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6.7 Modelling with objects

Object-oriented software development is very much focused on representing the world of the problem domain as a set of interacting objects. If the classes of the objects are chosen to correspond to natural categories of things in the world, such as customer invoice, payment, bill, there will be a structural similarity between the world and the software. This can lead to good traceability from requirements through to code.

Domain, analysis and design are the three modelling perspectives t
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6.3 Networks of objects

No serious program consists of a single object. Instead there will be a network of objects, which collaborate to achieve the functionality of the whole system. Figure 4 shows a network of objects representing a hotel, some guests and some rooms. This sort of diagram is called an object diagram or a snaps
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4 Emergent approaches to software development

Iterative and incremental methods have been widely adopted in software development. Nowadays, high competitiveness, reduced time-to-market and pressure to develop flexible enterprise software together with the rapid change of technology have led to the emergence of new approaches to building, deploying and maintaining software. At the time of writing (2005), several new approaches to software development have been established that may become significant during the lifetime of this course. The
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References

Michael Jackson, Software Requirements & Specifications, Addison-Wesley, 1995. ISBN 0–201–87712–0.
Suzanne Robertson and James Robertson, Mastering the Requirements Process, Addison-Wesley, second edition, 2006. ISBN 0–321–41949–9

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6.2 Actors

Iteration is a natural part of the modelling process. It does not matter whether you start by looking for the actors or the use cases. We have chosen to begin with the actors, since it is a way of expressing the system boundary implicitly and identifying the different views that need to be taken into account. In practice, you are likely to find that the actors are to be found in the roles that people play as employees in the problem domain, such as the hotel's receptionist or manager.

A
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4.1 What is a data flow diagram?

A data flow diagram (DFD) is a graphical description of the ebb and flow of data in a given context. A DFD allows you to identify the transformations that take place on data as it moves from input to output in the system. (DFDs pre-date UML diagrams, but still have a complementary role to play in describing systems.)

The Case Study below provides an example of a DFD used to describe the Open University's eTMA system (electronic Tutor Marked Assignment system). It uses the
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3.1 Making consistent models

It would be preferable to have a consistent way of representing the different models that one might want to construct. The notion of a modelling language allows the developer to make useful connections between different models. For the most part, models are represented diagrammatically. There are two aspects of a diagram-based modelling language that you should be aware of:

  • a set of rules that defines what symbols can be used on a particular type of di
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3.1 Distributed objects technology

This technology virtually hides the network from the designer and programmer. A distributed object is an object which is resident on one computer and for which methods can be invoked associated with code resident on other computers. A good distributed objects technology should totally hide the underlying communication details from the programmer, for example when a programmer wants to invoke the method Author(s): The Open University

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2.4 The message passing idea

Figure 1 shows the central idea behind the message passing paradigm. It involves an architecture in which clients and servers communicate using communication lines. In this model, in contrast with the others that are to be presented in this unit, the underlying structure of the network is visible via the communication media used to connect servers and clients and devices such as sockets, ports and server sockets which are involved in the transfer of a message from one computer to another.


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2.3 Synchronous and asynchronous message passing

Synchronous message passing involves one entity (usually a client) in the message passing process sending a message and a second entity (usually a server) receiving it, carrying out some processing and then sending back some response which the first entity processes in some way. While the second entity is carrying out the processing the first entity pauses waiting for the response.

In asynchronous message passing each entity in the process does not have to wait for the next part
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2.2 Fixed and adaptive protocols

The protocol described above for a simple naming service is an example of a fixed protocol. This is a protocol whose vocabulary is fixed: it is embedded in the client and server's code and data and does not change. An adaptive protocol is one where the protocol changes. A fixed protocol could change over a period of time because the functionality provided by a server changes. However, this change will be over months or years rather than over seconds.

There are some instances wher
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2.1 Protocols

Message passing is the simplest form of development paradigm. For example, the way that a client running a browser communicates with a web server is via message passing.

Message passing is based on the idea of a protocol: a language which embodies the functions required by one entity in a distributed system (usually a client) which another entity provides (usually a server). As an example of a protocol consider Table 1. It shows the protocol associated with a naming servi
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7.2.4 E-mall

An electronic mall or e-mall is a collection of e-shops which are often devoted to a specific service or product, for example an e-mall might be devoted to selling goods associated with a leisure activity such as fishing. Usually e-malls are organised by a company which charges the e-shops for administering their presence: maintaining the website, hosting the e-mall, and providing payment and transaction facilities and marketing.

The e-mall operator gains revenue for charging the e-shop
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7.2.2 E-auction

This model describes sites which electronically simulate the bidding process in a conventional physical auction. Such sites can range in sophistication from those which present a simple catalogue of items to those which offer multimedia presentations. Most sites which are described by this business model are concerned with selling items to individual consumers. However, there are an increasing number of sites which provide facilities for businesses to auction products to other businesses.


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7.2.1 E-shop

This is the most ubiquitous form of commerce on the World Wide Web. It involves a company presenting a catalogue of its wares to internet users and providing facilities whereby such customers can purchase these products. Almost invariably such a site will contain facilities for ordering and paying for products by means of credit cards. The sophistication of sites described by this business model range from just the simple presentation of a static catalogue to the presentation of an interactiv
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2.2 Slavery reform

Some of the first international concerns over human rights, as they would now be recognised, were expressed about slavery at the end of the eighteenth century. Somerset's case in 1772 challenged the acceptance of slavery in the UK. This case is regarded as a turning point, as statutory abolition followed in the UK. Out of this changing social, political and legal attitude towards slavery grew a movement which sought to prohibit slavery internationally. It was not possible to secure the freedo
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4.1.3 Global positioning system (GPS)

These days, it is possible to buy a device known as a global positioning system (GPS) to tell you where you are. Receivers are made for aircraft, ships, ground vehicles, and (as the one shown in Figure 6) for carrying in the hand.

Figure 6

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2.3 Summary

This section showed that computers pervade our daily lives, but that many of them are invisible to us.

It investigated the information requirements of certain individuals, such as shoppers and doctors. You learned that their requirements can range from the simple and obvious to the complex and not so obvious.

You also learned that it is not just individuals who require information: it is also essential to the operation of organisations. The example of loyalty cards was used
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The ‘why’ and ‘what’ of educational leadership and management
This free course, The 'why' and 'what' of educational leadership and management, introduces you to researching educational leadership and management and how undertaking research can contribute to both good practice and the building of leadership capacity. First published on Wed, 17 Feb 2016 as Author(s): Creator not set

D Solving simple equations with one unknown

Now we wish to move on to equations where you can find a solution. The work that you have done so far on simplifying and rearranging equations will be used here.

Example 3

Consider the equation 4x = 24. We call x the unknown and we want to find the value of x which makes
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