Object Oriented Analysis & Design. Software Engineering Lab. Sharif University of Technology. Agenda. Object-Oriented Concepts and Principles Object Oriented Analysis Object Oriented Design. Object-Oriented Concepts and Principles. Introduction. We live in a world of objects.
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Software Engineering Lab.
Sharif University of Technology
We live in a world of objects.
They can be categorized, described, organized, combined, manipulated, and created.
Object-Oriented view is an abstraction that models the world in ways that help us to better understand and navigate it.
OO approach was first proposed in the late 1960s.
As time passes, object technologies are replacing classical software development approaches. Why?
The answer is not simple. Object technologies lead to reuse, OO software is easier to maintain, to adapt, and to scale.
For many years, the term OO was used to denote a software development approach that used one of a number of OO programming languages(e.g. Ada 95, C++, Eiffel, Smalltalk)
Today, the OO paradigm encompasses a complete view of software engineering.
Software engineers and their managers must consider OORA, OOD, OODA(domain anal.), OODBMS.
consider a chair.
It is a member of a much larger class of objects that we call furniture.
A set of generic attributes can be associated with every object in the class furniture (e.g. cost, dimensions, weight, location, color).
Chair inherits all attributes defined for the class furniture.
Every object in the class furniture can be bought, sold, modified, and moved.
The object Chair encapsulates data, operations, other objects (composite), constants, and other related information.
Encapsulations means that all of this information is packaged under one name and can be reused as one specification or program component.
Object-oriented = objects + classification + inheritance + communications.
classes and class hierarchies
abstraction and hiding
A class is an OO concept that encapsulates the data and procedural abstractions that are required to describe the content and behavior of some real world entity.
The only way to reach the attributes is to go through one of the methods.
This achieves information hiding and reduces the impact of side effects associated with changes.
The methods are cohesive and the class tends to be decoupled from other elements of a system. All of these design characteristics lead to high-quality software.
subclasses of the
instances of chair
Messages are the means by which objects interact.
The behavior is accomplished when an operation is executed.
An operation within a sender object generates a message of the form:
[destination, operation, parameters]
The receiver object responds to the message by first choosing the operation implements the message, executing this operation and then returning control to the caller.
It is a characteristic that greatly reduces the effort required to extend an existing OO system.
Polymorphism enables a number of different operations to have the same name.
For example: drawing different type of graphs( e.g. Line, Pie, Histogram )
It decouples objects from one another making each more independent.
General class graph and one subclass for each type.
Insert a voucher
Object oriented analysis is based upon concept that we first learned in kindergarten: objects and attributes, classes and members, wholes and parts.
To define all classes, the operations, the attributes, the relationships and the behavior the following tasks must be done:
Basic user requirements must be communicated between the customer and the software engineer.
Classes must be identified.
A class hierarchy must be specified.
Object to object relationships should be represented.
Object behavior must be modeled.
Above Tasks must be reapplied iteratively until the model is complete.
There is no universal agreement on the concepts that serve as a foundation for OOA.
Structured Analysis (SA):
takes a distinct input-process-output view of requirements.
Data are considered separately from the processes that transform the data.
System behavior tends to play a secondary role.
makes heavy use of functional decomposition.
Identification/classification of entities.
General-to-specific and whole-to-part entity relationships
Other entity relationships
Description of attributes of entities
Large-scale model partitioning
States and transitions between states
Detailed specification for functions
End-to-end processing sequences
Identification of exclusive services
Entity communications (via messages or events)
The modeling dimensions 8 and 9 are always present with SA.
Dozens of OOA method during the late 1980s and into the 1990s are introduced.
Each of them proposed:
A process for the analysis of the product or system
A set of diagrams that evolved out of the process.
A notation that enabled the software engineer to create the analysis model in a consistent manner.
The most widely use were:
The Booch method ( an evolutionary approach is maintained).
The Rumbaugh method (Object modeling technique (OMT))
The Jacobson method (OO Software Engineering (OOSE))
The Coad and Yourdon method (One of the easiest)
The Wirfs-Brock method (do not make clear distinction between design and analysis tasks)
Elicit customer requirements for the system.
Identify scenarios for use-cases.
Select classes and objects using basic requirements as a guide.
Identify attributes and operations for each system object.
Define structures and hierarchies that organize classes.
Build an object-behavior model.
Review the OO analysis model against use-cases or scenarios.
Grady Booch, James Rumbaugh and Ivar Jacobson combine the best features into a unified method called:
Unified Modeling Language (UML)
UML allows a software engineer to express and analysis model using a modeling notation that is governed by a set of Syntactic, Semantic, and Pragmatic rules.
The syntax tells us how the symbols should look and how they are combined. (Word in natural language)
The semantics tells us what each symbols means and how it should be interpreted. (Meaning of words in natural language)
The pragmatic rules define the intentions of the symbols through which the purpose of the model is achieved and become understandable. (The rules for constructing sentences that are clear and understandable in natural language)
User model view. This view represents the system (product) from the user’s (called “actors” in UML) perspective.
Structural model view. Data and functionality is viewed from inside the system. That is, static structure (classes, objects, and relationships) is modeled.
Behavioral model view. This part of the analysis model represents the dynamic or behavioral aspects of the system.
Implementation model view. The structural and behavioral aspects of the system are represented as they are to be built.
Environment model view. The structural and behavioral aspects of the environment in which the system is to be implemented are represented.
UML analysis modeling focuses on the first two views of the system.
UML design modeling addresses the other three views.
OO Analysis can occur at many different levels of abstraction:
At the business or enterprise level
At the business area level
At an application level
OOA at the middle level called Domain Analysis.
Domain Analysis is performed to create a library of reusable classes applicable to an entire category of applications.
Using a robust class library produces the system faster, cheaper and more reliable.
But where did such a library come from? By applying domain analysis.
The OOA process begins with an understanding of the manner in which the system will be used by:
People, if the system is human-interactive.
Machines, if the system is involved in process control.
Programs, if the system coordinates and controls applications
Once the scenario of usage has been defined, the modeling of the software begins.
A series of techniques may be used to gather basic customer requirements.
1. System intelligence should be evenly distributed.
2. Each responsibility should be stated as generally as possible.
3. Information and the behavior that is related to it should reside within the same class.
4. Information about one thing should be localized with a single class, not distributed across multiple classes.
1. Evaluate all use-cases to fully understand the sequence of interaction within the system.
2. Identify events that drive the interaction sequence and understand how these events relate to specific objects.
3. Create an event trace [RUM91] for each use-case.
4. Build a state transition diagram for the system
5. Review the object-behavior model to verify accuracy and consistency
OOD transforms the analysis model created using OOA into a design model that serves as a blueprint for software construction.
OOD results in a design that achieves a number of different levels of modularity.
Subsystems: Major system components.
Objects: Data and the operations.
Four important software design concepts:
OOD provides a mechanism that enables the designer to achieve all four with less complexity and compromise.
The design pyramid focuses exclusively on the design of a specific product or system.
Another layer of design which forms the foundation on which the pyramid rests, exists.
The foundation layer focuses on the design of domain objects.
Domain objects play a key role in building the infrastructure for the OO system by providing support for:
Human/computer interface activities,
Problem domain component—the subsystems that are responsible for implementing customer requirements directly;
Human interaction component —the subsystems that implement the user interface (this included reusable GUI subsystems);
Task Management Component—the subsystems that are responsible for controlling and coordinating concurrent tasks that may be packaged within a subsystem or among different subsystems;
Data management component—the subsystem that is responsible for the storage and retrieval of objects.
• Partition the analysis model into subsystems.
• Identify concurrency that is dictated by the problem.
• Allocate subsystems to processors and tasks.
• Develop a design for the user interface.
• Choose a basic strategy for implementing data management.
• Identify global resources and the control mechanisms required to access them.
• Design an appropriate control mechanism for the system, including task management.
• Consider how boundary conditions should be handled.
• Review and consider trade-offs.
• The subsystem should have a well-defined interface through which all communication with the rest of the system occurs.
• With the exception of a small number of “communication classes,” the classes within a subsystem should collaborate only with other classes within the subsystem.
• The number of subsystems should be kept small.
• A subsystem can be partitioned internally to help reduce complexity.
A protocol description establishes the interface of an object by defining each message that the object can receive and the related operation that the object performs
An implementation description shows implementation details for each operation implied by a message that is passed to an object.
information about the object's private part
internal details about the data structures that describe the object’s attributes
procedural details that describe operations
... you’ll find recurring patterns of classes and communicating objects in many object-oriented systems. These patterns solve specific design problems and make object-oriented design more flexible, elegant, and ultimately reusable. They help designers reuse successful designs by basing new designs on prior experience. A designer who is familiar with such patterns can apply them immediately to design problems without having to rediscover them.
Gamma and his colleagues [GAM95]
The design pattern name is an abstraction that conveys significant meaning about it applicability and intent.
The problem description indicates the environment and conditions that must exist to make the design pattern applicable.
The pattern characteristics indicate the attributes of the design that may be adjusted to enable the pattern to accommodate into a variety of problems.
The consequences associated with the use of a design pattern provide an indication of the ramifications of design decisions.