Object Oriented Modelling Super Types

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Object Oriented ModellingSuper Types Sub Types

• HIIB62
• Databases and Data Modelling
• Contributions by Arthur Adamopoulos, Hossein S.
  Zadeh, Ian Searle and Ian Storey

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What is Object Oriented?

• The object-oriented approach models a system as a
  collection of interacting objects - that the
  objects in a computer system, like the objects in
  the world around us, are viewed as things.

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What is Object Oriented?

• These things (or objects) have certain features,
  or attributes, and they can exhibit certain
  behaviours. Similar objects in a system can be
  grouped and classified as a specific type of
  thing (or class).

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What is Object Oriented?

• Another important concept is that objects
  interact, that one object can tell another object
  to do something via a message.

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What is an Object?

• An object is an entity that has state, behaviour,
  and identity.
• State includes an objects properties (attributes
  and relationships) and the values those
  properties have.
• The objects under discussion could be
  electronic objects in a computer system or real
  objects found in a real human system.

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What is an Object?

• The following are all objects that each have a
  type, and encapsulate characteristics and
  behaviour

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UMLUnified Modelling Language

• The Unified Modelling Language (UML) is a
  language for specifying, visualising and
  constructing the artefacts of software systems,
  as well as for business modelling.
• UML provide notation for graphically depicting
  object-oriented analysis and design models. It
  allows for
• Specification of system requirements.
• Capture of design decisions.
• The promotion of communication among the key
  persons involved in system development.

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UML Models

• Class diagrams, which show the static structure
  of data, and the operations that act on the data.
• Use cases, which represent the functional
  requirements, or the what of the system.
• State diagrams, which represent dynamic models of
  how objects change their states in response to
  events.
• Sequence diagrams, which represent dynamic models
  of interactions (message flow) between objects.

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What is a Class?

• A collection of like objects.
• A template for defining new object instances.

• The behaviours reside in the class.
• Behaviour is implemented by methods (which can
  also be called services).

Class name
Student name dateOfBirth address phone calc-age()
register-for(course)
Course courseCode courseTitle creditHours enrolmen
t()
List of attributes
List of methods
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Relationships

• Relationships as defined in E-R modelling also
  apply to OO modelling.
• UML represents relationship with a line, just as
  in E-R diagrams.
• UML, however, divides relationships into a number
  of different categories, each with distinct
  symbols at the ends of the lines.

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Association

• An association is a meaningful connection between
  two classes, just as in E-R modelling.
• Associations are said to be loose
  relationships, in that if the relationship was
  broken, none of the objects would be deleted
  each can continue to exist in its own right.

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Association
Student
Course
StudentNo Surname Given
CrsCode Name
1

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Cardinality / Multiplicity

• one-to-one
• One-to-many
• Many-to-Many
• One-to-three

1
1

1


3
1
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Aggregation / Composition

• An aggregation is a stronger form of
  relationship.
• Aggregations are usually termed has a or part
  of relationships.
• One of the objects is said to belong to the
  other. If the main object is deleted then the
  related objects would also be deleted.

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Aggregation / Composition
Window
Button
Title Position
Label Position

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Inheritance Introduction

• Assume we are designing a system for a university
  library.
• We may define the following classes
• Student
• Staff
• Book
• Loan

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Inheritance Introduction
Student
Staff
BorrowerNo Surname Given StudentNo CrsCode
BorrowerNo Surname Given StaffNo Extension
setBorrowerNo() setSurname() setGiven() setStudent
No() setCrsCode()
setBorrowerNo() setSurname() setGiven() setStaffNo
() setExtension()
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Inheritance Introduction

• You may notice that the Student and Staff classes
  have some things in common
• Common fields
• Common methods
• We can take these similarities and put them into
  a higher level class (Borrower).
• We can then make the Student and Staff classes be
  sub-classes of this higher class.

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Borrower
SuperClass
BorrowerNo Surname Given
setBorrowerNo() setSurname() setGiven()
SubClasses
Student
Staff
StudentNo CrsCode
StaffNo Extension
setStudentNo() setCrsCode()
setStaffNo() setExtension()
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Inheritance

• Sub-class inherits everything from super-class.
• An object of type Student is also an object of
  type Borrower.
• It inherits all fields and methods defined in
  Borrower.
• Can be used any place a Borrower object can be
  used.

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Enhanced ER Model (EER) has Super and Sub Types

• The ER model has been enhanced by various people
  to include inheritance.
• Concepts have simply been borrowed from the OO
  model but drawn differently.
• In practice if you wanted to create a model
  involving inheritance you would probably go
  directly to an OO model.

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EER Model Super/Sub Types
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CASE Tools
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OBJECT/RELATIONALMAPPING
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Mapping Classes to Tables

• We can use tables to store the values of objects.
  Roughly, each object is represented by a row
  (record).
• In the same way that we map an E-R model to the
  relational model, we need to create a mapping
  from the class model to the relational/SQL model.
• For the most part this is identical to mapping an
  E-R model.

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Mapping Classes

• For the most part, a class in the class model
  will be mapped to a table in the relational model
  (see handling inheritance later).
• A single record in the table will represent the
  contents of one object.
• However, as in dealing with E-R models, every
  table in the relational model does not
  necessarily map back to a class.

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Mapping Attributes

• Native type attributes (ie. Numbers, text, dates
  etc) are simply fields in the table.
• Some native types may need translation (eg.
  Boolean may be stored as Y/N char)
• Attributes that reference other objects are
  actually storing relationships as per an E-R
  model see next slide.

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Mapping Relationships

• A relationship is implemented as a foreign key
  just like with an E-R model.
• 1-1 Relationship foreign key either side
• 1-M Relationship foreign key on many side
• In Java, a M-M relationship would usually be
  constructed with a hash table, so the entries in
  the hash table need to be mapped out to their own
  table.

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Mapping Inheritance

• Most relational databases do not support
  inheritance (some actually do).
• There are three different options to map
  inherited classes
• Keep All Levels
• Roll Up
• Roll Down
• This choice is made for every individual
  inheritance relationship.

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Sample classes
Borrower
BorrowerNo Surname Given
Student
StudentNo CrsCode
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Keep All Levels

• This is simply representing every class in the
  inheritance hierarchy as a table.
• This means, however, that one physical instance
  object of a sub-class will be saved across
  multiple rows in multiple tables.
• A common key may need to be introduced to
  maintain the link between the super and sub-class
  tables so objects can be re-built from their
  parts.

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Keep All Levels

• Borrower Student Staff
• BorrNo BorrNo BorrNo
• Surname StudentNo StaffNo
• Given CrsCode Extension

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Roll Up

• All the attributes from the sub-class are rolled
  up into the super-class.
• One table is then used to represent the combined
  attributes of both the super and sub-class
  objects.
• An extra field is usually needed to flag which
  type of object the row represents (ie. Which
  sub-class does it belong to).

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Roll Up

• Borrower
• BorrNo
• Surname
• Given
• BorrType
• StudentNo
• CrsCode
• StaffNo
• Extension

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Roll Down

• All the attributes from the super-class are
  rolled down into each sub-class.
• A table is then used to represent each sub-class,
  with no table for the super-class.
• The attributes from the super-class are, in
  effect, repeated in each sub-class table.

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Roll Down

• Student Staff
• BorrNo BorrNo
• Surname Surname
• Given Given
• StudentNo StaffNo
• CrsCode Extension