COMP 6471 Software Design Methodologies

1
COMP 6471Software Design Methodologies

• Winter 2006
• Dr Greg Butler
• http//www.cs.concordia.ca/gregb/home/comp647-w20
  06.html

2
Ch 11 Operation Contracts

• Aim Define system operations via contracts
• Operation
• Method
• Invariant
• Precondition
• Postcondition

3
Context within artefacts
4
Context with SSDs
5
UML Definitions

• Event
• A significant or noteworthy occurrence.
• Operation
• An operation is a specification of a
  transformation or query that an object may be
  called to execute. RJB1999
• Signature of an operation specifies the name,
  parameters, and return type (and exceptions
  thrown).
• Pre-conditions and post-conditions are UML
  constraints specified using OCL (Object
  Constraint Language).
• Method
• A method is the implementation of an operation.
  It specifies the algorithm or procedure
  associated with an operation OMG 2003

6
Definitions

• Contract
• A contract specifies detailed changes, as a
  result of a system operation, to objects in the
  domain model using pre-conditions and
  post-conditions.
• Contract Format
• Operation name and parameters of operation.
• Cross References use cases that involve the
  operation.
• Preconditions noteworthy assumptions about the
  state of the system or object in the domain model
  before execution of the operation.
• Postconditions The state of objects in the
  domain model after completion of the operation.
• State
• A state is the condition of an object (or system)
  at a moment in time.

7
Describing the State of a System

• Describe the objects in the system
• Describe the links (relationships) between the
  objects
• Describe the properties of each object (ie the
  state of the object)
• the (abstract) values of the object attributes
• as in a state machine

8
Example State of CUWME

• Objects
• users is the set of all User objects
• roles is the set of all Role objects
• projects is the set of all Project objects
• Plus files, documents,
• Links
• u works_on p for User and Project objects
• p owns d for Project and Document objects
• d is_stored_as f for Document and File objects
• Object States
• Document d has state reviewed
• p.activity3.status completed

9
Invariant of a System or Object

• Invariant
• Is a condition which is always true about the
  state of the system (or object)
• Note the state is only defined in between
  execution of operations
• Hence, invariant only has to be true before and
  after each operation, not during an operation

10
Example Invariants of CUWME(These depend on your
abstractions/assumptions.)

• Each document is owned by exactly one project.
• Each project owns exactly one document.
• Each user has precisely one role in a given
  project.
• A user may only access documents owned by the
  projects that a user is working on.
• A document may be stored in more than one file.
• A file has only a single location.

11
Postcondition

• Definition
• The postconditions describe changes in the state
  of objects in the domain model. Domain model
  state changes include instances created,
  associations formed or broken, and attributes
  changed.
• Note postconditions are not actions to be
  performed during the operation
• They are the effect, ie observations about state
  of domain objects when the operation is finished.
• Ie, what not how

12
Writing Postconditions

• Document
• Instance creation and deletion
• A SalesLineItem sli was created
• Attribute change of value
• sli.quantity became quantity
• Note quantity is an operation parameter
• Association links formed and broken
• sli was linked to the current Sale
• sli was linked with a productDescription based
  on itemID match
• Use past tense.

13
Guidelines

• Identify system operations from the SSDs.
• For system operations that are complex and
  perhaps subtle in their results, or which are not
  clear in the use case, construct a contract.
• To describe the postconditions, document
• Instance creation and deletion
• Attribute modification
• Links formed and broken
• Use past tense for postconditions.
• Remember to document the forming of links!