3100 SYSC - System Analysis and Design Basic Concepts

SYSC 3100 - System Analysis and Design
Basic Concepts
Introduction to OO Development and
Software Engineering Principles
Everything is an Object!
• You, me, your neighbour, desks, chairs, doors,
etc.
• Some of these things are the same, and some
of them are different.
• Some of them are “active”, and others are
“passive”.
Objectives
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Define objects, attributes and operations.
Differentiate between OOA, OOD and OOP (!)
Describe a Class and an object.
Define information hiding and encapsulation.
Define generalization and specialization and
the isA rule.
• Define polymorphism and over-riding.
• Describe active objects and persistence.
Object-Oriented Development (OOD)
• Objects are abstractions of real-world or system
entities and manage themselves
• Objects are independent and encapsulate state
and representation information (attributes).
• System functionality is expressed in terms of
object services (operations)
• Shared data areas are eliminated. Objects
communicate by message passing
• Objects may be distributed and may execute
sequentially or in parallel
Objects, Attributes, Operations
Bicycle objects
Bicycle class
abstract
into
Polygon objects
Attributes
frame size
wheel size
number of gears
material
Operations
shift
move
repair
Polygon class
abstract
into
Attributes
vertices
border color
fill color
Operations
draw
erase
move
O-O World View
• An object-oriented system is regarded as
a network of cooperating objects which
- interact by sending each other messages
- maintain their own state
- have an individual identity
o4: C4
o1: C1
o3:C3
state o1
ops1()
state o3
state o4
ops3 ()
ops4 ()
o2: C3
o6: C1
state o2
ops3 ()
state o6
ops1 ()
o5:C5
state o5
ops5 ()
Advantages of OOD
• Easier maintenance. Classes may be
understood as stand-alone entities, including
their own functionality and data
• Classes are appropriate reusable components
• For some systems, there may be an obvious
mapping from real world entities to system
classes, e.g., accounts in banking system
Object-oriented development
• Object-oriented analysis (OOA), design (OOD)
and programming (OOP) are related but distinct
• OOA is concerned with developing an object
model of the application domain
• OOD is concerned with developing an objectoriented system model (structure, behaviour) to
implement requirements
• OOP is concerned with realising an OOD using an
OO programming language such as Java or C++
Principal Concepts
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Objects, classification
encapsulation and information hiding
modularity
generalization and inheritance
typing
polymorphism
dynamic binding
concurrency
persistence
Classes and Objects
•Object
–A chunk of structured data in a running software
system
–Has properties
• Represent its state
–Has behaviour
• How it acts and reacts
• May simulate the behaviour of an object in the real world
–Has an identity: each object is unique
High-level Examples
Class
Identity
Behaviour State
Person.
‘Hussain Pervez.’
Speak, walk, read.
Studying, resting,
qualified.
Shirt.
My favourite button
white denim shirt.
Shrink, stain, rip.
Pressed, dirty,
worn.
Sale.
Sale no #0015,
16/06/02.
Earn loyalty points.
Invoiced,
cancelled.
Bottle of
ketchup.
This bottle of
ketchup.
Spill in transit.
Unsold, opened,
empty.
Objects
Jane:
date of birth: 1955/02/02
address: 99 UML St.
position: Manager
Greg:
Savings Account 12876:
balance: 1976.32
opened: 1997/03/03
date of birth: 1970/01/01
address: 75 Object Dr.
Margaret:
Mortgage Account 29865:
date of birth: 1980/03/03
address: 150 C++ Rd.
position: Teller
balance: 198760.00
opened: 2000/08/12
property: 75 Object Dr.
Transaction 487:
Instant Teller 876:
amount: 200.00
time: 2001/09/01 14:30
location: Java Valley Cafe
Classes
•A class:
–Is a unit of abstraction in an object oriented analysis,
design, or program
–Represent instances of real-world and system
entities
–Represents similar objects
• Its instances
–Is a kind of software module
• Describes its instances’ structure (properties)
• Contains operations/methods to implement their
behaviour
Classes and Objects: Example
Person
JoeSmith:Person
MarySharp:Person
name: string
birthdate: date
name=“Joe Smith”
birthdate=21 October 1983
name=“Mary Sharp”
birthdate=16 March 1950
Class with Attributes
Objects with Values
File
Person
name
birthdate
changeJob
changeAddress
fileName
sizeInBytes
lastUpdate
print
GeometricObject
color
position
move (delta : Vector)
select (p : Point): Boolean
rotate (in angle : float = 0.0)
Encapsulation and Information Hiding
Encapsulation: The technique of hiding details which are not needed by the
user of an abstraction
Applied in object-oriented systems by the separation of object interfaces and
bodies
A_STACK
client’s view
interface
PUSH
POP
body
Message-passing and Encapsulation
‘Layers of an onion’ model of
an object:
Message from another object
requests a service.
Operation called only via valid
operation signature.
An outer layer of
operation signatures…
…gives access to middle
layer of operations…
…which can access
inner core of data
Data accessed only by
object’s own operations.
An object’s data
is hidden
(encapsulated).
Object-Oriented Computation Model
system operation
control flow
data flow
#1
draw
#2
insert
print
Attributes #1
remove
update
Attributes #4
#3
insert
#4
clear
Attributes #2
Attributes #3
remove
draw-line
Organizing Classes into Inheritance Hierarchies
•Superclasses
–Contain features common to a set of subclasses
•Inheritance hierarchies
–Show the relationships among superclasses and
subclasses
•Inheritance
–The implicit possession by all subclasses of
features defined in its superclasses
An Example Inheritance Hierarchy
Account
SavingsAccount
ChequingAccount MortgageAccount
•Inheritance
–The implicit possession by all subclasses of
features defined in its superclasses
Generalization and Specialization
• Classification is hierarchic in nature
• Generalization: the technique of factoring out
common properties into shared classes
– facilitates reuse and change control
– the reverse operation is the specialization
• For example, a person may be an employee, a
customer, a supplier of a service
• An employee may be paid monthly, weekly or
hourly
• An hourly paid employee may be a driver, a
cleaner, a sales assistant
Specialization Hierarchy
More general
(superclasses)
Person
Employee
monthly paid
Customer
weekly paid
Driver
Cleaner
Supplier
hourly paid
Sales
assistant
More specialized
(subclasses)
The Isa Rule
•Always check generalizations to ensure they
obey the “isa” rule
–“A checking account is an account”
–“A village is a municipality”
•Should ‘Province’ be a subclass of ‘Country’?
–No, it violates the isa rule
• “A province is a country” is invalid!
Inheritance hierarchy of mathematical objects
MathematicalObject
Shape
Shape2D
Ellipse
Polygon
Circle
Quadrilateral
Rectangle
Point
Matrix
Shape3D
Line
Plane
Make Sure all Inherited Features Make Sense in
Subclasses
Account
ba lance
op ened
creditOrOverdra ftLi mit
credit
de bit
calculateInterest
SavingsAccount
ChequingAccount
hi ghes tChequ eNumbe r
with drawUsing Ch eque
calculateServiceCharge
MortgageAccount
colla teral Property
colla teral Valu e
se tColla teral Valu e
Methods, Operations and Polymorphism
•Operation
–A higher-level procedural abstraction that specifies a
type of behaviour
–Independent of any code which implements that
behaviour
• E.g., calculating area (in general)
–Operations are defined during Analysis and Design
–They are implemented with methods
Methods, Operations and Polymorphism
•Method
–A procedural abstraction used to implement the
behaviour of a class.
–Several different classes can have methods with the
same name
• They implement the same abstract operation in ways
suitable to each class
• E.g, calculating area in a rectangle is done differently from
in a circle
Polymorphism
•A property of object oriented software by
which an abstract operation may be performed
in different ways in different classes.
–Requires that there be multiple methods of the
same name
–The choice of which one to execute depends on the
object (i.e., type) that is in a variable, on which the
method is executed
–Reduces the need for programmers to code many
if-else or switch statements
Overriding
•A method would be inherited, but a subclass
contains a new version instead
–For restriction
• E.g. scale(x,y) would not work in Circle
–For extension
• E.g. SavingsAccount might charge an extra fee
following every debit
–For optimization
• E.g. The getPerimeterLength method in Circle is
much simpler than the one in Ellipse
How a decision is made about which method to run
1.
2.
3.
4.
If there is a concrete method for the operation
in the current class, run that method.
Otherwise, check in the immediate superclass
to see if there is a method there; if so, run it.
Repeat step 2, looking in successively higher
superclasses until a concrete method is found
and run.
If no method is found, then there is an error
– In Java and C++ the program would not have
compiled
Dynamic binding
•Occurs when decision about which method to
run can only be made at run time
–Needed when:
• A variable is declared to have a superclass as its type, and
• There is more than one possible polymorphic method that
could be run among the type of the variable and its
subclasses
Abstract Classes and Methods
•An operation should be declared to exist at the highest class in the
hierarchy where it makes sense
– The operation may be abstract (lacking implementation) at that level
– If so, the class also must be abstract
• No instances can be created
• The opposite of an abstract class is a concrete class
– If a superclass has an abstract operation then its subclasses at some
level must have a concrete method for the operation
• Leaf classes must have or inherit concrete methods for all operations
• Leaf classes must be concrete
Concurrency
•The nature of objects as self-contained entities
make them suitable for concurrent
implementation.
•There are two forms of objects in concurrent
systems.
•Passive:
–operations execute only in response to external stimuli
–call other objects only in response to external stimuli
•Active
–operations may execute asynchronously
–has an independent thread of control
–call other objects “spontaneously”
–change state “spontaneously”
Active transponder object
• Active objects may have their attributes
modified by operations but may also update
them autonomously using internal operations
• Transponder object broadcasts an aircraft’s
position. The position may be updated using
a satellite positioning system. The object
periodically update the position by
triangulation from satellites
An active transponder object
class Transponder extends Thread {
Position currentPosition ;
Coords c1, c2 ;
Satellite sat1, sat2 ;
Navigator theNavigator ;
public Position givePosition ()
{
return currentPosition ;
}
public void run ()
{
while (true)
{
c1 = sat1.position () ;
c2 = sat2.position () ;
currentPosition = theNavigator.compute (c1, c2) ;
}
}
} //Transponder
Persistence
• The ability of an object to exist after the termination of the program
which created it
• Allows data storage in terms of objects (e.g. files are objects)
• Provides a model for interaction with a DBMS or FMS
• E.g., Java serialization, Object-oriented DBMS
OO program
Database
Summary
• An object is some “thing” which has attributes
(variables) and operations that they can perform
• A class is the blueprint or template that describes
objects that have similar characteristics.
• Big idea: hide implementation of objects to make
it easy to change without affecting the world.
• Polymorphism allows different types to
implement common operations differently.
• “Active objects” operate spontaneously and
independently of each other (like us!).