Transcript UML - Vrije Universiteit Brussel

Objects and Components
The adaptive organization
The competitive environment of businesses continuously
changing , and the pace of that change is increasing at
an accelerating rate.
Where it was once possible for a company to stake out
its marketing turf and defend its position for years, static
positioning is now viable only in a few isolated industries.
For most companies today, the only constant is change.
Object Orientation
Basic building blocks of a system are the
OBJECTS
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black boxes encapsulating data and procedures
messages can be send and received by objects
object libraries and templates
Importance
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reusability
maintainability
flexibility
Database Evolution
File System
Database
OO - system
Why objects ?
Grady Booch:
simply because there appears to be no other way to
economically produce an enduring and resilient
programming system.
David A. Taylor
objects are the enabling technology for adaptive
business systems
Planning !!!
The waterfall model
Project proposal
report
project
definition
Functional
specifications
Feasibility report
system
study
design specifications
design
program specs
code
programming
- intermediate reports
- go/nogo intervals
Installation
tests of system
performance
audit , feed-back
Post Implementation
System reliability evolution
Number of
problems
time
Why OO ? Good reasons
With the increasing complexity of the systems,
the waterfall model suffers from two illusions:
The analyst knows everything and
understands the problem completely before
implementation starts
The users read the system analysis report
and approve it
Project management
Iterative style
develop a series of solutions to a problem ,
each of them closer to satisfying the requirements
( also called : evolutionary development )
Incremental style
Builds system functionality a little at a time.
The results are not entire solutions.
Matthew Pittman proposes iterative analysis and design
combined with incremental development
OO - Life cycle
Facts:
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System requirements are not fully known at the start
knowledge of the system grows during development
better develop a system incrementally
start with some core functions
OMG
Life Cycle
analysis
object
modeling
design
construction
coordination and reuse
full
system
definition
What is an object ?
An object is a software package that contains a collection of related
procedures ( methods ) and data ( variables )
An object is a data abstraction with a state, a behavior and an identity
whereby operations are encapsulated together with the data structures on
which they are defined
B0923C7
change
address
12403
Peters
Brussels
1 y comp
give
grade
enroll
The object paradigm
Five principles:
Abstraction
Encapsulation
Object communication , message passing
Inheritance
Polymorphism
Abstraction
an abstraction is a simplified view of some part of reality,
focusing on some aspects , suppressing others
good abstraction emphasizes on those details that are
important for the actual observer
the use of abstraction makes it possible to postpone
decisions regarding details
an object is an abstraction of both data and functionality,
with a focus on the outside view , separating the
implementation of the object from the essential
Abstraction
From: Grady Booch
Abstraction focuses upon the essential characteristics of some object,
relative to the perspective of the user
Encapsulation
From: Grady Booch
Encapsulation hides the details of the
implementation of an object
Encapsulation
Encapsulation is the mechanism by which an object is
made to look like a black box to an external observer
It is the packaging of a set of ideas into one logical unit,
which can be referred to as a single unit
Good encapsulation hides design decisions .
An external observer sees what the object does, not how
it does it.
It is the basis for reusability
Classes
A Class is a software template that defines the methods
and variables to be included in a particular kind of object.
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objects of the same structure and behavior belong to the same
class
an object belongs to a class or is an instance of a class
objects only exist at run-time
classes are set up at design time
a class can contain rules that all instances must satisfy
Messages passing
A message starts an operation on an object
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A message is simply the name of an object, followed by the name
of the method the object knows how to execute, and eventually
followed by parameters
Objects can pass messages to other objects
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Signature: The message signature contains the stipulation of the
message format
a message consists of the name of a method and the
required arguments
the set of messages to which an object responds is the
behavior of the object
Message passing
The task of software development is to engineer the illusion of simplicity.
Message Interface
Message interface
The set of messages an object commits to respond to.
A class must specify the messages that objects of this
type will make available to other objects
Interfaces
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protect objects from being corrupted by other objects
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protect other objects from depending on its internal structure
Interfaces may be segmented by combining them into
composite interfaces
Anatomy of a message
A message consists of three parts
1. A receiver object
2. A method the receiver knows how to execute
3. An ordered set of parameters that this method requires to
carry out this function (optional)
Vehicle
Turn :
receiver
method
90
parameter
Messages conform to signatures
Inheritance
the ability of an instance
of a class C to use not
only the methods and
variables defined for C but
also those designed for
an ancestor of C
Inheritance allows new
classes to be build
incrementally on existing
classes
Message interfaces are
also inherited
Person
change
address
student
grade
Staff
member
pay
students and staff members
are sub-classes of person
Composite objects
Objects can contain other objects(Composite objects)
Collection class is a special kind of class
Benefits of Composition
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even a deeply nested structure can be treated as a single,
unified object
this helps manage complexity
Delegation
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when an object assigns a task to another object
Method overriding
it is possible to
override methods
Polygon
Give area
Triangle
Rectangle
Give area
Give area
Hexagon
Attention
Person
Head
Leg
This is not inheritance (but Aggregation)
Polymorphism
The ability of different objects to respond
differently to the same message is called
POLYMORPHISM
e.g.
DRAW
NEW
A variable can point to an object whose class is
not absolutely known
OO-System Reliability
Number
of
problems
With OO development techniques
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A system is never replaced entirely
continuous evolution
lower implementation risk
time