Answer
Create Contract

• Identify system operations from the SSDs.
• For system operations that are complex, or which are not clear in the use case, construct a contract.
• To describe the post-conditions, use its categorization

• As mentioned, write in a declarative, passive past tense form (was …)
• Remember to establish an association between existing objects or those newly created.
• The SalesLineItem was associated with the Sale (association formed).

Answer

• Express them in the past tense to emphasize on observations about state changes e.g. "A Sales LineItem was created" Rather than "Create SalesLineItemComplete"
  
• It is not important to write all post-conditions for all system operations.
  
• Write contracts for the most complex and subtle system operations
• Treat their creation as an initial best guess
  
• When new conceptual classes, attributes, or associations are discovered then update the domain model accordingly.
  
• Enhance it as you make new discoveries while thinking through operation contracts
  
• Avoid common mistakes:
• Forgetting to include the forming of associations when new instances are created,
• it is very likely that associations to several objects need be established

Answer
The post condition of an operation will belongs to one of the following category:

• Instance creation and deletion.
  
• Attribute change of value.
  
• Associations (to be precise, UML links) formed and broken.
  

Answer

• Contract ID: enterItem
  
• Operation: enterItem(itemID: ItemID, quantity: integer)
  
• Cross References: Use Cases: Process Sale
  
• Pre-Condition: There is a sale underway.
  
• Post Conditions:
• A SalesLineItem (SLI) instance was created (instance creation).
• SLI was associated with the current Sale (association formed).
• SLI quantity became quantity (attribute modification).
• SLI was associated with a ProductDescription, based on itemID match (association formed).

Answer
Following are the main sections:

1. Contract ID: A Brief Name of the contract
2. Operation: Name of operation, and parameters
3. Cross References:Name of use cases in which this operation can occur
4. Pre-conditions: 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 told.
5. Post-conditions:This is the most important section. This section describes the state changes of objects, their association and construction/destruction.

Answer

• Define system operations.
• Create contracts for system operations.