OOP in Smalltalk

Lecture Notes

Smalltalk Overview

• developed at XEROX PARC in early 1970's by Alan Kay and colleagues as an environment to teach programming to children

• influenced by both Simula and LISP

• Smalltalk-72, Smalltalk-76, Smalltalk-80 (simply referred to as Smalltalk)

• in Smalltalk, everything is an object (not so in C++ or Java)

  • even a class is an object

  • for instance, class FinancialHistory is the only object of class Meta-FinancialHistory

• each object is an abstraction of a computer

• was the first pure object-oriented language

  • it is completely consistent with the OO paradigm

  • Ruby is also pure

  • Java is almost pure

• adopts an anthropomorphic programming model [PLP] p. 491 (i.e., the only way to affect computation is to pass messages to objects which then execute code (a method) in response to the message)

• not just a language, a complete software development environment

• requires a mouse which was developed at PARC for this

• uses a simple, uniform model of the world akin to LISP

• uses a simple uniform syntax, again, akin to LISP

• uniformity and simplicity: everything is an object

• typeless (akin to LISP): any name can be bound to any object; class membership is the only typing

• supports dynamic polymorphism (i.e., code is associated with no particular type)

• automatic garbage collection

• only reference variables which are implicitly dereferenced

• only supports single inheritance

• strongly typed (akin to ML and Haskell), but at runtime

• dynamic nature

  • typeless language (akin to LISP)

  • dynamic binding

  • supports dynamic polymorphism

  • supports reflection: ability to inspect and modify a program while it is running!

• contributions:

  • a new paradigm (not just a new language)

  • influential in personal computing revolution and windowing systems

• inspired and pioneered

  • the object-oriented paradigm,

  • the idea of personal computing, and

  • windowing systems and GUI's (mouse)

Smalltalk syntax

• class names must begin with an UPPER CASE letter and method names begin with a lower case letter; convention in C++ and Java, but not enforced

• be careful to distinguish between

  • Object: root class (capital O)

  • object: just some object

• period (.) is the statement separator

• up arrow is return (typed ^); if omitted, then current object returned

• left arrow (←) is assignment (typed _)

• text between single quotes is treated as a string

• text between double quotes are considered comments

• local variables introduced with pipe symbols (e.g., | temp |)

• blocks: [ ... ] can be thought of as a closure and call-by-name parameter; allows Smalltalk to handle control in a purely OO fashion

Smalltalk messages

• sometimes called selectors

• messages are written in postfix form

• a colon precedes the argument to a message

• unary

  • // C++ q.front() "Smalltalk" q front

• keyword

  • // C++ q.enqueue(x) "Smalltalk" q enqueue: x // C++ table.insert(anItem, anIndex) "Smalltalk" table insert: anItem at: anIndex

• binary: 2 + 3

• in this manner Smalltalk programs read like English

Class activity/Quiz

Given the following class diagram, answer the Meta-Madness Quiz.