Chapter 10 mapping models to code
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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.

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Chapter 10 mapping models to code

Chapter 10,Mapping Models to Code


Overview

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.

    • Operations on the object model:

      • Optimizations to address performance requirements

    • Implementation of class model components:

      • Realization of associations

      • Realization of operation contracts

    • Realizing entity objects based on selected storage strategy

      • Mapping the class model to a storage schema


Characteristics of object design activities

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.


State of the art of model based software engineering

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.


Model transformation example

LeagueOwner

Advertiser

Player

+email:Address

+email:Address

+email:Address

User

+email:Address

LeagueOwner

Advertiser

Player

Model Transformation Example

Object design model before transformation

Object design model after transformation:


Forward engineering example

public class User {

private String email;

public String getEmail() {

return email;

}

public void setEmail(String value){

email = value;

}

publicvoid 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 */

}

Forward Engineering Example

Object design model before transformation

LeagueOwner

User

+maxNumLeagues:int

+email:String

+notify(msg:String)

Source code after transformation


Other mapping activities

Other Mapping Activities

  • Optimizing the Object Design Model

  • Mapping Associations

  • Mapping Contracts to Exceptions

  • Mapping Object Models to Tables


Collapsing an object without interesting behavior

Person

SSN:String

Collapsing an object without interesting behavior

Object design model before transformation

Person

SocialSecurity

number:String

?

Object design model after transformation


Delaying expensive computations

Image

filename:String

data:byte[]

paint()

Image

filename:String

paint()

image

RealImage

ImageProxy

1

0..1

filename:String

data:byte[]

paint()

paint()

Delaying expensive computations

Object design model before transformation

?

Object design model after transformation


Other mapping activities1

Other Mapping Activities

  • Optimizing the Object Design Model

  • Mapping Associations

  • Mapping Contracts to Exceptions

  • Mapping Object Models to Tables


Implement associations

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


Unidirectional 1 to 1 association

Unidirectional 1-to-1 Association

Usually this transformation is automatically applied by the CASE tool

in the code model


Bidirectional 1 to 1 association

Bidirectional 1-to-1 Association

Object design model before transformation

ZoomInAction

MapArea

1

1

Object design model after transformation

ZoomInAction

MapArea

-targetMap:MapArea

-zoomIn:ZoomInAction

+getTargetMap()

+getZoomInAction()

+setTargetMap(map)

+setZoomInAction(action)


1 to many association

1-to-Many Association

Object design model before

transformation

Layer

LayerElement

1

*

Object design model after transformation

Layer

LayerElement

-layerElements:Set

-containedIn:Layer

+elements()

+getLayer()

+addElement(le)

+setLayer(l)

+removeElement(le)


Bidirectional many to many association

public class Tournament {

private List players;

public Tournament() {

players = new ArrayList();

}

publicvoid 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);

}

}

}

Bidirectional, many-to-many association

Object design model before transformation

{ordered}

*

*

Tournament

Player

Source code after transformation


Transformation of an association class

Statistics

+

getAverageStat(name)

+

getTotalStat(name)

+

updateStats(match)

1

1

Tournament

Player

*

*

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


Other mapping activities2

Other Mapping Activities

  • Optimizing the Object Design Model

  • Mapping Associations

  • Mapping Contracts to Exceptions

  • Mapping Object Models to Tables


Exceptions as building blocks for contract violations

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


The try throw catch mechanism in java

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());

}

}

}

}


Implementing a contract

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.


A complete implementation of the tournament addplayer contract

«invariant»

getMaxNumPlayers() > 0

Tournament

-maxNumPlayers: int

«precondition»

!isPlayerAccepted(p)

+getNumPlayers():int

+getMaxNumPlayers():int

+isPlayerAccepted(p:Player):boolean

+addPlayer(p:Player)

«precondition»getNumPlayers() <getMaxNumPlayers()

«postcondition»isPlayerAccepted(p)

A complete implementation of the Tournament.addPlayer() contract


Heuristics for mapping contracts to exceptions

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.


Other mapping activities3

Other Mapping Activities

  • Optimizing the Object Design Model

  • Mapping Associations

  • Mapping Contracts to Exceptions

  • Mapping Object Models to Tables


Mapping an object model to a relational database

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

    • Aone-to-many association is implemented as buried foreign key

  • Methods are not mapped


Mapping the user class to a database table

Mapping the User class to a database table

User

+firstName:String

+login:String

+email:String

User table

id:long

firstName:text[25]

login:text[8]

email:text[32]


Primary and foreign keys

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 candidatekey.

  • 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.


Example for primary and foreign keys

Primary key

fi

r

stName

login

email

“alice”

“am384”

[email protected]

“john”

“js289”

[email protected]

“bob”

“bd”

[email protected]

Candidate key

Candidate key

name

login

“tictactoeNovice”

“am384”

“tictactoeExpert”

“am384”

“chessNovice”

“js289”

Foreign key referencing User table

Example for Primary and Foreign Keys

User tab

le

League table


Buried association

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.


Buried association1

1

*

LeagueOwner

League

LeagueOwner table

League table

id:long

...

id:long

...

o

wner:long

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.


Another example for buried association

Transaction Table

Portfolio Table

transactionID

portfolioID

portfolioID ...

Another Example for Buried Association

Portfolio

portfolioID

...

Transaction

transactionID

*

Foreign Key


Mapping many to many associations

City

cityName

Airport

airportCode

airportName

*

Serves

*

Airport Table

Serves Table

City Table

airportCode

IAH

HOU

ALB

MUC

HAM

airportCode

IAH

HOU

ALB

MUC

HAM

cityName

Houston

Houston

Albany

Munich

Hamburg

airportName

Intercontinental

Hobby

Albany County

Munich Airport

Hamburg Airport

cityName

Houston

Albany

Munich

Hamburg

Mapping Many-To-Many Associations

In this case we need a separate table for the association

Separate table for

“Serves” association

Primary Key


Mapping the tournament player association as a separate table

Tournament table

Player table

TournamentPlayerAssociation

table

id

name

...

id

name

...

23

no

vice

56

alice

tournament

player

24

e

xper

t

79

john

23

56

23

79

Mapping the Tournament/Player association as a separate table

*

*

Tournament

Player


Realizing inheritance

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


Realizing inheritance with a separate table

User

name

Player

LeagueOwner

credits

maxNumLeagues

User table

id

name

...

r

ole

56

z

oe

LeagueOwner

79

john

Pla

y

er

LeagueOwner table

Player table

maxNumLeagues

id

...

id

credits

...

56

12

79

126

Realizing inheritance with a separate table


Realizing inheritance by duplicating columns

LeagueOwner table

Player table

id

...

id

...

name

maxNumLeagues

name

credits

56

z

oe

12

79

john

126

Realizing inheritance by duplicating columns

User

name

LeagueOwner

Player

maxNumLeagues

credits


Comparison separate tables vs duplicated columns

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


Heuristics for transformations

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.


Documenting the object design the object design document odd

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


Criteria for odd

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.


Approaches to documenting object design

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

      • Consistency between the object design model and the source code


Embedded odd approach

Analysis

Document

Analysis model

RAD

analysis

Generate class stubs

System design

Initial class

Subsystem

Design goals

stubs

decomposition

Object design

Implementation

Initial object

Document

ODD

Commented code

design model

object design

Embedded ODD approach


Documenting object design odd conventions

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.


Javadoc

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

    */


More on java doc

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.


Class and interface doc tags

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).


Constructor and method doc tags

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.


Example specifying a service in java

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();

....

}


Summary

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


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