CSCE 431: Testing Some material from Bruegge, Dutoit, Meyer et al.
Lecture for Chapter 10, Mapping Models To Codechate/2110634/07-Mapping-Model-to-Code.pdf · Bernd...
Transcript of Lecture for Chapter 10, Mapping Models To Codechate/2110634/07-Mapping-Model-to-Code.pdf · Bernd...
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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
Model
transformation
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:Address
LeagueOwner
+email:Address
Player Advertiser LeagueOwner
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
Account Advertiser 1 1
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;
}
}
Account Advertiser 1 1
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 Account 1 *
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
Player nickName
0..1 * League
Player * *
Object design model before transformation
League
nickName
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 console
ErrorConsole.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 tab le
Candidate key
login email
“am384” “[email protected]”
“js289” “[email protected]”
fi r stName
“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
League LeagueOwner * 1
id:long
LeagueOwner table
... o wner: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
Airport
airportCode
airportName
* * Serves
cityName
Houston
Albany
Munich
Hamburg
City Table
airportCode
IAH
HOU
ALB
MUC
HAM
Airport Table
airportName
Intercontinental
Hobby
Albany County
Munich Airport
Hamburg Airport
Primary Key
cityName
Houston
Houston
Albany
Munich
Hamburg
Serves Table
airportCode
IAH
HOU
ALB
MUC
HAM
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
Player Tournament * *
id
Tournament table
23
name ...
no vice
24 e xper t tournament 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 ...
z oe
79 john
r ole
LeagueOwner
Pla y er
Player
User
LeagueOwner
maxNumLeagues 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
z oe
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 mapping
We can add attributes to the superclass easily by adding a column to the superclass table
Searching for the attributes of an object requires a join operation.
Duplicated columns
Modifying the database schema is more complex and error-prone
Individual objects are not fragmented across a number of tables, resulting in faster queries
Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 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 Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 43
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 44
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 45
Unidirectional 1-to-1 Association
MapArea ZoomInAction
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 46
Bidirectional 1-to-1 Association
MapArea ZoomInAction 1 1
Object design model before transformation
MapArea ZoomInAction
-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 47
1-to-Many Association
Layer LayerElement 1 *
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 48
Qualification
SimulationRun simname 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 49
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 50
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 51
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 52
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 53
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 54
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 55
Optimization Activities: Collapsing Objects
Student
Matrikelnumber
ID:String
Student
Matrikelnumber:String
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 56
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 57
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 58
Optimization Activities: Delaying Complex Computations
Image
filename:String
width() height() paint()
Image
filename:String
width() height() paint()
RealImage
width() height() paint()
data:byte[]
data:byte[]
ImageProxy
filename:String
width() height() paint()
image
1 0..1
Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 59
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 60
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 61
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