1. Chapter Five–Part I Object Oriented Design
How to Construct?

2. Introduction
The object oriented analysis part does not do well on how the system should be build.
This gap is filled by the design phase that provides such details for the implementation.
There are various artifacts that would be used to model the design of the system that evolve from the analysis phase.

3. What it is? Designing of object oriented design requires
the definition of a multilayered software architecture,
the specification of subsystems that perform required functions and provide infrastructure support such interface, business logic and database layer
a description of objects (classes) that form the building blocks of the system such as main menu, form, etc and
a description of the communication mechanisms that allow data to flow between layers, subsystems, and objects.

4. Cont…
The input for object-oriented design is provided by the output of object-oriented analysis.
Realize that an output artifact does not need to be completely developed to serve as input of object-oriented design.
Some typical input artifacts for object-oriented design are; Class diagram, Conceptual model, Use case, Sequences Diagram and User interface.

5. Object-Oriented Analysis and Design
Construction
Investigation
of problem
Logical solution
Code

6. Object oriented design ..
a meaningful engineering representation of something that is to be built.
It is both “the process” that defines the architecture, components, interfaces, and other characteristics of a system or component” and “the result” of that process.”
As a process, software design is the software engineering life cycle activity in which software requirements are analyzed in order to produce a description of the software’s internal structure that will serve as the basis for its construction.

7. Cont…
the software architecture  how software is decomposed and organized into components and the interfaces between those components.
It must also describe the components at a level of detail that enable their construction.
In the software engineering context, design focuses on four major areas of concern:
data, architecture, interfaces, and components.

8. At each stage, software design work products are reviewed for
Design quality ----
At each stage, software design work products are reviewed for
Clarity,
Correctness,
Completeness, and
Consistency
with the requirements and with one another.

9. The software design process
Software design is
an iterative process through which requirements are translated into a “blueprint” for constructing the software.
is generally considered a two-step process:
Architectural design  describes how software is decomposed and organized into components (the software architecture)
, Class type architecture, Component, Deployment, persistent diagrams
Detailed design  describes the specific behavior of these components.
Refined class model ,Statechart, collaboration

10. Software Design Principles
The design should be traceable to the analysis model.
The design should not reinvent the wheel.
The design should “minimize the intellectual distance” between the software and the problem as it exists in the real world.
The design should exhibit uniformity and integration.
The design should be structured to accommodate change.
Tunnel vision  A good designer should consider alternative approaches, judging each based on the requirements of the problem, the resources available to do the job, and the design concepts.
Traceability  Because a single element of the design model often traces to multiple requirements
Don’t reinvent  Time is short and resources are limited!  previously used design patterns and integrating with new ones
Uniformity and integration  A design is uniform if it appears that one person developed the entire thing. Rules of style and format should be defined for a design team before design work begins. A design is integrated if care is taken in defining interfaces between design components.
Design ≠ Coding  Even when detailed procedural designs are created for program components, the level of abstraction of the design model is higher than source code.
Design quality  A variety of design concepts and design measures are available to assist the designer in assessing quality.
Conceptual/Semantic errors  There is sometimes a tendency to focus on minutiae when the design is reviewed, missing the forest for the trees. A design team should ensure that major conceptual elements of the design (omissions, ambiguity, inconsistency) have been addressed before worrying about the syntax of the design model.
When these design principles are properly applied, the software engineer creates a design that exhibits both external and internal quality factors.
External quality factors are those properties of the software that can be readily observed by users (e.g., speed, reliability, correctness, usability).
Internal quality factors are of importance to software engineers. They lead to a high-quality design from the technical perspective. To achieve internal quality factors, the designer must understand basic design concepts and principles.

11. Cont…
The design should be structured to degrade gently, even when aberrant data, events, or operating conditions are encountered.
Design is not coding, coding is not design.
The design should be assessed for quality as it is being created, not after the fact.
The design should be reviewed to minimize conceptual (semantic) errors.

12. Thus What to design? Some examples are…
– Process and association
--User Interfaces
– Storage
– Networking/Distribution
• Each requires the addition of extra classes and associations.

13. Software Architecture
Software architecture is the process of designing the global organization of a software system, including:
Dividing software into subsystems.
Deciding how these will interact.
Determining their interfaces.
The architecture is the core of the design, so all software engineers need to understand it.
The architecture will often constrain the overall efficiency, reusability and maintainability of the system.

14. The importance of software architecture
Why you need to develop an architectural model:
To enable everyone to better understand the system
To allow people to work on individual pieces of the system in isolation
To prepare for extension of the system
To facilitate reuse and reusability

15. Contents of a good architectural model
A system’s architecture will often be expressed in terms of several different views
The logical breakdown into subsystems
The interfaces among the subsystems
The dynamics of the interaction among components at run time
The data that will be shared among the subsystems
The components that will exist at run time, and the machines or devices on which they will be located

16. Describing an architecture using UML
All UML diagrams can be useful to describe aspects of the architectural model
Some UML diagrams are particularly suitable for architecture modelling and for implementation issues:
Class Type architecture (not in UML)
Component diagrams
Deployment diagrams
Persistent diagram
Package/subsystem diagram
Evolving UI

17. Class Type Architecture (not in UML)
A common architectural strategy, some might call it a pattern, is to layer the architecture of a system into several layers
Some strategies simply define N layers stacked on top of each other where layer J interacts only with layers J-1 and J+1. 
That's an interesting theory, and it clearly makes sense from a logical point of view, but in practice it is found that it isn't quite so simple. 

18. Cont…
The following slide Presents a high-level layering strategy for a software application.  
The various layers are represented by the rectangles and collaboration between layers by the arrows. 
The primary name of a layer is indicated first, and other common names in parenthesis

19. Layered class type architecture

20. Cont… Interface: Domain
There are two categories of interface class – user interface (UI) classes that provide people access to your system and system interface (SI) classes that provide access to external systems to your system
Domain
This layer implements the concepts pertinent to your business domain such as Student or purchase, focusing on the data aspects of the business objects, plus behaviors specific to individual objects

21. Cont… Process Persistence System
The process layer implements business logic that involves collaborating with several domain classes or even other process classes. Example – student registration, purchasing
Persistence
Persistence layers encapsulate the capability to store, retrieve, and delete objects/data permanently without revealing details of the underlying storage technology.   Often implement  between your object schema and your database schema and there are various available to you. 
System
System classes provide operating-system-specific functionality for your applications, isolating your software from the operating system (OS) by wrapping OS-specific features, increasing the portability of your application

22. Class Modeling
The class model at the design level will add some additional details than that of the analysis level class model.
Here the focus will be the solution domain rather than the problem domain.
In practice, the analysis level class model will evolve into a design level class model.
This gives the developers the chance to make amendments and modification to improve the quality of the system.

23. Cont…
The design level class model will concentrate on how to implement attributes methods, inheritance, association, aggregation, composition and the likes.
Modeling Methods
Methods, also called operations or member functions, are the object-oriented equivalent of functions and procedures.
The design level will model more information about methods than the analysis.

24. The design level may include:
Cont…
The design level may include:
Visibility: the level of access to methods by external objects.
To reduce the effect of coupling within a system, more restrictions on access of methods should be set.
In other words, if a method does not have to be public then make it protected and if it does not have to be protected then make it private.

25. Cont… + # - Visibility Symbol Description Proper Usage Public
A public method can be invoked by any other method in any other object or class.
When the method must be accessible by objects and classes outside of the class hierarchy in which the method is defined.
Protected #
A protected method can be invoked by any method in the class in which it is defined or any subclasses of that class.
When the method provides behavior needed internally within the class hierarchy, but not externally.
Private -
A private method can only be invoked by other methods in the class in which it is defined, but not in the subclasses.
When the method provides behavior specific to the class. Private methods are often the result of refactoring.

26. Cont…
Name: Descriptive name for the method. A good name is the one that is capable of explaining the purpose of the methods just by looking at its name.
In giving a name to methods the designer needs to know what programming language will be used for the development so that the naming convention of that language will be used here.
Parameters: The names of parameters, and optionally their types and default values (if any);
Return value type: The data type of the return value (if available)

27. Modeling Attributes Cont… Attributes are the data aspects of objects.
The design level will model more information about attributes than the analysis.
The design level may include:
Visibility: This is the level of access external objects have to an attribute.

28. Cont… + # - Visibility Symbol Description Public
A public attribute can be accessed by any other method in any other object or class.
Protected #
A protected attribute can be accessed by any method in the class in which it is declared or by any method defined in subclasses of that class.
Private -
A private attribute can only be accessed by method in the class in which it is declared, but not in the subclasses.

29. Cont…
Type: The data type of an attribute should be determined (could be a primitive type, such as string or int, or a class such as Address.)
Initial value: The initial value for an attribute should also be indicated (if available).

30. Cont… Modeling Association
Objects in any system cannot exist and work alone. For this reason they need to depend one another or collaborate with each other.
The dependency and collaboration will help the development team to define how they interact with each other.
The collaboration is important as an object needs to know about another object to work with it.
For each association multiplicity should be modeled, one on each end of the association line

31. In Design Minimize coupling and Maximize cohesion
Coupling is a measure of how much two items, such as classes or methods, are interrelated. When one class depends on another class, we say they are coupled.
When one class interacts with another class, but does not know any of the implementation details of the other class, we say they are loosely coupled.
A class is coupled to another class when it has knowledge of that other class.
Low Coupling is important to reduce maintenance.
In high coupling Class A and Class B, a change in B could necessitate a change in A.

32. Cont…
Cohesion is a measure of how much an item, such as a class or method, makes sense.
A good measure of the cohesiveness of something is how long describing it takes using only one sentence: the longer it takes, the less cohesive it likely is. You want to design methods and classes that are highly cohesive.

33. Cont…
In other words, it should be completely clear what a method or class is all about.
A good rule of thumb is if you cannot describe a class or method with one sentence in less than 15 seconds, then it probably is not cohesive.
Classes should represent only one kind of object, and methods should do one thing and one thing well.

34. Following is an example to compare Analysis and design versions of a class
Analysis Level
Design Level

35. Collaboration Diagrams
Collaboration Diagrams show the same information as a sequence diagram.
The emphasis is on the organization of the objects.
Sequence is shown by including a sequence number on the message.

36. Collaboration Diagram Example

37. Building Collaboration Diagram
place participating objects on the diagram
draw the links between the objects using the class diagrams as your guide
add each event by placing the message arrow parallel between the two objects
position the arrow to point from the sender to the receiver
number the messages in order of execution
repeat steps 3 and 4 until the entire scenario has been modeled

38. Statechart Diagrams
Statechart diagrams show class states and the events that cause them to transition between states.
It is also called a state transition diagram
An event happens at a specific time and place.
They cause a change of state, or the transition “fires”

39. Statechart Diagrams (Continued)
Each time an object changes state, some of its attributes must change.
There must be a method to change the attributes.
Often there is a display screen or Web form to enter the attributes.

40. Statechart Diagrams (Continued)
Statechart diagrams are not created for all classes.
They are created when:
A class has a complex life cycle.
An instance of a class may update its attributes in a number of ways through the life cycle.
A class has an operational life cycle.
The object’s current behavior depends on what happened previously.

41. Statechart Example

42. State chart …
A statechart diagram defines the behaviour of a single object
or of a set of objects related by a collaboration.
It captures the changes in an object throughout its lifecycle as
they occur in response to internal and external events.
The scope of a statechart is the entire life of one object.

43. State … Statecharts are composed of: 1) states 2) events
a) initial state
b) final state
2) events
a) guard conditions
b) actions
c) event syntax
3) complex states
a) activities
b) entry actions
c) exit actions

44. State
State is the current condition of an object reflected by the values of its attributes and its links to other objects.
The initial state identifies the state in which an object is created or constructed.
The initial state is called a pseudo-state because it does not really have the features of an actual state, but it helps clarify the purpose of another state of the diagram.

45. The final state is the state in which once reached, an object can never do a transition to another state.
The final state may also mean that the object has actually been destroyed and can no longer be accessed.

46. Event
Definition
Event is an occurrence of a stimulus that can trigger a state transition.
An event may be:
1) the receipt of a signal, e.g. the reception of an exception
2) the receipt of a call, that is the invocation of an operation,
e.g. for changing the expiration date of a license
An event on a statechart diagram corresponds to a message on a sequence diagram.

47. End of Part I