Unit 6-Section 2

Question 1

contract

Answer 1

object sends a message to another object

Question 2

How contract comes into

Answer 2

• the client (caller) expects the supplier to perform the service correctly.
  and
• the supplier expects to be asked to perform only those services.
  knows how to supply.
• If either of these expectations is not met, the contract has been
  broken.

Question 3

what is a Pre-Condition in use case diagram?

Answer 3

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

Question 4

What is a post-condition in the use case diagram?

Answer 4

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

Question 5

What are the Benefits from identifying roles in terms of clients and suppliers? ?

Answer 5

Enabling designers to specify the responsibilities of objects more
precisely;
−Allowing clearer software to be built, and, in turn,
−Leading to greater confidence in the correctness of the software.

Question 6

What is Design by Contract (DbC) ?

Answer 6

is the process of
developing software based on contract between
objects, where in this process:
* Operations appropriate for each class
* Pre- and post-conditions are specified

Question 7

Assertions?

Answer 7

are statements that are used for placing constraints on the
relationship between classes.
details of the requirements that cannot be represented
graphically.

Question 8

Four main advantages to the DbC approach for adding
assertions?

Answer 8

1. Assertions provide accurate documentation for the implemented
   classes
2. Provided they are executable,
3. Assertions provide a way of controlling inheritance
4. provided the programming language has an exception
   mechanism

Question 9

How can DbC help to improve quality?

Answer 9

DbC complements many of the other tools:
*. Static analysis tools: that are used to analyse code to detect defects before
execution such as a compiler that finds all syntax errors.
*.Dynamic analysis tools: such as testing, this occupies 40% of total project
effort

Question 10

critical system

Answer 10

are software systems
where failure may have catastrophic
consequences such as:
−Loss of life (safety-critical systems),
−Loss of business (business-critical systems) and
−Failure to meet significant objectives (mission
critical systems).

Question 11

How to use develop a critical systems

Answer 11

Formal methods – based on mathematical notations, logic and proof.
−Analysis tools to make the systems as error-free as possible.
−Design by Contract where the most important feature of DbC is allowing the
development of a software system to be traced from requirements through to
code.

Question 12

Contracts in real world

Answer 12

In business, contracts are specifications of the obligations of, and benefits to, the
parties involved in an agreement.

Question 13

To weaken a precondition

Answer 13

means generalizing the situation in which a
service can be provided.

Question 14

To strengthen a post-condition

Answer 14

means making the service that is
requested ‘better’ in terms of time, precision or some other measurable
item.

Question 15

view of contract

Answer 15

The form of contract should be written as below:
A description of the task, activity or operation in question;
2.
3.
Precondition: a set of constraints that are assumed to be
true before you can start
Post-condition: a set of constraints that must be true
afterwards.

Question 16

subcontracts in software

Answer 16

1. Through associations (composition, aggregation);
2. Through generalizations (inheritance).
   This relationship can be expressed in terms of the pre and post
   conditions associated with the equivalent methods in the two
   classes.

Question 17

Constraints on behavior

Answer 17

is the process of translating pre and post
conditions (from use cases) into your design, for Object Oriented
systems.(behaviour)
identifying the possible operations for a class:
1. Instances of a class (its objects) that have been created or deleted;
2. Instances of associations (links) that have been formed or broken;
3. Attributes that have been modified.