Unit 7 Flashcards
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 – leading to the possibility of non-deterministic behaviour.
How can conditional behaviour be shown on a sequence diagram?
By prefixing a message send with a guard that specifies the condition.
How can a repeated message send be shown on a sequence diagram?
UML uses the * notation for a repeated message send, which may be attached to a guard. For example, *[i := 1..10] would be used to one message ten times.
Does UML offer a more structured way of indicating conditional, alternative and iterative behaviour beyond the use of guards and branches?
Yes, there is a general notation known as a fragment or frame that can be used to show optional, alternative or looped behaviour.
Where does the responsibility for checking the precondition lie with defensive programming?
The checking of the precondition is done by the operation itself, that is, the receiver of the message.
Where does the responsibility for checking the precondition lie with design by contract?
The checking of the precondition is done by the caller of the operation, that is, the sender of the message.
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.
Describe the ‘one central class’ strategy for implementing use cases.
This is where the interface will send all messages to a single object. This central object will then forward messages to the appropriate object.
What are the advantages an disadvantages of the ‘one central class’ strategy?
From the interface’s point of view, it has to know the existence of only one object. There is good traceability from use cases to code because every use case links to a method in the same central class.
The main disadvantage arises when the one central class becomes overloaded with use cases. One possible solution to this problem is to divide the software system into several packages. Each package could still make use of one class to respond to messages from the interface. The focus of each package would be on the key concepts within each domain.
Describe the ‘actor classes’ strategy for implementing use cases.
This is where the interface will send a message to an object corresponding to the real-world actor who initiated the operation.
What is a disadvantage of the ‘actor classes’ strategy?
Traceability is rather different from that for one central class. To locate the code for a use case, the class corresponding to the initiating actor must be searched. There is a complication when there are two actors that can initiate an interaction
Describe the ‘use cases as classes’ strategy for implementing use cases.
This is where the interface messages are sent to a object of a class that has been specifically defined for the use case. There might be classes such as CheckerIn, CheckerOut and ReservationMaker. Each would have a method with a name like run, with a suitable number of arguments. The user interface would then create a single instance of the appropriate class, initialise it suitably and then send the run(…) message.
What are the advantages an disadvantages of the ‘use cases as classes’ strategy?
Working with use case objects lets you change or even replace the software to implement a given scenario, minimising the effects upon the core concepts.
Advocates of such use case objects appreciate the traceability they provide from each use case to a class. Each use case class contains information relevant to the use case alone, so each use case can be understood in isolation. If you want to collect management information about the use case, such as frequency of use or timing histories, having an object provides a natural place to put such functionality.
A possible disadvantage of the approach is the large number of extra classes that must be defined – one for each use case.
Another possible disadvantage is that many of the use case classes can be similar, resulting in duplicated code and more difficult maintenance.
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.
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.
