Unit 5 - Classes and Associations Flashcards
(a) Why do object modellers concentrate on nouns?
(b) What are the main criteria for filtering a list of nouns in order to remove inappropriate ones and settle upon a more suitable set of candidate classes?
(a) The nouns represent the things in the domain being modelled, and things are more stable than actions, which are expressed as verbs.
(b) There are three basic criteria that can be applied as follows:
- redundancy;
- not important or independent enough, such as an attribute of another class rather than a class in its own right;
- lack of relevance to the problem domain; either beyond the scope of the desired system, or part of the language used for modelling.
Explain why object diagrams cannot form the basis for a software specification.
Object diagrams represent particular states of the system at particular moments in time, whereas a specification must describe all valid states of the system, at all possible times.
(a) In a windowing system, a window may be converted to an icon, and back to a full window. What operations can be performed on full windows but not on iconised windows? Would a model containing the classes Icon and FullWindow be able to capture the distinction adequately?
(b) In connection with rooms, the hotel manager’s vocabulary includes the words ‘occupied’ and ‘free’. How might such words be represented in a class diagram?
(c) In your model in part (b), will your decision about occupancy change if you have to include the fact that a room must be cleaned before the next guest occupies it?
(a) Scrolling and maximising can be done on full windows but not on iconised ones. It will be difficult to model the distinction between a full window and its iconised version adequately by using two different classes, since an object (in this case, the window) cannot dynamically change its class. A solution to this problem might be to have a single class in which an attribute makes the distinction.
(b) Two ways come immediately to mind:
- as an attribute of the Room class;
- as an association between the Room and Guest classes. Either is quite acceptable as a way of recording the information.
(c) No. The cleaning of a room certainly depends upon whether or not it is occupied, but not on how we choose to model occupancy. (You would include this requirement relating to cleaning in a dynamic model, such as a sequence diagram or state diagram. This will be discussed in a later unit.)
(a) Does invoking an operation on an instance of a class always change the object’s state?
(b) What does an attribute of a class represent?
(a) No. Not all operations are intended to change an object’s state. For example, you might provide an operation on the Guest class to respond with the address for any particular instance (object) of that class.
(b) An attribute represents a particular property (a named value) of the class that each instance of that class will have. Whatever else the attributes of a class are used for, at any one time they collectively define the state of an instance of the class.
(a) Does a multiplicity of 1 indicate that there can be no change in the object to which the multiplicity relates?
(b) If an airline system models flights and pilots, and each flight needs two pilots, would you use a multiplicity of 2?
(c) Suppose that each person has a number of wardrobes, and each wardrobe contains an even number of shoes. How would you model the evenness of the shoes?
(d) If a model contains role names, do you also need to use association names?
(e) What is a navigation expression used for?
(f) What is a recursive association?
(a) No. It merely means that at any one time there will be exactly one object at that end of the relationship. The attributes, or even the identity, of this object may change over time.
(b) Probably not. There are probably times during the life of a Flight object when fewer than two pilots are allocated, such as when the flight has been scheduled but crew details have not yet been settled.
(c) You might use a multiplicity on the association between the classes Wardrobe and Shoe, indicating that valid values were 0, 2, 4, 6, 8 and so on up to some reasonable limit. Alternatively, you could say that a Wardrobe contains an arbitrary number of instances of a class called ShoePair, where each ShoePair contains one left shoe and one right shoe. This approach generalises more easily to situations where the groups are not homogeneous. For example, a table setting contains one knife, one fork and one spoon.
(d) No, but it is sometimes convenient to have a name for the association as a whole. For example, you might focus on what is meant by works for, rather than the need to consider both the role employer and the role employee (at the same time).
(e) It provides a way of naming another object or its attributes relative to a starting object, by referring to intermediate role names.
(f) A recursive association is an association where both ends terminate at the same class.
Imagine you are drawing up the specification for a software system for a petrol station to administer the dispensing of petrol and its associated billing. Consider all the interactions that are involved between driving into the petrol station and departing. What makes the petrol flow? How does the cashier know how much to charge? How does the manager know when to refill the storage tanks?
Make a list of all the nouns that you use in describing how various people make use of the system. (You are not required to make judgements about relevance or implementation at this stage.) After you have done that, filter your list of candidate classes. You need to be clear about the reasons for rejecting them.
Here is our list, but yours may be different. arrival bill car car’s petrol tank cashier change cheque credit card customer delivered volume departure fuel cost fuel delivery fuel quantity sold fuel type holster manager nozzle petrol tank cap price display pump sale signature storage tank for fuel trigger unit fuel cost volume display
You can group the rejected nouns from our candidate class list using the following
reasons:
- out of scope – customer, cashier, petrol tank cap, cheque and signature;
- probably out of scope – change and credit card;
- an operation – sale, arrival, departure and fuel delivery;
- attributes of something else – car’s petrol tank, unit fuel cost and fuel type.
In Figure 1, suppose that Jack checks out of the hotel. What changes would you make to the diagram?
The link between jack and room r123 should be deleted. Whether or not the link between jack and the Ritz should be removed needs to be checked with the domain expert, because the hotel may wish to retain some link with its guests after they have checked out.
Give examples of different possible interpretations of classes with the names Room and Guest. When would new instances be created, and when would they be destroyed?
A Room class might represent physical rooms, which might be created and destroyed as building operations change the number of physical rooms. The instances might represent lettable rooms, whose existence might be related to whether the room was unlettable because of cleaning or repair works. A Guest class could represent a person currently staying in the hotel, which would be created at check in and destroyed at check out. It might represent a person who has stayed at least once, so that they would be created on the first check in and not destroyed on check out, but kept on the books. It might represent a potential guest, such as someone who has reserved but perhaps did not stay. Instances would be created on first contact with the hotel, and perhaps would not be destroyed, or not until they had generated no business for some years.
Build a model to show the relationships between a person and their natural parents. Does your model prohibit someone from being his or her own mother? Is your model true for the whole of humanity or just for the people represented in some software system?
Figure 17 shows one possible model. There is nothing to say that the people at the end of the relationships are different. Since the non-identity of parent and child is part of the meaning of being a parent, you will need to capture that constraint in some other way. We have shown that every person has a father and a mother. That is true if we include dead people as instances of Person, but is not true if Person represents a living person, or a person in some finite set such as those represented in a computer. This is another example of how vital a project glossary is to relate terms in the class model, such as Person, to one particular meaning in the world. Simply naming a class Person is never enough.
When considering attributes, what is the effect of moving from a conceptual model to a specification model?
The conceptual model records attributes of classes that will be familiar to a domain expert. For example, a hotel manager will be familiar with the daily rate for a room and whether or not it is occupied. In the specification model, the developer must consider the representation of attributes within a software system. For instance, daily rates for rooms involve money, and you can use a true/false (Boolean) expression to represent the occupancy of rooms.
Why is a class model not sufficient to describe a system?
A class model is a static model that describes the elements of a system (the classes) and their relationships (the associations), but does not describe the behaviour of the system over time. For this you will need one or more dynamic models. In particular, you need to model the life histories of objects and the interactions between them. The model needs to capture when instances of classes should be created and destroyed.
(a) What is meant by navigability? When is this idea useful?
(b) In a multi-user operating system, users are allocated passwords. Draw a fragment of a class model to represent this association. Bear in mind that you do not want to be able to identify the corresponding user for a given password. What does this tell you about the representation of the association?
(a) Navigability means that it is possible to identify (or ‘reach’) objects in one class from objects in an associated class. The usefulness of this idea is realised during implementation when navigability in one direction alone (unidirectional navigability) can lead to simpler code.
(b) Users will want to ‘know about’ their passwords, not the other way round. Figure 23 shows that each instance of the class User will have a collection of references to the appropriate Password objects.
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(a) What is a qualified association?
(b) Suppose that, in the invoices example shown in Figure 18, invoices have unique numerical identifiers known as invoice numbers. How would you capture this information in a class diagram?
(a) A qualified association is an association at one end of which there is a qualifier, consisting of one or more attributes. The values of the attributes (taken together) uniquely identify the objects in the class at the other end of the association.
(b) You could use a qualified association whose attribute is named invoiceNo in a manner similar to that shown in Figure 25, by attaching the qualifier to the Customer end of the association between Customer and Invoice.
Under what circumstances would you want to show an association that is not independent of others in the diagram?
If a word describing an association is part of the natural vocabulary of the domain expert, it will be sensible to include it in the model, as otherwise a linguistic gulf will start to open between the domain expert and the system designer. However, if you know that an association is not essential, because it can be derived from other associations, you will also need to record that fact.
Build a class model associating companies, libraries and families with people, identifying people by suitable qualified associations. When considering a family, be sure that your model handles twins (multiple children with the same date of birth) and sharing of names between generations (for example, mother and daughter with the same name).
Looking from a company’s and a library’s perspective, you can find a way to identify a single person who may be a worker or a member, respectively, as shown in Figure 28. In the case of a family, two or more members of that family may have the same birthday or the same name, but the combination of name and date of birth should be unique. Using two qualifiers (name and birthdate) should uniquely identify a person.
