Using UML, Patterns, and Java
Object-Oriented Software Engineering
Chapter 10,
Mapping Models to Code
Overview

Operations on the object model:
 Optimizations to address performance requirements

Implementation of class model components:
 Realization of associations
 Realization of operation contracts

Self reading
Realizing entity objects based on selected storage strategy
 Mapping the class model to a storage schema
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
2
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.
Self reading
In C++? In Java?
Developers often revise the interface specification to
accommodate new requirements from the client.
All these activities are intellectually not challenging
 However, they have a repetitive and mechanical flavor that makes
them error prone.
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
3
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.
Recall our discussion

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.
And lack of traceability?
 Many improvised code changes and workarounds are made ad hoc,
which eventually leads to a severe degradation of the system.
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
4
Model transformations
Forward
engineering
Refactoring
Model
transformation
Reverse
engineering
Model space
Source code space
1-1 mapping?
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
5
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
Bernd Bruegge & Allen H. Dutoit
Advertiser
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Player
6
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;
//...
}
Bernd Bruegge & Allen H. Dutoit
public class Player
extends User {
//...
}
public class LeagueOwner extends
User {
//...
}
public class Advertiser extends
User {
//...
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
7
Refactoring Example: Pull Up Constructor Body
public class User {
public class User {
public User(String email) {
private String email;
this.email = email;
}
public class Player extends
User {
public Player(String email)
{
}
}
public class Player extends
User {
public Player(String email){
this.email = email;
super(email);
}
}
}
public class LeagueOwner
extends User{
public LeagueOwner(String
email) {
this.email = email;
}
public class LeagueOwner
extends User {
public LeagueOwner(String
email) {
super(email);
}
}
Bernd Bruegge & Allen H. Dutoit
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
8
Other Mapping Activities

Optimizing the Object Design Model
 Collapsing objects
 Delaying expensive computations



Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Bernd Bruegge & Allen H. Dutoit
Self reading
Object-Oriented Software Engineering: Using UML, Patterns, and Java
9
Collapsing an object without interesting behavior
Object design model before transformation
Person
SocialSecurity
number:String
Object design model after transformation
?
Person
SSN:String
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Object-Oriented Software Engineering: Using UML, Patterns, and Java
10
Self reading
Delaying expensive computations
Object design model before transformation
Image
filename:String
data:byte[]
paint()
Object design model after transformation
Image
filename:String
paint()
?
A good model?
Proxy Pattern
ImageProxy
filename:String
paint()
Bernd Bruegge & Allen H. Dutoit
image
1
0..1
RealImage
data:byte[]
paint()
Object-Oriented Software Engineering: Using UML, Patterns, and Java
11
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
12
Realization of a unidirectional, one-to-one association
Which direction?
Object design model before transformation
Advertiser
1
1
Source code after transformation
Account
Role name?
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
13
Bidirectional one-to-one association
Object design model before transformation
Advertiser
1
1
Source code after transformation
Account
Role name?
public class Account {
public class Advertiser {
/* The account field is initialized /* The owner field is initialized
* during the constructor and
* in the constructor and never
* never modified. */
* modified. */
private Advertiser owner;
private Account account;
public Account(owner:Advertiser) {
public Advertiser() {
this.owner = owner;
account = new Account(this);
}
}
public Advertiser getOwner() {
public Account getAccount() {
return owner;
return account;
}
}
}
}
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
14
Bidirectional, one-to-many association
Object design model before transformation
Advertiser
Source code after transformation
1
*
Account
constructor?
public class Account {
private Advertiser owner;
public void setOwner(Advertiser
newOwner) {
public Advertiser() {
if (owner != newOwner) {
accounts = new HashSet();
Advertiser old = owner;
}
owner = newOwner;
public void addAccount(Account
if (newOwner != null)
a) {
accounts.add(a);
newOwner.addAccount(this);
a.setOwner(this);
if (oldOwner != null)
}
public void
old.removeAccount(this);
removeAccount(Account a) {
}
accounts.remove(a);
}
complete?
a.setOwner(null);
Object-Oriented Software }
Engineering: Using UML, Patterns, and Java
15
}Bernd Bruegge & Allen H. Dutoit
public class Advertiser {
private Set accounts;
Bidirectional, many-to-many association
Object design model before transformation
Tournament
* {ordered}
*
Player
Source code after transformation
public class Tournament {
private List players;
public class Player {
private List tournaments;
/* ordered collection = sequence */
public Tournament() {
public Player() {
players = new ArrayList();
tournaments = new ArrayList();
/* implements List */
}
}
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);
}
}
}
}
}
}
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Object-Oriented Software Engineering: Using UML, Patterns, and Java
16
Transformation of an association class
Object design model before transformation
Statistics
+getAverageStat(name)
+getTotalStat(name)
+updateStats(match)
Tournament
*
Player
*
What’s the change?
Object design model after transformation: 1 class and two binary associations
Statistics
+getAverageStat(name)
+getTotalStat(name)
+updateStats(match)
1
1
Tournament
Bernd Bruegge & Allen H. Dutoit
*
*
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Player
17
Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions Self
Mapping Object Models to Tables
Bernd Bruegge & Allen H. Dutoit
reading
Object-Oriented Software Engineering: Using UML, Patterns, and Java
18
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.

Mapping of classes, attributes and associations






1-1 mapping?
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
19
Mapping the User class to a database table
User
+firstName:String
+login:String
+email:String
User table
id:long
firstName:text[25]
Bernd Bruegge & Allen H. Dutoit
login:text[8]
Object-Oriented Software Engineering: Using UML, Patterns, and Java
email:text[32]
20
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
21
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
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
22
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
...
1-1 mapping?
Bernd Bruegge & Allen H. Dutoit
League
League table
id:long
...
What
about
the
1 Patterns,
(*)andside?
Object-Oriented
Software Engineering:
Using UML,
Java
owner:long
23
Another Example for Buried Association
Transaction
Portfolio
*
portfolioID
...
transactionID
Transaction Table
transactionID
Portfolio Table
portfolioID ...
portfolioID
Foreign Key
1-1 mapping?
Bernd Bruegge & Allen H. Dutoit
What
about
the
1 Patterns,
(*)andside?
Object-Oriented
Software Engineering:
Using UML,
Java
24
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
the association “Serves”
Primary Key
City Table
cityName
Houston
Albany
Munich
Hamburg
Bernd Bruegge & Allen H. Dutoit
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
What are these?
25
Another Many-to-Many Association Mapping
We need the Tournament/Player association as a separate table
Tournament *
*
Player
Tournament table
id
name
23
novice
24
expert
Bernd Bruegge & Allen H. Dutoit
...
Player table
TournamentPlayerAssociation
table
tournament
player
23
56
23
79
id
name
56
alice
79
john
...
What
about
the
keys
– Javaprimary & foreign?
Object-Oriented
Software Engineering:
Using UML,
Patterns, and
26
Realizing Inheritance


Relational databases do not support inheritance
Two possibilities to map an inheritance association 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”)
/* flattening
 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
27
Realizing inheritance with a separate table
(”vertical mapping”)
User
name
Player
credits
LeagueOwner
maxNumLeagues
User table
id
name
56
zoe
79
john
...
role
LeagueOwner
Player
LeagueOwner table
id
56
maxNumLeagues
12
Bernd Bruegge & Allen H. Dutoit
Player table
...
id
credits
79
126
...
What should
toUML,retrieve
player info?
Object-Oriented be
Software done
Engineering: Using
Patterns, and Java
28
Realizing inheritance by duplicating columns
(”horizontal mapping”)
User
name
LeagueOwner
maxNumLeagues
Player
credits
Player table
LeagueOwner table
id
name
56
zoe
maxNumLeagues ...
id
name
credits
12
79
john
126
What
be doneObject-Oriented
when Software
the
“name”
of andaJavauser changes?
Bernd Bruegge should
& Allen H. Dutoit
Engineering:
Using UML, Patterns,
29
...
Comparison: Separate Tables vs Duplicated Columns

The trade-off is between modifiability and response time
 How likely is a change of the superclass?
If high, which one?
 What are the performance requirements for queries?

Separate table mapping
(”vertical 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
(”horizontal mapping”)
e.g., a hierachy
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
30
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
31
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
32
Additional Slides
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
33
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 */
}
Bernd Bruegge & Allen H. Dutoit
public class LeagueOwner extends
User {
private int maxNumLeagues;
public int getMaxNumLeagues() {
return maxNumLeagues;
}
public void setMaxNumLeagues
(int value) {
maxNumLeagues = value;
}
/* Other methods omitted */
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
34
Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions Self
Mapping Object Models to Tables
Bernd Bruegge & Allen H. Dutoit
reading
Object-Oriented Software Engineering: Using UML, Patterns, and Java
35
Self reading
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
36
Self reading
Bidirectional qualified association (continued)
Object design model before forward engineering
League
nickName
*
0..1
Player
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);
}
}
}
Bernd Bruegge & Allen H. Dutoit
public void addLeague
(String nickName, League l) {
if (!leagues.containsKey(l)) {
leagues.put(l, nickName);
l.addPlayer(nickName,
this);
}
}
}
Object-Oriented Software Engineering: Using UML, Patterns, and Java
37
Self reading
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.
 UML model
 Source code
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
38
Self reading
UML Model (Slide 22 in Object Design Lecture)
TournamentForm
1
1
+applyForTournament()
*
TournamentControl
*
+selectSponsors(advertisers):List
+advertizeTournament()
+acceptPlayer(p)
+announceTournament()
+isPlayerOverbooked():boolean
1
1
Tournament
* *
players
Player
*
matches *
Match
+start:Date
+status:MatchStatus
-maNumPlayers:String
* +start:Date
+end:Date
+acceptPlayer(p)
+removePlayer(p)
+getMaxNumPlayers()
*
sponsors * *
Advertiser
matches
*
+playMove(p,m)
+getScore():Map
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
39
Self reading
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());
}
}
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
40
}
}
Self reading
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
41
Self reading
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()
Bernd Bruegge & Allen H. Dutoit
«postcondition»
isPlayerAccepted(p)
Object-Oriented Software Engineering: Using UML, Patterns, and Java
42
Heuristics for Mapping Contracts to Exceptions
Be pragmatic, if you don’t have enough time, change your tests in
the following order:
1. Omit checking code for postconditions and invariants.

Often redundant, if you are confident that the code accomplishes
the functionality of the method
 Not likely to detect many bugs unless written by a separate tester
2.
3.
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.
4.
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
43
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
44
Statistics as a product in the Game Abstract Factory
Game
Tournament
createStatistics()
ChessGame
TicTacToeGame
Statistics
update()
getStat()
TTTStatisticsChessStatisticsDefaultStatistics
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
45
N-ary association class Statistics
Statistics relates League, Tournament, and Player
Statistics
1
*
1 1
0..1
Game
Bernd Bruegge & Allen H. Dutoit
0..1
League
0..1
Tournament
Object-Oriented Software Engineering: Using UML, Patterns, and Java
0..1
Player
46
Realization of the Statistics Association
TournamentControl
StatisticsView
StatisticsVault
update(match)
getStatNames(game)
getStat(name,game,player)
getStat(name,league,player)
getStat(name,tournament,player)
Statistics
update(match,player)
getStatNames()
getStat(name)
Game
createStatistics()
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
47
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()
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
48
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
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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
...
Bernd Bruegge & Allen H. Dutoit
League table
id:long
...
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Tournament table
id:long
...
50
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
51
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
52
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
53
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
54
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
55
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
56
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
57
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
58
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
59
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
60
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
61
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
62
Optimization Activities: Collapsing Objects
Matrikelnumber
Student
ID:String
Student
Matrikelnumber:String
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
63
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
64
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
65
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
66
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
67
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
68
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
69
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Lecture for Chapter 10, Mapping Models To Code