Course Aims - Creating Beautiful Systems
This course aims to provide you with
A simple, clear, analysis and design notation.
A good basic understanding of the concepts of object oriented systems.
A method for construction of analyses and designs.
Some discussion of the implementation of designs.
Why Design?
Even the most professional programmers feel the temptation to sit down and produce code at the earliest possible moment. Therein lie many of the ills of the software engineering industry. Design is a process which involves
communication
creativity
negotiation
agreement
It is a human process, to produce products for human consumption. Too often the communication, negotiation and agreement aspects are left out.
Object modelling provides a notation which is clear, consistent, and which can be used to communicate within a software development team, and with the clients and other third-parties which the team need to deal with.
Design methods
Traditionally systems were constructed using the waterfall method. This was based on the idea that clients would formally agree a requirements document. A design would then be put together, which would be further agreed. Then the system would be implemented, and then there would be an endless process of maintenance.
Modern ideas move away from this. Iterative methods are considered more appropriate for many system development approaches. This still follows the notion of analysis, design and implementation, but on a cyclical basis, where subsequent cycles build on earlier cycles. There are variants on this, and we will discuss later how to drive and control this process.
Objects
We begin at the beginning. The world is made of objects. Just open your eyes and ears. They are out there. Bank customers, students, cats, elephants, cars, balls of string, atoms, molecules, tubs of ice cream, Madonna, stars, bureaucrats, Robin Hood. The world is built of objects. Objects are built of smaller objects, and so ad infinitum. Objects combine to make bigger objects. We already live in an object-oriented world.
The first thing an object analyst must do is to remove the scales from his or her eyes. Object modelling consists of looking for objects. Of course, there has to be some boundary. Even sitting at my desk I can see more objects than I could reasonably list. But that is where the beauty of object modelling comes in. It uses observation.
Objects can be described by their attributes and operations. Attributes are the changeable characteristics of an object. Cats have colour, size, weight and a preference for either Kit-E-Kat or Whiskers. Operations are the things an object does or can have done to it. Cats can catch mice, eat, miaow, worm up to owners, and be stroked. In our notation we draw an object such as a cat like this.
The name is shown at the top. The attributes are listed underneath. The operations are listed below that. Actually, strictly speaking, this is a class diagram. But we will explain that later.
In an object model, all data is stored as attributes of some object. The attributes of an object are manipulated by the operations. The only way of getting at the attributes is through an operation. Attributes may sometimes be objects in their own right (more of that later).
In an object model, all functionality is defined by operations. Objects may use each others operations, but the only legal way one object can manipulate another object is through an operation. An operation may inform, say "mass of ball", or change the state of an object, say "throw ball".
Object modelling is about finding objects, their attributes and their operations, and tying them together in an object model. Nothing more. Here are some more objects:
Some of these objects may seem jokey, but they could reasonably be part of a system, be it a computer game or a multimedia story. Do not be constrained to be those dull systems that most software engineers drag out. Object modelling can be used to design lots of things.
By now you should be getting the idea that object modelling is, at its simplest level, very straightforward. The trick comes in knowing what objects are appropriate, and what their appropriate attributes and operations are. It is a question of focus. We will consider some ways of controlling focus later in the course.