Chapter 9: Object Design: Specifying Interfaces

Object Design Object design is the process of adding details to the requirements analysis and making implementation decisions The object designer must choose among different ways to implement the analysis model with the goal to minimize execution time, memory and other measures of cost. Requirements Analysis: The functional model and the dynamic model deliver operations for the object model Object Design: We decide on where to put these operations in the object model Object design serves as the basis of implementation

Object Design: Closing the Gap

Plan for the next Lectures 1. Reuse: Identification of existing solutions Use of inheritance Off-the-shelf components and additional solution objects Design patterns 2. Interface specification Describes precisely each class interface 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. Object Design lectures Mapping Models to Code lecture

Interface Specification Motivation How can developers communicate clearly and precisely the details of the classes so that they can make reliable assumptions about the classes they did not implement?

Developers play different Roles during Object Design Call Class Class Extender Class Implementor Class User Realize Class Refine Class

Class user versus Class Extender Developers responsible for the implementation of Game are class implementors Developers responsible for the implementation of League are class users of Game League Game 1 * TicTacToe Chess Tournament The developer responsible for the implementation of TicTacToe is a class extender of Game

Specifying Interfaces Requirements analysis activities Identifying attributes and operations without specifying their types or their parameters. Object design: Three activities Add visibility information Add type signature information Add contracts

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.

Implementation of UML Visibility in Java Tournament - maxNumPlayers: int + acceptPlayer(p:Player) + removePlayer(p:Player) + getMaxNumPlayers():int + getPlayers(): List + 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) {…};

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) Information hiding is not a major issue during analysis, however during design it is a problem. Limit the effect of changes so that changes can be understood clearly and do not cause a major recompilation of the whole system (also called black box design)

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.

2. Add Type Signature Information Hashtable +put() +get() +remove() +containsKey() +size() -numElements:int Attributes and operations without type information are acceptable during analysis Hashtable +put(key:Object,entry:Object) +get(key:Object):Object +remove(key:Object) +containsKey(key:Object):boolean +size():int -numElements:int

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.

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 OCL expression Context is a class operation put Precondition: context Hashtable::put(key, entry) pre: !containsKey(key) Post-condition: context Hashtable::put(key, entry) post: containsKey(key) and get(key) = entry

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 HashTable <<precondition>> !containsKey(key) <<postcondition>> get(key) == entry numElements:int put(key,entry:Object) get(key):Object remove(key:Object) containsKey(key:Object):boolean size():int <<precondition>> containsKey(key) <<postcondition>> !containsKey(key) <<precondition>> containsKey(key)

Contract for acceptPlayer in Tournament context Tournament::acceptPlayer(p) pre: not isPlayerAccepted(p) getNumPlayers() < getMaxNumPlayers() context Tournament::acceptPlayer(p) post: isPlayerAccepted(p) getNumPlayers() = @pre.getNumPlayers() + 1 Value of the object or attribute before execution of the method

Contract for removePlayer in Tournament context Tournament::removePlayer(p) pre: isPlayerAccepted(p) context Tournament::removePlayer(p) post: not isPlayerAccepted(p) getNumPlayers() = @pre.getNumPlayers() - 1

Real and Integer Operators Steve Webster, dec.bournemouth.ac.uk Downloaded from http://neptune.irit.fr/Biblio/UML_OCL.PPT

String Operators

gender = #male implies title = ‘Mr. ‘ Using Enumerations Customer gender: enum{male, female} name: String title: String dateOfBirth: Date gender = #male implies title = ‘Mr. ‘

Boolean Operators

Other Special Keywords self An instance of the context result In postconditions, it refers to the return value of the context method

JavaDoc Annotation of Tournament class public class Tournament { /** The maximum number of players  * is positive at all times.  * @invariant 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.  * @pre !isPlayerAccepted(p)  * @pre getNumPlayers()<maxNumPlayers * @post isPlayerAccepted(p) * @post getNumPlayers() = * @pre.getNumPlayers() + 1 */ public void acceptPlayer (Player p) {…} /** The removePlayer() operation * assumes that the specified player * is currently in the Tournament. * @pre isPlayerAccepted(p) * @post !isPlayerAccepted(p) * @post getNumPlayers() = @pre.getNumPlayers() - 1 */ public void removePlayer(Player p) {…} }

Constraints can involve more than one class How do we specify constraints on more than one class?

3 Types of Navigation through a Class Diagram 1. Local attribute 2. Directly related class 3. Indirectly related class Tournament League League start:Date end:Date * * * Tournament Player * * Player Any OCL constraint for any class diagram can be built using a combination of these three navigation types

Association Navigation in OCL Association navigation is realized using the opposite rolename. object.rolename – the set of objects on the other side of the association If the rolename is missing, then the name of the class with lowercase first letter is used. If there are multiple types of associations between two classes, then rolenames are required. League From context of League self.tournaments refers to the set of objects of class Tournament associated with this league. From context of Tournament self.player refers to the set of objects of class Player associated with this tournament tournaments * Tournament * * Player

ARENA Example: League, Tournament and Player players * tournaments {ordered} Tournament +start:Date +end:Date +acceptPlayer(p:Player) League +getActivePlayers() Player +name:String +email:String

Model Refinement with 3 additional Constraints A Tournament’s planned duration must be under one week. Players can be accepted in a Tournament only if they are already registered with the corresponding League. The number of active Players in a League are those that have taken part in at least one Tournament of the League. To better understand these constraints we instantiate the class diagram for a specific group of instances 2 Leagues, 2 Tournaments and 5 Players

Instance Diagram: 2 Leagues, 2 Tournaments, and 5 Players alice:Player bob:Player marc:Player winter:Tournament tttExpert:League joe:Player xmas:Tournament chessNovice:League start=Dec 21 end=Dec 22 start=Dec 23 end=Dec 25 zoe:Player

Specifying the Model Constraints Local attribute navigation context Tournament inv: end - start <= Calendar.WEEK players * tournaments {ordered} Tournament +start:Date +end:Date +acceptPlayer(p:Player) League +getActivePlayers() Player +name:String +email:String Directly related class navigation context Tournament::acceptPlayer(p) pre: league.players->includes(p)

Specifying the Model Constraints Local attribute navigation context Tournament inv: end - start <= Calendar.WEEK Directly related class navigation context Tournament::acceptPlayer(p) pre: league.players->includes(p) Indirectly related class navigation context League::getActivePlayers post: result = tournaments.players->asSet players * tournaments {ordered} Tournament +start:Date +end:Date +acceptPlayer(p:Player) League +getActivePlayers() Player +name:String +email:String

OCL Collections Sets Sequences Bags Used when navigating a single association Sequences Used when navigating a single ordered association Bags Unlike sets, bags can contain the same object multiple times Used to accumulate objects when accessing indirectly related objects

Operations on Collections

Collection hierarchy Collection Set Bag Sequence minus symmetricDifference asSequence asBag asSequence asSet first last at(int) append prepend asBag asSet

OCL supports Quantification OCL forall quantifier /* All Matches in a Tournament occur within the Tournament’s time frame */ context Tournament inv: matches->forAll(m:Match | m.start.after(t.start) and m.end.before(t.end)) OCL exists quantifier /* Each Tournament conducts at least one Match on the first day of the Tournament */ context Tournament inv: matches->exists(m:Match | m.start.equals(start))

Summary There are three different roles for developers during object design Class user, class implementor and class extender During object design - and only during object design - we specify visibility rules Constraints are boolean expressions on model elements Contracts are constraints on a class enable class users, implementors and extenders to share the same assumption about the class (“Design by contract”) OCL is a language that allows us to express constraints on UML models Complicated constraints involving more than one class, attribute or operation can be expressed with 3 basic navigation types.

Additional Slides

ARENA’s object model identified during the analysis TournamentForm 1 1 applyForTournament() TournamentControl * selectSponsors() advertizeTournament() acceptPlayer() announceTournament() * 1 1 Tournament name * start * * * players end sponsors * * Player Advertiser acceptPlayer() removePlayer() schedule() * matches * Match matches * start status playMove() getScore()

Adding type information to ARENA’s object model TournamentForm 1 1 +applyForTournament() TournamentControl * +selectSponsors(advertisers):List +advertizeTournament() +acceptPlayer(p) +announceTournament() +isPlayerOverbooked():boolean * 1 1 Tournament +name:String +start:Date +end:Date * * * * players sponsors * * Player Advertiser +acceptPlayer(p) +removePlayer(p) +schedule() * matches * Match matches * +start:Date +status:MatchStatus +playMove(p,m) +getScore():Map

Pre- and post-conditions for ordering operations on TournamentControl +selectSponsors(advertisers):List +advertizeTournament() +acceptPlayer(p) +announceTournament() +isPlayerOverbooked():boolean Pre- and post-conditions for ordering operations on TournamentControl context TournamentControl::selectSponsors(advertisers) pre: interestedSponsors->notEmpty and tournament.sponsors->isEmpty context TournamentControl::selectSponsors(advertisers) post: tournament.sponsors.equals(advertisers) context TournamentControl::advertiseTournament() pre: tournament.sponsors->isEmpty and not tournament.advertised context TournamentControl::advertiseTournament() post: tournament.advertised context TournamentControl::acceptPlayer(p) pre: tournament.advertised and interestedPlayers->includes(p) and not isPlayerOverbooked(p) context TournamentControl::acceptPlayer(p) post: tournament.players->includes(p)

Specifying invariants on Tournament and Tournament Control All Matches of in a Tournament must occur within the time frame of the Tournament context Tournament inv: matches->forAll(m| m.start.after(start) and m.start.before(end)) No Player can take part in two or more Tournaments that overlap context TournamentControl inv: tournament.players->forAll(p| p.tournaments->forAll(t| t <> tournament implies not t.overlap(tournament)))

Specifying invariants on Match players * Player Tournament * tournaments players * matches Match * A match can only involve players who are accepted in the tournament context Match inv: players->forAll(p| p.tournaments->exists(t| t.matches->includes(self))) context Match inv: players.tournaments.matches.includes(self)