Implementation

In principle, the phase of implementation follows on from the design phase. In practice, however, the products of design may often only be regarded as providing a post hoc justification of the actual system. As noted, for instance, in  [HOB87], an object-oriented approach may blur the distinction between design and implementation, even to the extent of reversing their actual order. The most important distinction between design and implementation is hence the level of abstraction at which the structure of the system is described. Design is meant to clarify the conceptual structure of a system, whereas the implementation must include all the details needed for the system to run. Whatever approach is followed, in the end the design must serve both as a justification and clarification of the actual implementation.

Design is of particular importance in projects that require long-term maintenance. Correcting errors or adapting the functionality of the system on the basis of code alone is not likely to succeed. What may help, though, are tools that extract explanatory information from the code.

Testing and maintenance

Errors may (and will) occur during the implementation as well as later when the system is in operation. Apart from the correction of errors, other maintenance activities may be required, as we have seen previously. In  [Knuth92], an amusing account is given of the errors Knuth detected in the TeX program over a period of time. These errors range from trivial typos to errors on an algorithmic level. See slide 1-errors.

Errors, bugs

TeX


slide: TeX errors and bugs
 An interesting and important question is to what extent an object-oriented approach, and more specifically an object-oriented implementation language, is of help in avoiding and correcting such errors. The reader is encouraged to make a first guess, and to verify that guess later. As an interesting aside, the TeX system has been implemented in a language system called Web. The Web system allows one to merge code and explanatory text in a single document, and to process that document as either code or text. In itself, this has nothing to do with object orientation, but the technique of documentation supported by the Web system is also suitable for object-oriented programs. We may note that the javadoc tool realizes some of the goals set for the Web system, for Java.

Object-oriented language support

 Operationally, encapsulation and inheritance are considered to be the basic mechanisms underlying the object-oriented approach. These mechanisms have been realized in a number of languages. (See slide 1-languages. See also chapter 5 for a more complete overview.) Historically, Smalltalk is often considered to be the most important object-oriented language. It has served as an implementation vehicle for a variety of applications (see, for instance, Pope, 1991). No doubt, Smalltalk has contributed greatly to the initial popularity of the object-oriented approach, yet its role is being taken over by C++ and Java, which jointly have the largest community of users. Smalltalk is a purely object-oriented language, which means that every entity, including integers, expressions and classes, is regarded as an object. The popularity of the Smalltalk language may be attributed partly to the Smalltalk environment, which allows the user to inspect the properties of all the objects in the system and which, moreover, contains a large collection of reusable classes. Together with the environment, Smalltalk provides excellent support for fast prototyping.

The language Eiffel, described by  [Meyer88], may also be considered as a pure object-oriented language, pure in the sense that it provides classes and inheritance as the main device with which to structure a program. The major contribution of Eiffel is its support for correctness constructs. These include the possibility to specify pre- and post-conditions for methods, as well as to specify a class invariant, that may be checked before and after each method invocation. The Eiffel system comes with a number of libraries, including libraries for graphics and window support, and a collection of tools for browsing and the extraction of documentation.

The C++ language (Stroustrup, 1991) has a somewhat different history. It was originally developed as an extension of C with classes. A primary design goal of C++ has been to develop a powerful but efficient language. In contrast to Smalltalk and Eiffel, C++ is not a pure object-oriented language; it is a hybrid language in the sense that it allows us to use functions in C-style as well as object-oriented constructs involving classes and inheritance.

Smalltalk -- a radical change in programming

languages


Eiffel -- a language with assertions

C++ -- is much more than a better C

Java -- the dial-tone of the Internet

DLP -- introduces logic into object orientation

slide: Object-oriented languages

The newest, and perhaps most important, object-oriented language around is Java, which owes its popularity partly to its tight connection with the Internet. Java comes with a virtual machine that allows for running Java programs (applets) in a browser, in a so-called sandbox, which protects the user from possibly malicious programs.

As the final language in this brief overview, I wish to mention the distributed logic programming language DLP (see Eliëns, 1992). The DLP language combines logic programming with object-oriented features and parallelism. I mention it, partly because the development of this language was my first involvement with OOP. And further, because it demonstrates that other paradigms of programming, in particular logic programming, may be fruitfully combined with OOP. The language DLP provides a high level vehicle for modeling knowledge-based systems in an object-oriented way.

A more extensive introduction to the Smalltalk, Eiffel, C++, Java and DLP languages is given in the appendix.