Object-Oriented Programming and Design

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Object-Oriented Programming and Design

• Ralph Johnson johnson_at_cs.uiuc.edu

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Goals to learn

• Principles
• Patterns
• Practices
• Object think
• Design for reuse

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History of Objects

• Simula67
• Smalltalk 72,74,76,78,80
• 1985 C (Objective C, Object Pascal)
• 1986 OOPSLA
• 1990 COM, CORBA
• 1994 Design Patterns
• 1995 Java
• 2001 .NET

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Design Principles

• Encapsulate what varies
• Code to an interace rather than to an
  implementation
• Open-closed Principle Classes should be open to
  extension and closed to modification
• Dont Repeat Yourself (DRY)
• Single Responsibility Principle
• Liskov Substitution Principle Subtypes must be
  substitutable for their base types

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Smalltalk

• Extreme object-oriented language
• Live objects
• Readable library
• Influencial, unknown
• Fun!

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From Smalltalk

• WIMP - Windows, Mice, Pointing
• Model/View/Controller
• Browsers
• CRC cards
• eXtreme Programming
• Automated refactoring tools

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Before objects

• Computer system consists of data and programs.
• Programs manipulate data.
• Programs organized by
• functional decomposition
• dataflow
• modules

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Object Paradigm

• Computer system consists of a set of objects.
• Objects are responsible for knowing and doing
  certain things.
• Objects collaborate to carry out their
  responsibilities.
• Programs organized by classes, inheritance
  hierarchies and subsystems

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What is an object, anyway?

• Mystical view
• Computing systems are made up of objects that
  communicate only by sending messages between each
  other. All computation is message sending.

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What is an object, anyway?

• Scandinavian view
• A program is a simulation. Each entity in the
  system being simulated is represented by an
  entity in the program.

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What is an object, anyway?

• Programming language view
• An object-oriented system is characterized by
• data abstraction
• inheritance
• polymorphism by late-binding of procedure calls

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Heart of Object-Oriented Programming

• Don't make a new language, extend your old one.
• Objects should be abstractions of problem domain.

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Modeling

• All phases of software life-cycle are modeling
• analysis - modeling of problem
• design - modeling of solution
• implementation - making model run on a computer
• maintenance - fixing/extending your model

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Assumption about Modeling

• Basing system design on structure of problem
  makes system
• more reusable
• more extensible

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Modeling

• Claim people model the world with "objects"
• objects
• classes
• relationships between objects
• relationships between classes

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Modeling

• Advantages of object-oriented software
  development
• more natural - matches the way people think
• single notation - makes it easy to move between
  software phases

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Objects and Relationships

• John is Mary's father. Mary is John's
  daughter.
• Bob is Mary's dog. Mary is Bob's owner.
• Ann is John's employer. John is Ann's employee.

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Objects and Attributes

• John's name is "John Patrick O'Brian".
• John's age is 27.
• John's address is 987 N. Oak St, Champaign IL
  61820
• What about John's job? John's wife?
• What is an attribute, and what is a relationship?

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Objects and Behavior

• John goes on a trip.
• John makes reservations.
• John buys tickets.
• John travels by airplane.
• John checks into hotel.

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What Really is an Object?

• Reservation -- a promise to give service to a
  customer
• Ticket -- proof that customer has paid for
  service in advance
• Flight
• Payment -- an event (transaction?) in which money
  is exchanged

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What Really is an Object?

• Anything we can talk about can be an object,
  including relationships ("the husband of the
  first party", "first-born son").
• What are we trying to model?
• Models should be as simple as possible, but no
  simpler.

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Classification

• We naturally put objects into classes that have
  similar characteristics.
• John is a man.
• Mary is a woman.
• Bob is a dog.
• All women are people.
• All people are mammals.

Mammals
Dogs
People
Women
Men
Bob
Mary
John
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Classification

• John is an employee.
• John is a father.
• John is a sky-diver.
• John is under 30.
• Is John an instance of Employee, Father,
  Sky-diver, and Under30?

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Summary

• Objects
• have identity
• have attributes
• have behavior
• have relationships with other objects

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Summary

• Classes
• describes the attributes, behavior, and
  relationships of a set of objects
• subclasses/superclasses form graph of
  generalizations

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The class

• First month is Smalltalk
• Homework, often several per week
• Second half of the semester is group project
• See http//swiki.cs.uiuc.edu/cs598rej/MP0
• Make a page for yourself
• Download Squeak and play with it

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Smalltalk in a Nutshell

• Objects classes
• Messages methods
• Inheritance metaclasses

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Smalltalk Everything is an Object

• Application objects customer, inventory
• GUI objects button, text editor
• Foundation string, set, numbers, booleans
• Tools browser, debugger, compiler, GUI builder
• Language implementation class, method, context,

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Communicating with an Object

• All computation is done by objects.
• The only way to interact with an object is to
  "send a message" to it.
• Smalltalk has three kinds of syntax for sending a
  message.
• All messages have same implementation.

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Three Kinds of Message Syntax

• Unary messages
• aSalaryChange date
• Keyword messages
• earned at date add money
• Binary messages
• (worked - 40) max 0

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Sending a Message

• Object responds to message by looking in its
  class for a method with the same selector.
• If it doesn't find the method, it looks in its
  superclass.
• Repeat until it finds the method. If it never
  does, there is an error.

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Smalltalk in a nutshell

• Classes have methods and variables.
• Methods are a sequence of messages, assignments,
  returns.
• Variables, literals, psuedovariables
• Blocks
• Metaclasses - classes have classes

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Message Lookup and Inheritance

• EmployeeTransaction has subclasses Paycheck,
  SalaryChange, and Timecard.
• EmployeeTransaction defines the instance variable
  date and the method
• date
• date

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EmployeeTransaction date
check1 07/09/95
Paycheck
aPaycheck date
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Smalltalk Expression Syntax

• Literals
• 3.675, 14, 'hello', weight, d,
• ( foo 'bar' 92)
• Assignments and variables
• v v 1
• Messages

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Smalltalk Expression Syntax

• Sequences. Blocks.
• x lt y ifTrue z x ifFalse z y.
• paychecks do each each post
• Cascades
• aSet add blue add red

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Smalltalk Method Syntax

• Returns
• socialSecurity federalTaxes stateTaxes

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Variables in Smalltalk

• blockArguments and temporaries
• methodArguments and temporaries
• instanceVariables
• Can be accessed only by the object's methods.
• Globals, class variables
• Any method in the scope can access it
• Variable is object of class Association

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Using Variables
printOnCheckStream aStream aStream cr
cr. aStream next 40 put (Character
space). DateFormat print date on
aStream. aStream cr. ...
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More variables

• test
• "PayrollSystem test"
• payroll day1 ralph
• day1 Date newDay 5 year 1996.
• payroll self new.
• ralph Employee new name
• 'Ralph Johnson'.

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(continued)

• ralph changeSalaryFor day1 to 20.
• payroll addEmployee ralph.
• self
• employee ralph
• hours self aLittleOvertime
• starting day1.
• payroll

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Initializing an Object

• Every object needs to be initialized.
  Uninitialized variables are nil.
• The initialization method often defines the type
  of a variable.
• Two methods one class and one instance.

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Class Methods

• Class is an object. It can have methods, too.
• For class Date class
• newDay dayInteger year yearInteger
• self new day dayInteger year yearInteger

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Instance initializing method

• For class Date
• day dayInteger year yearInteger
• day dayInteger.
• year yearInteger

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Creating a Date
Date newDay 3 year 1995 Date new day 3 year
1995
day
day 3 year 1995
year
3
1995
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Complete Smalltalk

• Expressions (method definition)
• message, assignment, sequence, block, return
• Variables, literals
• Classes, inheritance
• Class methods / instance methods
• Pseudovariables
• nil, true, false
• self, super, thisContext

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Smalltalk (the language) is trivial

• Complexity is class library.
• New language extensions fit in as well as numbers
  and control structures.
• Language extension gt core language is trivial

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Implications

• class library language extension
• gt
• must know class library
• must standardize class library
• merging class libraries is like merging language
  extensions
• hard to make class libraries

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Applications

• No programs, just objects
• No main routine
• Most applications create new classes, reuse a lot
  of existing ones