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Contracts (Ch 13)

• Used to help understand requirements more completely

• based on assertions; assertions are applicable to any element of the UML

• text discusses contracts for system operations; contracts are applicable to execution of any software component

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Domain Model

Use Case Model

•text

•diagram

•SSD

•System operation contracts

Design Model

Figure 13.3 Relationship between Contracts and other artifacts

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Contracts

A contract is a technique for describing system operations in terms of state changes to objects in a Domain Model

Major proponent - Bertrand Meyer

Based on concept of assertion

•a statement, a constraint or declaration, that must be true

•a false value indicates a bug

•may be expressed informally, or in the UML we can (optionally) use the Object Constraint Language (OCL; 1999) to specify constraints rigorously

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Constraints

• Three types

• pre-condition: must be true before a part of the system executes

• post-condition: must be true after a part of the system executes

• invariant: must be true before and after any part of the system is executed.

• Constraints can be

• enclosed in braces and placed in a note on a diagram

• appear as guards on a diagram

• kept in a separate file

• managed by a CASE tool

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Constraints

• e.g. Suppose each instance of a Student class has an ageFirstEntry attribute and the age must always be 16 or more.

– OCL

context Student inv:

ageFirstEntry >=16

informal

precise

Student

ageFirstEntry {ageFirstEntry >=16}

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Consider the execution of a routine:

•The called routine provides a service - it is the supplier

•The caller is the client requesting the service.

•A contract will spells out precisely the obligations of the caller (client) and the callee (supplier).

•The contract serves as an interface specification for the routine.

Example: consider a routine that merges two sorted sequences. The merge routine is the supplier of the service; the calling routine is the caller. A contract will spell out the responsibilities of each party.

Contracts

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Responsibilities:

Client: Ensure that the both sequences to be merged are each already sorted

Supplier: Efficiently merge the two sorted sequences into one sorted sequence

Contracts

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Software contract:

•The responsibilities of the client will be called pre-conditions

•The responsibilities of the supplier will be called post-conditions

Pre-condition: both sequences to be merged are each already sorted

Post-condition: the two sorted sequences are merged into one sorted sequence

Contracts

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•During analysis developers must first understand the problem domain and identify domain objects, their relationships with other domain objects, and their constraints.

•If a contract is defined in terms of domain objects the constraints can be clear and explicit, easily understood

•Everyone understands the business contracting metaphor.

•Business rules (constraints) can become an integral part of the software from the very beginning.

Example: consider a withdraw method for an ACCOUNT class: withdraw (amount_to_withdraw: MONEY)

Contracts

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Example: consider a withdraw method for an ACCOUNT class:

withdraw (amount_to_withdraw: MONEY)

Pre-conditions:

•positive_amount: amount_to_withdraw > 0

•sufficient_balance: balance >= amount_to_withdraw

Post-conditions:

•correct_balance: balance =

initial balance - amount_to_withdraw

Contracts

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Contracts

Contract Components (Larman)

Operation - name and parameters

Cross References - where operation used

Preconditions - assumptions about the state of the system or Domain Model objects

Postconditions - state of objects after the operation completes

•objects:any new ones? any attributes modified?

•associations: any new or modified associations?

Larman’s version is very informal

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: Cashier:System

addLineItem(itemID, quantity)

endSale()

makePayment(amount)

description, total

total with taxes

change due, receipt

* [more items]

makeNewSale()

these input system eventsinvoke system operations

the system eventmakeNewSale invokes asystem operation calledmakeNewSale and so forth

this is the same as in object-oriented programming whenwe say the message fooinvokes the method (handlingoperation) foo

SSD for a samplePOS Use Case Figure 13.1Input Events invoke a system operation of the same name

same idea as in object-oriented programming when we say a message foo invokes the method foo

Referred to as the enterItem system operation

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SalesLineItem

quantity

Product Specification

itemID

Sale

1

1..*

*1Described by

Contained in

The part of the Domain Model relevant to enterItem( )

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Contracts

Example

Operation: enterItem (itemID : ItemID, quantity : integer)

Preconditions

•there is a Sale underway

Postconditions

•a SalesLineItem instance was created

•an association between the sale and the sales line item was created

•an attribute, quantity, was modified

•an association between the product specification and the sales line item was created

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Design By Contract (DBC) provides the basis for documenting software interfaces

Reusers of a software component need to understand what the software provides them and what they must do to obtain these benefits. This is the contract.

DBC is perhaps a best non-practice.

Example of a new DBC tool:

iContract: Design by Contract in Java

allows one to include runtime assertions for development purposes and suppress them for production

http://www.javaworld.com/javaworld/jw-02-2001/jw-0216-cooltools.html