Chapter 9, Object Design: Specifying Interfaces

Transcription

1 Chapter 9, Object Design: Specifying Interfaces Using UML, Patterns, and Java Object-Oriented Software Engineering

2 Specifying Interfaces Requirements analysis activities Identifying attributes and operations without specifying their types or their parameters. Object design: Three activities 1. Add visibility information 2. Add type signature information 3. Add contracts Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 6

3 1. Add Visibility Information UML defines three levels of visibility: Private (Class implementor): A private attribute can be accessed only by the class in which it is defined. A private operation can be invoked only by the class in which it is defined. Private attributes and operations cannot be accessed by subclasses or other classes. Protected (Class extender): A protected attribute or operation can be accessed by the class in which it is defined and on any descendent of the class. Public (Class user): A public attribute or operation can be accessed by any class. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 7

4 Implementation of UML Visibility in Java Tournament - maxnumplayers: int + getmaxnumplayers():int + getplayers(): List + acceptplayer(p:player) + removeplayer(p:player) + isplayeraccepted(p:player):boolean public class Tournament { private int maxnumplayers; public Tournament(League l, int maxnumplayers) public int getmaxnumplayers() { ; public List getplayers() { ; public void acceptplayer(player p) { ; public void removeplayer(player p) { ; public boolean isplayeraccepted (Player p) { ; Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 8

5 Information Hiding Heuristics Carefully define the public interface for classes as well as subsystems (façade) Always apply the Need to know principle. Only if somebody needs to access the information, make it publicly possible, but then only through well defined channels, so you always know the access. The fewer an operation knows the less likely it will be affected by any changes the easier the class can be changed Trade-off: Information hiding vs efficiency Accessing a private attribute might be too slow (for example in real-time systems or games) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 9

6 Information Hiding Design Principles Only the operations of a class are allowed to manipulate its attributes Access attributes only via operations. Hide external objects at subsystem boundary Define abstract class interfaces which mediate between system and external world as well as between subsystems Do not apply an operation to the result of another operation. Write a new operation that combines the two operations. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 10

7 2. Add Type Signature Information -numelements:int +put() +get() +remove() +containskey() +size() Hashtable Attributes and operations without type information are acceptable during analysis Hashtable -numelements:int +put(key:object,entry:object) +get(key:object):object +remove(key:object) +containskey(key:object):boolean +size():int Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 11

8 3. Add Contracts Contracts on a class enable caller and callee to share the same assumptions about the class. Contracts include three types of constraints: Invariant: A predicate that is always true for all instances of a class. Invariants are constraints associated with classes or interfaces. Precondition: Preconditions are predicates associated with a specific operation and must be true before the operation is invoked. Preconditions are used to specify constraints that a caller must meet before calling an operation. Postcondition: Postconditions are predicates associated with a specific operation and must be true after an operation is invoked. Postconditions are used to specify constraints that the object must ensure after the invocation of the operation. Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 12

9 Expressing constraints in UML Models OCL (Object Constraint Language) OCL allows constraints to be formally specified on single model elements or groups of model elements A constraint is expressed as an OCL expression returning the value true or false. OCL is not a procedural language (cannot constrain control flow). OCL expressions for Hashtable operation put(): Invariant: context Hashtable inv: numelements >= 0 Precondition: Context is a class operation put OCL expression context Hashtable::put(key, entry) pre:!containskey(key) Post-condition: context Hashtable::put(key, entry) post: containskey(key) and get(key) = entry Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 13

10 Expressing Constraints in UML Models A constraint can also be depicted as a note attached to the constrained UML element by a dependency relationship. <<invariant>> numelements >= 0 <<precondition>>!containskey(key) <<precondition>> containskey(key) <<precondition>> containskey(key) numelements:int HashTable put(key,entry:object) get(key):object remove(key:object) containskey(key:object):boolean size():int <<postcondition>> get(key) == entry <<postcondition>>!containskey(key) Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 14

11 Contract for acceptplayer in Tournament context Tournament::acceptPlayer(p) pre: not isplayeraccepted(p) context Tournament::acceptPlayer(p) pre: getnumplayers() < getmaxnumplayers() context Tournament::acceptPlayer(p) post: isplayeraccepted(p) context Tournament::acceptPlayer(p) post: getnumplayers() + 1 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 15

12 Contract for removeplayer in Tournament context Tournament::removePlayer(p) pre: isplayeraccepted(p) context Tournament::removePlayer(p) post: not isplayeraccepted(p) context Tournament::removePlayer(p) post: getnumplayers() - 1 Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 16

13 Annotation of Tournament class public class Tournament { /** The maximum number of players * is positive at all times. maxnumplayers > 0 */ private int maxnumplayers; /** The players List contains * references to Players who are * are registered with the * Tournament. */ private List players; /** Returns the current number of * players in the tournament. */ public int getnumplayers() { /** Returns the maximum number of * players in the tournament. */ public int getmaxnumplayers() { /** The acceptplayer() operation * assumes that the specified * player has not been accepted * in the Tournament yet. getnumplayers()<maxnumplayers isplayeraccepted(p) getnumplayers() = + 1 */ public void acceptplayer (Player p) { /** The removeplayer() operation * assumes that the specified player * is currently in the Tournament. isplayeraccepted(p) getnumplayers() - 1 */ public void removeplayer(player p) { Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 17

14 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

15 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());

16 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

17 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

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

19 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

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

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

22 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

23 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

24 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

25 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

26 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

27 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

28 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

29 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

30 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

31 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

32 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

33 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

34 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

35 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

36 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

37 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

38 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

39 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

40 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

41 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

42 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

43 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

44 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

45 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

46 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

47 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

48 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

49 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

50 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

51 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

52 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

53 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

54 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