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Usi
ng U
ML
, Pat
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s, a
nd J
ava
Ob
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-Ori
ente
d S
oftw
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En
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eeri
ng Chapter 10,
Mapping Models to Code
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
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.
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.
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
Modeltransformation
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:AddressLeagueOwner
+email:Address
PlayerAdvertiserLeagueOwner
User
+email:Address
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 {//...
}
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);}
}
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)
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
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 11
Collapsing an object without interesting behavior
Person SocialSecurity
number:String
Person
SSN:String
Object design model before transformation
Object design model after transformation?
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
1 0..1
paint()
paint() paint()
?
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
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 14
Realization of a unidirectional, one-to-one association
AccountAdvertiser11
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;
}
}
?
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;}
}
AccountAdvertiser 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;}
}
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);
}}
}
Advertiser Account1 *
Object design model before transformation
Source code after transformation
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);
}}
}
Tournament Player* *
Source code after transformation
{ordered}
Object design model before transformation
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 18
Bidirectional qualified association
Object design model before forward engineering
PlayernickName0..1*League
Player**
Object design model before transformation
LeaguenickName
Source code after forward engineering
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
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 20
Transformation of an association class
Tournament Player* *
Object design model before transformation
Object design model after transformation: 1 class and two binary associations
Statistics
+getAverageStat(name)+getTotalStat(name)+updateStats(match)
Tournament Player* *
1 1
Statistics
+getAverageStat(name)+getTotalStat(name)+updateStats(match)
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
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
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 consoleErrorConsole.log(e.getMessage());
}}
}}
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.
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)
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.
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
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
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:long firstName:text[25] login:text[8] email:text[32]
User table
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.
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
login email
“am384” “[email protected]”
“js289” “[email protected]”
firstName
“alice”
“john”
“bd” “[email protected]”“bob”
Candidate key
Primary key
League table login
“am384”
“am384”
name
“tictactoeNovice”
“tictactoeExpert”
“js289”“chessNovice”
Foreign key referencing User table
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.
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 33
Buried Association
LeagueLeagueOwner *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.
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
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 35
Mapping Many-To-Many Associations
City
cityName
AirportairportCodeairportName
* *Serves
cityNameHoustonAlbanyMunich
Hamburg
City Table
airportCodeIAHHOUALBMUCHAM
Airport Table
airportNameIntercontinental
HobbyAlbany CountyMunich Airport
Hamburg Airport
Primary Key
cityNameHoustonHoustonAlbanyMunich
Hamburg
Serves Table
airportCodeIAHHOUALBMUCHAM
In this case we need a separate table for the association
Separate table for“Serves” association
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
24 experttournament player
TournamentPlayerAssociation table
23 56
23 79
Player table
id
56
name ...
alice
79 john
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
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
79 john
role
LeagueOwner
Player
Player
User
LeagueOwnermaxNumLeagues credits
name
Player table
id
79
credits ...
126
id
LeagueOwner table
56
maxNumLeagues ...
12
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
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 mappingWe can add attributes to the superclass easily by adding a column
to the superclass tableSearching for the attributes of an object requires a join operation.
Duplicated columnsModifying the database schema is more complex and error-proneIndividual 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 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.
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
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 43
Additional Slides
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)
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()
ChessGameTicTacToeGame
TTTStatistics ChessStatistics
Tournament
Statistics
update()getStat()
DefaultStatistics
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 46
N-ary association class Statistics
Tournament
0..10..10..1
1 *
1
LeagueGame Player
Statistics
0..11
Statistics relates League, Tournament, and Player
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)
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)
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 {...}
}
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
id:long
StatisticCounters table
name:text[25] value:double
id:long
League table
...id:long
Game table
... id:long
Tournament table
...
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
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
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)
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 54
Bidirectional 1-to-1 Association
MapAreaZoomInAction11
Object design model before transformation
MapAreaZoomInAction
-targetMap:MapArea -zoomIn:ZoomInAction+getZoomInAction()+setZoomInAction(action)
+getTargetMap()+setTargetMap(map)
Object design model after transformation
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 55
1-to-Many Association
Layer LayerElement1 *
Object design model before transformation
LayerElement
-containedIn:Layer+getLayer()+setLayer(l)
Layer
-layerElements:Set+elements()+addElement(le)+removeElement(le)
Object design model after transformation
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 56
Qualification
SimulationRunsimname 0..1*Object design model before transformation
Scenario
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
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.
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
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.
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
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
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.
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 63
Optimization Activities: Collapsing Objects
StudentMatrikelnumber
ID:String
Student
Matrikelnumber:String
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().
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)
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 66
Optimization Activities: Delaying Complex Computations
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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.
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
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 possible2. 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