Unit 6 - Dynamic modelling – from analysis to design

Question 1

In the analysis phase of system development what are the assertions (preconditions, postconditions and invariants) used for?

Answer 1

Assertions were used for placing constraints on the relationships between classes.

SAQ 1

Question 2

Explain why pre- and postconditions express a contract between a client object and a supplier object.

Answer 2

The contract is expressed by:

• the precondition requiring something from the client object that is of benefit to the supplier object
• the postcondition requiring something from the supplier object that is of benefit to the client object.

SAQ 1

Question 3

What is meant by the term design by contract (DbC)?

Answer 3

DbC is the process of developing software based on the notion of a contract between objects.

SAQ 1

Question 4

To what extent can DbC help with traceability, and hence be used to improve the quality of a software system?

Answer 4

DbC allows the development of a software system to be traced from requirements through to code.

SAQ 2

Question 5

In DbC, what is the relationship between the client’s and supplier’s obligations and benefits?

Answer 5

A client’s obligations to constrain inputs provide benefits to a supplier in that fewer input cases need to be considered. A supplier’s obligations to produce outputs satisfying certain constraints mean that a client can expect to receive a clearly defined service.

SAQ 3

Question 6

What does weakening a precondition mean in terms of the provision of a service?

Answer 6

Weakening a precondition means generalising the situation in which a service can be provided. In general, this means that it is easier for the client to satisfy the precondition because there are ‘fewer’ conditions to be satisfied.

SAQ 4

Question 7

Similarly, what does strengthening a postcondition mean?

Answer 7

Strengthening a postcondition means making the service that is requested ‘better’ in terms of time, precision or some other measurable item. The precise notion of ‘better’ is not fixed, but must be considered in terms of the contract of which the postcondition is part. It can, however, make the postcondition more difficult for the supplier to satisfy because there are ‘more’ conditions to be satisfied.

SAQ 4

Question 8

Under what circumstances is one object, obj1 of class A, say, substitutable for another, obj2 of class B, say?

Answer 8

Class A must be a subclass of class B, and class A must respect all contracts agreed to by class B.

SAQ 5

Question 9

Once a class model that meets the contractual requirements of each use case has been developed, what three sets of items in this class model should you examine to help you find suitable postconditions when identifying the possible operations for a class?

Answer 9

You should investigate the following three sets of items when searching for the possible postconditions for an operation:

• instances of a class (its objects) that have been created or deleted
• instances of associations (links) that have been formed or broken
• attributes that have been modified.

SAQ 5

Question 10

How does the use of a pair of object diagrams help you prepare to build a sequence diagram?

Answer 10

The aim is to show how a given postcondition can be achieved in a sequence diagram. A pair of object diagrams, showing the states before and after the operation in question, identifies the changes in system state that take place in order to meet the postcondition.

SAQ 6

Question 11

Is the initial message on an interaction diagram always sent from an object representing the user interface?

Answer 11

No – we are not constrained to showing interactions with the user interface. Message sequences can originate from any object. The user interface is the origin for those messages that relate to a use case scenario, which we have described in the case of checking guests in to a hotel. However, interaction diagrams can become very complex if we try to show all the possible messages for a given configuration of objects. Following the principle of modularisation, we would split up a complex interaction into a number of smaller ones. In the new diagrams the starting point need not be the user interface.

SAQ 6

Question 12

What does GRASP stand for?

Answer 12

General responsibility assignment software patterns.

p. 96

Question 13

Describe the intent of the GRASP Expert pattern and how it works

Answer 13

Intent: To assign responsibility based on object properties.

How it works: The responsibility is assigned to the class that has the information necessary to fulfil that responsibility – the ‘information expert’; this information is represented by the properties of the object of the class.

p. 96

Question 14

Describe the intent of the GRASP Creator pattern and how it works

Answer 14

Intent. To assign responsibility for creating objects.

How it works. The responsibility for creating an instance of some class is assigned to the class that aggregates, maintains or records, or contains instances of the class of the newly created object, especially if the creator class provides the data required to initialise the newly created object.

Question 15

In an interaction diagram, which class must provide the operation indicated by a message passed from one object to another?

Answer 15

The class of the receiver object must provide the appropriate operation.

SAQ 7

Question 16

What does a lifeline represent?

Answer 16

A lifeline represents the portion of the life of an object covered by the sequence diagram.

SAQ 7

Question 17

What does the box at the top of a lifeline include?

Answer 17

It includes an instance name, optionally followed by a colon and a class name, or, for a generic object, just a colon and a class name. For example, objectname, objectName : ClassName and : Classname are all allowed.

SAQ 7

Question 18

What sort of arrowhead is used on an arrow depicting synchronous
message sending?

Answer 18

A solid black arrowhead, which indicates that the interaction is procedural. An arrowhead with a dashed shaft is used to signify the method’s return, although it is often omitted for clarity.

SAQ 7

Question 19

What is a procedural interaction? With what might it be contrasted?

Answer 19

It is an interaction in which the sender of a message is blocked until the receiver of the message has finished processing. This is exactly the same as what is sometimes called subroutine semantics. It is the usual policy when a single thread of control is allowed. If multiple threads are allowed, we might not want the sender of the message to block, in which case we could start a concurrent activity.

SAQ 7

Question 20

In a sequence diagram, what does the widening of a lifeline into a tall, thin rectangle mean?

Answer 20

This shows that the object is active. An object is said to be active if it is either performing an operation or awaiting completion of an operation that it has requested another object to perform.

SAQ 7

Question 21

What does to delegate mean in the context of object-oriented design?

Answer 21

One object is said to delegate behaviour to another when, instead of implementing some behaviour, it sends a message to another object that implements that behaviour. We considered the possibility that instead of the Hotel doing all the work of finding a free room and housing jill in it, the Hotel might delegate the work to a Room by passing it the message attemptToAccommodate(jill).

SAQ 8

Question 22

Would you describe a sequence diagram as a programming notation?

Answer 22

Not really, because all it shows is the inter-object message traffic. It does not represent the algorithms needed by senders in order to decide when and to which objects messages should be sent, or those needed by receivers in order to act on the messages. When you have finished drawing sequence diagrams you know the interfaces of the various classes but not how those interfaces will be implemented.

SAQ 8

Question 23

How is the sequencing of messages represented in a communication diagram?

Answer 23

Every message has a multi-stage number. The numbers specify the sequencing, replacing vertical position in a sequence diagram.

SAQ 9

Question 24

Sequence diagrams and communication diagrams show almost equivalent information. What are their respective strengths and weaknesses?

Answer 24

Sequence diagrams make the relative order of messages extremely easy to see by presenting time vertically. Communication diagrams are an extension of object diagrams, so there is less new notation. They make it easy to see the links and to show role names, at the cost of making the relative ordering of messages less immediately clear.

SAQ 9

Question 25

What is the difference in emphasis between sequence diagrams and communication diagrams?

Answer 25

Sequence diagrams emphasise the flow of messages from object to object over time. Communication diagrams emphasise the message traffic across the links in a particular configuration of objects.

SAQ 10

Question 26

How is time represented in a communication diagram?

Answer 26

Time is represented by the sequential numbering of messages.

SAQ 10

Question 27

What obligation is placed on an object that is sent a message?

Answer 27

The class of the receiving object is committed to implement an
operation with a particular name and parameter signature.

SAQ 10

Question 28

How does an assignment statement help construct a prototypical sequence diagram?

Answer 28

In a programming language, an assignment statement allows you to store the result of one message in a variable and then send messages to whatever object is currently referenced by that variable. You can use the same mechanism in UML, although the name used to store the message result is not the same as a programming variable. The name can then be used as a parameter to other messages. In a bank, for example, you might want to credit an account with an amount of money. You can use a name such as ac to store a reference to an Account object (at a particular branch) and then send a message, such as credit(amount), to that object.

SAQ 11

Question 29

How does a sequence diagram drawn for a use case scenario differ from one drawn to show how an internal operation is carried out?

Answer 29

A sequence diagram of a use case scenario will always have the very first message originating from an object which has been stimulated, directly or indirectly, by an actor (usually a user interface object). An internal operation will be invoked by a message that has been identified in the use case scenario. You can show how the recipient of that message achieves the required behaviour, for example how a free room is found. In all other respects they are the same.

SAQ 13

Question 30

Would you expect to use sequence diagrams in a conceptual model?

Answer 30

No. Sequence diagrams are about message passing between software objects and conceptual models are about things in the world, where the language of message passing makes no sense.

SAQ 13

Question 31

What are the advantages of recognising when an association is unidirectional?

Answer 31

Specifying an association in just one direction simplifies the implementation of the classes at each end and avoids the need to worry that both ends of the link are consistent when dealing with instances of those classes. But be aware that the overall flexibility is reduced in comparison with bidirectional associations.

SAQ 14

Question 32

Suppose that, in a Java implementation, a Company class represents the employment association with the Person class as a Vector of Person objects. What would be the disadvantage of providing a method Company::getAllEmployees that returned the Vector?

Answer 32

If Company::getAllEmployees is defined as returning an instance of the class Vector, changing the internal representation creates either a maintenance problem or a data-type conversion problem. Suppose the internal representation is changed to be an array. You then have to decide whether getAllEmployees should be changed to return the array (affecting all existing clients of the class) or a new Vector should be constructed from the array.
In addition, whichever Java class is used, encapsulation could be breached because some other object could modify the Vector object without using the methods supplied by the class Company, creating false links (the links would be in the copy of the Vector but not known by the Company). If clients of Company need to iterate over all employees, an interface should be provided that allows iteration without revealing internal implementation decisions, and that has defined semantics for what happens if the links change during iteration.

SAQ 15

Question 33

According to the Law of Demeter, an object should send messages only to a certain set of objects including the object itself. List the other objects.

Answer 33

The Law of Demeter allows an object to send messages to:

• any objects communicated as parameters of the current method
• any new objects that the object has created in the current method
• any objects to which the object has direct links – its neighbours
• itself.

Notice that the law does not allow sending messages to objects that are returned as a result of sending other messages.

SAQ 16

Question 34

What type of control does the Fork Pattern represent?

Answer 34

It represents a centralised form of control. In the fork pattern the central class A sends all the messages, which means that the A must know the interface of all the objects it is sending messages to.

Question 35

What type of control does the Cascade Pattern represent?

Answer 35

It represents control by delegation. In it, class A only needs to know its neighbouring class, which then passes messages to its neighbouring classes.