Object-Oriented Analysis and Design - PowerPoint Presentation

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Object-Oriented Analysis and Design

Chapter 11, 32: operation contracts & System Sequence diagrams

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What will we learn?

Operation Contracts – how to define the system operations, and how to create contracts for them

Define SSDs and Operation Contracts for the example case studies

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Operation Contracts

Often, the use cases and SSDs are enough to describe the systemSometimes, more detail of the system behavior is required, and that is where Operation Contracts are used

The Use-Case Model and Domain Model are the main OOA artifacts, but Operation Contracts and State Models are also helpful

Remember the SSDs are part of the Use-Case Model artifact

Actually, the Operation Contracts are considered part of the Use-Case Model, since they usually expand upon the SSD

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Operation Contracts

Operation Contracts describe pre- and post-condition changes to the objects in the

Domain Model

as a result of a system operation. They are text based documentation.

Before looking at them in detail, let’s consider an example (terms like (

instance creation

) are added for explanation in this example, and would not be part of the actual contract)

:Contract CO2:

enterItem

Operation:

enterItem

(

itemID: itemID, quantity: integer)Cross Reference: Use Cases: Process SalesPreconditions: There is a sale underwayPostconditions: -A SalesLineItem instance sli was created (instance creation) -sli was associated ith the current Sale (association formed) -sli.quantity became quantity (attribute modification) -sli was associated with a ProductDescription, based on itemID match (association formed)

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Operations Contracts - Sections

The sections of an Operations Contract are:Operation: Name of the operation, and parameters

Cross References:

Use cases this operation can occur within

Preconditions:

Noteworthy assumptions about the state of the system or objects in

the Domain Model before execution of the operation. These are

non-trivial assumptions the reader should be aware of.

Postconditions

:

The most important section. The state of objects in the Domain

Model after the completion of the operation.

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System Operations

System operations can be thought of as operations the system (as a black-box) offers to its usersThey are identified when the SSDs are created

The SSDs show

system events

which usually trigger system operations

Some of the system events are

input events

(see next slide)

The entire set of system operations, across all use cases, defines a

public interface

which views the system as one single component or class.

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Operation Contracts: Postconditions

The

postcondition

describes changes in the state of objects in the

Domain Model

. Such changes include instance creation, associations formed or broken, and attributes changed.

Note that

postconditions are

not

actions to be performed during the operation!

Postconditions

describe

the changes to the object, they do not explain how the changes are doneBreaking an association is rare, but can happen (cancel sale, for example)Instance deletion is also rare; remember, we are still in the Domain Model, not the actual software classes, so freeing up memory is not an issue9Slide10

Operation Contracts: Postconditions

The instances and associations that are created by an Operation Contract appear in the Domain Model – the contract should capture the behavior expressed in the Use Case Model as it relates to the objects and associations in the Domain Model

Note that

postconditions

(and in fact Operation Contracts) are optional – not every operation needs a contract, and often the

postconditions

are obvious and do not need to be documented

This is another tool to help understand the system behavior

Again, this is an

analysis

tool – it will help define

what

changes, not

how it changesUsually postconditions are written in the past tense – A SalesLineItem was created, not is created10Slide11

Example: enterItem

Postconditions

Look at the types of changes identified in the

enterItem

postcondition

First, recall the Main Scenario of the Process Sale use case (

pg 69 of the book):

Main Success Scenario:

…

3. Cashier enters item identifier

4. System records sale line item and presents item description, price, and running total. Price calculated from a set of price rules.

…

Also, recall the Domain Model and SSD for this example (next slides):11Slide12

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Operation Contracts

Contract CO2: enterItem

Operation:

enterItem

(

itemID

: itemID, quantity: integer)

Cross Reference:

Use Cases: Process Sales

Preconditions:

There is a sale underway

Postconditions

: -A SalesLineItem instance sli was created (instance creation) -sli was associated ith the current Sale (association formed) -sli.quantity became quantity (attribute modification) -sli was associated with a ProductDescription, based on itemID match (association formed)15Slide16

enterItem

Postcondition: Putting it Together

Instance Creation and Deletion:

After

itemID

and

quantity have been entered by the Cashier, the

SalesLineItem

object is created

This can be determined by the use case and Domain Model (composition of

Sales

)

Hence the postcondition: “A SalesLineItem instance sli was created”Attribute Modification:From the Domain Model, we see that the quantity attribute of the SalesLineItem is derived, or calculated. This value is entered by the Cashier (not mentioned in the use case, but shown in the SSD).This gives the postcondition: “sli.quantity became quantity”16Slide17

enterItem

Postcondition: Putting it Together

Associations Formed or Broken:

After

itemID

and

quantity are entered by the Cashier, and after we create the new

SalesLineItem

object, we must form an association with the

ProductDescription

object, as noted in the Domain Model. The new

SalesLineItem

is also associated with the Sale object. This gives the following postconditions:“sli was associated with the current Sale.”“sli was associated with a ProductDescription, based upon itemID match”Note that the original Domain Model did not show the SalesLineItem to ProductDescription association as being qualified by itemID, but this does not mean we can’t include text in the postcondition that describes how sli is associated with ProductDescription.17Slide18

Review

How to analyze a system:Use cases: Identify the main actors, and create the stories of how they interact. Add in details for each scenario

Domain Model: Identify the main conceptual classes (look for

nouns

in the use cases) and how they are associated with each other (look for

verbs

in the use cases)

System Sequence Diagrams: Create diagrams that show the system events that occur between the system (as one big black box) and the users

Operations Contracts – detail any system operations that occur to impart changes to the system objects as a result of the system events – relies on Domain Model, use cases, and SSDs

The use cases (text) and possible use case diagram, SSDs, and Operations Contracts make up the Use-Case Model artifact in UP

The Domain Model is a separate artifact, sometimes included in the Business Modeling artifact

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How to Create Operations Contracts

Identify the system operations from the SSDs

For complex system operations, or operations that may be subtle or not clearly understood, construct a contract

To describe the

postconditions

, use the following categories:

Instance creation or deletion

Attribute modification

Associations formed or broken

Tip: Always use past tense, and if an instance is created, make sure you add an association for the new instance.

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Back to the Process Sale use case SSD:Slide21

NextGen POS Operation Contracts

Contract CO1: makeNewSale

Operation:

makeNewSale

()

Cross References:

Uses cases: Process Sale

Preconditions: none

Postconditions

:

- A Sale instance

s

was created

- s was associated with a Register - Attributes of s were initializedNote that for this case, the contract is obvious and probably would not actually be written. Remember, these are system description tools, and we are not trying to document every change in the system; we just need to explain any operations that may not be obvious.21Slide22

NextGen POS Operation Contracts

Contract CO2: enterItem

Operation:

enterItem

(

itemID

: itemID, quantity: integer)

Cross Reference:

Use Cases: Process Sales

Preconditions:

There is a sale underway

Postconditions

: -A SalesLineItem instance sli was created -sli was associated with the current Sale s -sli.quantity became quantity -sli was associated with a ProductDescription, based on itemID matchWe saw this example earlier; this is more detailed and not as obvious as CO1, so it is reasonable to include this.22Slide23

NextGen POS Operation Contracts

Contract CO3:

endSale

Operation:

endSale

()

Cross Reference: Use Cases: Process SalesPreconditions:

There is a sale underway

Postconditions

:

-

s.isComplete became trueNote that according to the Domain Model, the Sale object does not have an isComplete attribute, but this is a suggested addition that occurred when we created the Operation Contract for endSale. We can then go back and modify the Domain Model to include this new attribute – an example of iterative development. 23Slide24

NextGen POS Operation Contracts

Contract CO4:

makePayment

Operation:

makePayment

(amount: Money)

Cross Reference: Use Cases: Process Sales

Preconditions:

There is a sale underway

Postconditions

:

-

A Payment instance p was created - p.amountTendered became amount - p was associated with the current Sale s - The current Sale s was associated with the StoreNote the last association that was formed creates the association “logs-completed” between Sale and Store in the Domain Diagram.24Slide25

NextGen POS: Expanding the SSDs

We now want to expand the basic Process Sales SSD we created earlier to include an expanded payment option

This would occur, for example, in a further iteration in the Elaboration phase

Recall the first iteration only included cash payments; then we enhanced the Domain Model to include credit and check, and added subclasses to the Domain Model to reflect this

We will enhance the existing SSD to include these new details, and also add Operation Contracts to explain them

Note that in this next iteration, the

makePayment

operation that we previously defined

would

be renamed “

makeCashPayment

”

First, let’s recall the expanded Domain Model for these types of payments

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The basic scenario we have been working with:Slide28

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Adding pay by credit; note that the

Accounts

object (Accounts Receivable in the Domain Model) gets updated.Slide29

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Adding pay by check:Slide30

NextGen POS Operation Contracts

Contract

CO5:

makeCreditPayment

Operation:

makeCreditPayment

(creditAccountNumber, expiryDate

)

Cross Reference:

Use Cases: Process Sales

Preconditions:

An underway sale exists and all items have been entered

Postconditions: - A CreditPayment pmt was created - pmt was associated with the current Sale s - a CreditCard cc was created, cc.number = creditAccountNumber, cc.expiryDate = expiryDate - cc was associated with pmt - a CreditCardPaymentRequest cpr

was created

-

pmt

was associated with

cpr

- a

ReceivableEntry

re

was created

-

re

was associated with the external Accounts Receivable

- The current sale

s

was associated with the Store as a completed sale

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NextGen POS Operation Contracts

Contract CO6:

makeCheckPayment

Operation:

makeCheckPayment

(

driversLicenseNumber)Cross Reference:

Use Cases: Process Sales

Preconditions:

An underway sale exists and all items have been entered

Postconditions

:

- a CheckPayment pmt was created - pmt was associated with the current sale s - a DriverLicense dl was created; dl.number = driversLicenseNumber - dl was associated with pmt - a CheckPaymentRequest cpr was created - pmt was associated with cpr - current sale s was associated with the Store as a completed sale

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Takeaways from Chapter 11, 32

Understand how to create Operations Contracts, based upon details from the use cases, SSDs, and Domain Models.

Know the three type of

postconditions

and how to write them

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