IS 8020

1
IS 8020

• Spring 2004
• Object Oriented Modelling

2
Views Of Objects and Classes

• In computer
• Representation of state
• Implementation of behaviors
• Real World
• Interpret the meanings

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Real World Objects

• Objects are distinctly identifiable
• We classify objects by grouping them according to
  common characteristics

4
Building Models

• In our model, objects are software entities
• Objects have these characteristics
• State
• Behavior
• Identity
• All the objects in a class share some
  characteristics of behavior and/or state

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Representation of State

• State is represented by attributes or fields
• Each field represents some aspect of the state of
  the object

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Representation of Behavior

• Behaviors are implemented by methods
• aka operations
• aka functions
• Behaviors are implemented by code that may access
  and change the state of the object

7
Characteristics of Fields

• Fields have
• Name
• Type
• Value
• The type indicates the kinds of values that the
  field may take

8
Characteristics of Methods

• Methods have
• Name
• Type
• Value
• Type type is interpreted as the kind of data the
  method returns
• The values is the code that carries out any
  activities of the method

9
Features

• The features of an object are its methods and
  attributes
• Or, its behavior and state

10
Identity

• Two ways of comparing if objects are the same
• Equals
• Equals means that objects have the same state
• had 24 sons and she named them all Dave
• Identity
• Identity means that the objects are actually the
  same object

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Mutators and Accessors

• Object fields are mutable, that is, changeable
• Mutator methods actually change the fields
• Accessor methods do not change the fields

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Immutable Objects

• Some objects have no behaviors that alter the
  state
• These objects are called immutable

13
Building Models

• The activity of OO modeling involves defining
  classes
• Classes specify templates or definitions
  according to which objects can be created
• The act of creating an object from a class is
  called instantiation
• An object is called an instance of a class

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Class Point

• Fields
• Named x and y
• Both type int
• Values assigned later (why?)
• Method
• Named move
• Its type is void, no value returned
• The implementation will be shown later

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Parameters

• Methods may require extra data to carry out their
  operation
• The extra data is specified using parameters
• Parameters each have a specified type

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Diagrams

• We are visual animals
• Diagrams are useful tools
• UML Unified Modeling Language
• Rectangles with three sub-parts represent classes

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UML Class
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Specification of Fields

• visibility name type
• Other possibilities are described in the text

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Visibility
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What Does Visibility Mean

• Who can access what features?
• Private restricts access to the class
• Public allows access to all
• Package allows access from the same package
  (defined later)
• Protected allows access by subclasses (defined
  later)

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Class Point
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Objects in UML

• These are less common
• Modeling usually focuses on describing classes
  rather than individual objects

23
Passing a Message

• A method is invoked or called to activate it
• The syntax is
• object.methodName(parameters)
• For example
• p1.move(10, 20)

24
Packages

• Packages are groups of related classes
• The relationship may be somewhat informal
• The Java library is organized into a large number
  of packages
• The Java library uses a hierarchical naming
  convention
• We probably will not create packages, just use
  them

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OO Design Principles

• Modularity
• Abstraction
• Encapsulation
• Polymorphism

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Modularity

• Decomposing a system into component parts
• The decomposition should exhibit
• High cohesion
• Low coupling

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Cohesion

• The parts of a module should be related
• A module that makes deposits to bank accounts and
  graphs stock-market activity is not cohesive
• This is a criterion sometimes applied to a class
  when critiquing a design
• Example A patent for a combination fly-trap and
  cheese grater

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Coupling

• Coupling refers to interdependence of modules
• High coupling can lead to problems when changing
  a system
• Changes will propagate more widely in highly
  coupled systems

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Abstraction

• In this context, means that a module is
  characterized by a service contract
• The services that the module are characterized
• Including the conditions that have to be met
  a-priori
• The methods by which the service are provided are
  not part of the contract

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Examples

• Telephones
• Telephone system is very complex
• The interface is much less complex
• sqrt(x)
• This gives us the square root
• We dont need to understand how it works

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Encapsulation

• Interface is separate from implementation and
  representation
• Information hiding The user only sees whats
  needed to be able to use the services

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Employee Example

• Attributes must be stored somewhere
• Might be in working memory
• Might be in a database
• Methods carry out some task
• Methods may simply operate on local data
• Methods may send requests to other systems to
  carry out the activity

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Polymorphism

• Different providers may honor the same interface

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Class Activity

• Design a Bank Account
• Determine attributes of the account
• Identify type
• Determine methods of the account
• Identify parameters
• Identify return

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Follow-up

• Consider issues of cohesion
• Should customer be represented?

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Class Relationships

• Inheritance
• Association
• Aggregation and Composition
• Dependency

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Interfaces

• Interfaces are like classes in that they support
  a contract for behavior
• However, interfaces do not actually implement any
  of that behavior

38
Inheritance

• Formally, one class inherits from another if all
  the objects from the first class are also in the
  second class
• We will usually say that the first class extends
  the second class
• A class implements an interface
• An interface extends another interface
• This is all Java jargon

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Inheritance Conceptually

• If one class inherits from another, we say that
  the first class is-a the second class
• We can substitute an element of the sub-class for
  an element of the super-class

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Student Example

• Relationships among groups
• Draw diagram

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Levels of Abstraction

• Higher levels of abstraction are more general
• Lower levels are more specific
• Super-classes are more general
• A super-class may be so general that no
  implementation makes sense, thus, an interface
• Sub-classes are more specific

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Multiple Inheritance

• Conceptually, a class may inherit from a number
  of other classes
• Java does not support multiple inheritance

43
Associations

• Binary relationships between classes
• The two classes play roles in the relation
• Names
• Role names
• Association name

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Multiplicities

• How many of a class will participate in a
  relation?
• Many students will be advisee to a faculty member
• Each student will have one advisee

45
Specifying Multiplicities

• i
• l..u
• Examples
• 0..
• 1..
• 1

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Aggregation and Composition

• Has-a relation
• Component/Aggregate
• Composition implies ownership and linking of
  lifetimes

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Dependency

• Uses relation

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Navigability

• Can you get from an instance of class A to an
  associated instance of class B?
• Part of reducing coupling is being specific about
  navigability

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Dynamic Behavior

• Sequence Diagram
• Read through diagram on page 37
• Read through state diagrams
• Theyll be less useful for us

50
Use Cases

• Actors
• Those who carry out actions
• Usually people
• May be other computers
• Use Cases
• Very high level actions of the total system
• Very undetailed

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E-Bookstore

• Create part of the use-case diagram
• Create an activity diagram
• Scenarios and activity diagram

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Identifying Objects

• Objects are often nouns
• Attributes are often adjectives
• Behaviors are often verbs