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Introduction to Object Orientation. Background: [behaviorally] object-oriented databases (OODBs) = OO Concepts + Persistency + Transaction support history & evolution of object orientation:

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introduction to object orientation
Introduction to Object Orientation
  • Background:
    • [behaviorally] object-oriented databases (OODBs) = OO Concepts + Persistency + Transaction support
    • history & evolution of object orientation:

> “Begin … End” blocks in Algol 60 (first attempt at providing encapsulation/protection within a PL)

> “Object” in Simula-67 (took the block concept one step further towards data encapsulation)

> “Object” , “Class”, “Inheritance” in Smalltalk (further towards data encapsulation & abstraction; ‘70s)

> theory of ADT (abstract data type) began to be developed in late ‘70s (eg, Ada)

introduction to object orientation1
Introduction to Object Orientation
    • history & evolution of object orientation (cont’d):

> DB languages emerged to support data abstraction & inheritance (eg, Taxis) from semantic modeling

> Concepts began to merge towards a unified concept strucutre -- object today! (early ‘80s --> )

E.g., Smalltalk-80, C++, Objective-C, CommonLisp,…

Gemstone, Orion, Vbase, Ontos, ObjectStore,...

  • Object Orientation

(1) three most fundamental/essential concepts:

- abstract data typing (encapsulation)

- inheritance

- object identity

introduction to object orientation2
Introduction to Object Orientation
  • Object Orientation (cont’d)

(2) Advantages of object orientation:

* better concepts and tools to model and represent the real world as closely as possible (=> model of reality!)

* OO languages provide expressive programming and/or manipulation primitives (=> behavior modeling!)

* better reusability & extensibility

=> reduce the time/cost of development

* enhanced maintainability & improved reliability

(eg, changes can be localized to the implementation of

one or more classes)

* better performance for complex applications

introduction to object orientation3
Introduction to Object Orientation
  • Object Orientation (cont’d)

(3) Approaches to OO Programming:

a) Novel languages: most radical approach

Eg, Smalltalk; Eiffel; Trellis/Owl

b) Language Extensions: less controversial approach

Eg., C++; Objective-C; Flavors; Object Pascal; …

c) Object-oriented style: as an enforced discipline

Eg., in C

to enforce ADT discipline

to simulate other OO features

slide5

Introduction to Object Orientation

  • OO Systems vs. Conventional ones
    • In Conventional Languages/Systems:

Programs: collections of procedures/subroutines that pass parameters

Procedure: manipulate its parameters, possibly return a value

=> the execution units (procedures) are the central focus!

    • In OO Systems/Languages:

the universe: a collection of independent objects communicating through message-passing

objects: the active entities; procedures (messages): passive entities

=> the data (ie, objects) are the central focus!

introduction to object orientation4
Introduction to Object Orientation
  • Features of Object Orientation
    • Abstract Data Type (ADT)

> ADT: defines a set of similar objects with an associated collection of operators

> Three criteria for a PL to support ADTs:

a) object classes: every datum (object) belongs to an

ADT (the object’s class)

b) information hiding:

- object only accessible through the external interface routines/operations defined by its ADT

- its internal implementation details, data structures, and storage elements are not visible outside

c) completeness: all required operations are defined.

Left to the user!

introduction to object orientation5
Introduction to Object Orientation
  • Abstract Data Type (cont’d)
    • Object Class: incorporates the definition of the structure as well as the operations of the ADTs

Instances: elements pertaining to the collection of objects described by a class

Minimally, a class definition should include:

a) the name of the class

b) the external interface (ie, operations for manipulating its instances; typically have a target object & some arguments)

c) the internal representation (ie, instance variables)

d) the internal implementation of the interface

introduction to object orientation6
Introduction to Object Orientation
  • Object Class (cont’d)

1) Instance Variables: to capture the internal representation of an object of the class

* declaration: Name: string

Address: string

Revenue: dollar

Employees: SetOf Employee

Subsidiaries: SetOf Company

Note: there are typeless PLs like Smalltalk, LISP, APL (where x can be bound to integer 5, and later to a

string ‘xy’ within the same session/program).

“type constraints”

(checked at compile

time for typed DB PL)

introduction to object orientation7
Introduction to Object Orientation
  • Object Class

1) Instance Variables (cont’d)

* Operator “overloading”:

E.g., in PASCAL,

“+” can be used for integers

as well as for floats!

2)Object Creation: done through invoking a “new” operator on the class

E.g. in Eiffel:

IBM:= new(Company, “IBM”, “111 success street, New York”)

in C++:

IBM = new Company (“IBM”, “111 success street, New York”)

introduction to object orientation8
Introduction to Object Orientation
  • Object Class

2) Object Creation (cont’d)

Note: instance variables are initialized either at object creation time (via “new”), or subsequently through calling specific operators

E.g., Set-Revenues(IBM, $5,000,000,000)

3) Methods & Messages

3.1) invoking an operation involves:

a) the target object

b) the name of the operator

c) the argument of the operation

d) calling the code that implement the operator, binding the parameters to the actual arguments

introduction to object orientation9
Introduction to Object Orientation
  • Object Class

3) Methods & Messages (cont’d)

E.g., in Smalltalk, a message looks like below:

GM(AddSubsidiary: Hughes)

in C++: GM.AddSubsidiary(Hughes)

in Eiffel: AddSubsidiary(GM, Hughes)

very similar to invoke procedures! (Difference: binding is done at run time, depending on the class of the target object!! )

The selector (name of a “method”)

AddSubsidiary(C: Company,

NewSubsj: Company)

C.Subsidiary[TotalSubsj]:= NewSubsi

TotalSubsi := TotalSubsi + 1

A message (to GM):

(i) a selector

(ii) one/more arguments

introduction to object orientation10
Introduction to Object Orientation
  • Object Class

3) Methods & Messages (cont’d)

On Dynamic Binding (“late binding”):

the system will bind at run-time a selector to the particular method (procedure) that implement it, based on the recipient object’s class.

Motivations:

1) the same message or method name could be used

by different classes (overloading!)

2) a variable’s object class might not be known till runtime (eg, in typeless languages)

Advantages:

flexibility & extensibility

(the cost is the performance penalty!)

introduction to object orientation11
Introduction to Object Orientation
  • Object Class

3) Methods & Messages (cont’d)

On Dynamic Binding (“late binding”):

E.g., there is a stack of heterogeneous objects, and we want to print every entry in the stack

in OOPLin conventional PLs:

for i:= 1 to Top for i:= 1 to Top do

do Print(Stack[i]) case Stack[i].type

integer: PrintInteger(Stack[i].object)

float: PrintFloat(Stack[i].object)

string: …

Boolean: …

If a new type X is added, then the OOPL system can be extended without affecting the code, whereas the other would need to add PrintX and recompile the while thing!

introduction to object orientation12
Introduction to Object Orientation
  • Object Class (cont’d)

4) Constraints

to help enforce correctness/completeness of ADT

Two approaches:

a) Constraints on objects and their instance variables

E.g., Student.age > 15

b) Pre- and Post-conditions of methods (as supported in, eg., Eiffle)

introduction to object orientation13
Introduction to Object Orientation
  • Inheritance Hierarchy
    • the 2nd powerful concept of OOPL and systems
    • similar concept found in semantic networks (Minsky’68) and semantic data models
    • useful in organizing/structuring information, and facilitating reusablity (rather than designing everything from scratch)

E.g.

superclass

Company

subclass

Commercial Company

Non-profit Organization

introduction to object orientation14
Introduction to Object Orientation
  • Inheritance Hierarchy (cont’d)
    • subclass and superclass relations are “transitive”
    • there is the notion of inheritance applicable
    • two main aspects of inheritance:

1) structural (instance variables)

2) behavioral (methods)

1) Inheriting Instance Variables

- common in AI (KR) and semantic data models

- subclsses “inherit” their superclass’ instance variables

- different ways to achieve the goal of inheritance:

introduction to object orientation15
Introduction to Object Orientation

1) Inheriting Instance Variables (cont’d)

* in Smalltalk: visible directly & completely!

Problem: violates both uniformity & encapsulation

(eg, suppose A changes its “Subsidiaries” from an

array to a list of company, then B’s evaluate-profit

method has to be modified!)

A

B

introduction to object orientation16
Introduction to Object Orientation

1) Inheriting Instance Variables (cont’d)

* in CommonObjects & Encore: invisible completely!

* more general approach: (eg, in C++, Trellis/Owl)

- Public (any client can access)

- Private (no client can access)

- protected (subclass’ clients can access)

A

Instance variables in the hierarchy are independent from each other

B

introduction to object orientation17
Introduction to Object Orientation

2) Inheriting Methods

- unique to OO languages and systems

- methods defined for a class are inherited by its subclasses

E.g.

move-window()

resize-window()

Window

retate-window()

Bordered

Window

Text

Window

edit-font()

edit-font()

edit-size()

reset-size()

introduction to object orientation18
Introduction to Object Orientation

2) Inheriting Methods (cont’d)

- a subclass can “over-ride” an inherited method

E.g.

Where: Evaluate-profit(FC: ForeignCommercial)

return (CommercialCompany.evaluate-profit(FC)-

ForeignTax(FC))

Commercial Company

evaluate-profit()

Foreign Commercial

evaluate-profit()

introduction to object orientation19
Introduction to Object Orientation

2) Inheriting Methods (cont’d)

- multiple inheritance: combine several classes to produce

“conglomerate classes”

E.g.

Person

Employee

Student

Manager

StudentWorker

introduction to object orientation20
Introduction to Object Orientation

2) Inheriting Methods

- multiple inheritance (cont’d):

Suppose Employee : {name, age, salary, rank, dept}

Student : {name, age, GPA, program, advisor}

then StudentWorker: {name, age, salary, rank, dept,

GPA, program, advisor}

What happen when there is a “conflict” (ie, different methods/instance variables have the same name)?

Eg., assume both student & employee had a method called “bonus” (with totally different semantics), which bonus method should apply to StudentWorker?

introduction to object orientation21
Introduction to Object Orientation
  • Meta Classes

- Are classes instances of other classes?

- If Yes, what is the class describing them, and where does

the transitive closure end?

E.g., in Smalltalk:

* classes are instances of their metaclasses which are created by the system (the user distinguishes between class methods/attributes from instance methods/attributes)

* all metaclasses are instances of METACLASS;

* METACLASS is an instance of CLASS which is an instance of OBJECT ( ----- the root!)

introduction to object orientation22
Introduction to Object Orientation
  • Object Identity
    • the 3rd powerful concept of OO systems

Definition: an identity is a handle which distinguishes

one object (entity) from another

    • the idea: within a complete OO system, each object will be given an identity (Oid) permanently, immaterial of the object’s structural or state transitions
    • necessity: without Oids, it is

a) awkward (if not impossible) to assign self-contained objects to instance variables

b) impossible to make the same object a component of multiple objects

introduction to object orientation23
Introduction to Object Orientation
  • Object Identity (cont’d)
    • Conventional programming language’s way:

- use variable names to distinguish objects

 mixes addressability and identity!

- practical limitations:

no way to test if x and y refer to the same object (ie, x==y), as opposed to x=y (assignment).

External to object (a way

to access; typically through

pointers or virtual addresses)

Internal to object (to

represent the individuality;

should be independent of

how it is accessed!)

Supported in Smalltalk

introduction to object orientation24
Introduction to Object Orientation
  • Object Identity (cont’d)
    • Conventional DB Approach:

- uses record keys to distinguish objects

 confuses identity and data values (obj. state)!

- practical problems:

a) modifying key attributes?

b) non-uniformity:

Eg, Compaq uses emp# as key attribute

IBM uses ss# as key attribute

a merge of Compaq and IBM  one of these

keys to change!

introduction to object orientation25
Introduction to Object Orientation
  • Object Identity
    • Conventional DB Approach (cont’d)

c) unnatural join:

Eg, Employee Company

EmpNamess# salary CompName Name Budget Location

Query: “find for all employees the Employee name and the location that employee works in”.

In SQL: SELECT Employee.EmpName, Company.Location

FROM Employee, Company

WHERE Employee.CompName = Company.Name;

But if two companies

have the same name?

introduction to object orientation26
Introduction to Object Orientation
  • Conventional DB Approach

c) unnatural join (cont’d)

(in most cases, what the user wants is the actual company object (tuple), instead of the CompName!)

Part of the reason: due to normalization which loses the semantic connections amongst the objects

 relational systems have to use additional facilities such as foreign key constraints to recapture the lost semantics!!

What about relax the normalization constraints?

 nested relations (or, Non-1NF relations) allowed, so as to support a more direct & intuitive representation

introduction to object orientation27
Introduction to Object Orientation

But, non-1NF models only provide a partial solution…

E.g. Person

Nameage address spouse Education

Degree Univ. Year

John 35 31 parker st. Mary MSc MIT 1981

PhD CMU 1986

Mary 30 31 parker st. John BBA USC 1985

MBA USC 1987

Children Parent Child Cage

John Tim 5

Mary Tim 5

John Jane 3

Mary Jane 3

Note: children still need to be

represented separately, since

each child pertains to both parents

introduction to object orientation28
Introduction to Object Orientation

What’s missing?

----- the ability to share objects (to refer to objects directly and completely)

Answer: Oid !

* preserve identity throughout object evolution:

- strong support of identity is important for temporal data models (where objects can evolve!)

- Oid can serve as the common thread that ties together the historial versions of an object

Some Oid operations:

- merge / coerce

- copy: shallow vs. deep

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