Object-Oriented Software Development with UML - UNU-IIST ...

Contracts for System Operations 83describe what state changes that the operation commits to achieve.The contract of an operation is defined mainly in terms of its pre-conditions and post-conditions:Pre-conditions are the conditions that the state of the system is assumed to satisfy before the execu-=Post-conditions are the conditions that the system state has to satisfy when the execution operation=tion of the operation.has finished.In formal specification literature, the contract for an operation šŒM] ¤£ can be written as a Hoare-triple¬Pre-ConditionsšŒM¤£ ¬Post-Conditionsroughlymeaning that if the execution of šŒM] ¤£ starts from a state satisfying Pre-Condition, then when theexecution has finished, system will be in a state satisfying Post-Condition.Pre-Conditions and Post-condition are Boolean statements. A post-condition is a statement of what theworld should look like after the execution of an operation. For instance, if we define the operation “get-Square” on a number, the post condition would take the form E;NDASH…ZCr®¯CLbO°A1y±CLbO°A , where result is the outputand CLbO^A is the object on which the operation is invoked.A pre-condition is a statement of how we expect the world to be before the operation is executed. We mightdefine a pre-condition for the “getSquare” operation as CLbO°Â ²³®´ƒ . Such a pre-condition says that it is anerror to invoke “getSquare” on a negative number and the consequences of is undefined.Therefore, the pre-condition shall be checked either by the caller to ensure it is true before the execution,and/or checked the object executing the operation to flag an error when it is not true.As an example, consider the operation NDU_CXNDE:VWCXNDY‰ ¦H©M…Iµ m >›a{§HJRUsCTOTCTeªµVSU_CXǸ hNE£:Its pre-condition can be stated as UPC is known to the system.=its post-conditions include:=– If a new sale, a K+RZ[N was created (instance creation).– If a new sale, the new K+RZ[N was associated with the >—šxK œ (association formed).– A K+RZ[NWA:\ OLU+N9VWCXNY was created (instance creation).– The K+R§Z†NDA;\ OLU+NSVWCXNDY·„°§H•R§UsCTOLCTe was set to §HJRUsCTOTCTe (attribute modification).– The K+R§Z†NDA;\ OLU+NSVWCXNDY was associated with a >¥E¦?G§H•ICLK§M•N9IOPQI9R§CTÕ ?U , based on m >›amatch (associationformed).It is recommended to express post-conditions in the past tense, in order to emphasise they are declarationsabout a past state change, rather than about actions. For example, we had better to writeReport No. 259, July 2002UNU/IIST, P.O. Box 3058, Macau