Transcript Lecture for Chapter 10, Mapping Models To Code - ICAR-CNR

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 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
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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.
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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.
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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
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Advertiser
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Player
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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 */
}
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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
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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
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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()
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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
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Implement 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:
 Translate to Vector
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Unidirectional 1-to-1 Association
Object desig n mo d el b efore tran sfo rmation
ZoomInAction
1
1
MapArea
Object desig n mo d el after tran sfo rmation
ZoomInAction
targetMap:MapArea
MapArea
Usually this transformation is automatically applied by the CASE tool
in the code model
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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)
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MapArea
-zoomIn:ZoomInAction
+getZoomInAction()
+setZoomInAction(action)
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1-to-Many Association
Object design model before transformation
Layer
1
LayerElement
*
Object design model after transformation
Layer
-layerElements:Set
+elements()
+addElement(le)
+removeElement(le)
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LayerElement
-containedIn:Layer
+getLayer()
+setLayer(l)
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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
tournaments = new
ArrayList();
ArrayList();
}
}
public void addPlayer(Player public void
p) {
addTournament(Tournament t) {
if (!players.contains(p))
if
{
(!tournaments.contains(t)) {
players.add(p);
tournaments.add(t);
t.addPlayer(this);
p.addTournament(this);
}
Bernd}
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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
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*
*
Object-Oriented Software Engineering: Using UML, Patterns, and Java
Player
25
Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
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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
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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.addPlayer((Player)i.next());
} catch (KnownPlayerException e) {
// If an exception was caught, log it to the console
ErrorConsole.log(e.getMessage());
}
}
}
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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.
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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()
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«postcondition»
isPlayerAccepted(p)
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30
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.
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Other Mapping Activities




Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables
Bernd Bruegge & Allen H. Dutoit
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32
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
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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]
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login:text[8]
Object-Oriented Software Engineering: Using UML, Patterns, and Java
email:text[32]
34
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.
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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
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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.
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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
Bernd Bruegge & Allen H. Dutoit
...
League
League table
id:long
...
Object-Oriented Software Engineering: Using UML, Patterns, and Java
owner:long
38
Another Example for Buried Association
Transaction
Portfolio
*
transactionID
Foreign Key
Transaction Table
transactionID
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portfolioID
Object-Oriented Software Engineering: Using UML, Patterns, and Java
portfolioID
...
Portfolio Table
portfolioID ...
39
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
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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
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Mapping 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
Object-Oriented Software Engineering: Using UML, Patterns, and Java
id
name
56
alice
79
john
41
...
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
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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
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Player table
...
id
credits
79
126
Object-Oriented Software Engineering: Using UML, Patterns, and Java
...
43
Realizing inheritance by duplicating columns
User
name
LeagueOwner
maxNumLeagues
Player
credits
LeagueOwner table
id
name
56
zoe
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Player table
maxNumLeagues ...
id
name
credits
12
79
john
126
Object-Oriented Software Engineering: Using UML, Patterns, and Java
...
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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
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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.
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Documenting the Object Design: The Object Design
Document (ODD)

Object design document
 Same as RAD +...
 … + additions to object, functional and dynamic models (from solution
domain)
 … + Navigational map for object model
 … + Javadoc documentation for all classes
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Criteria for ODD

Restrictiveness
 A specification should be precise enough that it excludes unwanted
implementations.
 Preconditions and postconditions specifying border cases is one way
to achieve restrictive specifications.

Generality
 A specification, however, should not restrict its implementation.

Clarity
 A specification should be easily and unambiguously understandable
by developers.
 Certain behaviors are more easily described in natura1 language,
whereas boundary cases can be described with constraints and
exceptions.
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Approaches to documenting object design

Self-contained ODD generated from model.
 We write and maintain a new UML model using a tool and generate the document
automatically. This document would duplicate any application objects identified during
analysis.
 Disadvantages:
 redundancy with the Requirements Analysis Document (RAD)
 high level of effort for maintaining consistency with the RAD.
 This often leads to an RAD and an ODD that are inaccurate or out of date.

ODD as extension of the RAD
 the object design is considered as the set of application objects augmented with solution objects
(we add solution objects to the RAD)
 Advantage
 maintaining consistency between the RAD and the ODD becomes much easier
 Disadvantages
 Polluting the RAD with information that is irrelevant to the client and the user.

ODD embedded into source code.
 We embed the ODD into the source code.
 We first represent the ODD using a modeling tool.
 Once the ODD becomes stable, we use the modeling tool to generate class stubs. We
describe each class interface using tagged comments that distinguish source code
comments from object design descriptions.
 Once the object design model is documented in the code, we abandon the initial object
design model.
 Advantage
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 Consistency between the object design model and the source code
Analysis
Embedded ODD approach
Analysis model
Document
analysis
RAD
System design
Subsystem
decomposition
Design goals
Object design
Initial object
design model
Generate class stubs
Initial class
stubs
Implementation
Commented code
Bernd Bruegge & Allen H. Dutoit
Document
object design
ODD
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Documenting Object Design: ODD Conventions

Each subsystem in a system provides a service (see Chapter on
System Design)
 Describes the set of operations provided by the subsystem

Specifying a service operation as
 Signature: Name of operation, fully typed parameter list and return
type
 Abstract: Describes the operation
 Pre: Precondition for calling the operation
 Post: Postcondition describing important state after the execution of the
operation
Use JavaDoc for the specification of service operations.
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JavaDoc





Add documentation comments to the source code.
A doc comment consists of characters between /** and */
When JavaDoc parses a doc comment, leading * characters on
each line are discarded. First, blanks and tabs preceding the
initial * characters are also discarded.
Doc comments may include HTML tags
Example of a doc comment:
/**
* This is a <b> doc </b> comment
*/
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More on Java Doc


Doc comments are only recognized when placed immediately
before class, interface, constructor, method or field
declarations.
When you embed HTML tags within a doc comment, you
should not use heading tags such as <h1> and <h2>, because
JavaDoc creates an entire structured document and these
structural tags interfere with the formatting of the generated
document.
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Class and Interface Doc Tags
@author name-text
 Creates an “Author” entry.
@version version-text
 Creates a “Version” entry.
@see classname
 Creates a hyperlink “See Also classname”
@since since-text
 Adds a “Since” entry. Usually used to specify that a feature or
change exists since the release number of the software specified in
the “since-text”
@deprecated deprecated-text
 Adds a comment that this method can no longer be used.
Convention is to describe method that serves as replacement
 Example: @deprecated Replaced by setBounds(int, int, int, int).
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Constructor and Method Doc Tags

Can contain @see tag, @since tag, @deprecated as well as:
@param parameter-name description
Adds a parameter to the "Parameters" section. The description may
be continued on the next line.
@return description
Adds a "Returns" section, which contains the description of the
return value.
@exception fully-qualified-class-name description
Adds a "Throws" section, which contains the name of the exception
that may be thrown by the method. The exception is linked to its
class documentation.
@see classname
Adds a hyperlink "See Also" entry to the method.
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Example: Specifying a Service in Java
/** Office is a physical structure in a building. It is possible to create an
instance of a office; add an occupant; get the name and the number of
occupants */
public class Office {
/** Adds an occupant to the office */
* @param NAME name is a nonempty string */
public void AddOccupant(string name);
/** @Return Returns the name of the office. Requires, that Office
has been initialized with a name */
public string GetName();
....
}
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Summary


Undisciplined changes => degradation of the system model
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
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