Modelling object-oriented software – an introduction
Modelling object-oriented software – an introduction

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Modelling object-oriented software – an introduction

8 Conceptual models

8.1 Conceptual model for the Hospital System

We are now in a position to produce a conceptual model for the Hospital System.

In this course a conceptual model comprises the following:

  1. a class diagram showing:

    • the classes that have been identified as appropriate;

    • any generalisation relationships between the classes;

    • any abstract classes;

    • the associations between the classes, together with their multiplicities;

    • any derived associations;

  2. associated text comprising:

    • for each class, a class description listing the attributes, together with the identification of any derived attributes, and brief comments indicating:

      – the purpose of the class and of each attribute;

      – if the class is abstract;

      – any generalisation/specialisation relationships;

      – any restrictions on attribute values where relevant, including uniqueness constraints;

    • any invariants not evident from the class diagram or class descriptions.

In producing the conceptual model we have aimed to create an appropriate representation of the system domain. However, we have had to make a number of decisions, for example, about which attributes and associations to label as derived. If we had made different decisions we would have produced a slightly different model, but one which might have been equally good at this stage. There is no one correct model (except perhaps in extremely trivial cases), and there are many debates, even among experts, about whether one model is better than another. Our concern is only that you should understand the concepts well enough to be able to produce a reasonable conceptual model.

All the aspects of the conceptual model for the Hospital System have now been considered. However, before presenting this conceptual model, recall the earlier discussion about the role of the conceptual model in forming a starting point for constructing the system. The conceptual model represents the system domain -the part of the real world relevant to the system. Since a key object-oriented principle is that software objects can usefully mirror real-world entities, the structure set out in the conceptual model is a sensible starting point for the structure of the system. Thus the conceptual model, given below, can be taken as the initial structural model: an initial representation of the software. That is, we will initially view the Hospital System as having classes such as and , whose instances and the links between them:

  • represent real-world entities (patients and doctors etc.), and the connections between them;

  • are subject to invariants, just as their real-world equivalents are subject to real-world constraints.

Here is the complete model, which is also set out in Section 4 of the PDF document below, under the heading Hospital System initial structural model.

Click on the link below to view Section 4 of the pdf.

Section 4 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

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