Unit 6

Question 1

In the analysis and design phases of system development that you studied in Units 3 and 5, what were the assertions (preconditions, postconditions and invariants) used for?

Answer 1

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

Question 2

What is meant by the term Design by Contract (DbC)?

Answer 2

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

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, which is of benefit to the supplier object;
• the postcondition requiring something from the supplier object, which is of benefit to the client object.

Question 4

What is the important feature of DbC, which shows that it can 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.

Question 5

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 not so many 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 6

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

Answer 6

To weaken 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 7

Similarly, what does strengthening a postcondition mean?

Answer 7

To strengthen 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 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.

Question 9

What three sets of items 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.

Question 10

Figure 1 shows a class model for the lending of books where there is a requirement to record both past and current loans. Describe the contract to borrow a book in terms of objects and links created. Your answer should differentiate between the pre-and the postconditions.

Answer 10

The contract between the library member and the library to borrow a book is constrained as follows.

Precondition:

there must be an instance of the class LibraryMember that corresponds to the real-world member;
there must be an instance of the class Book that corresponds to the real-world book that the member wants to borrow;
the instance of the class LibraryMember must be linked to fewer than 3 instances of the class Loan in the role of currentLoans.

Postcondition:

a new instance of the class Loan will have been created;

the instance of the class LibraryMember will have been linked to the new instance of Loan in the role of currentLoans;
the instance of the class Book will have been linked to the same new instance of the class Loan.

Question 11

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

Answer 11

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 12

How is time represented in a communication diagram?

Answer 12

Time is represented by the sequential numbering of messages (see Section 4).

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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 15

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

Answer 15

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.

Question 16

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

Answer 16

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

Question 17

What does a lifeline represent?

Answer 17

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

Question 18

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

Answer 18

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 19

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

Answer 19

A solid black arrowhead, which indicates that the interaction is procedural. Note that an open or stick arrowhead with a dashed shaft is used to signify the method’s return, although it is often omitted for clarity. (See Unit 7 for a list of permitted arrow styles.)

Question 20

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

Answer 20

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 21

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

Answer 21

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 22

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

Answer 22

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).

Question 23

Would you describe a sequence diagram as a programming notation?

Answer 23

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 24

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

Answer 24

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

Question 25

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

Answer 25

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 26

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

Answer 26

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 account 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 27

Figure 14 shows UML notation for creating new Guest objects in a communication diagram. Has this cured the problem of apparently sending messages to objects that do not yet exist?

Answer 27

No – it is still a notation based on message sending. As Figure 14 shows, marking the instance {new} has not captured exactly when it comes into existence. We are still dependent on the pseudo-message newGuest() to express creation.

Question 28

How does a sequence diagram drawn for a use case scenario differ from one drawn to show how an internal operation, such as findAFreeRoom in Figure 12, is carried out?

Answer 28

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 (how a free room is found, for example). In all other respects they are the same.

Question 29

Other than the simple realisation of a use case scenario, what are the other uses of interaction diagrams?

Answer 29

An interaction diagram can be drawn at any level of detail for a software system; use cases are just one example. Other uses of interaction diagrams are as follows.

• They show how a class provides an operation. This is sometimes called a subcommunication. An example might be documenting how findAFreeRoom works in the hotel example of Figure 6, as this isjust one operation on a class rather than a particular use case scenario.
• They show how a component can be used. A component might need various operations to be invoked in a particular sequence in order to achieve some goal. An interaction diagram can document a typical interchange of messages.
• They show how a design pattern works. We described the possibility of a Hotel delegating its room allocation to a RoomAllocator class, and mentioned that this is a case of the Strategy pattern in which behaviour is moved out to a separate object. Alternatively, we could describe this pattern in general terms using a sequence diagram. The instances would be general (prototypical) objects with names such as theServer and theDelegate.

Question 30

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

Answer 30

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 31

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

Answer 31

Representing an association injust 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 note that the overall flexibility is reduced in comparison with bidirectional associations.

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.

Question 33

According to the Law of Demeter, an object should only send messages 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.

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

Question 34

In Figure 23, which pattern would be outlawed by the Law of Demeter?

Answer 34

The Fork pattern would be outlawed. Here, it would involve sending a message to the Job to get a Person, and then sending messages to the resultant Person object. The law bans the sending of messages to objects that are returned as a result of sending other messages.
The Cascade implementation involves the Company talking only to Job, and Job talking only to Person. Both of these are direct associations, so the Law of Demeter will allow this. One advantage of using the Cascade pattern is that the Company is independent of how the age of an employee is represented. You could store the age directly within Job, or you could store it in an associated Person object. The Company will be oblivious to such choices.