Principles of ObjectOriented Software Development

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Principles of Object-Oriented Software Development

• Object-oriented programming languages

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Object-oriented programming languages
Introduction The object paradigm
Comparing Smalltalk, Eiffel, C and Java
Design dimensions of object-oriented languages
Prototypes -- delegation versus inheritance
Meta-level architectures Summary Q/A
Literature
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Object-orientedprogramming languages

• the object paradigm
• language design dimensions
• classless prototypes
• meta-level architectures

Additional keywords and phrases programming
languages, orthogonality, reliability,
complexity, types, delegation, multiple
paradigms, prototypes, reflection
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The object paradigm
Subsections A classification of
object-oriented languages Alternative object
models Object extensions of Lisp, C and
Prolog Script languages -- integration with
Java
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The notion of object
Simula

• abstract data types -- software engineering
• frames -- artificial intelligence
• semantic data models -- database system
  development
• capability-based computing -- distributed systems
  

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object-oriented
structurally capability of representing
arbitrarily structured complex objects operationa
lly the ability to operate on complex
objects through generic operators behaviorally
the specification of types and operations
(data abstraction)
Perspectives of object orientation
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A classification of object-oriented languages

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Objects
language characteristics

• object creation facility
• message passing capability
• class capability
• inheritance features

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Classification

• hybrid -- C, Lisp, Pascal, Prolog
• frame-based -- knowledge-based reasoning
• distributed, concurrent, actor -- parallel
  computing
• alternative object models -- prototypes,
  delegation

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Alternative object models

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Object extensions of Lisp, C and Prolog
Object extensions Lisp -- LOOPS, FLAVORS,
CLOS, FOOPS C -- Objective C, C
Prolog -- SPOOL, VULCAN, DLP Commercial
products -- languages Smalltalk, Eiffel,
C, Objective C, Object Pascal, Java
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Object structure -- efficient mapping


C struct A ...
class A public ...
class A ... struct A private

The
equivalence between class and struct
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Script languages
Java embedding
Javascript -- Dynamic HTML Perl
-- CGI/Web library
JPL
Tcl/Tk -- tclets
Jacl, Tcl Blend
Python -- Grail
JPython
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Objects in Javascript

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ltscript languageJavascriptgt
javascript
function object_display(msg)
object method
return msg ' (' this.variable ')'
function object()
object
constructor this.variable0
this.display object_display return
this var a new object()

create object document.write(a.display("a
message")) document.write(a.display("another
message")) lt/scriptgt
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Comparing Smalltalk, Eiffel, C and Java
Subsections Criteria for comparison
Language characteristics
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Criteria for comparison

• class libraries
• programming environment
• language characteristics

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Language characteristics

• uniformity of data structures
• documentation value
• reliability
• inheritance mechanisms
• efficiency
• memory management
• language complexity

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Smalltalk
Eiffel C Java
uniformity high
medium low medium
documentation value medium high
medium high reliability
medium medium
low high protected operations
no no yes
yes multiple inheritance no
yes yes
no efficiency low
medium high
low garbage collection yes
yes no yes
language complexity low
medium high medium
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Design dimensions of object-oriented
languages
Subsections Object-based versus
object-oriented Towards and orthogonal
approach -- type extensions Multi-paradigms
languages -- logic Active objects --
synchronous Java/C
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Object Oriented LanguageDesign

• object state operations
• class template for object creation
• inheritance super/base and subclasses

object-oriented objects classes
inheritance
data abstraction -- state accessible by
operations strong typing -- compile time checking
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Orthogonal approach

• objects -- modular computing agents
• types -- expression classification
• delegation -- resource sharing
• abstraction -- interface specification

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Multi-paradigm languages

• logic

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Open systems reactive -- flexible (dynamic)
choice of actions modular -- (static)
scalability Dimensions of modularity
encapsulation boundary -- interface to client
distribution boundary -- visibility from within
objects concurrency boundary -- threads per
object, synchronization
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Active objects

• synchronous Java/C

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Object-based concurrency

• add processes -- synchronization
• multiple active objects -- rendezvous
• asynchronous communication -- message buffers

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Synchronous C/Java
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active class S
sC public
m () ... private _at_S ()

pseudo-constructor select
01 -gt m()
external call
instructions ...
accept m
accept internal method instructions
... waituntil
(date)
time-out instructions ...
default

default instructions ...

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Prototypes

• delegation versus inheritance

Subsections Alternative forms of sharing
Implementation techniques -- Self
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Prototypes
exemplars

• cloning -- creation time sharing
• delegation -- lifetime sharing

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State slots -- parents, variables and
methods Creation shallow cloning deep
cloning Delegation implicit delegation
explicit delegation
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Improving performance

• special purpose hardware
• hybrid languages
• static typing
• dynamic compilation

Implementation techniques -- Self
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Self -- prototypes objects, cloning,
delegation Dynamic compilation -- type
information customized compilation
message inlining lazy compilation
message splitting
Implementation techniques -- Self
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Meta-level architectures

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The class concept

• abstract data type -- interface description
• object generator -- template for creation
• repository -- for sharing resources
• object -- instance of a metaclass

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Postulates
class-based languages

• everything is an object
• every object belongs to a class
• every class inherits from the class Object
• class variables of an object are instance
  variables of its class

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Reflective definition of Class
name Class supers
(Object) iv
(name supers iv methods) methods
(new ...)
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Summary
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The object paradigm
1

• notion of object -- viewpoints
• classification -- object extensions

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Comparing Smalltalk,Eiffel, C and Java
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• criteria -- libraries, environments, language
  characteristics
• comparison -- language characteristics

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Design dimensions ofobject-oriented languages
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• object-oriented -- object-based inheritance
• orthogonal dimensions -- objects, types,
  delegation, abstraction
• open systems -- dimensions of modularity

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Prototypes -- delegationversus inheritance
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• prototypes -- cloning and delegation
• performance -- dynamic compilation

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Meta-level architectures
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• class -- the concept of class
• meta architecture -- subclass and instance
  hierarchy
• reflection -- postulates

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Questions
1. What are the basic characteristics of
object-oriented languages? 2. How would you
classify object-oriented languages? Name a few
representatives of each category. 3. What do you
consider to be the major characteristic of the
object model supported by C? Explain. 4. Why
would you need friends? 5. How would you
characterize the difference between object-based
and object-oriented? 6. Along what orthogonal
dimensions would you design an object-oriented
language? Explain. 7. Give a characterisation of
active objects. In what situations may active
objects be advantageous? 8. How would you
characterize prototype-based languages?
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9. What are the differences between inheritance
and delegation? Does C support delegation?
Explain. And Java? 10. How would you
characterize the concept of a class? 11. Can you
sketch the meta architecture of Smalltalk? 12.
How would you phrase the postulates underlying
class-based languages? Can you give a reflective
version of these postulates?
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Further reading
A concise treatment of programming languages is
given in BG94. Further, you may want to
consult Wegner87, which contains the original
presentation of the discussion concerning the
distinction between object-based and
object-oriented. For Java, read the original
white paper, Java. An interesting extension of
C is described in Petitpierre98. At the
corresponding web site, http//ltiwww.epfl.ch/sCxx
, there is much additional material. Finally,
for an account of the design and evolution of
C, read Stroustrup97. For more information on
C, visit http//www.accu.org , and for Java,
http//www.javasoft.com .