Block 2 Unit 7 Flashcards
(33 cards)
Why must the conditions on a message send be mutually exclusive in a sequential system?
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 –
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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.
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leading to the possibility of non-deterministic behaviour.
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?
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();
Suggest a reason why an object’s lifeline must branch in cases such as Figure 2. How is this handled in Figure 5?
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.
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?
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.
What is the main disadvantage of defensive programming?
The main disadvantage of defensive programming is that the same constraints within each precondition may be checked repeatedly in different operations.
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?
We are following the strategy of using one central class here.
Are use case objects consistent with the twin aims of high cohesion and low coupling (terms introduced in Block 1 Unit 1)?
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.
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?
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().
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)?
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.
Give an example of an architectural decision that would provide a general solution to the problem of unexpected messages.
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.
What does a final state signify in a state machine?
Final states are used to show the point or points where the object in question has finished processing. Its activity has been completed.
In what ways does a final state differ from an initial state?
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.
Why might two objects of the same class respond differently to the same message?
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.
What is the most common form of event that causes a transition between two states? How is it shown in a state machine diagram?
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.
What is the difference between an event and an action?
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.
What is the main constraint on the kinds of action that may be shown in a state diagram?
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.
What is an action sequence?
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.
What is a guard and how does it protect a transition?
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.
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?
A guest who is checking out cannot make the hotel full, so no guard is needed for the checkOut(aGuest) event.
Why were guards introduced on the transitions for the checkOut(aGuest) event in Figure 18?
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.
What is an entry event, and how does it contribute to the maintenance of a state diagram?
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).
What is the benefit of using an internal event as opposed to a self-transition?
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.
Identify three problems associated with complex state diagrams that might arise when designing classes.
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.
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.
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.
