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2 n Software Design
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Class and object
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characteristic, attribute, quality or property In terms of object-oriented programming, this description, from the Collins English Dictionary, is ideal In object-oriented programming, we aim to develop a program by composing it of objects working together to do some job In the majority of cases, several objects with similar capabilities would be used to divide up the labour and to represent different, but similar, items For example, a word processor uses a number of documents All of these can be displayed on the screen, searched for speci c words or phrases, printed and stored on a disk With this in mind, we can see that it would be very inef cient to develop objectoriented programs by creating each individual object as a separate programming task Objects that share capabilities belong to the same class, and the best approach is therefore to develop classes of objects, rather than the objects themselves Figure 22 shows in diagram form a class and several objects of that class A class is a template for any number of objects that share the same general characteristics and capabilities When we develop a class, we describe the types of information that an arbitrary object of the class might contain, and the actions or operations that it can perform This does not mean, however, that all objects that belong to a class are identical In the same way that two word processed documents can be operated on in the same way, even though their text and formatting are completely different, so two objects of a class can contain different information and yet perform the same types of operation We can make this distinction by saying that two objects of the same class have different states The state of an object is the pattern of data values, or properties, contained in it, so although every object of a class performs exactly the same operations, the operations will have different outcomes because they work with different data An example should clarify this Let s assume that in a given program, we have de ned a class of objects that can be displayed on the screen of a computer All of the objects of the class have the capabilities to display themselves They all also have a Colour property that indicates the colour that an object will display in In the program, we can create two of these objects, one in white and one in grey, as shown in Figure 23
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Two objects of the same class with a different Colour property
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21 Designing objects, classes and applications
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It should be easy to see that both objects belong to the same class, since they have the same capabilities and store the same type of information, but that both are also different, since the colours speci ed by their Colour properties are different
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212 Class Notation
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If we are to design classes of objects and indicate how they are to be inter-connected, we need some form of notation to save having to use text descriptions repetitively Software developers tend to work with diagrams, since they make very concise shorthand when describing the structure of systems Figure 24 shows a commonly used form, a UML class diagram, for depicting classes and objects in a system This can be re ned to depict not only the class or object, but also the properties and methods de ned for it:
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Class and object diagrams, showing properties and methods
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Figure 24 shows a picture of a class (in this case, a generic class class), and of an object of that class The class diagram shows the properties and methods of the class, while the object diagram shows what class the object belongs to and the speci c values taken on by each property There is no need to indicate the methods in the object, since methods are de ned for a whole class Note that in this form of diagram, we show classes in terms of their interfaces There is nothing in the diagram that indicates how information will be stored in an object of the class, or how a particular method actually works Figure 25 shows a speci c class (Appointment) and how its interface is de ned It also shows a speci c Appointment object with its property values in place as an example Note that there is no need to show the names of the class methods (AddToSchedule and SetAlarm) on the Appointment instance Since we know that the object is a member of the Appointment class, we are allowed to assume that these methods are available
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