Transcript Lecture for Chapter 10, Mapping Models To Code

Using UML, Patterns, and Java
Object-Oriented Software Engineering
Chapter 10:
Mapping Models
to Code
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
minimize 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
Modified from Bruegge & Dutoit’ originals
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2
Object Modeling Activities
1. Reuse: Identification of existing solutions
 Use of inheritance
 Off-the-shelf components and additional
solution objects
 Design patterns
Object
Design
lectures
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
Mapping
Models to Code
lecture
 Transforms the object design model to address
performance criteria such as response time or
memory utilization.
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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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.
Modified from Bruegge & Dutoit’ originals
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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.
Modified from Bruegge & Dutoit’ originals
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Model transformations
Forward engineering
Refactoring
Model
transformation
Reverse engineering
Model space
Modified from Bruegge & Dutoit’ originals
Source code space
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Model Transformation Example
Object design model before transformation
LeagueOwner
+email:Address
Object design model
after transformation:
Player
Advertiser
+email:Address
+email:Address
User
+email:Address
LeagueOwner
Modified from Bruegge & Dutoit’ originals
Advertiser
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Player
7
Refactoring Example: Pull Up Field
public class User {
private String email;
}
public class Player {
private String email;
//...
}
public class LeagueOwner {
private String eMail;
//...
}
public class Advertiser {
private String email_address;
//...
}
Modified from Bruegge & Dutoit’ originals
public class Player extends User {
//...
}
public class LeagueOwner extends
User {
//...
}
public class Advertiser extends User {
//...
}
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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;
}
}
Modified from Bruegge & Dutoit’ originals
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);
}
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Forward Engineering Example
Object design model before transformation
LeagueOwner
+maxNumLeagues:int
User
+email:String
+notify(msg:String)
Source code after transformation
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 */
}
Modified from Bruegge & Dutoit’ originals
public class LeagueOwner extends User {
private int maxNumLeagues;
public int getMaxNumLeagues() {
return maxNumLeagues;
}
public void setMaxNumLeagues
(int value) {
maxNumLeagues = value;
}
/* Other methods omitted */
}
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Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Modified from Bruegge & Dutoit’ originals
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Collapsing an object without interesting behavior
Object design model before transformation
Person
SocialSecurity
number:String
Object design model after transformation
?
Person
SSN:String
Modified from Bruegge & Dutoit’ originals
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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()
Modified from Bruegge & Dutoit’ originals
image
1
0..1
RealImage
data:byte[]
paint()
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Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Realization of a unidirectional, one-to-one association
Object design model before transformation
Advertiser
1
Source code after transformation
1
Account
?
public class Advertiser {
private Account account;
public Advertiser() {
account = new Account();
}
public Account getAccount() {
return account;
}
}
Modified from Bruegge & Dutoit’ originals
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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;
}
}
Modified from Bruegge & Dutoit’ originals
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;
}
}
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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);
}
}
Modified from Bruegge & Dutoit’ originals
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);
}
}
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Bidirectional, many-to-many association
Object design model before transformation
Tournament
* {ordered}
*
Player
Source code after transformation
public class Tournament {
public class Player {
private List players;
private List tournaments;
public Tournament() {
public Player() {
players = new ArrayList();
tournaments = new ArrayList();
}
}
public void addPlayer(Player p) {
public void
addTournament(Tournament t) {
if (!players.contains(p)) {
if (!tournaments.contains(t)) {
players.add(p);
tournaments.add(t);
p.addTournament(this);
t.addPlayer(this);
}
}
}
}
}
}
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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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
Modified from Bruegge & Dutoit’ originals
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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);
}
}
}
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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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
+getAverageStat(name)
+getTotalStat(name)
+updateStats(match)
1
1
Tournament
Modified from Bruegge & Dutoit’ originals
*
*
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Player
21
Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Modified from Bruegge & Dutoit’ originals
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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
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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The try-throw-catch Mechanism in Java
public class TournamentControl {
private Tournament tournament;
public void addPlayer(Player p) throws KnownPlayerException {
if (tournament.isPlayerAccepted(p)) {
throw new KnownPlayerException(p);
}
//... Normal addPlayer behavior
}
}
public class TournamentForm {
private TournamentControl control;
private ArrayList players;
public void processPlayerApplications() {
// Go through all the players
for (Iteration i = players.iterator(); i.hasNext();) {
try {
// Delegate to the control object.
control.acceptPlayer((Player)i.next());
} catch (KnownPlayerException e) {
// If an exception was caught, log it to the console
ErrorConsole.log(e.getMessage());
}
}
}
}
Modified from Bruegge & Dutoit’ originals
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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 violated. 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.
Modified from Bruegge & Dutoit’ originals
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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()
Modified from Bruegge & Dutoit’ originals
«postcondition»
isPlayerAccepted(p)
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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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.
Modified from Bruegge & Dutoit’ originals
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Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Modified from Bruegge & Dutoit’ originals
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
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Mapping the User class to a database table
User
+firstName:String
+login:String
+email:String
User table
id:long
firstName:text[25]
Modified from Bruegge & Dutoit’ originals
login:text[8]
Object-Oriented Software Engineering: Using UML, Patterns, and Java
email:text[32]
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.
Modified from Bruegge & Dutoit’ originals
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Example for Primary and Foreign Keys
Primary key
User table
firstName
login
email
“alice”
“am384”
“[email protected]”
“john”
“js289”
“[email protected]”
“bob”
“bd”
“[email protected]”
Candidate key
League table
name
Candidate key
login
“tictactoeNovice”
“am384”
“tictactoeExpert”
“am384”
“chessNovice”
“js289”
Foreign key referencing User table
Modified from Bruegge & Dutoit’ originals
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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.
Modified from Bruegge & Dutoit’ originals
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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.
LeagueOwner
1
*
LeagueOwner table
id:long
...
Modified from Bruegge & Dutoit’ originals
League
League table
id:long
...
Object-Oriented Software Engineering: Using UML, Patterns, and Java
owner:long
34
Another Example for Buried Association
Transaction
Portfolio
*
transactionID
Foreign Key
Transaction Table
transactionID
Modified from Bruegge & Dutoit’ originals
portfolioID
Object-Oriented Software Engineering: Using UML, Patterns, and Java
portfolioID
...
Portfolio Table
portfolioID ...
35
Mapping Many-To-Many Associations
In this case we need a separate table for the association
City
*
Serves
*
cityName
Airport
airportCode
airportName
Separate table for
“Serves” association
Primary Key
City Table
cityName
Houston
Albany
Munich
Hamburg
Modified from Bruegge & Dutoit’ originals
Airport Table
airportCode
IAH
HOU
ALB
MUC
HAM
airportName
Intercontinental
Hobby
Albany County
Munich Airport
Hamburg Airport
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Serves Table
cityName airportCode
IAH
Houston
HOU
Houston
ALB
Albany
MUC
Munich
HAM
Hamburg
36
Mapping the Tournament/Player association as a
separate table
Tournament *
*
Player
Tournament table
id
name
23
novice
24
expert
...
Modified from Bruegge & Dutoit’ originals
Player table
TournamentPlayerAssociation
table
tournament
player
23
56
23
79
Object-Oriented Software Engineering: Using UML, Patterns, and Java
id
name
56
alice
79
john
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
Modified from Bruegge & Dutoit’ originals
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Realizing inheritance with a separate table
User
name
Player
credits
LeagueOwner
maxNumLeagues
User table
id
name
56
zoe
79
john
...
role
LeagueOwner
Player
LeagueOwner table
id
56
maxNumLeagues
12
Modified from Bruegge & Dutoit’ originals
Player table
...
id
credits
79
126
Object-Oriented Software Engineering: Using UML, Patterns, and Java
...
39
Realizing inheritance by duplicating columns
User
name
LeagueOwner
maxNumLeagues
Player
credits
LeagueOwner table
id
name
56
zoe
Player table
maxNumLeagues ...
id
name
credits
12
79
john
126
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
...
40
Comparison: Separate Tables vs Duplicated Columns

The trade-off is between modifiability and response time
 How likely is a change of the superclass?
 What are the performance requirements for queries?

Separate table mapping
We can add attributes to the superclass easily by adding a column to
the superclass table
Searching for the attributes of an object requires a join operation.

Duplicated columns
Modifying the database schema is more complex and error-prone
Individual objects are not fragmented across a number of tables,
resulting in faster queries
Modified from Bruegge & Dutoit’ originals
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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.
Modified from Bruegge & Dutoit’ originals
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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
Modified from Bruegge & Dutoit’ originals
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Additional Slides
Modified from Bruegge & Dutoit’ originals
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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 Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
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Statistics as a product in the Game Abstract Factory
Game
Tournament
createStatistics()
ChessGame
TicTacToeGame
Statistics
update()
getStat()
TTTStatisticsChessStatisticsDefaultStatistics
Modified from Bruegge & Dutoit’ originals
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N-ary association class Statistics
Statistics relates League, Tournament, and Player
Statistics
1
*
1 1
0..1
Game
Modified from Bruegge & Dutoit’ originals
0..1
League
0..1
Tournament
Object-Oriented Software Engineering: Using UML, Patterns, and Java
0..1
Player
47
Realization of the Statistics Association
TournamentControl
StatisticsView
StatisticsVault
Statistics
update(match,playe
update(match)
getStatNames()
getStatNames(game)
getStat(name)
getStat(name,game,player)
getStat(name,league,player)
getStat(name,tournament,player)
Game
createStatistics()
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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StatisticsVault as a Facade
TournamentControl
StatisticsView
StatisticsVault
update(match)
getStatNames(game)
getStat(name,game,player)
getStat(name,league,player)
Statistics
update(match,player)
getStatNames()
getStat(name)
Game
getStat(name,tournament,player)
createStatistics()
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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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 {...}
}
Modified from Bruegge & Dutoit’ originals
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Database schema for the Statistics Association
Statistics table
id:long
scope:long scopetype:long
player:long
StatisticCounters table
id:long
name:text[25] value:double
Game table
id:long
...
Modified from Bruegge & Dutoit’ originals
League table
id:long
...
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Tournament table
id:long
...
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
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
54
Bidirectional 1-to-1 Association
Object design model before transformation
ZoomInAction
1
MapArea
1
Object design model after transformation
ZoomInAction
-targetMap:MapArea
+getTargetMap()
+setTargetMap(map)
Bernd Bruegge & Allen Dutoit
MapArea
-zoomIn:ZoomInAction
+getZoomInAction()
+setZoomInAction(action)
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
55
1-to-Many Association
Object design model before transformation
Layer
1
*
LayerElement
Object design model after transformation
Layer
LayerElement
-layerElements:Set
+elements()
+addElement(le)
+removeElement(le)
Bernd Bruegge & Allen Dutoit
-containedIn:Layer
+getLayer()
+setLayer(l)
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
56
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)
Bernd Bruegge & Allen Dutoit
SimulationRun
-scenarios:Vector
+elements()
+addScenario(s:Scenario)
+removeScenario(s:Scenario)
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
Matrikelnumber
Student
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
Image
filename:String
data:byte[]
width()
height()
paint()
Image
filename:String
width()
height()
paint()
ImageProxy
filename:String
width()
height()
paint()
Bernd Bruegge & Allen Dutoit
image
1
0..1
RealImage
data:byte[]
width()
height()
paint()
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 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 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
70