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1、Suggested answer for Exercise 8.1(a) The instanceOf stereotypes are only shown here for illustration. The fact that the objects are instances of the Position class is already shown in the label of each object. (b) We assume that the counter maintains a single integer value, which is initialized to z
2、ero. Set and reset have the same effect, and so are modelled as a single operation, setToZero. (c) An enumeration is used below to specify the possible states of the switch. In this particular case, a Boolean state variable could be used, but defining an enumeration gives greater scope for extensibi
3、lity. Alternatively, the class interface could be specified without specifying how the state is recorded. (d) An attribute with many multiplicity is used below to record the set of illuminated lights, using the Colour enumeration illustrated in Figure 8.3. The OCL notation for collection literals ha
4、s been used to show the values of this attribute in the object icon. Suggested answer for Exercise 8.2(a) A person can have zero or more books out on loan, and each book can be borrowed by at most one person at any given time. (b) A person can have read many books, and each book may have been read b
5、y many people. (c) A piece can only occupy one square at a time, and if it has been captured, it occupies no square. Each square can have at most one piece on it. If the aim is to model only pieces on the board, the multiplicity at the right of this association should be exactly 1. (d) The multiplic
6、ities below refer to who is married to whom at a given time, assuming no polygamy or polyandry. As the spouse relation is symmetric, it can simply be labelled with an association name. (e) A person can have zero or more children, but each person has exactly two (biological) parents. This relation is
7、 not symmetric, so adding role names makes the intended interpretation clearer. Suggested answer for Exercise 8.7The job titles and salaries can most naturally be modelled by attributes in the Contract class, as shown in the diagram below. Suggested answer for Exercise 8.9If the abstract draw operat
8、ion was omitted from the Shape class, it would be impossible to invoke the draw operation on a shape without knowing what subclass it belonged to. In other words, code such as the following would not work: Shape s = new Rectangle() ;s.draw() ;It would be necessary to cast the object to its specific
9、subclass first: Shape s = new Rectangle() ;(Rectangle) s).draw() ;This would frustrate a major benefit of using generalization, namely the ability to abstract from the specific runtime class of objects and manipulate them through a common interface defined in a superclass. It is not really a failure
10、 of substitutability, as instances of subclasses such as Rectangle are still substitutable for instances of Shape. However, once this is done, they can only be manipulated through the shared interface defined in the Shape class. The principle required is that abstract superclasses must define all th
11、e operations in the interface shared by all their subclasses. Suggested answer for Exercise 8.10The association as written does not have the properties of antisymmetry and transitivity specified. The relationship being modelled, that between parents and children, is antisymmetric, as nobody can be t
12、heir own parent. The relationship is not transitive, however: the fact that the relationship of parent holds between your grandmother and your mother, and also between your mother and you does not mean that it holds between your grandmother and you. As the relationship is not transitive, it would be
13、 misleading to make the association in Figure 8.20 into an aggregation. Suggested answer for Exercise 8.11The class diagram is shown below. Note that only the classes linked by compositions are nested. Simple aggregations do not have the lifetime dependency of composites, and must still be shown as
14、associations. Object diagrams based on this reflect the same structure, as shown below. Suggested answer for Exercise 8.12The object diagram below shows a port on one computer connected to the processor of another. Suggested answer for Exercise 8.13The diagram in Figure 8.44 does not place any restr
15、ictions on the number of student or modules a given Attempt instance is related to. It would be consistent with the diagram to have an Attempt object that was not linked to any student, for example, and also to have one that was linked to more than one student. In the latter case, however, it would
16、have also to be linked to more than one module, as any given attempt and module objects can be linked to at most one student. There is no easy way of imposing the required multiplicity restrictions on this relationship when it is expressed as an n-ary association. Phenomena like this explain why in practical modelling, the overwhelming majority of associations used are bina
