1. Chapter 10, Mapping Models to Code

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.
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

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.

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.

5. Four types of transformations
Model transformations – operate on object models
Refactoring – source code transformations
Forward Engineering – changes in an object (e.g., attributes or associations) map into corresponding changes in source code
Reverse Engineering – the model space is defined based on existing source code

6. Model transformations
Model space
Source code space
Forward engineering
Refactoring
Model transformation
Reverse engineering

7. Model Transformation Example
Object design model before transformation
LeagueOwner
+ Address
Advertiser
+ Address
Player
+ Address
Object design model after transformation:
Player
Advertiser
LeagueOwner
User
+ Address

8. Refactoring by pulling up a field
Create a parent class that will hold the field that is factored out

9. Refactoring Example: Pull Up Field
public class User {
private String ; }
public class Player extends User {
//...
public class LeagueOwner extends User {
public class Advertiser extends User {
public class Player {
private String ;
//... }
public class LeagueOwner {
private String ;
public class Advertiser {
private String _address;

10. Refactoring by pulling up a constructor body
Create a parent class that will assign a value to the field in common

11. Refactoring Example: Pull Up Constructor Body
public class User {
private String ; }
public class Player extends User {
public Player(String ) {
this. = ;
public class LeagueOwner extends User{
public LeagueOwner(String ) {
public class Advertiser extendsUser{
public Advertiser(String ) {
public class User {
public User(String ) {
this. = ; }
public class Player extends User {
public Player(String ) {
super( );
public class LeagueOwner extends User {
public LeagueOwner(String ) {
public class Advertiser extends User {
public Advertiser(String ) {

12. Refactoring by pulling up methods
The field and initialization in the constructor have been pulled up to the parent class
Now see if any methods associated with can be moved to the parent class

13. Forward Engineering Example
Object design model before transformation
User
LeagueOwner
+ String
+maxNumLeagues:int
+notify(msg:String)
Source code after transformation
public class User {
private String ;
public String get () {
return ; }
public void set (String value){
= 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 */

14. Transformation principles
Each transformation must address a single criteria
Each transformation must be local
Each transformation must be applied in isolation to other changes
Each transformation must be followed by a validation step

15. Other Mapping Activities
Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables

16. Some optimizations Avoid frequent repeated association traversals
Multiple association traversals can lead to code such as method1().method2(). …. methodn()
The sequence diagram can be used to identify such traversals
See if a direct connection is possible to improve efficiency
Try to make “many” associations more efficient
Try to use a qualified association to reduce multiplicity to one
Try to use ordering or indexing to decrease access time
Try to avoid misplaced attributes
Inefficient due to the need for get and set methods
Try bringing the attribute into the calling class
Catch the result of expensive computations
Example, statistics in the Arena example are only updated after a match is completed
This only has to be calculated once at the end of each match and not repeatedly until another match is completed

17. Collapsing an object without interesting behavior
Object design model before transformation
Person
SocialSecurity
number:String
Object design model after transformation
Person
SSN:String

18. Delaying expensive computations
Object design model before transformation
Image
filename:String
paint()
data:byte[]
Object design model after transformation
Image
filename:String
RealImage
data:byte[]
ImageProxy
image 1
0..1
paint()

19. Other Mapping Activities
Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables

20. Realization of a unidirectional, one-to-one association
Object design model before transformation 1 1
Advertiser
Account
Source code after transformation
public class Advertiser {
private Account account;
public Advertiser() {
account = new Account(); }
public Account getAccount() {
return account;

21. Bidirectional one-to-one association
Object design model before transformation 1 1
Advertiser
Account
Source code after transformation
public class Advertiser {
/* The account field is initialized
* in the constructor and never
* modified. */
private Account account;
public Advertiser() {
account = new Account(this); }
public Account getAccount() {
return account;
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;

22. Bidirectional, one-to-many association
Object design model before transformation 1 *
Advertiser
Account
Source code after transformation
public class Advertiser {
private Set accounts;
public Advertiser() {
accounts = new HashSet(); }
public void addAccount(Account a) {
accounts.add(a);
a.setOwner(this);
public void removeAccount(Account a) {
accounts.remove(a);
a.setOwner(null);
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); }

23. Bidirectional, many-to-many association
Object design model before transformation *
{ordered} *
Tournament
Player
Source code after transformation
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);

24. Bidirectional qualified association
Object design model before transformation * *
League
Player
nickName
Object design model before forward engineering
Player
nickName
0..1 *
League
Source code after forward engineering

25. Bidirectional qualified association (continued)
Source code after forward engineering
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); }

26. 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
Tournament
Player * 1
Statistics +
getAverageStat(name)
getTotalStat(name)
updateStats(match)

27. Other Mapping Activities
Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables

28. 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

29. 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());
Example of exception handling in Java. TournamentForm catches exceptions raised by Tournament and TournamentControl and logs them into an error console for display to the user.

30. 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.

31. A complete implementation of the Tournament.addPlayer() contract
«invariant»
getMaxNumPlayers() > 0
Tournament
+isPlayerAccepted(p:Player):boolean
+addPlayer(p:Player)
+getMaxNumPlayers():int
-maxNumPlayers: int
+getNumPlayers():int
«precondition»
!isPlayerAccepted(p)
«precondition» getNumPlayers() < getMaxNumPlayers()
«postcondition» isPlayerAccepted(p)

32. The code

33. 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.

34. Heuristics for mapping contracts to exceptions

35. Other Mapping Activities
Optimizing the Object Design Model
Mapping Associations
Mapping Contracts to Exceptions
Mapping Object Models to Tables

36. 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

37. Mapping the User class to a database table
+firstName:String
+login:String
+ String
User table
id:long
firstName:text[25]
login:text[8]
text[32]

38. 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.

39. Example for Primary and Foreign Keys
Primary key
User tab le
login
“am384”
“js289” fi r
stName
“alice”
“john”
“bd”
“bob”
Candidate key
Candidate key
login
“am384”
name
“tictactoeNovice”
“tictactoeExpert”
“js289”
“chessNovice”
League table
Foreign key referencing User table

40. 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.

41. 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.
League
LeagueOwner * 1
id:long
LeagueOwner table
... o
wner:long
League table
...
id:long

42. Another Example for Buried Association
Portfolio
portfolioID
...
Transaction
transactionID *
Foreign Key
portfolioID
Portfolio Table
transactionID
Transaction Table
portfolioID

43. Mapping Many-To-Many Associations
In this case we need a separate table for the association
City
cityName
Airport
airportCode
airportName *
Serves
Separate table for
“Serves” association
Primary Key
cityName
Houston
Albany
Munich
Hamburg
City Table
airportCode
IAH
HOU
ALB
MUC
HAM
Airport Table
airportName
Intercontinental
Hobby
Albany County
Munich Airport
Hamburg Airport
cityName
Houston
Albany
Munich
Hamburg
Serves Table
airportCode
IAH
HOU
ALB
MUC
HAM

44. Mapping the Tournament/Player association as a separate table* *
Tournament
Player id
Tournament table 23
name
... no
vice 24 e
xper t
Player table id 56
name
...
alice 79
john
tournament
player
TournamentPlayerAssociation
table 23 56 79

45. 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

46. Realizing inheritance with a separate table
Player
User
LeagueOwner
maxNumLeagues
credits
name
User table id 56
name
... z oe 79
john r
ole
LeagueOwner
Pla y er id
LeagueOwner table 56
maxNumLeagues
... 12
Player table id 79
credits
...
126

47. Realizing inheritance by duplicating columns
User
name
LeagueOwner
Player
maxNumLeagues
credits id
LeagueOwner table 56
maxNumLeagues
... 12
name z oe
Player table id 79
credits
...
126
name
john

48. 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

49. 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.

50. 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

51. Statistics as a product in the Game Abstract Factory
Tournament
Game
createStatistics()
TicTacToeGame
ChessGame
Statistics
update()
getStat()
TTTStatistics
ChessStatistics
DefaultStatistics

52. N-ary association class Statistics
Statistics relates League, Tournament, and Player
Statistics 1 * 1 1
0..1
0..1
0..1
0..1
Game
League
Tournament
Player

53. Realization of the Statistics Association
TournamentControl
StatisticsView
StatisticsVault
Statistics
update(match)
update(match,player)
getStatNames(game)
getStatNames()
getStat(name)
getStat(name,game,player)
getStat(name,league,player)
getStat(name,tournament,player)
Game
createStatistics()

54. StatisticsVault as a Facade
TournamentControl
StatisticsView
StatisticsVault
Statistics
update(match)
update(match,player)
getStatNames(game)
getStatNames()
getStat(name)
getStat(name,game,player)
getStat(name,league,player)
Game
getStat(name,tournament,player)
createStatistics()

55. 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)
public double getStat(String name, Tournament t, Player p) }

56. 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
League table T
ournament table
id:long
...
id:long
...
id:long
...

57. Restructuring Activities
Realizing associations
Revisiting inheritance to increase reuse
Revising inheritance to remove implementation dependencies

58. 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
Not covered in Fall 91 class

59. Unidirectional 1-to-1 Association
Object design model before transformation
ZoomInAction
MapArea
Object design model after transformation
ZoomInAction
MapArea
-zoomIn:ZoomInAction
+getZoomInAction()
+setZoomInAction(action)

60. 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()
+setTargetMap(map)
+getZoomInAction()
+setZoomInAction(action)

61. 1-to-Many Association Object design model before transformation
Layer
LayerElement 1 *
Object design model after transformation
Layer
-layerElements:Set
+elements()
+addElement(le)
+removeElement(le)
LayerElement
-containedIn:Layer
+getLayer()
+setLayer(l)

62. Qualification Object design model before transformation
Scenario *
0..1
SimulationRun
simname
Object design model after transformation
Scenario
SimulationRun
-runs:Hashtable
-scenarios:Vector
+elements()
+addRun(simname,sr:SimulationRun)
+removeRun(simname,sr:SimulationRun)
+elements()
+addScenario(s:Scenario)
+removeScenario(s:Scenario)

63. 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.

64. 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

65. Object Design Areas 1. Service specification 2. Component selection
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.

66. 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
Not covered in Fall 91 class

67. 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
Not covered in Fall 91 class

68. 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.

69. Optimization Activities: Collapsing Objects
Student
Matrikelnumber
ID:String
Matrikelnumber:String

70. To Collapse or not to Collapse?
Collapse a class into an attribute if the only operations defined on the attributes are Set() and Get().

71. 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)

72. Optimization Activities: Delaying Complex Computations
Image
filename:String
width()
height()
paint()
RealImage
data:byte[]
ImageProxy
image 1
0..1

73. 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.

74. 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

75. 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
Not covered in Fall 91 class