Unit 5 - Structural modelling Flashcards
What are the agile modelling principles?
- Lightness and simplicity take precedence over completeness and complexity.
- Models should only be pursued while they are of use.
- Ease of change and adaptability also need to be part of the modelling process.
p. 7
The four useful sources when looking for objects are…?
- tangible objects – the physical things in the domain, such as rooms, bills, books and vehicles
- roles – the roles played by people in the domain, such as employees, guests and members of some organisation
- business transactions – the activities, episodes and interactions, such as room reservations, vehicle registrations, orders, deliveries and transactions
- organisational units – the groups to which people belong, such as accounts departments, production teams and maintenance crews.
Why do object modellers concentrate on nouns?
The nouns represent the things in the domain being modelled, and things are more stable than actions, which are expressed as verbs.
SAQ 1
When modelling objects, what are the basic filtering criteria that can be applied to reduce the number of objects?
- different concepts that are expressed using the same name
- redundancy, the same concept, given different names
- not important or independent enough, such as an attribute of another concept rather than a concept in its own right
- lack of relevance to the problem domain, being either beyond the scope of the desired system
SAQ 1
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.
SAQ 2
In connection with rooms in a hotel, a hotel manager’s vocabulary might include the words ‘occupied’ and ‘free’. How might such words be represented in a class diagram?
- as an attribute of the Room class
- as an association between the Room and Guest class.
SAQ 3
What characterises the state of an object at a particular point in time?
The state of an object is characterised by the value of each of its attributes at that point in time.
SAQ 4
What does an attribute of a class represent?
An attribute represents a particular property (a named value) of the class that each instance of that class will have. At any one time they collectively define the state of an instance of the class.
SAQ 4
Does a multiplicity of 1 indicate that there can be no change in the object to which the multiplicity relates?
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.
SAQ 5
If an airline system models flights and pilots, and each flight needs two pilots, would you use a multiplicity of 2?
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.
SAQ 5
Suppose that a shoe shop has a number of cupboards and each cupboard contains an even number of shoes. How could you model the evenness of the shoes?
You might use a multiplicity on the association between the classes Cupboard and Shoe, indicating that valid values are 0, 2, 4, 6, 8 and so on up to some reasonable limit. Alternatively you could say that a Cupboard 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.
SAQ 5
If a model contains role names, do you also need to use association names?
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).
SAQ 5
What is a navigation expression used for?
It provides a way of naming another object or its attributes relative to a starting object by referring to intermediate role names.
SAQ 5
What is a recursive association?
A recursive association is an association where both ends terminate at the same class.
SAQ 5
When considering attributes, what is the effect of moving from a conceptual model to an analysis model?
The conceptual model records attributes of classes that will be familiar to a domain expert. In the analysis model, the developer must consider the representation of attributes within a software system.
Example
A hotel manager will be familiar with the daily rate for a room and whether or not it is occupied. For a developer, daily rates for rooms involve money and you can use a true/false (Boolean) expression to represent the occupancy of rooms.
SAQ 6
The model below expresses the fact that a train must be associated with two employees: a driver and a guard. By considering the loop of associations identify a problem with the model, and suggest a constraint that solves the problem.
You need to capture the fact that a given employee cannot be both the driver and the guard for a particular train. You can formulate this as a constraint on any class round the loop.
For example, the following constraint can be located in the class Train:
context Train inv:
self.driver <> self.guard
SAQ 14
The model shows that a person takes out a mortgage to buy a house. A person must offer that house as security against the mortgage. A person can take out more mortgages, each secured by a house. By considering the loop of associations, identify a problem with the model and suggest a suitable constraint using OCL.
The model would allow one person to have a mortgage that uses someone else’s home as security. You need to capture the fact that a house’s owner is the same person who offered that house as security for a given mortgage. When there are multiplicities in the loop that can be greater than one, you need to take care in writing the invariant. In one direction around the loop an individual house has only one owner. In the opposite direction each house is security for only one mortgage, and that mortgage is associated with only one person. This makes it easy to place an appropriate constraint on either the House class or the Mortgage class. For the House class:
- *context** House inv:
self. owner = self.mortgage.person
Similarly for the Mortgage class:
context Mortgage inv:
self.person = self.security.owner
However expressing an equivalent constraint on the class Person is more problematic because you would have to find the intersection of two sets: the houses owned by a particular person and the houses secured by mortgages taken out by the same person. We suggest that you use English to formulate a suitable constraint in such circumstances. You could place the constraint as text inside a note and attach it to the appropriate class.
