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Use Case Model Operation Contracts. Chapter 11 Applying UML and Patterns Craig Larman. Objectives. Create contracts for each system operation. UP Artifacts. Domain Model. Vision. Use Case Model. Use Case Text. Glossary. Operation Contract. System Sequence Diagrams. - PowerPoint PPT Presentation
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Use Case Model Operation Contracts
Chapter 11Applying UML and Patterns
Craig Larman
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Objectives Create contracts for each system
operation
UP Artifacts
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Unified Process Artifacts
Business Model
Requirements
Design
Domain Model
Use Case Model
Operation Contract
Use Case Text
System Sequence Diagrams
Interaction Diagrams
Supplementary Specifications
Vision
Glossary
See Figure 6.1 in text for more detail
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Why Write Operation Contracts Use cases are the primary mechanism
in the UP to describe system behavior They are usually sufficient.
Sometimes a more detailed description of system behavior has value.
Contracts for operations can help define system behavior.
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Domain Model And Contracts A Domain Model is a visual
representation of conceptual classes or real-world objects in a domain of interest.
Contracts describe detailed system behavior in terms of state changes to objects in the Domain Model, after a system operation has executed.
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Keep it Agile In many, or even most software
development projects, operation contracts may be unnecessary
Use them only when necessary to add additional detail and understanding
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System Operations andthe System Interface Contracts may be defined for system
operations Operations treat the system as a black
box which uses its public interface to handle incoming system events.
System operations can be identified by discovering these system events.
The entire set of system operations, defines the public system interface, views system as single component/class
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Example Contract: enterItem
Operation: enterItem(itemID : ItemID,quantity : integer)
Cross References: Use Cases: Process SalePreconditions:There is a Sale Underway.Postconditions: -A SalesLineItem instance sli
was created (instance creation)-sli was associated with the current Sale (association
formed)-sli.quantity became quantity (attribute modification)-sli was associated with a ProductSpecification, based on itemID match (association
formed)
Contract CO2: enterItem
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Contract Sections
Operation: Name Of operation, and parameters.Cross References: (optional) Use cases this
can occur within.Preconditions:Noteworthy assumptions about the
state of the system or objects in the Domain Model before execution of the operation.
Postconditions: -The state of objects in the Domain Model after completion
of the operation.
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Postconditions Five changes in Domain Model include
1. instance created 2. instance destroyed 3. association formed 4. association broken, 5. attribute value modified.
Postconditions are declarations about the Domain Model objects that are true when the operation has finished
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Naming Postconditions Express postconditions in past tense, as
they describe a previous state change
-A SalesLineItem instance sli was created (instance creation)
-sli was associated with the current Sale (association formed)
-sli.quantity became quantity (attribute modification)
-sli was associated with a ProductSpecification, based on itemID match (association formed)
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Writing Contracts Leads to Domain Model Updates New conceptual classes, attributes, or
associations in the Domain Model are often discovered by writing contracts
Change the Domain Model as you make new discoveries while thinking through the operation contracts
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Contracts vs. Use Cases The use cases are the main repository
of requirements for the project. Provide most or all of the detail
necessary to know what to do in design. If the details and complexity of required
state changes are awkward to capture in use cases, then write operation contracts.
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Too Many Contracts If…
If a team is making contracts for every system operation: the use cases are poorly done, or there is not enough collaboration or access to a
subject matter expert, or the team is doing too much unnecessary
documentation
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Guidelines: Contracts To make contracts:
Identify system operations from the SSDs For system operations that are complex and
perhaps subtle in their own results, or which are not clear in the use case, construct a contract.
To describe the postconditions, use ONLY: instance creation and deletion attribute modification associations formed and broken
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The Most Common Mistake In Creating Contracts The most common problem in creating
contracts is forgetting to include the forming of associations.
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.
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Contracts, Operations, and the UML
The UML formally defines operations. To quote: An operation is a specification of a
transformation or query that an object may be called to execute [RJB99]
An operation is an abstraction, not an implementation. By contrast, a method (in the UML) is an implementation of an operation.
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Contracts, Operations, and the UML
UML operation has signature (name and parameters), and a specification
Specification describes the effects produced by executing the operation (the postconditions)
UML operation specification does not show an algorithm or solution
It only shows state changes or operation effects
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Operation ContractsWithin the UP A pre- and postcondition contract is a
well-known style to specify an operation.
In UML, operations exist at many levels, from top level classes down to fine-grained classes.
Operation specification contracts for the top level classes are part of the Use-Case Model.
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Summary Contracts describe detailed system
behavior in terms of state changes to objects in the Domain 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 Domain Model.
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Summary Only write contracts for the most difficult
to understand or complicated system operations. Dr. Bob’s Rule of Thumb: If an operation
has Postconditions, write a contract Domain Model state changes include
instances created, associations formed or broken, and attributes changed.