Transcript Contract

1
Chapter 9:
Operation Contracts
Chapter 13 in Applying UML and Patterns Book.
2
Overview

What is contract ?

The guidelines for writing contracts for the system operations.

Use Case realizations.

Writing contracts leads to domain model updates.
3
Contracts

Contracts are documents that describe system behavior.

Contracts may be defined for system operations.


Operations that the system (as a black box) offers in its public interface to handle
incoming system events.
The entire set of system operations across all use cases, defines the public
system interface.
4
SSD for BuyItem use case
5
System Operations and the System Interface

In the UML the system as a whole can be
represented as a class.

Contracts are written for each system
operation to describe its behavior.
6
Guidelines for Contracts
7
Use Case Realizations

A use case realization describes how a use case is
realized in terms of collaborating objects.

UML interaction diagrams are used to illustrate
use case realizations.

Recall Process Sale: from main scenario we
identified a number of system events
(operations).

Each system event was then described by a
contract.
8
Contracts

A contract is a document that describes what an operation commits to achieve.


Emphasizing what will happen rather than how it will be achieved.
Contracts can be expressed by pre- and post- conditions.
9
Conceptual Model and Contracts

A Conceptual Model is a visual representation of conceptual classes or realworld objects in a domain of interest.

Contracts describe detailed system behavior in term of state changes to objects
in the Domain Model, after a system operation has executed.
10
Utilize Conceptual Model

To make contracts you have to use the conceptual model.

We added the conceptual model from previous slides to assist the
comprehension of post conditions in the contracts.
11
Domain Model
12
Example: AddLineItem Contract
Contract CO2: AddLineItem
Operation:
addLineItem (itemID: ItemID, quantity: integer)
Cross References:
Use Cases: Process Sale.
Pre-conditions:
There is a sale underway.
Post-conditions:

A SalesLineItem instance sli was created. (instance creation)

sli was associated with the current Sale. (association formed)

sli.quantity was set to quantity. (attribute modification)

sli was associated with a ProductSpecification, based on itemID match. (association
formed)
13
Pre- & Post- Conditions

Preconditions are assumptions about the state of the system before execution
of the operation.

A Postcondition is an assumption that refers to the state of the system after
completion of the operation.

The postconditions are not actions to be performed during the operation.

Describe changes in the state of the objects in the Domain Model (instances
created, associations are being formed or broken, and attributes are changed).
14
How to Make a Contract
1. Identify system operations from the system sequence diagrams.
2. For each system operation, construct a contract.
3. Write the preconditions section.
4. Write the postconditions section describing the state changes that occur to
concepts in the conceptual model. Postconditon categories:

Instance creation and deletion.

Attribute modification.

Associations formed and broken.
15
AddLineItem Postconditions
Instance Creation and Deletion

After the itemID and quantity of an item have been entered by the cashier,
what new objects should have been created?

A SaleLineItem instance sli was created.
16
AddLineItem Postconditions
Attribute Modification

After the itemID and quantity of an item have been entered by the cashier,
what attributes of new or existing objects should have been modified?

sli.quantity was set to quantity.
17
AddLineItem Postconditions
Associations Formed and Broken
After the itemID and quantity of an item have been entered by the cashier,
what associations of new or existing objects should have been formed or broken?


sli was associated with the current Sale.

sli was associated with a ProductSpecification, based on itemID match.
18
Make New Sale Enumeration Example

Assume a new sale is started, we will create a new object from the conceptual
class Sale, with an ID=5.

If this sale started on the register number 3, then we have to create an
association between the sale number 5 with the register number 3
(associations are established by updating foreign keys).
19
Writing Contracts leads to Domain Model
Updates

It is also common to discover the need to record new concepts, attributes or
associations in the Domain Model.

For example:

If in one of the postconditions, you wanted to create an instance to store some
values in it, and it was not presented by any Concept in the domain model.

Then you should go back and correct your domain model by adding this
concept.
20
The most common mistake in creating
contract

The most common problem in creating contracts is forgetting to include the
forming of associations. Particularly, when new instances are created, it is very
likely that associations to several objects need be established.

Don’t forget to include all the associations formed and broken.
21
Example 1: End Sale Contract
Contract: EndSale
Operation:
EndSale ()
Cross References:
Use Cases: Process Sale.
Pre-conditions:
There is a sale underway and all items have been entered.
Post-conditions:

Sale.isComplete became true. (attribute modification)
22
Example 2: Make Payment Contract
Contract: MakePayment
Operation:
MakePayment (amount: Number or Quantity)
Cross References:
Use Cases: Process Sale.
Pre-conditions:
There is a sale underway and all items have been entered.
Post-conditions:

A Payment instance p was created. (instance creation)

p.amountTendered was set to amount. (attribute modification)

p was associated with the current Sale. (association formed)

The current Sale was associated with the Store; to add it to the historical log of
completed sales. (association formed)
23
Example 3: Start Up Contract
Contract: StartUp
Operation:
StartUp ()
Cross References:
Use Cases: Process Sale.
Pre-conditions:
None.
Post-conditions:

A Store, POST, ProductCatalog and ProductSpecifications have been created. (instance
creation)

ProductCatalog was associated with ProductSpecifications. (association formed)

Store was associated with ProductCatalog. (association formed)

Store was associated with POST. (association formed)

POST was associated with ProductCatalog. (association formed)
24
Summary

Contracts describe detailed system behavior in terms of state changes to
objects in the Conceptual model after a system operation.

Contracts have sections of Operations, Cross references, Preconditions and
Postconditions. Postconditions are the most important section.

Postconditions describe changes in the state of objects in the Conceptual
model.