Module 3

Operation Contract

• Operation contract identifies system state changes when an operation happens.
• Define what each system operation does
• An operation is a single event from the system sequence diagram
• A domain model is used to help generate an operation contract
• When making an operation contract, think of the state of the system before the action and the state of the system after the action.
• The conditions both before and after the action should be described in the operation contract.
• The pre and post conditions describe state, not actions.

Sections of an Operation Contract

• Operation: Name of the operation and parameters
• Cross references: Use cases and scenarios this operation can occur within
• Preconditions: Noteworthy assumptions about the state of the system or objects in the domain model before execution of the operation
• Postconditions: This is the most important section. The state of objects in the domain model after completion of the operation

Example of operation contracts

Main Success Scenario

1. Customer arrives at POS checkout with goods and/or services to purchase.
2. Cashier starts a new sale.
3. Cashier enters item identifier.
4. System records sale line item and presents item description, price, and running total.
   1. Price calculated from a set of price rules.
   2. Cashier repeats steps 3-4 until indicates done.
5. System presents total with taxes calculated.
6. Cashier tells Customer the total, and asks for payment.
7. Customer pays and System handles payment.

POS Domain Model

Operation Contract for makeNewSale operation

• Operation: makeNewSale()
• Cross References:
  • Use case: Process sale
  • Scenario: Process sale
• Preconditions: none
• Postconditions:
  • a sale instance "s" was created (instance creation)
  • s was associated with the register (association formed)
  • attributes of s were initialized
    

Operation Contract for enterItem operation

• Operation: enterItem(itemID,quantity)
• Cross References:
  • Use Case: Process Sale
  • Scenario: Process Sale
• Preconditions: 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 ProductDescription, based on itemId match (association formed)
    

Operation Contract for endSale operation

• Operation: endSale()
• Cross references:
  • Use case: Process sale
  • Scenario: Process sale
• Preconditions: There is a sale underway
• Postconditions:
  • s.isComplete became true (attribute modification)
    

Operation Contract for makePayment operation

• Operation: makePayment(amount:Money)
• Cross References:
  • Use Case: Process Sale
  • Scenario: Process Sale
• Preconditions: There is a sale underway
• Postconditions
  • a payment instance “p” was created (instance creation)
  • p.amountTendered became amount (attribute modification)
  • p was associated with s:Sale (association former)
  • s:Sale was associated with the Store (association formed)
    

---

Tags: !OOADIndex