Unit 6

Question 1

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

Answer 1

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

Question 2

In terms of DbC, what is meant by an assertion?

Answer 2

In DbC, an assertion is a precondition, a postcondition or an invariant.

Question 3

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

Answer 3

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.

Question 4

What are the main advantages of the DbC approach for adding assertions?

Answer 4

1. Assertions provide accurate documentation for the implemented classes so that a programmer knows how to use the classes and what to expect from them.
2. Provided that they are executable, assertions are an important aid to testing, but without being an obstacle to efficiency.
3. Assertions provide a way of controlling inheritance in which substitutability and redefinition of methods are allowed.
4. Provided that the programming language has an exception mechanism that accords with the principles of DbC, assertions together with the exception-handling mechanism can be an important aid to developing mission-critical systems.

Question 5

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

Answer 5

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

Question 6

What is the relationship between the client’s and supplier’s obligations and benefits?

Answer 6

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.

Question 7

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

Answer 7

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.

Question 8

What does strengthening a postcondition mean in terms of the provision of a service?

Answer 8

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.

Question 9

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

Answer 9

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

Question 10

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 10

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.

It is crucial of course that the postconditions reflect the requirements, and some practitioners would refer more directly to the requirements.

Question 11

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

Answer 11

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.

Question 12

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

Answer 12

No – we are not constrained to showing interactions with the user interface. Message sequences can originate from any object.

Question 13

What does the acronym, GRASP, stand for?

Answer 13

General responsibility assignment software patterns.

Question 14

How does the GRASP Expert pattern work?

Answer 14

This pattern assigns the responsibility in a system to a class that has the information necessary to fulfill that responsibility. This information is represented by the properties of the object of the class.

Question 15

When would you use the GRASP Expert pattern?

Answer 15

This pattern should be used when you need to decide which of a number of interacting objects a responsibility.

Question 16

What are the benefits of the GRASP Expert pattern?

Answer 16

The pattern maximises encapsulation, as objects use their own information to fulfil a task. Therefore it enhances low coupling and high cohesion within a system.

Question 17

How does the GRASP Creator pattern work?

Answer 17

This pattern assigns the responsibility for creating an instance of some class 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 18

When would you use the GRASP Creator pattern?

Answer 18

Use this pattern whenever you need to assign responsibilities for object creation.

Question 19

What are the benefits of the GRASP Creator pattern?

Answer 19

Low coupling is maintained between the creator class and the class of the created object, because the latter is probably already linked to the creator class owing to the existing associations that motivated its choice as the creator.

Question 20

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

Answer 20

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

Question 21

In a sequence diagram, what does a lifeline represent?

Answer 21

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

Question 22

In a sequence diagram, what does the box at the top of a lifeline include?

Answer 22

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.

Question 23

In a sequence diagram, what sort of arrowhead is used on an arrow depicting synchronous message sending?

Answer 23

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.

Question 24

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

Answer 24

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.

Question 25

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

Answer 25

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.

Question 26

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

Answer 26

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.

Question 27

Would you describe a sequence diagram as a programming notation?

Answer 27

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.

Question 28

What is a class-responsibility-collaboration (CRC) card?

Answer 28

A CRC card has three boxes that can be filled in with the name of the card, its responsibilities (what data it holds and what it can do) and its collaborators (other classes it interacts with directly).

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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.

Question 31

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

Answer 31

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.

Question 32

How is time represented in a communication diagram?

Answer 32

Time is represented by the sequential numbering of messages.

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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.

Question 35

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

Answer 35

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. In all other respects they are the same.

Question 36

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

Answer 36

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.

Question 37

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

Answer 37

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.

Question 38

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 38

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.

Question 39

Would the fork or cascade pattern be outlawed by the Law of Demeter?

Answer 39

The fork pattern would be outlawed as it bans the sending of messages to objects that are returned as a result of sending other messages.