Chapter 10, Mapping Models to Code

Transcription

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

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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2

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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 3

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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 4

5 Four types of transformations Model transformations operate on object models Refactoring source code transformations Forward Engineering changes in an object (e.g., attributes or associations) map into corresponding changes in source code Reverse Engineering the model space is defined based on existing source code Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 5

6 Model transformations Forward engineering Refactoring Model transformation Reverse engineering Model space Source code space Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 6

7 Model Transformation Example Object design model before transformation LeagueOwner + address Advertiser + address Player + address Object design model after transformation: User + address LeagueOwner Advertiser Player Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 7

8 Refactoring by pulling up a field Create a parent class that will hold the field that is factored out Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 8

9 Refactoring Example: Pull Up Field public class Player { private String ; //... public class LeagueOwner { private String ; //... public class Advertiser { private String _address; //... public class User { private String ; public class Player extends User { //... public class LeagueOwner extends User { //... public class Advertiser extends User { //... Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 9

10 Refactoring by pulling up a constructor body Create a parent class that will assign a value to the field in common Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 10

11 Refactoring Example: Pull Up Constructor Body public class User { private String ; public class Player extends User { public Player(String ) { this. = ; public class LeagueOwner extends User{ public LeagueOwner(String ) { this. = ; public class Advertiser extendsuser{ public Advertiser(String ) { this. = ; public class User { public User(String ) { this. = ; public class Player extends User { public Player(String ) { super( ); public class LeagueOwner extends User { public LeagueOwner(String ) { super( ); public class Advertiser extends User { public Advertiser(String ) { super( ); Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 11

12 Refactoring by pulling up methods The field and initialization in the constructor have been pulled up to the parent class Now see if any methods associated with can be moved to the parent class Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 12

13 Forward Engineering Example Object design model before transformation User + string +notify(msg:string) LeagueOwner +maxnumleagues:int Source code after transformation public class User { private String ; public String get () { return ; public void set (string value){ = 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 */ Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 13

14 Transformation principles Each transformation must address a single criteria Each transformation must be local Each transformation must be applied in isolation to other changes Each transformation must be followed by a validation step Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 14

15 Other Mapping Activities Optimizing the Object Design Model Mapping Associations Mapping Contracts to Exceptions Mapping Object Models to Tables Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 15

16 Some optimizations Avoid frequent repeated association traversals Multiple association traversals can lead to code such as method1().method2().. methodn() The sequence diagram can be used to identify such traversals See if a direct connection is possible to improve efficiency Try to make many associations more efficient Try to use a qualified association to reduce multiplicity to one Try to use ordering or indexing to decrease access time Try to avoid misplaced attributes Inefficient due to the need for get and set methods Try bringing the attribute into the calling class Catch the result of expensive computations Example, statistics in the Arena example are only updated after a match is completed This only has to be calculated once at the end of each match and not repeatedly until another match is completed Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 16

17 Collapsing an object without interesting behavior Object design model before transformation Person SocialSecurity number:string Object design model after transformation Person SSN:String Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 17

18 Delaying expensive computations Object design model before transformation Image filename:string data:byte[] paint() Object design model after transformation Image filename:string paint() ImageProxy filename:string paint() image RealImage data:byte[] paint() Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 18

19 Other Mapping Activities Optimizing the Object Design Model Mapping Associations Mapping Contracts to Exceptions Mapping Object Models to Tables Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 19

20 Realization of a unidirectional, one-to-one association Object design model before transformation Advertiser 1 1 Account Source code after transformation public class Advertiser { private Account account; public Advertiser() { account = new Account(); public Account getaccount() { return account; Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 20

21 Bidirectional one-to-one association Object design model before transformation Advertiser 1 1 Account Source code after transformation 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; 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; Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 21

22 Bidirectional, one-to-many association Object design model before transformation 1 Advertiser * Account Source code after transformation 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); Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 22

23 Bidirectional, many-to-many association Object design model before transformation * {ordered * Tournament Player Source code after transformation 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); Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 23

24 Bidirectional qualified association Object design model before transformation League * * Player nickname Object design model before forward engineering League nickname * 0..1 Player Source code after forward engineering Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 24

25 Bidirectional qualified association (continued) Source code after forward engineering public class League { private Map players; public class Player { private Map leagues; public void addplayer (String nickname, Player p) { if (!players.containskey(nickname )) { players.put(nickname, p); p.addleague(nickname, this); public void addleague (String nickname, League l) { if (!leagues.containskey(l)) { leagues.put(l, nickname); l.addplayer(nickname, this); Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 25

26 Transformation of an association class Object design model before transformation Statistics +getaveragestat(name) +gettotalstat(name) +updatestats(match) Tournament * * Player Object design model after transformation: 1 class and two binary associations Statistics Tournament +getaveragestat(name) +gettotalstat(name) +updatestats(match) 1 1 * * Player Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 26

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

28 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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 28

29 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 Bernd Bruegge the & Allen console H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 29 ErrorConsole log(e getmessage());

30 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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 30

31 A complete implementation of the Tournament.addPlayer() contract «invariant» getmaxnumplayers() > 0 «precondition»!isplayeraccepted(p) Tournament -maxnumplayers: int +getnumplayers():int +getmaxnumplayers():int +isplayeraccepted(p:player):boolean +addplayer(p:player) «precondition» getnumplayers() < getmaxnumplayers() «postcondition» isplayeraccepted(p) Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 31

32 The code Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 32

33 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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 33

34 Heuristics for mapping contracts to exceptions Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 34

35 Other Mapping Activities Optimizing the Object Design Model Mapping Associations Mapping Contracts to Exceptions Mapping Object Models to Tables Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 35

36 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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 36

37 Mapping the User class to a database table User +firstname:string +login:string + string User table id:long firstname:text[25] login:text[8] text[32] Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 37

38 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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 38

39 Example for Primary and Foreign Keys User table Primary key firstname alice john bob login am384 js289 bd Candidate key Candidate key League table name tictactoenovice tictactoeexpert chessnovice login am384 am384 js289 Foreign key referencing User table Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 39

40 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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 40

41 Buried Association 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. 1 LeagueOwner * League LeagueOwner table League table id:long... id:long... owner:long Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 41

42 Another Example for Buried Association Transaction transactionid * Foreign Key Portfolio portfolioid... transactionid Transaction Table portfolioid Portfolio Table portfolioid... Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 42

43 Mapping Many-To-Many Associations In this case we need a separate table for the association City cityname * Serves * Airport airportcode airportname Separate table for Serves association Primary Key City Table Airport Table Serves Table cityname Houston Albany Munich Hamburg airportcode IAH HOU ALB MUC HAM airportname Intercontinental Hobby Albany County Munich Airport Hamburg Airport cityname Houston Houston Albany Munich Hamburg airportcode IAH HOU ALB MUC HAM Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 43

44 Mapping the Tournament/Player association as a separate table Tournament * * Player Tournament table id name novice 24 expert TournamentPlayerAssociation table tournament player Player table id name alice 79 john Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 44

45 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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 45

46 Realizing inheritance with a separate table name User LeagueOwner maxnumleagues Player credits id User table name... role 56 zoe LeagueOwner 79 john Player LeagueOwner table Player table id maxnumleagues... id credits Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 46

47 Realizing inheritance by duplicating columns name User LeagueOwner maxnumleagues Player credits LeagueOwner table Player table id name maxnumleagues... id name credits zoe john 126 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 47

48 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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 48

49 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. Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 49

50 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 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 50

51 Statistics as a product in the Game Abstract Factory Tournament Game createstatistics() TicTacToeGame ChessGame Statistics update() getstat() TTTStatisticsChessStatisticsDefaultStatistics Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 51

52 N-ary association class Statistics Statistics relates League, Tournament, and Player Statistics 1 * Game League Tournament Player Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 52

53 Realization of the Statistics Association TournamentControl StatisticsView StatisticsVault Statistics update(match) update(match,playe getstatnames(game) getstatnames() getstat(name,game,player) getstat(name) getstat(name,league,player) getstat(name,tournament,player) Game createstatistics() Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 53

54 StatisticsVault as a Facade TournamentControl StatisticsView StatisticsVault Statistics update(match) getstatnames(game) getstat(name,game,player) getstat(name,league,player) getstat(name,tournament,player) update(match,player) getstatnames() getstat(name) Game createstatistics() Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 54

55 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 {... Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 55

56 Database schema for the Statistics Association Statistics table id:long scope:long scopetype:long player:long id:long StatisticCounters table name:text[25] value:double Game table League table Tournament table id:long... id:long... id:long... Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 56

57 Restructuring Activities Realizing associations Revisiting inheritance to increase reuse Revising inheritance to remove implementation dependencies Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 57

58 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 58

59 Unidirectional 1-to-1 Association Object design model before transformation ZoomInAction MapArea Object design model after transformation ZoomInAction MapArea -zoomin:zoominaction +getzoominaction() +setzoominaction(action) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 59

60 Bidirectional 1-to-1 Association Object design model before transformation ZoomInAction 1 1 MapArea Object design model after transformation ZoomInAction -targetmap:maparea +gettargetmap() +settargetmap(map) MapArea -zoomin:zoominaction +getzoominaction() +setzoominaction(action) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 60

61 1-to-Many Association Object design model before transformation Layer 1 * LayerElement Object design model after transformation Layer -layerelements:set +elements() +addelement(le) +removeelement(le) LayerElement -containedin:layer +getlayer() +setlayer(l) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 61

62 Qualification Object design model before transformation Scenario simname * 0..1 SimulationRun Object design model after transformation Scenario -runs:hashtable +elements() +addrun(simname,sr:simulationrun) +removerun(simname,sr:simulationrun) SimulationRun -scenarios:vector +elements() +addscenario(s:scenario) +removescenario(s:scenario) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 62

63 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. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 63

64 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 64

65 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. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 65

66 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 66

67 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 67

68 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. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 68

69 Optimization Activities: Collapsing Objects Student Matrikelnumber ID:String Student Matrikelnumber:String Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 69

70 To Collapse or not to Collapse? Collapse a class into an attribute if the only operations defined on the attributes are Set() and Get(). Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 70

71 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) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 71

72 Optimization Activities: Delaying Complex Computations Image filename:string data:byte[] width() height() paint() Image filename:string width() height() paint() ImageProxy filename:string width() height() paint() image RealImage data:byte[] width() height() paint() Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 72

73 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. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 73

74 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 74

75 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 Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 75