9.oo languages

Object-Oriented Software Systems Engineering – Chapter 9

OO Languages

Chapter 9


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++SmalltalkEiffelJava


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 BrowserClass BrowserWorkspaceSystem TranscriptDisk BrowserWalkback; Debugger


Environment Classes

System Transcript:reports events in the system as they occur

Class Hierarchy Browserallows users to browse, edit, and add new classes to the

running system

Workspaceallows 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 extensionlanguage is semi-compiled then interpretedhas dynamic binding anddynamic 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 executionnot very interactivestrong static typing to catch programming errors before

execution


C++ Summary continued

choice of static / dynamic binding for speed => confusiondebugging less well integrated, though often goodspecial run-time error recovery mechanism = exceptions“dirty” design to support existing C code + OO

extensionsconstructor and destructor functions to speed up object

creation and deletion


Comparison

C++:edit-compile-executecompiled (interpreters exist)language separate from environmentmany variations of environment from different vendorswidely ported to different platforms


...Comparison

Smalltalk:interactiveinterpreted (compilers exist)language integrated with environmentonly one vendorlimited number of hardware platforms


...Comparison

C++:statically typedstatic or dynamic binding (user chooses)environment is not part of programsstandardised syntax and semanticsconstructor / destructor functions - no garbage collection


...Comparison

Smalltalk:dynamically typed (“doesNotUnderstand” message)dynamic binding alwaysenvironment can confuse - part of all programsstandardisation not really a major problem yet (ParcPlace

and Digitalk have produced different versions)automatic garbage collection


Strengths

C++: Systems Engineeringportablequicklarge librariesC compatible


...Strengths

Smalltalk - Rapid Prototyping & Developmentinteractiveinterpretedeasy to understandeasy to learnhighly 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-conditionspost-conditionsassertionsinvariants


...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 lowit’s proprietarythere were to few good environments


Eiffel Summary

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

automatic garbage collectionstatic typingdynamic binding (but compiler can optimize to static)


...Eiffel Summary

checkable assertionsdocumentation tools“new” design with no old compatibility featurespoor 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

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