Block 2 Unit 7

Question 1

Why must the conditions on a message send be mutually exclusive in a sequential system?

Answer 1

If conditions were not mutually exclusive, more than one condition might be true. This would mean that multiple messages would be sent at once, resulting in multiple receiving objects being active at the same time –
[
Non-deterministic behaviour is behaviour where one of a number of outcomes is possible, for example when a dice is thrown. It is something we wish to avoid introducing where it is not intended.
]
leading to the possibility of non-deterministic behaviour.

Question 2

Figure 2 shows two conditional message sends originating at the same moment on the timeline, meaning that all the guards are evaluated before any message is sent. Could the same behaviour be implemented if the messages (and their conditions) were separated on the time line?

Answer 2

Yes, provided that the conditions were mutually exclusive, so that the first message could not affect the state of the receiver before the second message arrived. We might use code such as the following:
if (room.ready) room.accept(jill);
if (!room.ready) room.requestCleaner();

Question 3

Suggest a reason why an object’s lifeline must branch in cases such as Figure 2. How is this handled in Figure 5?

Answer 3

The Room object will receive different messages depending on which branch is executed in the Hotel object. It makes no sense for the messages to arrive at just one lifeline, as that would imply that the Room object received all of the messages in a single interaction. Instead, we show a fork in the lifeline of the receiving object to reflect the fork of control in the sending object in Figure 2.
In Figure 5 the alternatives are shown using the alternative fragment. Although the figure doesn’t show a branching lifeline, this same branching is implied by the alternatives in the fragment, in their separate boxes.

Question 4

How would you change the sequence diagrams in Figures 3 and 6 to sound the car’s horn five times each time it moved forward?

Answer 4

To get the horn to sound five times for every forward movement in the sequence diagram in Figure 3 you would add an iteration clause to the second, nested message: *[j := 1..5] sound(). In Figure 6 you would add the extra iteration by including a fragment within the existing fragment.

Question 5

What is the main disadvantage of defensive programming?

Answer 5

The main disadvantage of defensive programming is that the same constraints within each precondition may be checked repeatedly in different operations.

Question 6

Which of the three strategies for implementing use cases is being used when we place methods such as createGuest(lastname : String, firstname : String, address : String) in the Hotel class?

Answer 6

We are following the strategy of using one central class here.

Question 7

Are use case objects consistent with the twin aims of high cohesion and low coupling (terms introduced in Block 1 Unit 1)?

Answer 7

Yes and no. Yes, because high cohesion asks us to ‘do one thing and do it well’. A CheckerIn class should only support the one activity of checking in. No, because having an extra class for each use case introduces more coupling through the extra associations involved. However the introduction of these new classes limits the impact of a change to a software system.

Question 8

Suppose you are designing the graphical user interface (GUI) part of a hotel system. You need to be able to check in people known to the system as well as new people. What operations on the business model might be needed to support a usable GUI?

Answer 8

Checking in unknown people would involve the creation of a new Guest object, requiring an operation createGuest(firstName, lastName, address).
To check in a known person, you need to find the business object that corresponds to that person. This mapping might be done by having a pick list of all people known to the system, which would require a business-model operation that returns a collection of guests, for example getAllGuests().

Question 9

Figure 15 shows a simple state machine diagram for the class Copy in a library model. What should happen if the copy returned message arrives when an object of the class Copy is in the state on shelf (assuming that we are not using design by contract)?

Answer 9

Something has gone wrong. The state of the system does not describe the fact that the copy of the book is on loan, so it cannot be returned. There must be some means of indicating or recording an error, such as a message to a log file.

Question 10

Give an example of an architectural decision that would provide a general solution to the problem of unexpected messages.

Answer 10

At an architectural level we might introduce a single object of a class Error, which is globally accessible to objects in the software system. Such an object would be responsible for reporting errors due to unexpected messages, for example.

Question 11

What does a final state signify in a state machine?

Answer 11

Final states are used to show the point or points where the object in question has finished processing. Its activity has been completed.

Question 12

In what ways does a final state differ from an initial state?

Answer 12

There can be zero, one or more final states but at most one initial state. A final state can have several incoming transitions and no outgoing transitions, but an initial state has no incoming transitions and only one outgoing transition.

Question 13

Why might two objects of the same class respond differently to the same message?

Answer 13

An object’s behaviour will in general be affected by the values of its attributes, which are part of the object’s state. If two objects have different values for the same attributes, they are in different states, and therefore might respond differently to the same message.

Question 14

What is the most common form of event that causes a transition between two states? How is it shown in a state machine diagram?

Answer 14

The receipt of a message is the most common form of event that causes a transition between states. An event is used to label the transition between the states. In a state machine diagram, a transition is labelled with the name of the relevant message, which includes any arguments for that message.

Question 15

What is the difference between an event and an action?

Answer 15

An event is something done to the object, such as sending it a message. An action is something that the object does, such as sending a message to itself or to another object. An action is an object’s reaction to an event.

Question 16

What is the main constraint on the kinds of action that may be shown in a state diagram?

Answer 16

Actions should only refer to things that the object ‘knows’ about. For example, they can refer to attributes, operations and links of the object, as well as to the parameters of the message that caused the transition. An action cannot refer to the state or attributes of another object unless there is some way for these other attributes to be known in a short time without requiring any state changes.

Question 17

What is an action sequence?

Answer 17

An action sequence is an ordered series of individual actions that are associated with a particular event. They are written as a list separated by semicolons, and are performed sequentially in left-to-right order. Like actions, action sequences are atomic.

Question 18

What is a guard and how does it protect a transition?

Answer 18

A guard is a Boolean condition that is applied to a transition – the guard must be either true or false. A guarded transition can only take place when the specified guard is true.

Question 19

Look at Figure 17, which only considers whether an object of class Hotel is full. Why are there no guards on the transitions for the checkOut(aGuest) event?

Answer 19

A guest who is checking out cannot make the hotel full, so no guard is needed for the checkOut(aGuest) event.

Question 20

Why were guards introduced on the transitions for the checkOut(aGuest) event in Figure 18?

Answer 20

The example in Figure 17 is incomplete. Figure 18 deals with both extremes – when the hotel is full and when it is empty – as well as the possible configurations between full and empty. This results in checkOut(aGuest) occurring in three transitions, two of which (from not full to empty and from not full to not full) require guards to ensure mutual exclusion between them.

Question 21

What is an entry event, and how does it contribute to the maintenance of a state diagram?

Answer 21

An entry event can be used where there are multiple transitions, with the same actions, leading to a particular state in a given diagram. An entry event occurs every time an object enters the state that it annotates. Entry events reduce the risk of introducing errors, because the action sequence is written once (associated with the entry event of the state) rather than many times (on each of the transitions leading to that state).

Question 22

What is the benefit of using an internal event as opposed to a self-transition?

Answer 22

When there are entry and exit events that might interfere with a self-transition, an internal event is useful because the entry and exit events are not triggered.

Question 23

Identify three problems associated with complex state diagrams that might arise when designing classes.

Answer 23

Classes with complex state machines can cause three kinds of problem:
it is harder to write the eventual code for such classes because there are likely to be many conditional tests to identify the actual state
it is harder to test the classes because of the number of choices of pathway through the conditional tests
it is much harder for external code to use a class correctly without some means of ascertaining the actual state of objects belonging to complex classes.

Question 24

Suppose the class Copy included the attributes returnDate, libraryNumber and classification. Which of these attributes are significant for a state machine for the class Copy that has two states called on shelf and on loan? Explain your choice.

Answer 24

State machines help model what might happen when a particular object receives a given message because the behaviour of an object is influenced by the values of its attributes. A state machine tells you about the life history of an object. Therefore you can use the frequency of change of an attribute during an object’s lifetime as a means of choosing which attributes are significant when constructing a state machine.
In the class Copy, for example, the attribute returnDate is likely to change often, as it reflects the transition between the states on shelf and on loan. However attributes such as libraryNumber and classification will only change as a result of some reorganisation within the library (probably set during the creation of instances of the class Copy). The borrowing and returning of each copy by the library’s members do not affect them. Among other things, you might consider an additional attribute that records the actual date when a copy is returned (actualReturnDate, say) if you need to investigate the notion of fines for overdue books.

Question 25

Suppose you implemented your design for a class without using a state diagram. Are there any subsequent activities where a state machine might help you as a developer? Briefly explain your answer.

Answer 25

There are situations where you might need to develop state machines retrospectively. For example, you might benefit by preparing a state machine when testing individual classes to demonstrate that the behaviour of an object over its lifetime satisfies the relevant requirements for its class – usually for the purposes of verification. In addition, a proposed change to a software system might introduce a need to prepare one or more state machine diagrams to show how an object’s state would be affected.

Question 26

What characteristics of events suggest that an object should respond to them when they occur?

Answer 26

An object should normally respond to:

1. events that are external, such as those from a point-of- sale terminal
2. change or time events that require some response
3. certain changes, such as a high or low temperature in a process-control system, to avoid problems or even disasters.

Question 27

UML lets us describe five things about a transition. What are they?

Answer 27

The five parts of a transition are:
1. source state
2. target state
3. event trigger
4. action (or action sequence)
5. guard.
In practice, you may not need all five parts in every case.

Question 28

Do packages contain only classes?

Answer 28

No – they can contain any model element from UML. Among other things, you can include classes, use cases, associations and even other packages.

Question 29

Which modelling principle would lead you to form packages?

Answer 29

You should use a package to raise the abstraction level, suppressing any detail in a particular diagram that is not necessary to understand what the model is showing.

Question 30

Explain how it is possible to have two separate elements that share the same name..

Answer 30

If you place the two elements in separate packages, they can share the same name. This is because each model element is identified by its name and the name of the smallest package that contains it.

Question 31

The stereotypes «import» and «access» allow you to ‘see’ through the ‘wall’ around a package. Which one can give rise to a naming problem? Which one provides the simpler traceability?

Answer 31

The «import» stereotype can give rise to a naming problem, because an «import» adds the names of the elements in the target package to the source package. The result is that, if an imported name clashes with an existing name, you have to qualify the imported elements with their package name in order to distinguish them. The «access» stereotype allows simpler traceability, because other packages must use the full name of an accessed element, such as PackageName::ElementName.

Question 32

Suppose you use «access» to ‘see’ into a package. Can you see everything inside that package? Briefly explain your answer

Answer 32

There is no guarantee that you can ‘see’ all the elements of the package. It is possible to control the visibility of elements within a package in the same way that you can control the visibility of the attributes of a class. An element of the accessed package can be marked as private, making it invisible to other packages that import this package and indicated with a ‘–’ character. Alternatively, it can be marked as protected, making it invisible to importers of the package but visible to child packages, and indicated with a ‘#’ character.

Question 33

Which parties are involved in satisfying the postcondition of a system operation? How can you identify them?

Answer 33

The parties are:

1. the actor that initiates the corresponding use case
2. the system object, which orchestrates the collaboration
3. a set of objects within the system that are required to satisfy the postcondition; these can be identified by looking at the operation specification.