9. oo languages

OO Languages

Chapter 9

Object-Oriented Software Systems Engineering – Chapter 9 Slide 1

Objectives

In this chapter we will:
 Introduce some Object Oriented Programming
Languages
 Compare some aspects of these languages


Major OOPL

 Four of the major Object Oriented Programming
languages are:
C++
Smalltalk
Eiffel
Java


Smalltalk

 Smalltalk was for most purposes the first object-
oriented language, although it was based on ideas
first developed in a simulation language called
Simula in the 1960s

 Smalltalk was designed to support a (then)
radically new type of user interface - the graphical
user interface


Smalltalk environment

 Then Smalltalk was synonymous with GUIs - now
they are commonplace

 Smalltalk’s environment is graphical, based on
windows and browsers, and is controlled by a
pointing device


Environment...

 The development environment is an integral part
of Smalltalk, and all aspects of it are available for
inspection and change within a running Smalltalk
system, conversely a Smalltalk system can be
extended by writing new classes which are then
available in the environment for immediate use, so
it is possible to add your own code-writing or
debugging tools, although these are already
present in Smalltalk


...Environment

 Smalltalk has several classes of interactive code
writing and debugging tools. They are:
 Class Hierarchy Browser
 Class Browser
 Workspace
 System Transcript
 Disk Browser
 Walkback; Debugger


Environment Classes

 System Transcript:
 reports events in the system as they occur
 Class Hierarchy Browser
 allows users to browse, edit, and add new classes to the
running system
 Workspace
 allows users to edit and evaluate expressions and
displays the results


Debugging

 Whenever an error occurs Smalltalk displays a
window containing a “walkback” (=backtrace)
which shows the messages that led to the error
 It is possible to resume or to start an interactive
debugging tool to trace messages through the
objects leading up to the error
 Objects can also initiate error recovery whenever
a message they send is not understood by the
receiver


Smalltalk Summary

 Designed for interactive and extensible use,
therefore:
 environment is available for modification and extension
 language is semi-compiled then interpreted
 has dynamic binding and
 dynamic typing


Smalltalk Summary

 everything is an object
 conceptually consistent
 run-time error recovery
 well-integrated debugging facilities
 “clean” design to support quick learning
 automatic garbage collection for ease of use


C++

 C++ is not tightly integrated with its development
environment and aspects of the environment are
not usually available to the programmer
 Unlike Smalltalk it is compiled, and so is much
less interactive, all changes and evaluations being
made in the traditional edit-compile-execute cycle


C++

 C++ was designed as a production software
language, so much of it is designed to be fast at
the expense of simplicity


C++ Programming

 C++ is a hybrid of two languages, C and an object-
oriented extension - so it can also accept existing
C programs
 C++ has been used to “object-orientize” existing C
programs, a major factor in its popularity
 But this means that it is harder to use than
Smalltalk because the language is much more
complex


C++ Error Handling

 C++ can handle run-time errors by writing
exception handlers which trap and deal with the
error

 They fulfill a role similar to that of the
“doesNotUnderstand” message in Smalltalk,
although in C++ an object can never fail to
understand a message because C++ is statically
typed


C++ Environment

 A typical C++ environment contains program
editing, debugging (tracing and breakpointing)
and library facilities
 Debugging tools depend on what the vendor can
supply - there is no standard - but many good
debugging and single-stepping tools are available
(e.g. Borland C++)


C++ Environment

 Libraries are also non-standard, although many
libraries conform closely to a de-facto standard


C++ Summary

 C++ was designed for production programming
and to be compatible with C:
 compiled for speed of execution
 not very interactive
 strong static typing to catch programming errors before
execution


C++ Summary continued

 choice of static / dynamic binding for speed => confusion
 debugging less well integrated, though often good
 special run-time error recovery mechanism = exceptions
 “dirty” design to support existing C code + OO
extensions
 constructor and destructor functions to speed up object
creation and deletion


Comparison

 C++:
 edit-compile-execute
 compiled (interpreters exist)
 language separate from environment
 many variations of environment from different vendors
 widely ported to different platforms


...Comparison

 Smalltalk:
 interactive
 interpreted (compilers exist)
 language integrated with environment
 only one vendor
 limited number of hardware platforms


...Comparison

 C++:
 statically typed
 static or dynamic binding (user chooses)
 environment is not part of programs
 standardised syntax and semantics
 constructor / destructor functions - no garbage collection


...Comparison

 Smalltalk:
 dynamically typed (“doesNotUnderstand” message)
 dynamic binding always
 environment can confuse - part of all programs
 standardisation not really a major problem yet (ParcPlace
and Digitalk have produced different versions)
 automatic garbage collection


Strengths

 C++: Systems Engineering
 portable
 quick
 large libraries
 C compatible


...Strengths

 Smalltalk - Rapid Prototyping & Development
 interactive
 interpreted
 easy to understand
 easy to learn
 highly integrated


Eiffel

Eiffel was designed by Bertrand Meyer as a systems
engineering language
 It has a clean, efficient design with many features
to ensure the correctness of code:
 pre-conditions
 post-conditions
 assertions
 invariants


...Eiffel

 Eiffel has automatic garbage collection which can
be programmer-controlled
 It was designed to be compatible with C library
code, but this feature is never emphasized
 Eiffel is a proprietary language and has never had
a good programming environment
 Like C++ there is provision for exception handlers


...Eiffel

 Eiffel has failed to make a major impact because:
 compatibility with C syntax is low
 it’s proprietary
 there were to few good environments


Eiffel Summary

 Eiffel was designed as a clean systems
engineering language, so:

 automatic garbage collection
 static typing
 dynamic binding (but compiler can optimize to static)


...Eiffel Summary

 checkable assertions
 documentation tools
 “new” design with no old compatibility features
 poor backward compatibility with C


Java

 Java is the most trendy computer programming
language ever
 Also the only programming language ever to make
the front cover of Time magazine
 Likely to become the major WWW programming
language
 Could achieve a critical mass for OOP in the as yet
non converted industry


Background

 Java comes form the Oak project at Sun
Microsystems to develop embedded applications
and set top boxes for telecommunications
services
 Java is a clean OO language designed for wide
scale distribution
 Java compiles to ByteCodes which then
interpreted by a Virtual Machine on a host
computer (like Smalltalk 20 years ago)


Language features

 Syntax is based on C and C++
“C++ without the guns, knives and clubs”: James Gosling
 However it has single inheritance, dynamic linking
and is fully OO, more like Smalltalk than C++
 Strongly typed like C++ and Eiffel
 No (visible) pointers unlike C++ and like Smalltalk


Immediate Future

 Long range predictions are futile
 Java will have significant impact
 It is still fairly new
 Its popularity could cause problems if
expectations can’t be met
 Object thinking is more important than language
 Look out for C# and .NET


Web

 universally employed
 first use of Web was relatively static, lacking
component-based infrastructure
 could be standards-based infrastructure
 diverse object application models that conform to
a core set of standards
 ActiveX components, Java and CORBA objects,
Agents


Web object-based developments

 IIOP (Internet Inter-ORB Protocol) - a CORBA
standard that defines network interfaces for OO
applications over the Internet
 will serve as basis for Netscape ONE (Open
Network Environment)
 will provide developers with an open standards-
based framework for building interoperable
Internet applications


Interoperability among diverse platforms

 Microsoft is member of OMG in name only so far
 never supported their standards
 Microsoft now shifting more towards Internet
 DCOM (distributed common object model) of
Microsoft
 .NET coming


Benefits through Web

 support modularity, reusability and integration
 together objects plus infrastructure =
improvements
 people are becoming object developers
 object marketplace has started up quickly
 financial services, telecommunications industries
need: rapid deployment, reliability, modularity and
flexibility of OT


Summary

In this chapter we have:
 Introduced some Object Oriented Programming
Languages
 Compared some aspects of these languages


9. oo languages

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