a) { accounts.remove(a); a.setOwner(null); } } public class Account { private Advertiser owner; public void setOwner(Advertiser newOwner) { if (owner != newOwner) { Advertiser old = owner; owner = newOwner; if (newOwner != null) newOwner.addAccount(this); if (oldOwner != null) old.removeAccount(this); } } } AdvertiserAccount 1 * Object design model before transformation Source code after transformation

17. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 17 Bidirectional, many-to-many association public class Tournament { private List players; public Tournament() { players = new ArrayList(); } public void addPlayer(Player p) { if (!players.contains(p)) { players.add(p); p.addTournament(this); } public class Player { private List tournaments; public Player() { tournaments = new ArrayList(); } public void addTournament(Tournament t) { if (!tournaments.contains(t)) { tournaments.add(t); t.addPlayer(this); } TournamentPlayer ** Source code after transformation {ordered} Object design model before transformation

18. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 18 Bidirectional qualified association Object design model before forward engineering PlayernickName 0..1 * League Player * * Object design model before transformation League nickName Source code after forward engineering

19. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 19 Bidirectional qualified association (continued) public class League { private Map players; public void addPlayer (String nickName, Player p) { if (!players.containsKey(nickName )) { players.put(nickName, p); p.addLeague(nickName, this); } public class Player { private Map leagues; public void addLeague (String nickName, League l) { if (!leagues.containsKey(l)) { leagues.put(l, nickName); l.addPlayer(nickName, this); } Source code after forward engineering

20. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 20 Transformation of an association class TournamentPlayer ** Object design model before transformation Object design model after transformation: 1 class and two binary associations Statistics +getAverageStat(name) +getTotalStat(name) +updateStats(match) TournamentPlayer ** 11 Statistics +getAverageStat(name) +getTotalStat(name) + updateStats(match)

21. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 21 Other Mapping Activities Optimizing the Object Design Model Mapping Associations  Mapping Contracts to Exceptions  Mapping Object Models to Tables

22. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 22 Exceptions as building blocks for contract violations  Many object-oriented languages, including Java do not include built-in support for contracts.  However, we can use their exception mechanisms as building blocks for signaling and handling contract violations  In Java we use the try-throw-catch mechanism  Example:  Let us assume the acceptPlayer() operation of TournamentControl is invoked with a player who is already part of the Tournament.  In this case acceptPlayer() should throw an exception of type KnownPlayer.  See source code on next slide

23. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 23 The try-throw-catch Mechanism in Java public class TournamentControl { private Tournament tournament; public void addPlayer(Player p) throws KnownPlayerException { if (tournament.isPlayerAccepted(p)) { throw new KnownPlayerException(p); } //... Normal addPlayer behavior } public class TournamentForm { private TournamentControl control; private ArrayList players; public void processPlayerApplications() { // Go through all the players for (Iteration i = players.iterator(); i.hasNext();) { try { // Delegate to the control object. control.acceptPlayer((Player)i.next()); } catch (KnownPlayerException e) { // If an exception was caught, log it to the console ErrorConsole.log(e.getMessage()); }

24. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 24 Implementing a contract For each operation in the contract, do the following  Check precondition: Check the precondition before the beginning of the method with a test that raises an exception if the precondition is false.  Check postcondition: Check the postcondition at the end of the method and raise an exception if the contract is violoated. If more than one postcondition is not satisfied, raise an exception only for the first violation.  Check invariant: Check invariants at the same time as postconditions.  Deal with inheritance: Encapsulate the checking code for preconditions and postconditions into separate methods that can be called from subclasses.

25. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 25 A complete implementation of the Tournament.addPlayer() contract «precondition» !isPlayerAccepted(p) «invariant» getMaxNumPlayers() > 0 «precondition» getNumPlayers() < getMaxNumPlayers() Tournament +isPlayerAccepted(p:Player):boolean +addPlayer(p:Player) +getMaxNumPlayers():int -maxNumPlayers: int +getNumPlayers():int «postcondition» isPlayerAccepted(p)

26. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 26 Heuristics for Mapping Contracts to Exceptions Be pragmatic, if you don’t have enough time.  Omit checking code for postconditions and invariants.  Usually redundant with the code accomplishing the functionality of the class  Not likely to detect many bugs unless written by a separate tester.  Omit the checking code for private and protected methods.  Focus on components with the longest life  Focus on Entity objects, not on boundary objects associated with the user interface.  Reuse constraint checking code.  Many operations have similar preconditions.  Encapsulate constraint checking code into methods so that they can share the same exception classes.

27. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 27 Other Mapping Activities Optimizing the Object Design Model Mapping Associations Mapping Contracts to Exceptions  Mapping Object Models to Tables

28. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 28 Mapping an object model to a relational database  UML object models can be mapped to relational databases:  Some degradation occurs because all UML constructs must be mapped to a single relational database construct - the table.  UML mappings  Each class is mapped to a table  Each class attribute is mapped onto a column in the table  An instance of a class represents a row in the table  A many-to-many association is mapped into its own table  A one-to-many association is implemented as buried foreign key  Methods are not mapped

29. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 29 Mapping the User class to a database table User +firstName:String +login:String +email:String id:longfirstName:text[25]login:text[8]email:text[32] User table

30. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 30 Primary and Foreign Keys  Any set of attributes that could be used to uniquely identify any data record in a relational table is called a candidate key.  The actual candidate key that is used in the application to identify the records is called the primary key.  The primary key of a table is a set of attributes whose values uniquely identify the data records in the table.  A foreign key is an attribute (or a set of attributes) that references the primary key of another table.

31. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 31 Example for Primary and Foreign Keys User table Candidate key loginemail “am384”“am384@mail.org” “js289”“john@mail.de” firstName “alice” “john” “bd”“bobd@mail.ch”“bob” Candidate key Primary key League table login “am384” name “tictactoeNovice” “tictactoeExpert” “js289”“chessNovice” Foreign key referencing User table

32. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 32 Buried Association  Associations with multiplicity one can be implemented using a foreign key.  For one-to-many associations we add a foreign key to the table representing the class on the “many” end.  For all other associations we can select either class at the end of the association.

33. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 33 Buried Association League LeagueOwner * 1 id:long LeagueOwner table...owner:long League table...id:long  Associations with multiplicity “one” can be implemented using a foreign key. Because the association vanishes in the table, we call this a buried association.  For one-to-many associations we add the foreign key to the table representing the class on the “many” end.  For all other associations we can select either class at the end of the association.

34. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 34 Another Example for Buried Association Transaction transactionID Portfolio portfolioID... * portfolioID... Portfolio Table transactionID Transaction Table portfolioID Foreign Key

35. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 35 Mapping Many-To-Many Associations City cityName Airport airportCode airportName * * Serves cityName Houston Albany Munich Hamburg City Table airportCode IAH HOU ALB MUC HAM Airport Table airportName Intercontinental Hobby Albany County Munich Airport Hamburg Airport Primary Key cityName Houston Albany Munich Hamburg Serves Table airportCode IAH HOU ALB MUC HAM In this case we need a separate table for the association Separate table for “Serves” association

36. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 36 Mapping the Tournament/Player association as a separate table PlayerTournament ** id Tournament table 23 name... novice 24expert tournamentplayer TournamentPlayerAssociation table 2356 2379 Player table id 56 name... alice 79john

37. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 37 Realizing Inheritance  Relational databases do not support inheritance  Two possibilities to map UML inheritance relationships to a database schema  With a separate table (vertical mapping)  The attributes of the superclass and the subclasses are mapped to different tables  By duplicating columns (horizontal mapping)  There is no table for the superclass  Each subclass is mapped to a table containing the attributes of the subclass and the attributes of the superclass

38. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 38 Realizing inheritance with a separate table User table id 56 name... zoe 79john role LeagueOwner Player Player User LeagueOwner maxNumLeagues credits name Player table id 79 credits... 126 id LeagueOwner table 56 maxNumLeagues... 12

39. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 39 Realizing inheritance by duplicating columns Player User LeagueOwner maxNumLeagues credits name id LeagueOwner table 56 maxNumLeagues... 12 name zoe Player table id 79 credits... 126 name john

40. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 40 Comparison: Separate Tables vs Duplicated Columns  The trade-off is between modifiability and response time  How likely is a change of the superclass?  What are the performance requirements for queries?  Separate table mapping We can add attributes to the superclass easily by adding a column to the superclass table  Searching for the attributes of an object requires a join operation.  Duplicated columns  Modifying the database schema is more complex and error-prone Individual objects are not fragmented across a number of tables, resulting in faster queries

41. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 41 Heuristics for Transformations  For a given transformation use the same tool  If you are using a CASE tool to map associations to code, use the tool to change association multiplicities.  Keep the contracts in the source code, not in the object design model  By keeping the specification as a source code comment, they are more likely to be updated when the source code changes.  Use the same names for the same objects  If the name is changed in the model, change the name in the code and or in the database schema.  Provides traceability among the models  Have a style guide for transformations  By making transformations explicit in a manual, all developers can apply the transformation in the same way.

42. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 42 Summary  Undisciplined changes => degradation of the system model  Four mapping concepts were introduced  Model transformation improves the compliance of the object design model with a design goal  Forward engineering improves the consistency of the code with respect to the object design model  Refactoring improves the readability or modifiability of the code  Reverse engineering attempts to discover the design from the code.  We reviewed model transformation and forward engineering techniques:  Optiziming the class model  Mapping associations to collections  Mapping contracts to exceptions  Mapping class model to storage schemas

43. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 43 Additional Slides

44. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 44 More Terminology  Roundtrip Engineering  Forward Engineering + reverse engineering  Inventory analysis: Determine the Delta between Object Model and Code  Together-J and Rationale provide tools for reverse engineering  Reengineering  Used in the context of project management:  Provding new functionality (customer dreams up new stuff) in the context of new technology (technology enablers)

45. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 45 Statistics as a product in the Game Abstract Factory Game createStatistics() ChessGame TicTacToeGame TTTStatisticsChessStatistics Tournament Statistics update() getStat() DefaultStatistics

46. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 46 N-ary association class Statistics Tournament 0..1 1* 1 LeagueGame Player Statistics 0..1 1 Statistics relates League, Tournament, and Player

47. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 47 Realization of the Statistics Association TournamentControl Statistics update(match,player) getStatNames() StatisticsVault update(match) getStatNames(game) getStat(name,game,player) getStat(name,league,player) getStat(name,tournament,player) StatisticsView Game createStatistics() getStat(name)

48. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 48 StatisticsVault as a Facade TournamentControl Statistics update(match,player) getStatNames() StatisticsVault update(match) getStatNames(game) getStat(name,game,player) getStat(name,league,player) getStat(name,tournament,player) StatisticsView Game createStatistics() getStat(name)

49. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 49 Public interface of the StatisticsVault class public class StatisticsVault { public void update(Match m) throws InvalidMatch, MatchNotCompleted {...} public List getStatNames() {...} public double getStat(String name, Game g, Player p) throws UnknownStatistic, InvalidScope {...} public double getStat(String name, League l, Player p) throws UnknownStatistic, InvalidScope {...} public double getStat(String name, Tournament t, Player p) throws UnknownStatistic, InvalidScope {...} }

50. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 50 Database schema for the Statistics Association scope:long Statistics table player:longscopetype:longid:long StatisticCounters table name:text[25] value:double id:long League table...id:long Game table...id:long T ournament table...

51. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 51 Restructuring Activities  Realizing associations  Revisiting inheritance to increase reuse  Revising inheritance to remove implementation dependencies

52. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 52 Realizing Associations  Strategy for implementing associations:  Be as uniform as possible  Individual decision for each association  Example of uniform implementation  1-to-1 association:  Role names are treated like attributes in the classes and translate to references  1-to-many association:  "Ordered many" : Translate to Vector  "Unordered many" : Translate to Set  Qualified association:  Translate to Hash table

53. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 53 Unidirectional 1-to-1 Association MapAreaZoomInAction Object design model before transformation ZoomInAction Object design model after transformation MapArea -zoomIn:ZoomInAction +getZoomInAction() +setZoomInAction(action)

54. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 54 Bidirectional 1-to-1 Association MapAreaZoomInAction 11 Object design model before transformation MapAreaZoomInAction -targetMap:MapArea-zoomIn:ZoomInAction +getZoomInAction() +setZoomInAction(action) +getTargetMap() +setTargetMap(map) Object design model after transformation

55. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 55 1-to-Many Association LayerLayerElement 1* Object design model before transformation LayerElement -containedIn:Layer +getLayer() +setLayer(l) Layer -layerElements:Set +elements() +addElement(le) +removeElement(le) Object design model after transformation

56. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 56 Qualification SimulationRun simname 0..1 * Object design model before transformation Scenario -runs:Hashtable +elements() +addRun(simname,sr:SimulationRun) +removeRun(simname,sr:SimulationRun) -scenarios:Vector +elements() +addScenario(s:Scenario) +removeScenario(s:Scenario) Object design model after transformation SimulationRun

57. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 57 Increase Inheritance  Rearrange and adjust classes and operations to prepare for inheritance  Abstract common behavior out of groups of classes  If a set of operations or attributes are repeated in 2 classes the classes might be special instances of a more general class.  Be prepared to change a subsystem (collection of classes) into a superclass in an inheritance hierarchy.

58. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 58 Building a super class from several classes  Prepare for inheritance. All operations must have the same signature but often the signatures do not match:  Some operations have fewer arguments than others: Use overloading (Possible in Java)  Similar attributes in the classes have different names: Rename attribute and change all the operations.  Operations defined in one class but no in the other: Use virtual functions and class function overriding.  Abstract out the common behavior (set of operations with same signature) and create a superclass out of it.  Superclasses are desirable. They  increase modularity, extensibility and reusability  improve configuration management  Turn the superclass into an abstract interface if possible  Use Bridge pattern

59. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 59 Object Design Areas 1. Service specification  Describes precisely each class interface 2. Component selection  Identify off-the-shelf components and additional solution objects 3. Object model restructuring  Transforms the object design model to improve its understandability and extensibility 4. Object model optimization  Transforms the object design model to address performance criteria such as response time or memory utilization.

60. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 60 Design Optimizations  Design optimizations are an important part of the object design phase:  The requirements analysis model is semantically correct but often too inefficient if directly implemented.  Optimization activities during object design: 1. Add redundant associations to minimize access cost 2. Rearrange computations for greater efficiency 3. Store derived attributes to save computation time  As an object designer you must strike a balance between efficiency and clarity.  Optimizations will make your models more obscure

61. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 61 Design Optimization Activities 1. Add redundant associations:  What are the most frequent operations? ( Sensor data lookup?)  How often is the operation called? (30 times a month, every 50 milliseconds) 2. Rearrange execution order  Eliminate dead paths as early as possible (Use knowledge of distributions, frequency of path traversals)  Narrow search as soon as possible  Check if execution order of loop should be reversed 3. Turn classes into attributes

62. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 62 Implement Application domain classes  To collapse or not collapse: Attribute or association?  Object design choices:  Implement entity as embedded attribute  Implement entity as separate class with associations to other classes  Associations are more flexible than attributes but often introduce unnecessary indirection.  Abbott's textual analysis rules  Every student receives a number at the first day in in the university.

63. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 63 Optimization Activities: Collapsing Objects Student Matrikelnumber ID:String Student Matrikelnumber:String

64. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 64 To Collapse or not to Collapse?  Collapse a class into an attribute if the only operations defined on the attributes are Set() and Get().

65. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 65 Design Optimizations (continued) Store derived attributes  Example: Define new classes to store information locally (database cache)  Problem with derived attributes:  Derived attributes must be updated when base values change.  There are 3 ways to deal with the update problem:  Explicit code: Implementor determines affected derived attributes (push)  Periodic computation: Recompute derived attribute occasionally (pull)  Active value: An attribute can designate set of dependent values which are automatically updated when active value is changed (notification, data trigger)

66. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 66 Optimization Activities: Delaying Complex Computations Image filename:String width() height() paint() Image filename:String width() height() paint() RealImage width() height() paint() data:byte[] ImageProxy filename:String width() height() paint() image 10..1

67. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 67 Increase Inheritance  Rearrange and adjust classes and operations to prepare for inheritance  Generalization: Finding the base class first, then the sub classes.  Specialization: Finding the the sub classes first, then the base class  Generalization is a common modeling activity. It allows to abstract common behavior out of a group of classes  If a set of operations or attributes are repeated in 2 classes the classes might be special instances of a more general class.  Always check if it is possible to change a subsystem (collection of classes) into a superclass in an inheritance hierarchy.

68. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 68 Generalization: Building a super class from several classes  You need to prepare or modify your classes for generalization.  All operations must have the same signature but often the signatures do not match:  Some operations have fewer arguments than others: Use overloading (Possible in Java)  Similar attributes in the classes have different names: Rename attribute and change all the operations.  Operations defined in one class but no in the other: Use virtual functions and class function overriding.  Superclasses are desirable. They  increase modularity, extensibility and reusability  improve configuration management  Many design patterns use superclasses  Try to retrofit an existing model to allow the use of a design pattern

69. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 69 Implement Associations  Two strategies for implementing associations: 1. Be as uniform as possible 2. Make an individual decision for each association  Example of a uniform implementation (often used by CASE tools)  1-to-1 association:  Role names are treated like attributes in the classes and translate to references  1-to-many association:  Always Translate into a Vector  Qualified association:  Always translate into to Hash table