2' Object Model

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2. Object Model
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Benefits Of OO Development

• Models System using Objects
• Small Semantic gap between reality model
• Understanding the system is easier
• Modifications are localized

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Object-Oriented Paradigm

• Collection of Discrete Objects
• Data Behavior
• OO Paradigm
• Abstraction
• Encapsulation
• Hierarchy
• Inheritance hierarchy (is-a)
• Part of hierarchy (has a)
• Polymorphism

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Abstraction

• A simplified description ...
• of a system that emphasizes some of the
  systems details ... while suppressing others

An abstraction denotes the essential
characteristics of an object that distinguish it
from all other kind of objects and thus provide
crisply defined conceptual boundaries, relative
to the perspective of the viewer
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Encapsulation

• Information hiding
• Interface - Implementation
• Behavior Data
• Encapsulation is the process of
  compartmentalizing the elements of an abstraction
  that constitute its structure and behavior
  encapsulation serves to separate the contractual
  interface of an abstraction and its
  implementation

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What is an Object?

• Concept, abstraction, or thing
• with crisp boundary meaning for a problem
• An Object has state and behavior
• Objects receive stimuli/messages respond
• Receiving a stimulus, Object may change state
• Examples

charlie.fly() stimulus
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What is a Class?

• Group of Objects with similar
• properties (attributes)
• behavior
• relationships to other objects
• semantics
• Blueprints of Objects

Parrot
Example
Girl
Boy
charlie
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Hierarchy

• Hierarchy is a ranking of abstractions
• Inheritance
• expresses is-a or Kind-of relationship
• Extensibility Reusability
• Part-of
• expresses that object is an aggregate of another

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Polymorphism

• Hiding alternative procedures
• behind a Common Interface
• Send a Message to an object - Polymorphism
  guarantees that the correct/proper implementation
  is invoked.
• Sender does not know specific class of receiver

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Object Model

• Captures static structure of system
• Objects, relationships, attributes
• operations
• Most important
• Intuitive graphic representation
• Valuable for communication documentation

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Objects

• Decomposing problem into objects
• depends on judgement nature of problem
• No one correct representation
• Objects have identity

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Class

• Often appear as nouns in problem descriptions
• Has semantic
• Interpretation of semantics
• depends on application and matter of judgement
• Each class may have zero, one or more objects
• Each object knows it class

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Class Diagrams

• Provide formal graphic notation for modeling
• Concise, easy to understand, practical
• Describes many possible instances

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Object Diagrams

• Describes how set of objects relate
• Useful for
• documenting test cases
• Clarification of complex class diagrams
• Class Diagram corresponds to infinite set of
  object diagrams

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Notation for Classes Objects (UML)
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Attributes

• Data value held by objects of a class
• Objects may have same/different values for
  attribute
• Attribute name unique within a class
• Adjectives often represent specific enumerated
  attribute values red car

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Attributes...

• Attribute is a pure data value - not an object
• Internal identifiers must not be shown as
  attributes
• Show only important attributes

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Derived Attributes

• Base Attribute
• primitive, not dependent on other attributes
• Derived Attribute
• computing not considered to change state of an
  object
• dependent on base attributes
• may be stored or computed upon a query operation

Example Area of a circle, age of a person
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Attributes Notation
Underline in revised notation
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Operations Methods

• Operation Function that may be applied to or by
  objects
• Same Operations applying
  to different
  classes Polymorphic
• Method is implementation of an operation for a
  class
• Operation has a target object and may have
  arguments
• Same operations on different classes should have
• same signature and consistent intent
• Query Operation Does not affect the state of
  object
• Show only important methods

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Operations Notation
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Example with Attributes Operations
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Associations and Links

• Link is physical or conceptual connection
  between objects
• Link is an instance of an Association

Example Link Susan is-wife-of
Robert Julie is-wife-of John Association Woma
n is-wife-of Man
Associations and Links appear as Verbs
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Associations Links...

• Associations are bi-directional
• may be implemented as unidirectional
• Implemented usually as pointers
• important not to think as pointers
• Associations may be
• one-to-one
• one-to-many
• many-to-one
• many-to-many

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Ternary High Order Associations

• Binary Association Relates two classes
• Woman is-wife-of Man
• Ternary Association Relates three classes
• Nancy is-daughter-of Susan and Robert
• n-ary Association Relates n classes
• Higher Order Associations
• complicated to draw, implement and think
• try to avoid if possible

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Associations Notations
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Link Attributes
Association Classes

• Attributes that belong to
  association of object
  rather than one object
• Link Attributes belong in Association Classes
• Ex Salary received by Employee from Company
• In an one-to-one association you may try to make
  it attribute of one of the objects
• Leads to extensibility problems

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Association Class Notation
Person
Company


Employment
salarydouble
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Role Names

• Name given to either end of an association

• Helps to navigate from one object to related
  objects

husband
wife
Woman
Man
0..1
married
0..1

• Helps clarify when two classes have several
  associations between them

husband
wife
Man
Woman
married
0..1
0..1
1

daughter
children
father
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Qualifiers

• Distinguishes among set of associated objects
• Models associative arrays, dictionaries
• Qualifiers may be wrongly modeled as attribute of
  associated class

GradeList
Student


code
Unqualified
GradeList
1

code
Student
Qualified
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Aggregation Composition

• Aggregation
• Part-of or part-whole relationship (by reference)
• Example Car has Engine and Transmission
• Assembly of objects with aggregate and component
  parts
• Component existence may or may not depend on
  aggregate

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Aggregation
Composition...

• Composition
• Part belongs to only one whole (by Value)
• Part lives and dies with the whole
• Whole cannot replace the part

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Aggregation/Composition Notation
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Aggregation /Composition Example
Person
brain
Composition
Aggregation
Car
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Inheritance

• Models is-a relationship
• Relationship between a class and its refined
  versions
• Superclass or Base class
• Subclass or Derived class
• Inheritance is transitive
• Discriminator The property being abstracted by
  a particular inheritance
• Breath Vs. Depth of inheritance

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Inheritance Notation
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Inheritance Example
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Grouping Mechanism Package

• Grouping classes together into higher-level
  units
• Package diagram with dependency
• Dependency between packages exists if
• class in one package depends on a class in the
  other
• definition change of one package may change other

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Package Notation
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Aggregation Vs. Association

• Special form of Association
• Confusing
• Aggregation represents part-of relationship
• Some operations on whole automatically
  applied to its parts
• Aggregate is asymmetric part is subordinate
  to the whole
• Association is symmetric objects involved
  are of equal stature

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Aggregation Vs.
Inheritance

• Aggregation represents part-of relationship
• Inheritance represents kind-of relationship
• Aggregation refers to object relationships
• Inheritance refers to class relationships

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Fixed, Variable
Recursive Aggregates

• Fixed
• Fixed structure
• Number types of parts pre-defined
• Variable
• Finite number of levels - Number of parts vary
• Recursive
• Contains instances of the same kind of aggregate
• number of potential levels unlimited

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Example Fixed,
Variable, Recursive
Aggregation
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Operations Aggregation

• Operation or Triggering
• automatic application of an operation to network
  of objects when applied to some starting object
• ShallowCopy
• DeepCopy

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Abstract Classes

• Representing an Abstraction that is Abstract.
• Abstract classes represent
• concepts
• not real objects
• ABCs used only to create other Concrete classes

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Abstract Classes ...

• Example Shape, Employee, Animal
• Whether a class in Abstract or not depends on
• judgement
• application on hand

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Inheritance Extension
Restriction

• Extension
• Subclass adds new features
• Subclass inherits all properties operations of
  ancestor
• Restriction
• Subclass constrains ancestor attributes
• Subclass may not inherit all properties
  operations of its ancestor
• Often leads to trouble (Liskov Substitutability
  Principle)

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Inheritance Extension
Restriction ...

• Proper Extension
• A Subclass may override the internal
  implementation of an operation
• No problem as long as external protocol remains
  the same

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Metadata Metaclass

• Metadata Data that is description of other data
• Metaclass Class descriptor objects that
  describe other classes

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Constraints

• Restricts values that entities can assume
• Entity objects, classes, attributes,
  associations
• Better to capture constraints in structure rather
  than using constraints constructs

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Multiple Inheritance

• Allows a class to have more than one Superclass
• Join class

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Multiple Inheritance
Poses Problems

• Ambiguous function collisions
• Class proliferation

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Workarounds for MI

• Delegation using aggregation of roles
• Single inherit important class delegate rest
• Nested Generalization

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Delegation using Aggregation
of Roles

• Inheritance of operations across aggregation not
  automatic
• Join class must handle that operations and
  delegate to appropriate component

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Single Inherit
Important Class
Delegate Rest

• Preserves identity and inheritance across at
  least one generalization

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Nested Generalization

• Violates the OO spirit
• Not practical for large number of combinations