Unit 7 - More on dynamic modelling – states and activities

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 – leading to the possibility of non-deterministic behaviour.

SAQ 1

Question 2

What is the main disadvantage of defensive programming?

Answer 2

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

SAQ 2

Question 3

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 3

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

SAQ 2

Question 4

Are use case objects consistent with the twin aims of high cohesion and low coupling?

Answer 4

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.

SAQ 2

Question 5

What is a state machine and what does it do?

Answer 5

A state machine is a model that shows how an object changes from state to state in response to events.

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Question 6

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

Answer 6

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.

SAQ 3

Question 7

What does a final state signify in a state machine?

Answer 7

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

SAQ 4

Question 8

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

Answer 8

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.

SAQ 4

Question 9

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

Answer 9

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.

SAQ 5

Question 10

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 10

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.

SAQ 5

Question 11

What is the difference between an event and an action?

Answer 11

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.

SAQ 5

Question 12

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

Answer 12

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.

SAQ 5

Question 13

What is an action sequence?

Answer 13

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.

SAQ 5

Question 14

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

Answer 14

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.

SAQ 6

Question 15

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

Answer 15

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).

SAQ 7

Question 16

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

Answer 16

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.

SAQ 7

Question 17

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

Answer 17

• 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.

SAQ 8

Question 18

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?

Answer 18

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.

SAQ 8

Question 19

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

Answer 19

An object should normally respond to:

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

SAQ 9

Question 20

The separation of a software system into layers or tiers has the which advantages?

Answer 20

• developers can be allocated to different layers to facilitate parallel development or the use of particular skills such as interface design
• reuse becomes possible in certain areas, such as business services
• distribution of the resultant software can be considered in response to
  certain performance requirements.
• a well-structured package displays high cohesion
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Question 21

Do packages contain only classes?

Answer 21

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

SAQ 10

Question 22

Which modelling principle would lead you to form packages?

Answer 22

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.

SAQ 10

Question 23

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

Answer 23

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.

SAQ 10

Question 24

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 24

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.

SAQ 10

Question 25

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

Answer 25

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.

SAQ 10

Question 26

How can packages help you?

Answer 26

• Packages help you organise models in a way that lets you better represent and communicate your understanding of a problem.
• Packages can help you construct large software systems by splitting them up into smaller, more manageable chunks.
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