1. Chapter 10, Mapping Models to Code

2. Lecture Plan Part 1 Part 2 Operations on the object model:
Optimizations to address performance requirements
Implementation of class model components:
Realization of associations
Realization of operation contracts
Part 2
Realizing entity objects based on selected storage strategy
Mapping the object model to a storage schema
Mapping class diagrams to tables
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

3. Problems with implementing an Object Design Model
Programming languages do not support the concept of UML associations
The associations of the object model must be transformed into collections of object references
Many programming languages do not support contracts (invariants, pre and post conditions)
Developers must therefore manually transform contract specification into source code for detecting and handling contract violations
The client changes the requirements during object design
The developer must change the contracts in which the classes are involved
All these object design activities cause problems, because they need to be done manually.

4. Let us get a handle on these problems
To do this we distinguish two kinds of spaces
the model space and the source code space
and 4 different types of transformations
Model transformation
Forward engineering
Reverse engineering
Refactoring.

5. 4 Different Types of Transformations
Program
(in Java)
Yet Another
System Model
Another
System Model
Forward
engineering
Model space
Source code space
Refactoring
Model transformation
Another
Program
Reverse
engineering
System Model
(in UML)

6. Model Transformation
Takes as input a model conforming to a meta model (for example the MOF metamodel) and produces as output another model conforming to the metamodel
Model transformations are used in MDA (Model Driven Architecture).

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. Model space Source code space Forward engineering Refactoring
Program
(in Java)
Yet Another
System Model
Another
System Model
Forward
engineering
Model space
Source code space
Refactoring
Model transformation
Another
Program
Reverse
engineering
System Model
(in UML)

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

10. Refactoring Example: Pull Up Constructor Body
public class User {
public User(String ) {
this. = ; }
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 extends User{
public Advertiser(String ) {
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 ) { }.

11. 4 Different Types of Transformations
Program
(in Java)
Yet Another
System Model
Another
System Model
Forward
engineering
Model space
Source code space
Refactoring
Model transformation
Another
Program
Reverse
engineering
System Model
(in UML)

12. Forward Engineering Example
Object design model before transformation:
LeagueOwner
-maxNumLeagues:int
+getMaxNumLeagues():int +setMaxNumLeagues(n:int)
User
- String
+get ():String +set (e:String) +notify(msg:String)
Source code after transformation:
public class User {
private String ;
public String get () {
return ; }
public void set (String e){
= e;
public void notify(String msg) {
// ....
public class LeagueOwner extends User {
private int maxNumLeagues;
public int getMaxNumLeagues() {
return maxNumLeagues; }
public void setMaxNumLeagues(int n) {
maxNumLeagues = n;

13. More Forward Engineering Examples
Model Transformations
Goal: Optimizing the object design model
Collapsing objects
Delaying expensive computations
Forward Engineering
Goal: Implementing the object design model in a programming language
Mapping inheritance
Mapping associations
Mapping contracts to exceptions
Mapping object models to tables

14. Collapsing Objects Object design model before transformation:
Person
SocialSecurity
number:String
Object design model after transformation:
Person
SSN:String
Turning an object into an attribute of another object is usually
done, if the object does not have any interesting dynamic behavior (only get and set operations).

15. Examples of Model Transformations and Forward Engineering
Goal: Optimizing the object design model
Collapsing objects
Delaying expensive computations
Forward Engineering
Goal: Implementing the object design model in a programming language
Mapping inheritance
Mapping associations
Mapping contracts to exceptions
Mapping object models to tables

16. Delaying expensive operations
Object design model before transformation:
Image
filename:String
paint()
data:byte[]
Question: Which pattern is being used here? Answer: The proxy pattern!
Why: Image is an analysis domain object, the proxy pattern subclasses Imageproxy and RealImage are solution
domain objects!
Object design model after transformation:
Image
filename:String
RealImage
data:byte[]
ImageProxy
image 1
0..1
paint()
Proxy Pattern

17. Examples of Model Transformations and Forward Engineering
Goal: Optimizing the object design model
Collapsing objects
Delaying expensive computations
Forward Engineering
Goal: Implementing the object design model in a programming language
Mapping inheritance
Mapping associations
Mapping contracts to exceptions
Mapping object models to tables

18. Forward Engineering: Mapping a UML Model into Source Code
Goal: We have a UML-Model with inheritance. We want to translate it into source code
Question: Which mechanisms in the programming language can be used?
Let’s focus on Java
Java provides the following mechanisms:
Overwriting of methods (default in Java)
Final classes
Final methods
Abstract methods
Abstract classes
Interfaces.
For each of these mechanisms a project manager can specify heuristics to use them properly

19. Realizing Inheritance in Java
Realisation of specialization and generalization
Definition of subclasses
Java keyword: extends
Realisation of simple inheritance
Overwriting of methods is not allowed
Java keyword: final
Realisation of implementation inheritance
Overwriting of methods
No keyword necessary:
Overwriting of methods is default in Java
Realisation of specification inheritance
Specification of an interface
Java keywords: abstract, interface.
Java offers several techniques to realize the different types of inheritance:
Realisation of simple inheritance also called strict inheritance

20. Examples of Model Transformations and Forward Engineering
Goal: Optimizing the object design model
Collapsing objects
Delaying expensive computations
Forward Engineering
Goal: Implementing the object design model in a programming language
Mapping inheritance
Mapping associations
Mapping contracts to exceptions
Mapping object models to tables

21. Mapping Associations Unidirectional one-to-one association
Bidirectional one-to-one association
Bidirectional one-to-many association
Bidirectional many-to-many association
Bidirectional qualified association.

22. 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(); }
If Advertiser accesses only Account, but not vice versa, the class Account can be made a local attribute in the Advertiser class

23. Bidirectional one-to-one association
Object design model before transformation:
Advertiser 1 1
Account
+ getAcount (): Account
+ getOwner (): Advertiser
Source code after transformation:
public class Advertiser {
/* account 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 {
/* owner is initialized
* in the constructor and
* never modified. */
private Advertiser owner;
public Account(Advertiser owner) {
this.owner = owner; }
public Advertiser getOwner() {
return owner;

24. Bidirectional one-to-many association
Object design model before transformation: 1 *
Advertiser
Account
+ addAcount (a: Account)
+ setOwner (Advertiser: NewOwner)
+ removeAcount (a: 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); }
HastSet implements the Set interface, backed by a hash table (actually a HashMap instance (see
It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance's size (the number of elements) plus the "capacity" of the backing HashMap instance (the number of buckets). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

25. Bidirectional many-to-many association
Object design model before transformation
Tournament *
{ordered} *
Player *
+addTournament(t: Tournament)
+ addPlayer(p: 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);

26. Examples of Model Transformations and Forward Engineering
Goal: Optimizing the object design model
Collapsing objects
Delaying expensive computations
Forward Engineering
Goal: Implementing the object design model in a programming language
Mapping inheritance
Mapping associations
Mapping contracts to exceptions
Mapping object models to tables
Moved to Exercise Session
on Testing
Next!

27. Summary 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

28. Additional Slides

29. Heuristics for Transformations
For any given transformation always use the same tool
Keep the contracts in the source code, not in the object design model
Use the same names for the same objects
Have a style guide for transformations (Martin Fowler)
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.

30. Generalization: Finding the super class
You need to prepare or modify your classes for generalization
All operations must have the same signature but often the signatures do not match
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.
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

31. 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.

32. 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

33. Object 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

34. Object Design Optimizations (continued)
Store derived attributes
Example: Define new classes to store information locally (database cache, proxy pattern)
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 the active value is changed (observer pattern, data trigger).

35. 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.

36. Heuristics for Implementing 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 always treated like attributes in the classes and translated into references
1-to-many association:
Always translated into a Vector
Qualified association:
Always translated into to a Hash table.
Not covered in Fall 91 class

37. Model-Driven Engineering
Model-driven engineering refers to a range of development approaches that are based on the use of software modeling as a primary form of expression. Sometimes models are constructed to a certain level of detail, and then code is written by hand in a separate step.
Sometimes complete models are built including executable actions. Code can be generated from the models, ranging from system skeletons to complete, deployable products.
With the introduction of the Unified Modeling Language (UML), MDE has become very popular today with a wide body of practitioners and supporting tools. More advanced types of MDE have expanded to permit industry standards which allow for consistent application and results. The continued evolution of MDE has added an increased focus on architecture and automation.
MDE technologies with a greater focus on architecture and corresponding automation yield higher levels of abstraction in software development. This abstraction promotes simpler models with a greater focus on problem space. Combined with executable semantics this elevates the total level of automation possible.
The Object Management Group (OMG) has developed a set of standards called model-driven architecture (MDA), building a foundation for this advanced architecture-focused approach.

38. A Historical View on MDE
The first tools to support MDE were the Computer-Aided Software Engineering (CASE) tools developed in the 1980s
Companies like Integrated Development Environments (IDE - StP), Higher Order Software (now Hamilton Technologies, Inc., HTI), Cadre Technologies, Bachman Information Systems, and Logicworks (BP-Win and ER-Win) were pioneers in the field.
Except for HTI's 001AXES Universal Systems Language (USL) and its associated automation (001),
The government got involved in the modeling definitions creating the IDEF specifications.
Several modeling languages appeared (Grady Booch, Jim Rumbaugh, Iva Jacobson, Ganes, Sarson, Harel, Shlaer, Mellor, and many others) leading eventually to the Unified Modeling Language (UML).
Rational Rose was the first dominant product for UML implementation developed by Rational Corporation which was acquired by IBM in 2002.
CASE had the same problem that the current MDA/MDE tools have today: the model gets out of sync with the source code.
To be continued

39. 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 2 binary associations
Tournament
Player * 1
Statistics +
getAverageStat(name)
getTotalStat(name)
updateStats(match)

40. Review: Terminology Roundtrip Engineering Reengineering
Forward Engineering + reverse engineering
Inventory analysis: Determine the Delta between Object Model and Code
Together-J and Rationale provide tools for reverse engineering
Reengineering
Used in the context of project management:
Provding new functionality (customer dreams up new stuff) in the context of new technology (technology enablers)

41. Specifying Interfaces
The players in object design:
Class User
Class Implementor
Class Extender
Object design: Activities
Adding visibility information
Adding type signature information
Adding contracts
Detailed view on Design patterns
Combination of delegation and inheritance

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

43. 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

44. Realisation 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()

45. 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()

46. 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) }

47. 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
...

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

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

50. 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)

51. 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)

52. 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)

53. 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.

54. Building a super class from several classes
Prepare for inheritance. All operations must have the same signature but often the signatures do not match
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.
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

55. Exercise Session: Implementing Contract Violations
Many object-oriented languages do not have built-in support for contracts
However, if they support exceptions, we can use their exception mechanisms 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
UML model (see slide 35)
In this case acceptPlayer() in TournamentControl should throw an exception of type KnownPlayer
Java Source code (see slide 36).

56. Implementing Contract Violations
Many object-oriented languages do not have built-in support for contracts
However, if they support exceptions, we can use their exception mechanisms for signaling and handling contract violations
In Java we use the try-throw-catch mechanism
See exercise next week

57. UML Model for Contract Violation Example
TournamentControl
Player
players *
Tournament 1
+applyForTournament()
Match
+playMove(p,m)
+getScore():Map
matches
+start:Date
+status:MatchStatus
-maNumPlayers:String
+end:Date
TournamentForm
+acceptPlayer(p)
+removePlayer(p)
+isPlayerAccepted(p)
Advertiser
sponsors
+selectSponsors(advertisers):List
+advertizeTournament()
+announceTournament()
+isPlayerOverbooked():boolean

58. Implementation in Java
TournamentControl
Player
players *
Tournament 1
+applyForTournament()
Match
+playMove(p,m)
+getScore():Map
matches
+start:Date
+status:MatchStatus
-maNumPlayers:String
+end:Date
TournamentForm
+acceptPlayer(p)
+removePlayer(p)
+isPlayerAccepted(p)
Advertiser
sponsors
+selectSponsors(advertisers):List
+advertizeTournament()
+announceTournament()
+isPlayerOverbooked():boolean
public class TournamentForm {
private TournamentControl control;
private ArrayList players;
public void processPlayerApplications() {
for (Iteration i = players.iterator(); i.hasNext();) {
try { control.acceptPlayer((Player)i.next()); }
catch (KnownPlayerException e) {
// If exception was caught, log it to 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. 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());

59. 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() {
for (Iteration i = players.iterator(); i.hasNext();) {
try { control.acceptPlayer((Player)i.next()); }
catch (KnownPlayerException e) {
// If exception was caught, log it to 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. 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());

60. TournamentControl
Player
players *
Tournament 1
+applyForTournament()
Match
+playMove(p,m)
+getScore():Map
matches
+start:Date
+status:MatchStatus
-maNumPlayers:String
+end:Date
TournamentForm
+acceptPlayer(p)
+removePlayer(p)
+isPlayerAccepted(p)
Advertiser
sponsors
+selectSponsors(advertisers):List
+advertizeTournament()
+announceTournament()
+isPlayerOverbooked():boolean

61. Implementing a Contract
Check each precondition:
Before the beginning of the method with a test to check the precondition for that method
Raise an exception if the precondition evaluates to false
Check each postcondition:
At the end of the method write a test to check the postcondition
Raise an exception if the postcondition evaluates to false. If more than one postcondition is not satisfied, raise an exception only for the first violation.
Check each invariant:
Check invariants at the same time when checking preconditions and when checking postconditions
Deal with inheritance:
Add the checking code for preconditions and postconditions also into methods that can be called from the class.
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.

62. 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)

63. Heuristics: Mapping Contracts to Exceptions
Executing checking code slows down your program
If it is too slow, omit the checking code for private and protected methods
If it is still too slow, focus on components with the longest life
Omit checking code for postconditions and invariants for all other components.
Be pragmatic, if you don’t have enough time, change your tests in the following order:
Omit checking code for postconditions and invariants.
Often redundant, if you are confident that the code accomplishes the functionality of the method
Not likely to detect many bugs unless written by a separate tester
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.